Student Projects for UMass Chemistry 423 Spring 2012-6

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Contents 1 Catechol-O-Methyltransferase 1.1 Introduction 1.2 Overall Structure 1.3 Binding Interactions 1.4 Additional Features 1.5 Credits 1.6 References

Catechol-O-Methyltransferase

Introduction

COMT transfer of methyl group from SAM to catecholamine inactivates catecholamine Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

Catechol-O-Methyltransferase (COMT) is an enzyme, which can be either soluble or membrane-bound, that is responsible for the degradation of catecholamine neurotransmitters ^[1]. This inactivation is accomplished by transferring a methyl group from S-adenosyl methionine (SAM) to the catecholamine, seen ^[2].

One neurotransmitter in this catecholamine family targeted by COMT is dopamine, the neurotransmitter most closely associated with Parkinson's disease. Parkinson's Disease arises out of a lack of dopamine and is characterized by uncontrollable tremors, muscular rigidity, postural instability. At a functional synapse, the action potential prompts release of neurotransmitters like dopamine at the synapse. These neurotransmitters bind to receptors on the postsynaptic membrane, perpetuating the signal. Once the signal has been transmitted, the neurotransmitters are removed from the synapse via reuptake or degradation by enzymes such as COMT. In a person with Parkinson's Disease, dopamine levels are often too low to adequately continue the message to the next neuron. The disease is currently treated with L-DOPA, a dopamine precursor that is converted to dopamine within the brain. However the bioavailability and stability of L-DOPA when used alone is limited. COMT is being investigated as a target for therapeutic agents that would increase the efficacy of L-DOPA. Inhibition of COMT would prevent inactivation of dopamine, leaving higher levels of active dopamine at the synapse and increasing the likelihood of perpetuation of the message to the postsynaptic neuron^[3].

Overall Structure

The structural differences between Catechol-O-Methyltransferase with a coumarine inhibitor, pdb code 2zvj, compared to a carecholic inhibitor, pdb code 2cl5. Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

The Catechol-O-Methyltransferase complex, with a coumarine inhibitor, is a monomer that is made up of eight . These sheets are all parallel except for one. The monomer is also comprised of alpha helices. The beta sheets are on the inside of the complex where the alpha helices are on the outside and enclose the beta strands. The Catechol-O-Methyltransferase can either be membrane bound or soluble. Because it can be either, the polar and nonpolar residues are seen .^[4]

Catechol-O-Methyl Transferase in complex with a catcholic inhibitor which is a non-coumarine inihibitor can normally form a (pdb code 2cl5). For previous inhibitors that have been used and studied this is true but not for the coumarine inhibitor. This dimer can be formed because the ligand that is attached, which in this case in a bacterial inhibitor, BIE, allows the complex to be structurally flexible. The dimer that forms creates a , the ligands are enclosed in the center so that the two monomers fit together nicely and have flexibility. As you can see the two ligands slide pass each other and the two rings look as if they are stacked on top of each other. In this pocket the S-Adenosyl Methionine, , donates a methyl group to each of the ligands present that have the magnesium ion attached. The distance between the SAM molecules and the ligands are almost exactly equal distances apart from each other creating an even spacing pocket to form making the dimer very symmetric.^[5]

The complex with the coumarine ligand, pdb code 2zvj, does not allow for such a convenient between the SAM molecule and ligand to form when they are bound together. Also, with a coumarine inhibitor, the aromatic ring stacking that we saw with the non-coumarine inhibitor does not occur. This asymmetric structure has effects on the binding interactions and therefore the function of the complex.^[6]

Binding Interactions

Insert caption here Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

Met 40, Leu 198, and Tyr 200 define the for the 4-phenyl-7, 8-dihydroxycoumarine (4PCM) ligand binding site. Trp 38 and Pro 174 make interactions with the 4PCM. This also allows us to see how, unlike the ligand described in the Overall Structure section, 4PCM is sterically constrained and unable to form necessary interactions for a dimer configuration. The magnesium ion interacts with the two of the 4PCM. The magnesium ion also aids in the of Lys 144, causing an electrostatic interaction with a hydroxyl group of 4PCM. The other end of this Lys 144 then acts a hydrogen bond donor for a water molecule in the binding pocket. This water molecule then acts a hydrogen bond donor for the carbonyl group of 4PCM, creating an interesting network of hydrogen bonds. The above interactions stabilize the ligand in the binding pocket. These interactions are somewhat similar to other inhibitors previously used, however some differences do occur that make past inhibitors more stable.^[7] In our 4PCM, we can see a between the sulfur and carbon atoms of Met 40 to be significantly less than of the Met 40 in a COMT complex using 3,5-dinitrocatchetol^[8] as an inhibitor instead; the change is about 96 degrees. This interaction disrupts the favorable binding stabilization interactions of 4PCM with COMT. This interaction, as well as the constraining effects of Trp 38 and Pro 174 interactions, make 4PCM less stable than nitrocatchetol inhibitors currently being used to treat PD. However, nitrocatchetol inhibitors act as uncouplers, making 4PCM side effects less complex and more attractive.

Additional Features

The location at position 158 where valine replaces with methionine (Val148Met) in the COMT enzyme activity Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

In the gene for COMT, there is a functional single-nucleotide polymorphism that switches from valine to methionine mutation at position 158 (Va Met). The Val variations can breakdowns dopamine as high as four times the rate as its methionine. As a result, neurotransmitter is then release due to the lower dopamine levels. Since COMT's role is to degraded dopamine, the Val158Met polymorphism is told to utilize its effects on cognition by regulating dopamine signaling in the front area of the human brain. ^[9] There are drugs in the market that could focus only in the frontal lobes and prevent Val158Met to happen. The two common major inhibitor drugs (tolcapone and entacapone) that are sold on the market, inhibit the action of COMT. These drugs effectively reduce / inhibit COMT’s ability to degrade neurotransmitters. As mentioned earlier, these drugs are mainly used to combat Parkinson’s Disease.

Tolcapone , which inhibits COMT from immediately converting L-DOPA into 3-O-methyldopa, has the ability to facilitate higher levels of L-DOPA conversion in the central nervous system. Additionally, it allows dopamine to hang around longer by preventing its degradation. Unfortunately, this has also lead to the discovery that Tolcapone promotes high levels of hepatotoxicity (chemical-driven liver damage). This negative side effect has limited the usage of Tolcapone and promoted the selection of another inhibitor drug: Entacapone. ^[10]

Entacapone, unlike Tolcapone, does not easily cross the blood-brain barrier. As a result, it does not cause hepatotoxicity, and is therefore a very common choice. It is typically used to treat the “end-of-dose ‘wearing-off’ symptoms of Parkinson’s Disease. Entacapone is typically used with Levodopa and its role is to prevent the breakdown of L-DOPA OUTSIDE of the brain. Recall that L-DOPA can be broken down into Dopamine, but too much Dopamine throughout the body could be harmful. As a result, Entacapone restores normal cognitive function in patients but also limits the side effects of dopamine on the rest of the body. ^[11]

Credits

Introduction - Jessica Royal

Overall Structure - Stephanie Bristol

Drug Binding Site - Emily Brackett

Additional Features - Anh Huynh

References

1. ↑ Palma PN, Rodrigues ML, Archer M, Bonifacio MJ, Loureiro AI, Learmonth DA, Carrondo MA, Soares-da-Silva P. Comparative study of ortho- and meta-nitrated inhibitors of catechol-O-methyltransferase: interactions with the active site and regioselectivity of O-methylation. Mol Pharmacol. 2006 Jul;70(1):143-53. Epub 2006 Apr 17. PMID:16618795 doi:10.1124/mol.106.023119
2. ↑ Grossman, MH, Emanuel, BS, Budarf, ML. Chromosomal mapping of the human catechol-O-methyltransferase gene to 22q11.1----q11.2. (1992). Genomics, 12(4), 822-825.
3. ↑ Espinoza, S, Manago, F, Leo, D, Sotnikova, TD, Gainetdinov, RR. Role of catechol-O-methyltransferase (COMT)-dependent processes in Parkinson's Disease and L-DOPA treatment. (2012). CNS Neurological Disorder Drug Targets.
4. ↑ Tsuji E, Okazaki K, Takeda K. Crystal structures of rat catechol-O-methyltransferase complexed with coumarine-based inhibitor. Biochem Biophys Res Commun. 2009 Jan 16;378(3):494-7. Epub 2008 Dec 3. PMID:19056347 doi:S0006-291X(08)02267-5
5. ↑ Palma PN, Rodrigues ML, Archer M, Bonifacio MJ, Loureiro AI, Learmonth DA, Carrondo MA, Soares-da-Silva P. Comparative study of ortho- and meta-nitrated inhibitors of catechol-O-methyltransferase: interactions with the active site and regioselectivity of O-methylation. Mol Pharmacol. 2006 Jul;70(1):143-53. Epub 2006 Apr 17. PMID:16618795 doi:10.1124/mol.106.023119
6. ↑ Tsuji E, Okazaki K, Takeda K. Crystal structures of rat catechol-O-methyltransferase complexed with coumarine-based inhibitor. Biochem Biophys Res Commun. 2009 Jan 16;378(3):494-7. Epub 2008 Dec 3. PMID:19056347 doi:S0006-291X(08)02267-5
7. ↑ Palma PN, Rodrigues ML, Archer M, Bonifacio MJ, Loureiro AI, Learmonth DA, Carrondo MA, Soares-da-Silva P. Comparative study of ortho- and meta-nitrated inhibitors of catechol-O-methyltransferase: interactions with the active site and regioselectivity of O-methylation. Mol Pharmacol. 2006 Jul;70(1):143-53. Epub 2006 Apr 17. PMID:16618795 doi:10.1124/mol.106.023119
8. ↑ Vidgren J, Svensson LA, Liljas A. Crystal structure of catechol O-methyltransferase. Nature. 1994 Mar 24;368(6469):354-8. PMID:8127373 doi:http://dx.doi.org/10.1038/368354a0
9. ↑ Rakvåg TT, Klepstad P, Baar C, Kvam TM, Dale O, Kaasa S, Krokan HE, Skorpen F. "Molecular Pain | Full Text | Genetic Variation in the Catechol-O-Methyltransferase (COMT) Gene and Morphine Requirements in Cancer Patients with Pain." Molecular Pain. Web. 25 Apr. 2012. <http://www.molecularpain.com/content/4/1/64>.
10. ↑ "Why is this medication prescribed?" Entacapone. 18 Dec. -0001. U.S. National Library of Medicine. 25 Apr. 2012 <http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0000168/>.
11. ↑ "Why is this medication prescribed?" Entacapone. 18 Dec. -0001. U.S. National Library of Medicine. 25 Apr. 2012 <http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0000168/>.