Proteopedia - User contributions [en]

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T22:04:09Z

Greg Angelides:

== Introduction and Background Information ==
[[Image:rec.gif|thumb|right|200px|]]
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine, two opiate products, are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects of opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref> There are 17 known opioid receptors with the three primary types being the μ, κ, and δ. The μ opioid receptor (μ-OR) is the most related to opioid use. There are three subtypes μ1, μ2, μ3 and most is known about μ1.<ref>Cadet P, Mantione KJ, Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J. Immunol. 170 (10): 5118–23. PMID 12734358.</ref> The μ-ORs are part of the neurohormonal system. Within the body (sans drugs) endogenous small molecules naturally produced in the CNS and other glads act upon the GPCRs. Endogenous opioid peptides produce can range in action from pain reduction to regulating diarrhea.<ref> Janecka, Anna, Jakub Fichna, and Tomasz Janecki. "Opioid Receptors and Their Ligands." Current Topics in Medicinal Chemistry 4.1 (2004): 1-17.</ref>
<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the receptor in its inactivated form.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Ligand_pocket/1'>bound to β-funaltrexamine</scene> (β-FNA), a close relative of morphine that is bound in the pocket. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Asp147/2'>Asp147</scene> is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/His297/1'>His297</scene> indirectly interacts with the aromatic ring of the ligand. Rather than bind directly it is thought to employ two water molecules that line up and form a hydrogen bond chain to the hydroxyl group on the morphinan ring.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Glu229_lys303_and_trp318/1'>Glu229, Lys303 and Trp318</scene> are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref name="zad">Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Such a high selectivity is a property that is being examined and looked at for the possibility of certain therapies. Better understanding the manner in which an edogenous peptides bind could be clinically useful in addiction studies. Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.<ref name="zad"/>

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Dimer_in_membrane/1'>dimers</scene> and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref> Oligomers have been observed in other GPCRs which gives further support of this possibility.<ref>Fanelli, F. & De Benedetti, P. G. Update 1 of: computational modeling approaches to structure-function analysis of G protein-coupled receptors. Chem. Rev. 111, PR438–PR535 (2011) </ref>
The linking of multiple units is of interest because μ-OR as part of a homo or heterodimer or oligomer could affect morphine tolerance. Its possible that DAMGO and methadone (both agonists) could reduce tolerance to morphine through endocytosis by the μ-OR oligomer. <ref>Finn, Andrew K., and Jennifer L. Whistler. "Endocytosis of the Mu Opioid Receptor Reduces Tolerance and a Cellular Hallmark of Opiate Withdrawal." Neuron 32.5 (2001): 829-39.</ref> Expressing multiple forms of opioid receptors showed profiles that were different from either expressed alone.<ref>George, S. R. et al. Oligomerization of μ- and δ-opioid receptors. Generation of novel functional properties. J. Biol. Chem. 275, 26128–26135 (2000) </ref> More support for oligomerization as a direct function of the μ opioid receptors is the amino acid dimer interface had an extremely high degree of homology with the δ-OR.
[[Image:Fig7.jpg|thumb|left|300px|]]

== Other Aspects and Addiction==
μ-OR has been shown to be a great target for controlling pain. However such benefits do not come without side effects and drawbacks. The μ-OR is currently at the forefront of research in many addiction diseases as well as therapies.<ref name="can">Candice Contet, Brigitte L Kieffer, Katia Befort, Mu opioid receptor: a gateway to drug addiction, Current Opinion in Neurobiology, Volume 14, Issue 3, June 2004, Pages 370-378, ISSN 0959-4388, 10.1016/j.conb.2004.05.005.
(http://www.sciencedirect.com/science/article/pii/S0959438804000728)</ref> One study found that, addiction from non opioid drugs such as alcohol, cannabinoids, and nicotine are heavily dependent on the μ-OR (all of these act at a different receptor). In μ-OR deficient mutant mice addictions typically seen from these compounds were strongly diminished. This highlighted the possibility of the μ-OR as a convergent molecular switch that can act through direct or indirect activation.<ref name="can"/> Furthermore, it is possible that slight mutations in the µ-OR could be linked to addiction. In one study it was found that in 10% of the former heroine addict population they studied there was a variant receptor that contained a A118G SNP. This SNP causes an Asn40 to Asp40 amino acid substitution. Although this variant receptor has not been shown to have altered affinities to most opioid peptides, it did have a binding affinity approximately three times that of the normal receptor for β-endorphin, an endogenous opioid. β-endorphin is also three times more potent when bound to the receptor and will be important in the future for understanding addictive diseases.<ref>
Cherie Bond, K. Steven LaForge, Mingting Tian, Dorothy Melia, Shengwen Zhang, Lisa Borg, Jianhua Gong, James Schluger, Judith A. Strong, Suzanne M. Leal, Jay A. Tischfield, Mary Jeanne Kreek, and Lei Yu
Single-nucleotide polymorphism in the human mu opioid receptor gene alters β-endorphin binding and activity: Possible implications for opiate addiction PNAS 1998 95 (16) 9608-9613</ref>

== Conclusion ==
The μ-OR crystal structure elucidated in "Crystal structure of the µ-opioid receptor bound to a morphinan antagonist" is the first high resolution receptor of its kind. Although opiates have been used for centuries current opioid receptor agonist drugs are far from perfect. The new understanding of the structure will enable drug discovery programs to take a structure based approach to finding new drugs and possibly treating some of the underlying causes of addiction. Such a drug could prove helpful to the medical word as well as extremely lucrative; especially since the combined legal and illegal opiate market brought in revenues around $70 billion in 2009.<ref name="base"/>

== References ==
<references/>
</StructureSection>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T22:03:13Z

Greg Angelides:

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T20:19:27Z

Greg Angelides:

== Introduction and Background Information ==
[[Image:rec.gif|thumb|right|200px|]]
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine, two opiate products, are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects of opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref> There are 17 known opioid receptors with the three primary types being the μ, κ, and δ. The μ opioid receptor (μ-OR) is the most related to opioid use. There are three subtypes μ1, μ2, μ3 and most is known about μ1.<ref>Cadet P, Mantione KJ, Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J. Immunol. 170 (10): 5118–23. PMID 12734358.</ref> The μ-ORs are part of the neurohormonal system. Within the body (sans drugs) endogenous small molecules naturally produced in the CNS and other glads act upon the GPCRs. Endogenous opioid peptides produce can range in action from pain reduction to regulating diarrhea.<ref> Janecka, Anna, Jakub Fichna, and Tomasz Janecki. "Opioid Receptors and Their Ligands." Current Topics in Medicinal Chemistry 4.1 (2004): 1-17.</ref>
<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the receptor in its inactivated form.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Ligand_pocket/1'>bound to β-funaltrexamine</scene> (β-FNA), a close relative of morphine that is bound in the pocket. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Asp147/2'>Asp147</scene> is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/His297/1'>His297</scene> indirectly interacts with the aromatic ring of the ligand. Rather than bind directly it is thought to employ two water molecules that line up and form a hydrogen bond chain to the hydroxyl group on the morphinan ring.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Glu229_lys303_and_trp318/1'>Glu229, Lys303 and Trp318</scene> are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref name="zad">Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Such a high selectivity is a property that is being examined and looked at for the possibility of certain therapies. Better understanding the manner in which an edogenous peptides bind could be clinically useful in addiction studies. Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.<ref name="zad"/>

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref> Oligomers have been observed in other GPCRs which gives further support of this possibility.<ref>Fanelli, F. & De Benedetti, P. G. Update 1 of: computational modeling approaches to structure-function analysis of G protein-coupled receptors. Chem. Rev. 111, PR438–PR535 (2011) </ref>
The linking of multiple units is of interest because μ-OR as part of a homo or heterodimer or oligomer could affect morphine tolerance. Its possible that DAMGO and methadone (both agonists) could reduce tolerance to morphine through endocytosis by the μ-OR oligomer. <ref>Finn, Andrew K., and Jennifer L. Whistler. "Endocytosis of the Mu Opioid Receptor Reduces Tolerance and a Cellular Hallmark of Opiate Withdrawal." Neuron 32.5 (2001): 829-39.</ref> Expressing multiple forms of opioid receptors showed profiles that were different from either expressed alone.<ref>George, S. R. et al. Oligomerization of μ- and δ-opioid receptors. Generation of novel functional properties. J. Biol. Chem. 275, 26128–26135 (2000) </ref> More support for oligomerization as a direct function of the μ opioid receptors is the amino acid dimer interface had an extremely high degree of homology with the δ-OR.
[[Image:Fig7.jpg|thumb|left|300px|]]

== Other Aspects and Addiction==
μ-OR has been shown to be a great target for controlling pain. However such benefits do not come without side effects and drawbacks. The μ-OR is currently at the forefront of research in many addiction diseases as well as therapies.<ref name="can">Candice Contet, Brigitte L Kieffer, Katia Befort, Mu opioid receptor: a gateway to drug addiction, Current Opinion in Neurobiology, Volume 14, Issue 3, June 2004, Pages 370-378, ISSN 0959-4388, 10.1016/j.conb.2004.05.005.
(http://www.sciencedirect.com/science/article/pii/S0959438804000728)</ref> One study found that, addiction from non opioid drugs such as alcohol, cannabinoids, and nicotine are heavily dependent on the μ-OR (all of these act at a different receptor). In μ-OR deficient mutant mice addictions typically seen from these compounds were strongly diminished. This highlighted the possibility of the μ-OR as a convergent molecular switch that can act through direct or indirect activation.<ref name="can"/> Furthermore, it is possible that slight mutations in the µ-OR could be linked to addiction. In one study it was found that in 10% of the former heroine addict population they studied there was a variant receptor that contained a A118G SNP. This SNP causes an Asn40 to Asp40 amino acid substitution. Although this variant receptor has not been shown to have altered affinities to most opioid peptides, it did have a binding affinity approximately three times that of the normal receptor for β-endorphin, an endogenous opioid. β-endorphin is also three times more potent when bound to the receptor and will be important in the future for understanding addictive diseases.<ref>
Cherie Bond, K. Steven LaForge, Mingting Tian, Dorothy Melia, Shengwen Zhang, Lisa Borg, Jianhua Gong, James Schluger, Judith A. Strong, Suzanne M. Leal, Jay A. Tischfield, Mary Jeanne Kreek, and Lei Yu
Single-nucleotide polymorphism in the human mu opioid receptor gene alters β-endorphin binding and activity: Possible implications for opiate addiction PNAS 1998 95 (16) 9608-9613</ref>

== Conclusion ==
The μ-OR crystal structure elucidated in "Crystal structure of the µ-opioid receptor bound to a morphinan antagonist" is the first high resolution receptor of its kind. Although opiates have been used for centuries current opioid receptor agonist drugs are far from perfect. The new understanding of the structure will enable drug discovery programs to take a structure based approach to finding new drugs and possibly treating some of the underlying causes of addiction. Such a drug could prove helpful to the medical word as well as extremely lucrative; especially since the combined legal and illegal opiate market brought in revenues around $70 billion in 2009.<ref name="base"/>

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T20:00:46Z

Greg Angelides:

== Introduction and Background Information ==
[[Image:rec.gif|thumb|right|200px|]]
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine, two opiate products, are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects of opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref> There are 17 known opioid receptors with the three primary types being the μ, κ, and δ. The μ opioid receptor (μ-OR) is the most related to opioid use. There are three subtypes μ1, μ2, μ3 and most is known about μ1.<ref>Cadet P, Mantione KJ, Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J. Immunol. 170 (10): 5118–23. PMID 12734358.</ref> The μ-ORs are part of the neurohormonal system. Within the body (sans drugs) endogenous small molecules naturally produced in the CNS and other glads act upon the GPCRs. Endogenous opioid peptides produce can range in action from pain reduction to regulating diarrhea.<ref> Janecka, Anna, Jakub Fichna, and Tomasz Janecki. "Opioid Receptors and Their Ligands." Current Topics in Medicinal Chemistry 4.1 (2004): 1-17.</ref>
<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the receptor in its inactivated form.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Ligand_pocket/1'>bound to β-funaltrexamine</scene> (β-FNA), a close relative of morphine that is bound in the pocket. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Asp147/2'>Asp147</scene> is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/His297/1'>His297</scene> indirectly interacts with the aromatic ring of the ligand. Rather than bind directly it is thought to employ two water molecules that line up and form a hydrogen bond chain to the hydroxyl group on the morphinan ring.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Glu229_lys303_and_trp318/1'>Glu229, Lys303 and Trp318</scene> are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref name="zad">Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Such a high selectivity is a property that is being examined and looked at for the possibility of certain therapies. Better understanding the manner in which an edogenous peptides bind could be clinically useful in addiction studies. Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.<ref name="zad"/>

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref> Oligomers have been observed in other GPCRs which gives further support of this possibility.<ref>Fanelli, F. & De Benedetti, P. G. Update 1 of: computational modeling approaches to structure-function analysis of G protein-coupled receptors. Chem. Rev. 111, PR438–PR535 (2011) </ref>
The linking of multiple units is of interest because μ-OR as part of a homo or heterodimer or oligomer could affect morphine tolerance. Its possible that DAMGO and methadone (both agonists) could reduce tolerance to morphine through endocytosis by the μ-OR oligomer. <ref>Finn, Andrew K., and Jennifer L. Whistler. "Endocytosis of the Mu Opioid Receptor Reduces Tolerance and a Cellular Hallmark of Opiate Withdrawal." Neuron 32.5 (2001): 829-39.</ref> Expressing multiple forms of opioid receptors showed profiles that were different from either expressed alone.<ref>George, S. R. et al. Oligomerization of μ- and δ-opioid receptors. Generation of novel functional properties. J. Biol. Chem. 275, 26128–26135 (2000) </ref> More support for oligomerization as a direct function of the μ opioid receptors is the amino acid dimer interface had an extremely high degree of homology with the δ-OR.
[[Image:Fig7.jpg|thumb|left|300px|]]

== Other Aspects and Addiction==
μ-OR has been shown to be a great target for controlling pain. However such benefits do not come without side effects and drawbacks. The μ-OR is currently at the forefront of research in many addiction diseases as well as therapies.<ref name="can">Candice Contet, Brigitte L Kieffer, Katia Befort, Mu opioid receptor: a gateway to drug addiction, Current Opinion in Neurobiology, Volume 14, Issue 3, June 2004, Pages 370-378, ISSN 0959-4388, 10.1016/j.conb.2004.05.005.
(http://www.sciencedirect.com/science/article/pii/S0959438804000728)</ref> One study found that, addiction from non opioid drugs such as alcohol, cannabinoids, and nicotine are heavily dependent on the μ-OR (all of these act at a different receptor). In μ-OR deficient mutant mice addictions typically seen from these compounds were strongly diminished. This highlighted the possibility of the μ-OR as a convergent molecular switch that can act through direct or indirect activation.<ref name="can"/>

== Conclusion ==
The μ-OR crystal structure elucidated in "Crystal structure of the µ-opioid receptor bound to a morphinan antagonist" is the first high resolution receptor of its kind. Although opiates have been used for centuries current opioid receptor agonist drugs are far from perfect. The new understanding of the structure will enable drug discovery programs to take a structure based approach to finding new drugs and possibly treating some of the underlying causes of addiction. Such a drug could prove helpful to the medical word as well as extremely lucrative; especially since the combined legal and illegal opiate market brought in revenues around $70 billion in 2009.<ref name="base"/>

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T19:51:11Z

Greg Angelides:

== Introduction and Background Information ==
[[Image:rec.gif|thumb|right|200px|]]
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine, two opiate products, are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects of opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref> There are 17 known opioid receptors with the three primary types being the μ, κ, and δ. The μ opioid receptor (μ-OR) is the most related to opioid use. There are three subtypes μ1, μ2, μ3 and most is known about μ1.<ref>Cadet P, Mantione KJ, Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J. Immunol. 170 (10): 5118–23. PMID 12734358.</ref> The μ-ORs are part of the neurohormonal system. Within the body (sans drugs) endogenous small molecules naturally produced in the CNS and other glads act upon the GPCRs. Endogenous opioid peptides produce can range in action from pain reduction to regulating diarrhea.<ref> Janecka, Anna, Jakub Fichna, and Tomasz Janecki. "Opioid Receptors and Their Ligands." Current Topics in Medicinal Chemistry 4.1 (2004): 1-17.</ref>
<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure and the Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the receptor in its inactivated form.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Ligand_pocket/1'>bound to β-funaltrexamine</scene> (β-FNA), a close relative of morphine that is bound in the pocket. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Asp147/2'>Asp147</scene> is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/His297/1'>His297</scene> indirectly interacts with the aromatic ring of the ligand. Rather than bind directly it is thought to employ two water molecules that line up and form a hydrogen bond chain to the hydroxyl group on the morphinan ring.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Glu229_lys303_and_trp318/1'>Glu229, Lys303 and Trp318</scene> are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref name="zad">Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Such a high selectivity is a property that is being examined and looked at for the possibility of certain therapies. Better understanding the manner in which an edogenous peptides bind could be clinically useful in addiction studies. Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.<ref name="zad"/>

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref> Oligomers have been observed in other GPCRs which gives further support of this possibility.<ref>Fanelli, F. & De Benedetti, P. G. Update 1 of: computational modeling approaches to structure-function analysis of G protein-coupled receptors. Chem. Rev. 111, PR438–PR535 (2011) </ref>
The linking of multiple units is of interest because μ-OR as part of a homo or heterodimer or oligomer could affect morphine tolerance. Its possible that DAMGO and methadone (both agonists) could reduce tolerance to morphine through endocytosis by the μ-OR oligomer. <ref>Finn, Andrew K., and Jennifer L. Whistler. "Endocytosis of the Mu Opioid Receptor Reduces Tolerance and a Cellular Hallmark of Opiate Withdrawal." Neuron 32.5 (2001): 829-39.</ref> Expressing multiple forms of opioid receptors showed profiles that were different from either expressed alone.<ref>George, S. R. et al. Oligomerization of μ- and δ-opioid receptors. Generation of novel functional properties. J. Biol. Chem. 275, 26128–26135 (2000) </ref> More support for oligomerization as a direct function of the μ opioid receptors is the amino acid dimer interface had an extremely high degree of homology with the δ-OR.
[[Image:Fig7.jpg|thumb|left|300px|]]

== Other Aspects and Addiction==
μ-OR has been shown to be a great target for controlling pain. However such benefits do not come without side effects and drawbacks. The μ-OR is currently at the forefront of research in many addiction diseases as well as therapies.<ref name="can">Candice Contet, Brigitte L Kieffer, Katia Befort, Mu opioid receptor: a gateway to drug addiction, Current Opinion in Neurobiology, Volume 14, Issue 3, June 2004, Pages 370-378, ISSN 0959-4388, 10.1016/j.conb.2004.05.005.
(http://www.sciencedirect.com/science/article/pii/S0959438804000728)</ref> One study found that, addiction from non opioid drugs such as alcohol, cannabinoids, and nicotine are heavily dependent on the μ-OR (all of these act at a different receptor). In μ-OR deficient mutant mice addictions typically seen from these compounds were strongly diminished. This highlighted the possibility of the μ-OR as a convergent molecular switch that can act through direct or indirect activation.<ref name="can"/>

== Conclusion ==
The μ-OR crystal structure elucidated in "Crystal structure of the µ-opioid receptor bound to a morphinan antagonist" is the first high resolution receptor of its kind. Although opiates have been used for centuries current opioid receptor agonist drugs are far from perfect. The new understanding of the structure will enable drug discovery programs to take a structure based approach to finding new drugs and possibly treating some of the underlying causes of addiction. Such a drug could prove helpful to the medical word as well as extremely lucrative; especially since the combined legal and illegal opiate market brought in revenues around $70 billion in 2009.<ref name="base"/>

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T19:30:41Z

Greg Angelides:

== Introduction and Background Information ==
[[Image:rec.gif|thumb|right|200px|]]
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine, two opiate products, are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects of opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref> There are 17 known opioid receptors with the three primary types being the μ, κ, and δ. The μ opioid receptor (μ-OR) is the most related to opioid use. There are three subtypes μ1, μ2, μ3 and most is known about μ1.<ref>Cadet P, Mantione KJ, Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J. Immunol. 170 (10): 5118–23. PMID 12734358.</ref> The μ-ORs are part of the neurohormonal system. Within the body (sans drugs) endogenous small molecules naturally produced in the CNS and other glads act upon the GPCRs. Endogenous opioid peptides produce can range in action from pain reduction to regulating diarrhea.<ref> Janecka, Anna, Jakub Fichna, and Tomasz Janecki. "Opioid Receptors and Their Ligands." Current Topics in Medicinal Chemistry 4.1 (2004): 1-17.</ref>
<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure and the Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the receptor in its inactivated form.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Ligand_pocket/1'>bound to β-funaltrexamine</scene> (β-FNA), a close relative of morphine that is bound in the pocket. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Asp147/2'>Asp147</scene> is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/His297/1'>His297</scene> indirectly interacts with the aromatic ring of the ligand. Rather than bind directly it is thought to employ two water molecules that line up and form a hydrogen bond chain to the hydroxyl group on the morphinan ring.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Glu229_lys303_and_trp318/1'>Glu229, Lys303 and Trp318</scene> are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref name="zad">Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Such a high selectivity is a property that is being examined and looked at for the possibility of certain therapies. Better understanding the manner in which an edogenous peptides bind could be clinically useful in addiction studies. Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.<ref name="zad"/>

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref> Oligomers have been observed in other GPCRs which gives further support of this possibility.<ref>Fanelli, F. & De Benedetti, P. G. Update 1 of: computational modeling approaches to structure-function analysis of G protein-coupled receptors. Chem. Rev. 111, PR438–PR535 (2011) </ref>
The linking of multiple units is of interest because μ-OR as part of a homo or heterodimer or oligomer could affect morphine tolerance. Its possible that DAMGO and methadone (both agonists) could reduce tolerance to morphine through endocytosis by the μ-OR oligomer. <ref>Finn, Andrew K., and Jennifer L. Whistler. "Endocytosis of the Mu Opioid Receptor Reduces Tolerance and a Cellular Hallmark of Opiate Withdrawal." Neuron 32.5 (2001): 829-39.</ref> Expressing multiple forms of opioid receptors showed profiles that were different from either expressed alone.<ref>George, S. R. et al. Oligomerization of μ- and δ-opioid receptors. Generation of novel functional properties. J. Biol. Chem. 275, 26128–26135 (2000) </ref> More support for oligomerization as a direct function of the μ opioid receptors is the amino acid dimer interface had an extremely high degree of homology with the δ-OR.
[[Image:Fig7.jpg|thumb|left|300px|]]

== Other Aspects and Addiction==
μ-OR has been shown to be a great target for controlling pain. However such benefits do not come without side effects and drawbacks. The μ-OR is currently at the forefront of research in many addiction diseases as well as therapies.<ref name="can">Candice Contet, Brigitte L Kieffer, Katia Befort, Mu opioid receptor: a gateway to drug addiction, Current Opinion in Neurobiology, Volume 14, Issue 3, June 2004, Pages 370-378, ISSN 0959-4388, 10.1016/j.conb.2004.05.005.
(http://www.sciencedirect.com/science/article/pii/S0959438804000728)</ref> One study found that, addiction from non opioid drugs such as alcohol, cannabinoids, and nicotine are heavily dependent on the μ-OR (all of these act at a different receptor). In μ-OR deficient mutant mice addictions typically seen from these compounds were strongly diminished. This highlighted the possibility of the μ-OR as a convergent molecular switch that can act through direct or indirect activation.<ref name="can"/>

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T19:27:12Z

Greg Angelides:

== Introduction and Background Information ==
[[Image:rec.gif|thumb|right|200px|]]
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine, two opiate products, are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects of opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref> There are 17 known opioid receptors with the three primary types being the μ, κ, and δ. The μ opioid receptor (μ-OR) is the most related to opioid use. There are three subtypes μ1, μ2, μ3 and most is known about μ1.<ref>Cadet P, Mantione KJ, Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J. Immunol. 170 (10): 5118–23. PMID 12734358.</ref> The μ-ORs are part of the neurohormonal system. Within the body (sans drugs) endogenous small molecules naturally produced in the CNS and other glads act upon the GPCRs. Endogenous opioid peptides produce can range in action from pain reduction to regulating diarrhea.<ref> Janecka, Anna, Jakub Fichna, and Tomasz Janecki. "Opioid Receptors and Their Ligands." Current Topics in Medicinal Chemistry 4.1 (2004): 1-17.</ref>
<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure and the Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the receptor in its inactivated form.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Ligand_pocket/1'>bound to β-funaltrexamine</scene> (β-FNA), a close relative of morphine that is bound in the pocket. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Asp147/2'>Asp147</scene> is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/His297/1'>His297</scene> indirectly interacts with the aromatic ring of the ligand. Rather than bind directly it is thought to employ two water molecules that line up and form a hydrogen bond chain to the hydroxyl group on the morphinan ring.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Glu229_lys303_and_trp318/1'>Glu229, Lys303 and Trp318</scene> are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref name="zad">Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Such a high selectivity is a property that is being examined and looked at for the possibility of certain therapies. Better understanding the manner in which an edogenous peptides bind could be clinically useful in addiction studies. Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.<ref name="zad"/>

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref> Oligomers have been observed in other GPCRs which gives further support of this possibility.<ref>Fanelli, F. & De Benedetti, P. G. Update 1 of: computational modeling approaches to structure-function analysis of G protein-coupled receptors. Chem. Rev. 111, PR438–PR535 (2011) </ref>
The linking of multiple units is of interest because μ-OR as part of a homo or heterodimer or oligomer could affect morphine tolerance. Its possible that DAMGO and methadone (both agonists) could reduce tolerance to morphine through endocytosis by the μ-OR oligomer. <ref>Finn, Andrew K., and Jennifer L. Whistler. "Endocytosis of the Mu Opioid Receptor Reduces Tolerance and a Cellular Hallmark of Opiate Withdrawal." Neuron 32.5 (2001): 829-39.</ref> Expressing multiple forms of opioid receptors showed profiles that were different from either expressed alone.<ref>George, S. R. et al. Oligomerization of μ- and δ-opioid receptors. Generation of novel functional properties. J. Biol. Chem. 275, 26128–26135 (2000) </ref> More support for oligomerization as a direct function of the μ opioid receptors is the amino acid dimer interface had an extremely high degree of homology with the δ-OR.
[[Image:Fig7.jpg|thumb|left|300px|]]

== Other Aspects and Addiction==
μ-OR has been shown to be a great target for controlling pain. However such benefits do not come without side effects and drawbacks. The μ-OR is currently at the forefront of research in many addiction diseases as well as therapies.<ref name="can">Candice Contet, Brigitte L Kieffer, Katia Befort, Mu opioid receptor: a gateway to drug addiction, Current Opinion in Neurobiology, Volume 14, Issue 3, June 2004, Pages 370-378, ISSN 0959-4388, 10.1016/j.conb.2004.05.005.
(http://www.sciencedirect.com/science/article/pii/S0959438804000728) One study found that, addiction from non opioid drugs such as alcohol, cannabinoids, and nicotine are heavily dependent on the μ-OR (all of these act at a different receptor). In μ-OR deficient mutant mice addictions typically seen from these compounds were strongly diminished. This highlighted the possibility of the μ-OR as a convergent molecular switch that can act through direct or indirect activation.<ref name="can"/>

</ref>

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T19:25:39Z

Greg Angelides:

== Introduction and Background Information ==
[[Image:rec.gif|thumb|right|200px|]]
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine, two opiate products, are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects of opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref> There are 17 known opioid receptors with the three primary types being the μ, κ, and δ. The μ opioid receptor (μ-OR) is the most related to opioid use. There are three subtypes μ1, μ2, μ3 and most is known about μ1.<ref>Cadet P, Mantione KJ, Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J. Immunol. 170 (10): 5118–23. PMID 12734358.</ref> The μ-ORs are part of the neurohormonal system. Within the body (sans drugs) endogenous small molecules naturally produced in the CNS and other glads act upon the GPCRs. Endogenous opioid peptides produce can range in action from pain reduction to regulating diarrhea.<ref> Janecka, Anna, Jakub Fichna, and Tomasz Janecki. "Opioid Receptors and Their Ligands." Current Topics in Medicinal Chemistry 4.1 (2004): 1-17.</ref>
<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure and the Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the receptor in its inactivated form.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Ligand_pocket/1'>bound to β-funaltrexamine</scene> (β-FNA), a close relative of morphine that is bound in the pocket. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Asp147/2'>Asp147</scene> is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/His297/1'>His297</scene> indirectly interacts with the aromatic ring of the ligand. Rather than bind directly it is thought to employ two water molecules that line up and form a hydrogen bond chain to the hydroxyl group on the morphinan ring.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Glu229_lys303_and_trp318/1'>Glu229, Lys303 and Trp318</scene> are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref name="zad">Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Such a high selectivity is a property that is being examined and looked at for the possibility of certain therapies. Better understanding the manner in which an edogenous peptides bind could be clinically useful in addiction studies. Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.<ref name="zad"/>

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref> Oligomers have been observed in other GPCRs which gives further support of this possibility.<ref>Fanelli, F. & De Benedetti, P. G. Update 1 of: computational modeling approaches to structure-function analysis of G protein-coupled receptors. Chem. Rev. 111, PR438–PR535 (2011) </ref>
The linking of multiple units is of interest because μ-OR as part of a homo or heterodimer or oligomer could affect morphine tolerance. Its possible that DAMGO and methadone (both agonists) could reduce tolerance to morphine through endocytosis by the μ-OR oligomer. <ref>Finn, Andrew K., and Jennifer L. Whistler. "Endocytosis of the Mu Opioid Receptor Reduces Tolerance and a Cellular Hallmark of Opiate Withdrawal." Neuron 32.5 (2001): 829-39.</ref> Expressing multiple forms of opioid receptors showed profiles that were different from either expressed alone.<ref>George, S. R. et al. Oligomerization of μ- and δ-opioid receptors. Generation of novel functional properties. J. Biol. Chem. 275, 26128–26135 (2000) </ref> More support for oligomerization as a direct function of the μ opioid receptors is the amino acid dimer interface had an extremely high degree of homology with the δ-OR.
[[Image:Fig7.jpg|thumb|left|300px|]]

== Other Aspects and Addiction==
μ-OR has been shown to be a great target for controlling pain. However such benefits do not come without side effects and drawbacks. The μ-OR is currently at the forefront of research in many addiction diseases as well as therapies.<refname="can">Candice Contet, Brigitte L Kieffer, Katia Befort, Mu opioid receptor: a gateway to drug addiction, Current Opinion in Neurobiology, Volume 14, Issue 3, June 2004, Pages 370-378, ISSN 0959-4388, 10.1016/j.conb.2004.05.005.
(http://www.sciencedirect.com/science/article/pii/S0959438804000728) One study found that, addiction from non opioid drugs such as alcohol, cannabinoids, and nicotine are heavily dependent on the μ-OR (all of these act at a different receptor). In μ-OR deficient mutant mice addictions typically seen from these compounds were strongly diminished. This highlighted the possibility of the μ-OR as a convergent molecular switch that can act through direct or indirect activation.<ref name="can"/>

</ref>

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T19:14:47Z

Greg Angelides:

== Introduction and Background Information ==
[[Image:rec.gif|thumb|right|200px|]]
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine, two opiate products, are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects of opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref> There are 17 known opioid receptors with the three primary types being the μ, κ, and δ. The μ opioid receptor (μ-OR) is the most related to opioid use. There are three subtypes μ1, μ2, μ3 and most is known about μ1.<ref>Cadet P, Mantione KJ, Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J. Immunol. 170 (10): 5118–23. PMID 12734358.</ref> The μ-ORs are part of the neurohormonal system. Within the body (sans drugs) endogenous small molecules naturally produced in the CNS and other glads act upon the GPCRs. Endogenous opioid peptides produce can range in action from pain reduction to regulating diarrhea.<ref> Janecka, Anna, Jakub Fichna, and Tomasz Janecki. "Opioid Receptors and Their Ligands." Current Topics in Medicinal Chemistry 4.1 (2004): 1-17.</ref>
<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure and the Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the receptor in its inactivated form.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Ligand_pocket/1'>bound to β-funaltrexamine</scene> (β-FNA), a close relative of morphine that is bound in the pocket. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Asp147/2'>Asp147</scene> is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/His297/1'>His297</scene> indirectly interacts with the aromatic ring of the ligand. Rather than bind directly it is thought to employ two water molecules that line up and form a hydrogen bond chain to the hydroxyl group on the morphinan ring.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Glu229_lys303_and_trp318/1'>Glu229, Lys303 and Trp318</scene> are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref name="zad">Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Such a high selectivity is a property that is being examined and looked at for the possibility of certain therapies. Better understanding the manner in which an edogenous peptides bind could be clinically useful in addiction studies. Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.<ref name="zad"/>

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref> Oligomers have been observed in other GPCRs which gives further support of this possibility.<ref>Fanelli, F. & De Benedetti, P. G. Update 1 of: computational modeling approaches to structure-function analysis of G protein-coupled receptors. Chem. Rev. 111, PR438–PR535 (2011) </ref>
The linking of multiple units is of interest because μ-OR as part of a homo or heterodimer or oligomer could affect morphine tolerance. Its possible that DAMGO and methadone (both agonists) could reduce tolerance to morphine through endocytosis by the μ-OR oligomer. <ref>Finn, Andrew K., and Jennifer L. Whistler. "Endocytosis of the Mu Opioid Receptor Reduces Tolerance and a Cellular Hallmark of Opiate Withdrawal." Neuron 32.5 (2001): 829-39.</ref> Expressing multiple forms of opioid receptors showed profiles that were different from either expressed alone.<ref>George, S. R. et al. Oligomerization of μ- and δ-opioid receptors. Generation of novel functional properties. J. Biol. Chem. 275, 26128–26135 (2000) </ref> More support for oligomerization as a direct function of the μ opioid receptors is the amino acid dimer interface had an extremely high degree of homology with the δ-OR.
[[Image:Fig7.jpg|thumb|left|300px|]]

== Other Aspects ==
μ-OR has been shown to be a great target for controlling pain. However such benefits do not come without side effects and drawbacks. The μ-OR is currently at the forefront of research in many addiction diseases as well as therapies.<ref>Candice Contet, Brigitte L Kieffer, Katia Befort, Mu opioid receptor: a gateway to drug addiction, Current Opinion in Neurobiology, Volume 14, Issue 3, June 2004, Pages 370-378, ISSN 0959-4388, 10.1016/j.conb.2004.05.005.
(http://www.sciencedirect.com/science/article/pii/S0959438804000728)

</ref>

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T19:12:12Z

Greg Angelides:

== Introduction and Background Information ==
[[Image:rec.gif|thumb|right|200px|]]
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine, two opiate products, are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects of opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref> There are 17 known opioid receptors with the three primary types being the μ, κ, and δ. The μ opioid receptor (μ-OR) is the most related to opioid use. There are three subtypes μ1, μ2, μ3 and most is known about μ1.<ref>Cadet P, Mantione KJ, Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J. Immunol. 170 (10): 5118–23. PMID 12734358.</ref> The μ-ORs are part of the neurohormonal system. Within the body (sans drugs) endogenous small molecules naturally produced in the CNS and other glads act upon the GPCRs. Endogenous opioid peptides produce can range in action from pain reduction to regulating diarrhea.<ref> Janecka, Anna, Jakub Fichna, and Tomasz Janecki. "Opioid Receptors and Their Ligands." Current Topics in Medicinal Chemistry 4.1 (2004): 1-17.</ref>
<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure and the Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the receptor in its inactivated form.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Ligand_pocket/1'>bound to β-funaltrexamine</scene> (β-FNA), a close relative of morphine that is bound in the pocket. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Asp147/2'>Asp147</scene> is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/His297/1'>His297</scene> indirectly interacts with the aromatic ring of the ligand. Rather than bind directly it is thought to employ two water molecules that line up and form a hydrogen bond chain to the hydroxyl group on the morphinan ring.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Glu229_lys303_and_trp318/1'>Glu229, Lys303 and Trp318</scene> are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref>Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Such a high selectivity is a property that is being examined and looked at for the possibility of certain therapies. Better understanding the manner in which an edogenous peptides bind could be clinically useful in addiction studies. Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref> Oligomers have been observed in other GPCRs which gives further support of this possibility.<ref>Fanelli, F. & De Benedetti, P. G. Update 1 of: computational modeling approaches to structure-function analysis of G protein-coupled receptors. Chem. Rev. 111, PR438–PR535 (2011) </ref>
The linking of multiple units is of interest because μ-OR as part of a homo or heterodimer or oligomer could affect morphine tolerance. Its possible that DAMGO and methadone (both agonists) could reduce tolerance to morphine through endocytosis by the μ-OR oligomer. <ref>Finn, Andrew K., and Jennifer L. Whistler. "Endocytosis of the Mu Opioid Receptor Reduces Tolerance and a Cellular Hallmark of Opiate Withdrawal." Neuron 32.5 (2001): 829-39.</ref> Expressing multiple forms of opioid receptors showed profiles that were different from either expressed alone.<ref>George, S. R. et al. Oligomerization of μ- and δ-opioid receptors. Generation of novel functional properties. J. Biol. Chem. 275, 26128–26135 (2000) </ref> More support for oligomerization as a direct function of the μ opioid receptors is the amino acid dimer interface had an extremely high degree of homology with the δ-OR.
[[Image:Fig7.jpg|thumb|left|300px|]]

== Other Aspects ==
μ-OR has been shown to be a great target for controlling pain. However such benefits do not come without side effects and drawbacks. The μ-OR is currently at the forefront of research in many addiction diseases as well as therapies.<ref>Candice Contet, Brigitte L Kieffer, Katia Befort, Mu opioid receptor: a gateway to drug addiction, Current Opinion in Neurobiology, Volume 14, Issue 3, June 2004, Pages 370-378, ISSN 0959-4388, 10.1016/j.conb.2004.05.005.
(http://www.sciencedirect.com/science/article/pii/S0959438804000728)

</ref>

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T12:06:15Z

Greg Angelides:

== Introduction and Background Information ==
[[Image:rec.gif|thumb|right|200px|]]
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine, two opiate products, are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects of opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref> There are 17 known opioid receptors with the three primary types being the μ, κ, and δ. The μ opioid receptor (μ-OR) is the most related to opioid use. There are three subtypes μ1, μ2, μ3 and most is known about μ1.<ref>Cadet P, Mantione KJ, Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J. Immunol. 170 (10): 5118–23. PMID 12734358.</ref> The μ-ORs are part of the neurohormonal system. Within the body (sans drugs) endogenous small molecules naturally produced in the CNS and other glads act upon the GPCRs. Endogenous opioid peptides produce can range in action from pain reduction to regulating diarrhea.<ref> Janecka, Anna, Jakub Fichna, and Tomasz Janecki. "Opioid Receptors and Their Ligands." Current Topics in Medicinal Chemistry 4.1 (2004): 1-17.</ref>
<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure and the Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the receptor in its inactivated form.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Ligand_pocket/1'>bound to β-funaltrexamine</scene> (β-FNA), a close relative of morphine that is bound in the pocket. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Asp147/2'>Asp147</scene> is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/His297/1'>His297</scene> indirectly interacts with the aromatic ring of the ligand. Rather than bind directly it is thought to employ two water molecules that line up and form a hydrogen bond chain to the hydroxyl group on the morphinan ring.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Glu229_lys303_and_trp318/1'>Glu229, Lys303 and Trp318</scene> are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref>Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref> Oligomers have been observed in other GPCRs which gives further support of this possibility.<ref>Fanelli, F. & De Benedetti, P. G. Update 1 of: computational modeling approaches to structure-function analysis of G protein-coupled receptors. Chem. Rev. 111, PR438–PR535 (2011) </ref>
The linking of multiple units is of interest because μ-OR as part of a homo or heterodimer or oligomer could affect morphine tolerance. Its possible that DAMGO and methadone (both agonists) could reduce tolerance to morphine through endocytosis by the μ-OR oligomer. <ref>Finn, Andrew K., and Jennifer L. Whistler. "Endocytosis of the Mu Opioid Receptor Reduces Tolerance and a Cellular Hallmark of Opiate Withdrawal." Neuron 32.5 (2001): 829-39.</ref> Expressing multiple forms of opioid receptors showed profiles that were different from either expressed alone.<ref>George, S. R. et al. Oligomerization of μ- and δ-opioid receptors. Generation of novel functional properties. J. Biol. Chem. 275, 26128–26135 (2000) </ref> More support for oligomerization as a direct function of the μ opioid receptors is the amino acid dimer interface had an extremely high degree of homology with the δ-OR.
[[Image:Fig7.jpg|thumb|left|300px|]]

== Other Aspects ==
μ-OR has been shown to be a great target for controlling pain. However such benefits do not come without side effects and drawbacks. The μ-OR is currently at the forefront of research in many addiction diseases as well as therapies.<ref>Candice Contet, Brigitte L Kieffer, Katia Befort, Mu opioid receptor: a gateway to drug addiction, Current Opinion in Neurobiology, Volume 14, Issue 3, June 2004, Pages 370-378, ISSN 0959-4388, 10.1016/j.conb.2004.05.005.
(http://www.sciencedirect.com/science/article/pii/S0959438804000728)

</ref>

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T11:57:56Z

Greg Angelides:

== Introduction and Background Information ==
[[Image:rec.gif|thumb|right|200px|]]
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine, two opiate products, are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects of opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref> There are 17 known opioid receptors with the three primary types being the μ, κ, and δ. The μ opioid receptor (μ-OR) is the most related to opioid use. There are three subtypes μ1, μ2, μ3 and most is known about μ1.<ref>Cadet P, Mantione KJ, Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J. Immunol. 170 (10): 5118–23. PMID 12734358.</ref> The μ-ORs are part of the neurohormonal system. Within the body (sans drugs) endogenous small molecules naturally produced in the CNS and other glads act upon the GPCRs. Endogenous opioid peptides produce can range in action from pain reduction to regulating diarrhea.<ref> Janecka, Anna, Jakub Fichna, and Tomasz Janecki. "Opioid Receptors and Their Ligands." Current Topics in Medicinal Chemistry 4.1 (2004): 1-17.</ref>
<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure and the Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the receptor in its inactivated form.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Ligand_pocket/1'>bound to β-funaltrexamine</scene> (β-FNA), a close relative of morphine that is bound in the pocket. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Asp147/2'>Asp147</scene> is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/His297/1'>His297</scene> indirectly interacts with the aromatic ring of the ligand. Rather than bind directly it is thought to employ two water molecules that line up and form a hydrogen bond chain to the hydroxyl group on the morphinan ring.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Glu229_lys303_and_trp318/1'>Glu229, Lys303 and Trp318</scene> are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref>Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref> Oligomers have been observed in other GPCRs which gives further support of this possibility.<ref>Fanelli, F. & De Benedetti, P. G. Update 1 of: computational modeling approaches to structure-function analysis of G protein-coupled receptors. Chem. Rev. 111, PR438–PR535 (2011) </ref>
The linking of multiple units is of interest because μ-OR as part of a homo or heterodimer or oligomer could affect morphine tolerance. Its possible that DAMGO and methadone (both agonists) could reduce tolerance to morphine through endocytosis by the μ-OR oligomer. <ref>Finn, Andrew K., and Jennifer L. Whistler. "Endocytosis of the Mu Opioid Receptor Reduces Tolerance and a Cellular Hallmark of Opiate Withdrawal." Neuron 32.5 (2001): 829-39.</ref> Expressing multiple forms of opioid receptors showed profiles that were different from either expressed alone.<ref>George, S. R. et al. Oligomerization of μ- and δ-opioid receptors. Generation of novel functional properties. J. Biol. Chem. 275, 26128–26135 (2000) </ref> More support for oligomerization as a direct function of the μ opioid receptors is the amino acid dimer interface had an extremely high degree of homology with the δ-OR.
[[Image:Fig7.jpg|thumb|left|300px|]]

== Other Aspects ==

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T11:45:34Z

Greg Angelides:

== Introduction and Background Information ==
[[Image:rec.gif|thumb|right|200px|]]
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects of opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref> There are 17 known opioid receptors with the three primary types being the μ, κ, and δ. The μ opioid receptor (μ-OR) is the most related to opioid use. There are three subtypes μ1, μ2, μ3 and most is known about μ1.<ref>Cadet P, Mantione KJ, Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J. Immunol. 170 (10): 5118–23. PMID 12734358.</ref> The μ-OR is activated by an agonist and is inhibited by antagonists.

<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure and the Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the receptor in its inactivated form.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Ligand_pocket/1'>bound to β-funaltrexamine</scene> (β-FNA), a close relative of morphine that is bound in the pocket. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Asp147/2'>Asp147</scene> is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/His297/1'>His297</scene> indirectly interacts with the aromatic ring of the ligand. Rather than bind directly it is thought to employ two water molecules that line up and form a hydrogen bond chain to the hydroxyl group on the morphinan ring.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Glu229_lys303_and_trp318/1'>Glu229, Lys303 and Trp318</scene> are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref>Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref> Oligomers have been observed in other GPCRs which gives further support of this possibility.<ref>Fanelli, F. & De Benedetti, P. G. Update 1 of: computational modeling approaches to structure-function analysis of G protein-coupled receptors. Chem. Rev. 111, PR438–PR535 (2011) </ref>
The linking of multiple units is of interest because μ-OR as part of a homo or heterodimer or oligomer could affect morphine tolerance. Its possible that DAMGO and methadone (both agonists) could reduce tolerance to morphine through endocytosis by the μ-OR oligomer. <ref>Finn, Andrew K., and Jennifer L. Whistler. "Endocytosis of the Mu Opioid Receptor Reduces Tolerance and a Cellular Hallmark of Opiate Withdrawal." Neuron 32.5 (2001): 829-39.</ref> Expressing multiple forms of opioid receptors showed profiles that were different from either expressed alone.<ref>George, S. R. et al. Oligomerization of μ- and δ-opioid receptors. Generation of novel functional properties. J. Biol. Chem. 275, 26128–26135 (2000) </ref> More support for oligomerization as a direct function of the μ opioid receptors is the amino acid dimer interface had an extremely high degree of homology with the δ-OR.
[[Image:Fig7.jpg|thumb|left|300px|]]

== Other Aspects ==

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T11:41:55Z

Greg Angelides:

== Introduction and Background Information ==
[[Image:rec.gif|thumb|right|200px|]]
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects of opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref> There are 17 known opioid receptors with the three primary types being the μ, κ, and δ. The μ opioid receptor (μ-OR) is the most related to opioid use. There are three subtypes μ1, μ2, μ3 and most is known about μ1.<ref>Cadet P, Mantione KJ, Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J. Immunol. 170 (10): 5118–23. PMID 12734358.</ref> The μ-OR is activated by an agonist and is inhibited by antagonists.

<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure and the Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the receptor in its inactivated form.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Ligand_pocket/1'>bound to β-funaltrexamine</scene> (β-FNA), a close relative of morphine that is bound in the pocket. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Asp147/2'>Asp147</scene> is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/His297/1'>His297</scene> indirectly interacts with the aromatic ring of the ligand. Rather than bind directly it is thought to employ two water molecules that line up and form a hydrogen bond chain to the hydroxyl group on the morphinan ring.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Glu229_lys303_and_trp318/1'>Glu229, Lys303 and Trp318</scene> are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref>Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref> Oligomers have been observed in other GPCRs which gives further support of this possibility.<ref>Fanelli, F. & De Benedetti, P. G. Update 1 of: computational modeling approaches to structure-function analysis of G protein-coupled receptors. Chem. Rev. 111, PR438–PR535 (2011) </ref>
The linking of multiple units is of interest because μ-OR as part of a homo or heterodimer or oligomer could affect morphine tolerance. Its possible that DAMGO and methadone (both agonists) could reduce tolerance to morphine through endocytosis by the μ-OR oligomer. <ref>Finn, Andrew K., and Jennifer L. Whistler. "Endocytosis of the Mu Opioid Receptor Reduces Tolerance and a Cellular Hallmark of Opiate Withdrawal." Neuron 32.5 (2001): 829-39.</ref> Expressing multiple forms of opioid receptors showed profiles that were different from either expressed alone.<ref>George, S. R. et al. Oligomerization of μ- and δ-opioid receptors. Generation of novel functional properties. J. Biol. Chem. 275, 26128–26135 (2000) </ref> More support for oligomerization as a direct function of the μ opioid receptors is the amino acid dimer interface had an extremely high degree of homology with the δ-OR.
[[Image:Fig7.jpg|thumb|left|300px|]]

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

File:Fig7.jpg

2012-11-29T11:40:55Z

Greg Angelides: μ opioid receptors shown as associated pairs and oligomers. The different interfaces TM5-TM6 as well as TM1-TM2-H8 are shown as well. I do not own this image, it is property of Nature and the authors, used for academic purposes only: Aashish, Manglik, et

μ opioid receptors shown as associated pairs and oligomers. The different interfaces TM5-TM6 as well as TM1-TM2-H8 are shown as well. I do not own this image, it is property of Nature and the authors, used for academic purposes only: Aashish, Manglik, et al. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T11:38:47Z

Greg Angelides:

== Introduction and Background Information ==
[[Image:rec.gif|thumb|right|200px|]]
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects of opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref> There are 17 known opioid receptors with the three primary types being the μ, κ, and δ. The μ opioid receptor (μ-OR) is the most related to opioid use. There are three subtypes μ1, μ2, μ3 and most is known about μ1.<ref>Cadet P, Mantione KJ, Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J. Immunol. 170 (10): 5118–23. PMID 12734358.</ref> The μ-OR is activated by an agonist and is inhibited by antagonists.

<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure and the Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the receptor in its inactivated form.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Ligand_pocket/1'>bound to β-funaltrexamine</scene> (β-FNA), a close relative of morphine that is bound in the pocket. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Asp147/2'>Asp147</scene> is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/His297/1'>His297</scene> indirectly interacts with the aromatic ring of the ligand. Rather than bind directly it is thought to employ two water molecules that line up and form a hydrogen bond chain to the hydroxyl group on the morphinan ring.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Glu229_lys303_and_trp318/1'>Glu229, Lys303 and Trp318</scene> are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref>Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref> Oligomers have been observed in other GPCRs which gives further support of this possibility.<ref>Fanelli, F. & De Benedetti, P. G. Update 1 of: computational modeling approaches to structure-function analysis of G protein-coupled receptors. Chem. Rev. 111, PR438–PR535 (2011) </ref>
The linking of multiple units is of interest because μ-OR as part of a homo or heterodimer or oligomer could affect morphine tolerance. Its possible that DAMGO and methadone (both agonists) could reduce tolerance to morphine through endocytosis by the μ-OR oligomer. <ref>Finn, Andrew K., and Jennifer L. Whistler. "Endocytosis of the Mu Opioid Receptor Reduces Tolerance and a Cellular Hallmark of Opiate Withdrawal." Neuron 32.5 (2001): 829-39.</ref> Expressing multiple forms of opioid receptors showed profiles that were different from either expressed alone.<ref>George, S. R. et al. Oligomerization of μ- and δ-opioid receptors. Generation of novel functional properties. J. Biol. Chem. 275, 26128–26135 (2000) </ref> More support for oligomerization as a direct function of the μ opioid receptors is the amino acid dimer interface had an extremely high degree of homology with the δ-OR.
[[Image:Fig7.jpg]]

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T11:34:40Z

Greg Angelides:

== Introduction and Background Information ==
[[Image:rec.gif|thumb|right|200px|]]
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects of opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref> There are 17 known opioid receptors with the three primary types being the μ, κ, and δ. The μ opioid receptor (μ-OR) is the most related to opioid use. There are three subtypes μ1, μ2, μ3 and most is known about μ1.<ref>Cadet P, Mantione KJ, Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J. Immunol. 170 (10): 5118–23. PMID 12734358.</ref> The μ-OR is activated by an agonist and is inhibited by antagonists.

<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure and the Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the receptor in its inactivated form.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Ligand_pocket/1'>bound to β-funaltrexamine</scene> (β-FNA), a close relative of morphine that is bound in the pocket. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Asp147/2'>Asp147</scene> is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/His297/1'>His297</scene> indirectly interacts with the aromatic ring of the ligand. Rather than bind directly it is thought to employ two water molecules that line up and form a hydrogen bond chain to the hydroxyl group on the morphinan ring.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Glu229_lys303_and_trp318/1'>Glu229, Lys303 and Trp318</scene> are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref>Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref> Oligomers have been observed in other GPCRs which gives further support of this possibility.<ref>Fanelli, F. & De Benedetti, P. G. Update 1 of: computational modeling approaches to structure-function analysis of G protein-coupled receptors. Chem. Rev. 111, PR438–PR535 (2011) </ref>
The linking of multiple units is of interest because μ-OR as part of a homo or heterodimer or oligomer could affect morphine tolerance. Its possible that DAMGO and methadone (both agonists) could reduce tolerance to morphine through endocytosis by the μ-OR oligomer. <ref>Finn, Andrew K., and Jennifer L. Whistler. "Endocytosis of the Mu Opioid Receptor Reduces Tolerance and a Cellular Hallmark of Opiate Withdrawal." Neuron 32.5 (2001): 829-39.</ref> Expressing multiple forms of opioid receptors showed profiles that were different from either expressed alone.<ref>George, S. R. et al. Oligomerization of μ- and δ-opioid receptors. Generation of novel functional properties. J. Biol. Chem. 275, 26128–26135 (2000) </ref> More support for oligomerization as a direct function of the μ opioid receptors is the amino acid dimer interface had an extremely high degree of homology with the δ-OR.

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T11:26:05Z

Greg Angelides:

== Introduction and Background Information ==
[[Image:rec.gif|thumb|right|200px|]]
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects of opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref> There are 17 known opioid receptors with the three primary types being the μ, κ, and δ. The μ opioid receptor (μ-OR) is the most related to opioid use. There are three subtypes μ1, μ2, μ3 and most is known about μ1.<ref>Cadet P, Mantione KJ, Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J. Immunol. 170 (10): 5118–23. PMID 12734358.</ref> The μ-OR is activated by an agonist and is inhibited by antagonists.

<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure and the Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the receptor in its inactivated form.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Ligand_pocket/1'>bound to β-funaltrexamine</scene> (β-FNA), a close relative of morphine that is bound in the pocket. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Asp147/2'>Asp147</scene> is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/His297/1'>His297</scene> indirectly interacts with the aromatic ring of the ligand. Rather than bind directly it is thought to employ two water molecules that line up and form a hydrogen bond chain to the hydroxyl group on the morphinan ring.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors. Glu229, Lys303 and Trp318 are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref>Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref> Oligomers have been observed in other GPCRs which gives further support of this possibility.<ref>Fanelli, F. & De Benedetti, P. G. Update 1 of: computational modeling approaches to structure-function analysis of G protein-coupled receptors. Chem. Rev. 111, PR438–PR535 (2011) </ref>
The linking of multiple units is of interest because μ-OR as part of a homo or heterodimer or oligomer could affect morphine tolerance. Its possible that DAMGO and methadone (both agonists) could reduce tolerance to morphine through endocytosis by the μ-OR oligomer. <ref>Finn, Andrew K., and Jennifer L. Whistler. "Endocytosis of the Mu Opioid Receptor Reduces Tolerance and a Cellular Hallmark of Opiate Withdrawal." Neuron 32.5 (2001): 829-39.</ref> Expressing multiple forms of opioid receptors showed profiles that were different from either expressed alone.<ref>George, S. R. et al. Oligomerization of μ- and δ-opioid receptors. Generation of novel functional properties. J. Biol. Chem. 275, 26128–26135 (2000) </ref> More support for oligomerization as a direct function of the μ opioid receptors is the amino acid dimer interface had an extremely high degree of homology with the δ-OR.

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T11:24:08Z

Greg Angelides:

== Introduction and Background Information ==
[[Image:rec.gif|thumb|right|200px|]]
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects of opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref> There are 17 known opioid receptors with the three primary types being the μ, κ, and δ. The μ opioid receptor (μ-OR) is the most related to opioid use. There are three subtypes μ1, μ2, μ3 and most is known about μ1.<ref>Cadet P, Mantione KJ, Stefano GB (2003). "Molecular identification and functional expression of μ3, a novel alternatively spliced variant of the human μ opiate receptor gene". J. Immunol. 170 (10): 5118–23. PMID 12734358.</ref> The μ-OR is activated by an agonist and is inhibited by antagonists.

<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure and the Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the receptor in its inactivated form.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Ligand_pocket/1'>bound to β-funaltrexamine</scene> (β-FNA), a close relative of morphine that is bound in the pocket. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Asp147/1'>Asp147</scene> is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
<scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/His297/1'>His297</scene> indirectly interacts with the aromatic ring of the ligand. Rather than bind directly it is thought to employ two water molecules that line up and form a hydrogen bond chain to the hydroxyl group on the morphinan ring.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors. Glu229, Lys303 and Trp318 are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref>Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref> Oligomers have been observed in other GPCRs which gives further support of this possibility.<ref>Fanelli, F. & De Benedetti, P. G. Update 1 of: computational modeling approaches to structure-function analysis of G protein-coupled receptors. Chem. Rev. 111, PR438–PR535 (2011) </ref>
The linking of multiple units is of interest because μ-OR as part of a homo or heterodimer or oligomer could affect morphine tolerance. Its possible that DAMGO and methadone (both agonists) could reduce tolerance to morphine through endocytosis by the μ-OR oligomer. <ref>Finn, Andrew K., and Jennifer L. Whistler. "Endocytosis of the Mu Opioid Receptor Reduces Tolerance and a Cellular Hallmark of Opiate Withdrawal." Neuron 32.5 (2001): 829-39.</ref> Expressing multiple forms of opioid receptors showed profiles that were different from either expressed alone.<ref>George, S. R. et al. Oligomerization of μ- and δ-opioid receptors. Generation of novel functional properties. J. Biol. Chem. 275, 26128–26135 (2000) </ref> More support for oligomerization as a direct function of the μ opioid receptors is the amino acid dimer interface had an extremely high degree of homology with the δ-OR.

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T11:10:53Z

Greg Angelides:

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T11:06:55Z

Greg Angelides:

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T11:02:56Z

Greg Angelides:

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T10:37:56Z

Greg Angelides:

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T10:28:50Z

Greg Angelides:

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T10:24:31Z

Greg Angelides:

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T10:22:35Z

Greg Angelides:

File:Rec.gif

2012-11-29T10:00:03Z

Greg Angelides: uploaded a new version of "Image:Rec.gif": Very basic overview of μ-OR and how GPCR work in general. Image credit: http://www-personal.umich.edu/~timaster/biopsych/pharm.html

Very basic overview of μ-OR and how GPCR work in general. Image credit: http://www-personal.umich.edu/~timaster/biopsych/pharm.html

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T09:57:00Z

Greg Angelides:

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T09:47:43Z

Greg Angelides:

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T09:47:09Z

Greg Angelides:

== Introduction and Background Information ==
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects if opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref>[[Image:rec.gif|thumb|right|200px|]]

<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure and the Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the inactivated receptor.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is bound to β-funaltrexamine (β-FNA), a close relative of morphine. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. Asp147 is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors. Glu229, Lys303 and Trp318 are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref>Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref>

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

File:Rec.gif

2012-11-29T09:45:58Z

Greg Angelides: Very basic overview of μ-OR and how GPCR work in general. Image credit: http://www-personal.umich.edu/~timaster/biopsych/pharm.html

Very basic overview of μ-OR and how GPCR work in general. Image credit: http://www-personal.umich.edu/~timaster/biopsych/pharm.html

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T09:44:37Z

Greg Angelides:

== Introduction and Background Information ==
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects if opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref>[[Image:rec.gif]]

<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overview of Structure and the Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops and is 400 amino acids in length. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the inactivated receptor.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is bound to β-funaltrexamine (β-FNA), a close relative of morphine. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. Asp147 is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors. Glu229, Lys303 and Trp318 are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref>Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref>

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T09:32:13Z

Greg Angelides:

== Introduction and Background Information ==
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects if opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref>

<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overall Structure Of Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the inactivated receptor.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is bound to β-funaltrexamine (β-FNA), a close relative of morphine. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. Asp147 is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors. Glu229, Lys303 and Trp318 are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref>Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== Oligomeric Properties ==
As discussed the μ-OR has the ability to associate and pair with other μ-ORs to form dimers and likely larger groupings. Biological research has suggested that it is likely that the receptors aggregate together and the crystal structure reinforces this.<ref>Rozenfeld, R., Gomes, I. & Devi, L. in The Opiate Receptors Vol. 23 (ed. Pasternak, G. W.) Ch. 15 407–437 (Humana, 2011) </ref>

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T09:22:22Z

Greg Angelides:

== Introduction and Background Information ==
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects if opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref>

<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overall Structure Of Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the inactivated receptor.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is bound to β-funaltrexamine (β-FNA), a close relative of morphine. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. Asp147 is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors. Glu229, Lys303 and Trp318 are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref>Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>

== References ==
<references/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T09:20:11Z

Greg Angelides:

== Introduction and Background Information ==
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects if opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref>

<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overall Structure Of Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the inactivated receptor.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is bound to β-funaltrexamine (β-FNA), a close relative of morphine. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. Asp147 is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors. Glu229, Lys303 and Trp318 are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref> Small peptides named endomorphins 1 and 2 were shown to have the highest selectivity profile for the µ-OR. Endomorphin 1 shows a 15,000 fold increase in selectivity for µ-OR over κ-OR.<ref>Zadina, J. E., Hackler, L., Ge, L. J. & Kastin, A. J. A potent and selective endogenous agonist for the μ-opiate receptor. Nature 386, 499–502 (1997)</ref> Little is known about the manner in which they bind but it is clear that there is a large difference between different opioid receptors.

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

== References ==
<references/>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T08:57:03Z

Greg Angelides:

== Introduction and Background Information ==
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects if opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref>

<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overall Structure Of Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the inactivated receptor.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|300px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is bound to β-funaltrexamine (β-FNA), a close relative of morphine. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. Asp147 is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.

[[Image:Fig5.jpg|thumb|left|300px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors. Glu229, Lys303 and Trp318 are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref>

[[Image:Fig6.jpg|thumb|right|300px|]]<ref name="base"/>
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

== References ==
<references/>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T08:53:06Z

Greg Angelides:

== Introduction and Background Information ==
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects if opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref>

<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overall Structure Of Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the inactivated receptor.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|400px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is bound to β-funaltrexamine (β-FNA), a close relative of morphine. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. Asp147 is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.

[[Image:Fig5.jpg|thumb|left|400px|]]<ref name="base"/>

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors. Glu229, Lys303 and Trp318 are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.<ref>Bonner, G., Meng, F. & Akil, H. Selectivity of μ-opioid receptor determined by interfacial residues near third extracellular loop. Eur. J. Pharmacol. 403, 37–44 (2000) </ref>

[[Image:Fig6.jpg]]
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

== References ==
<references/>

File:Fig6.jpg

2012-11-29T08:47:24Z

Greg Angelides: The Trp318 residue selects against certain structures that could otherwise bind if it were a leucine as in the δ-OR. I do not own this image, it is property of Nature and the authors, used for academic purposes only: Aashish, Manglik, et al. "Crystal St

The Trp318 residue selects against certain structures that could otherwise bind if it were a leucine as in the δ-OR. I do not own this image, it is property of Nature and the authors, used for academic purposes only: Aashish, Manglik, et al. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T08:46:03Z

Greg Angelides:

== Introduction and Background Information ==
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects if opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref>

<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overall Structure Of Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the inactivated receptor.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|400px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is bound to β-funaltrexamine (β-FNA), a close relative of morphine. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. Asp147 is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.

[[Image:Fig5.jpg|thumb|left|400px|]]

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors. Glu229, Lys303 and Trp318 are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.

[[Image:Fig6.jpg]]
</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

== References ==
<references/>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T08:43:20Z

Greg Angelides:

== Introduction and Background Information ==
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects if opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref>

<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overall Structure Of Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the inactivated receptor.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|400px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is bound to β-funaltrexamine (β-FNA), a close relative of morphine. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. Asp147 is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.

[[Image:Fig5.jpg|thumb|left|400px|]]

The μ opioid receptor has three distinct differences in the binding domain from the δ receptors. Glu229, Lys303 and Trp318 are present in μ-OR which are Asp, Trp and Leu in the δ structure. The Trp318 could be a source of selectivity. When attempting to bind to certain naltirindole, a selective antagonist of the δ receptor, the Trp318 side chain clashes and does not allow for it while the leucine in the same position would accommodate this structure.

</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

== References ==
<references/>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T08:28:05Z

Greg Angelides:

== Introduction and Background Information ==
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects if opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref>

<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overall Structure Of Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the inactivated receptor.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|400px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is bound to β-funaltrexamine (β-FNA), a close relative of morphine. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. Asp147 is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.
[[Image:Fig5.jpg|thumb|left|400px|]]

</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

== References ==
<references/>

File:Fig5.jpg

2012-11-29T08:25:53Z

Greg Angelides: Interaction between β-FNA and the surrounding residues present in the ligand binding pocket. I do not own this image, it is property of Nature and the authors, used for academic purposes only: Aashish, Manglik, et al. "Crystal Structure of the µ-opioid

Interaction between β-FNA and the surrounding residues present in the ligand binding pocket. I do not own this image, it is property of Nature and the authors, used for academic purposes only: Aashish, Manglik, et al. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T08:15:44Z

Greg Angelides:

== Introduction and Background Information ==
Opium is one of the oldest drugs known to mankind and still has prevalence today. Morphine and codeine are heavily used both in clinical settings and illegally to to control pain and produce analgesic and unwanted side effects. Mu-opioid receptors are a group of G-protein-coupled receptors and have been found to be the target for the majority of effects if opioid alkaloids.<ref>Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996) </ref>

<StructureSection load='4dkl' size='400' side='right' caption='Crystal structure of the mu-opioid receptor bound to a morphinan antagonist (PDB entry [[4dkl]])' scene=4dkl/Overall_structure/1''>

== Overall Structure Of Mu Opioid Receptor ==
The structure of μ-opioid receptor consists of seven transmembrane alpha-helicies connected through three extra cellular loops and three intracellular loops. The greater multi-unit μ-opioid receptor has altering aqueous and lipid layers with receptor proteins arranged in parallel dimers. Transmembrane helicies <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6/1'>5 and 6</scene> tightly associate while TM1 and TM2 have more limited contact. These strands allow the single chain structures to polymerize. <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Tm5_and_tm6_dimer_form/1'>TM5 and TM6 Dimer Interface.</scene>

[[Image:Fig1.jpg|thumb|left|400px|]]<ref name="base">Aashish, Manglik. "Crystal Structure of the µ-opioid Receptor Bound to a Morphinan Antagonist." Nature 485.7398 (2012): 321-26. Nature.com. 21 Mar. 2012. Web. 27 Nov. 2012.</ref>

TM3 and ECL2 are connected through a <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Disulfide/3'>disulfide bridge</scene> between Cys140 and Cys217. This bridge is conserved across GPCRs as is the seven helix structure. It has been found that a mutation of Thr279 to a lysine group causes the protein to be constituatively active.<ref>Huang, P. et al. Functional role of a conserved motif in TM6 of the rat μ opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp. Biochemistry 40, 13501–13509 (2001</ref> A possible reason for this could be that the lack of an interaction between <scene name='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Arg_and_thr_interaction/1'>T279 and R165</scene> that is usually present and may help to stabilize the inactivated receptor.

== Ligand Binding Pocket and Specificity ==
The μ opioid receptor differs from many other GPCRs. The majority of GPCR have a binding area that is partially buried within the helical region and must get past side chains to bind in the pocket. An example of this can be seen in the M3 muscarinic receptor.<ref>Haga, K. et al. Structure of the human M2 muscarinic acetyl choline receptor bound to an antagonist. Nature 482, 547–551 (2012) </ref> The μ opioid receptor exhibits a much shallower and less protected binding pocket. This difference is likely the basis of the varying dissociation kinetics associated with these GPCRs.
This structure may also explain why some extremely potent opioids have such rapid dissociation half-lives. This unique feature allows for some drugs such as etorphine to be used as an extremely fast acting analgesic coupled with diprenophine as a reversing reagent. This drug combination is utilized in captive and free-range mammals.<ref>Alford, B. T., Burkhart, R. L. & Johnson, W. P. Etorphine and diprenorphine as immobilizing and reversing agents in captive and free-ranging mammals. J. Am. Vet. Med. Assoc. 164, 702–705 (1974) </ref>

[[Image:Fig2.jpg|thumb|right|400px|]]<ref name="base"/>

In this crystal structure of the μ opioid receptor it is bound to β-funaltrexamine (β-FNA), a close relative of morphine. When β-FNA binds there are nine residues that have direct interaction with the ligand while there are 14 total within 4Â of β-FNA. The nine residues in direct contact are conserved in the κ and δ opioid receptors. Asp147 is important because it forms a bond with the amine on the ligand. It is also notable because it is conserved across all opioid receptor subtypes.

</StructureSection>

<StructureSection load='4dkl' size='350' side='right' caption='Structure of mouse mu opioid receptor, complexed with an irreversible morphinan antagonist (PDB entry [[4dkl]]). Extracellular space is represented at the top above the red boundary of the lipid bi-layer.' scene='Mu_Opioid_Receptor_Bound_to_a_Morphinan_Antagonist/Start4dklopm/1'>

== References ==
<references/>

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T08:01:00Z

Greg Angelides:

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T06:58:05Z

Greg Angelides:

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T06:55:58Z

Greg Angelides:

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T06:46:38Z

Greg Angelides:

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T06:45:51Z

Greg Angelides:

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T06:44:23Z

Greg Angelides:

Mu Opioid Receptor Bound to a Morphinan Antagonist

2012-11-29T06:35:59Z

Greg Angelides: