Proteopedia - User contributions [en]

Histamine H1 receptor

2026-03-23T19:51:11Z

Ann Taylor:

==Histamine H1 Receptor==
<StructureSection load='3RZE' size='340' side='right' caption='Histamine H1 receptor with an antagonist doxepin, lipid and phosphate (PDB code [[3rze]])' scene='78/784820/2ndary_structure_color/1'>
Allergy symptoms are mostly caused by the release of histamine in response to allergens. The binding of histamine to the extracellular portion of the '''H1 receptor''' triggers a structural change of the transmembrane portion, leading to a change in the C terminal area. This c terminal region interacts with G proteins, leading to the activation of the Gq signalling pathway, which triggers allergy symptoms like itchy eyes and runny noses. Many allergy drugs are anti-histamines, in that they bind to the histamine receptor but do not cause the conformational change that leads to a response. See also [[Receptor]].

== Structural highlights ==
The structure of the H1 histamine receptor bound to an antihistamine, doxepin was published in 2011 <ref>PMID:21697825</ref>. A <scene name='78/784820/N_to_c_rainbow/1'>N-->C rainbow</scene> view colors the N terminus blue and the C terminus red, with the intervening segments paralleling the rainbow (blue, green, yellow, orange, red). This image is oriented with the transmembrane section at the top and the cytosolic portion below. The <scene name='78/784820/Hydrophobic/2'>hydrophobic residues</scene> are shown in grey, while hydrophilic amino acids are shown in purple.

<scene name='78/784820/Doxepin_ball_stick/1'>Doxepin</scene> was originally made as a tricyclic antidepressant, but it also is a potent antihistamine <ref>PMID: 39202</ref> binds among the transmembrane alpha helices. Binding is stabilized by a number of <scene name='78/784820/Interacting_amino_acids/4'>interactions with amino acids</scene>. Like many G protein coupled receptors, the bottom of the binding pocket contains a conserved <scene name='78/784820/Trp_428/1'>tryptophan</scene> residue. Interestingly, second generation antihistamines take advantage of an anion binding site formed by <scene name='78/784820/Lys/2'>two lysine residues</scene>; in this structure, they interact with a phosphate.

Like other [[G protein-coupled receptor]]s, the Histamine H1 Receptor contains a <scene name='78/784820/Dry_motif/1'>conserved DRY</scene> (aspartate (D), arginine (R), tyrosine (Y)) motif in the seven helix transmembrane surface near <scene name='78/784820/Dry_motif/4'>the cytosolic face</scene>. In some G protein receptors, an "ionic lock" interaction between the asparate and arginine in this motif stabilizes the inactive state<ref>PMID:17192495</ref>; however, in the Histamine H1 receptor, Arginine 125 forms a hydrogen bond with <scene name='78/784820/Arg125_gln_416_salt_bridge/1'>glutamine 416</scene>, which stabilizes the inactive state.

A cryo-EM structure of the <scene name='78/784820/G_protein_receptor_complex/1'>histamine-bound H1 receptor association with the Gq protein</scene> has been published. Histamine activates receptor via interacting with the key <scene name='78/784820/Histamine_interactions/1'>residues</scene> of both transmembrane domain 3 (TM3) and TM6 to squash the binding pocket on the extracellular side and to open the cavity on the intracellular side for Gq engagement. This can be seen by <scene name='78/784820/Gln416_arg125_distance_bound/1'>the farther distance</scene> between arginine 125 and Gln 416; they are now 2 angstroms (0.2 nm) farther apart.
See also:
* [[G protein-coupled receptor]]
*[[Receptor]]
*[[Transmembrane (cell surface) receptors]]
*[[Neurotransmitters]]
</StructureSection>
==3D structures of histamine H1 receptor==

Updated on {{REVISIONDAY2}}-{{MONTHNAME|{{REVISIONMONTH}}}}-{{REVISIONYEAR}}

[[3rze]] - hHHR + doxepin + lipid + phosphate - human 
[[7dfl]], [[8yn2]], [[9lrb]], [[9lrd]] - hHHR + guanine nucleotide-binding protein + scFv + histamine - Cryo EM 
[[8x5x]], [[8x5y]] – hHHR/B562- Cryo EM 
[[8x63]] – hHHR/B562 + mepyramine - Cryo EM 
[[8x64]] – hHHR/B562 + desloratadine - Cryo EM 

== References ==
<references/>
[[Category:Topic Page]]

Histamine H1 receptor

2026-03-23T19:46:01Z

Ann Taylor:

==Histamine H1 Receptor==
<StructureSection load='3RZE' size='340' side='right' caption='Histamine H1 receptor with an antagonist doxepin, lipid and phosphate (PDB code [[3rze]])' scene='78/784820/2ndary_structure_color/1'>
Allergy symptoms are mostly caused by the release of histamine in response to allergens. The binding of histamine to the extracellular portion of the '''H1 receptor''' triggers a structural change of the transmembrane portion, leading to a change in the C terminal area. This c terminal region interacts with G proteins, leading to the activation of the Gq signalling pathway, which triggers allergy symptoms like itchy eyes and runny noses. Many allergy drugs are anti-histamines, in that they bind to the histamine receptor but do not cause the conformational change that leads to a response. See also [[Receptor]].

== Structural highlights ==
The structure of the H1 histamine receptor bound to an antihistamine, doxepin was published in 2011 <ref>PMID:21697825</ref>. A <scene name='78/784820/N_to_c_rainbow/1'>N-->C rainbow</scene> view colors the N terminus blue and the C terminus red, with the intervening segments paralleling the rainbow (blue, green, yellow, orange, red). This image is oriented with the transmembrane section at the top and the cytosolic portion below. The <scene name='78/784820/Hydrophobic/2'>hydrophobic residues</scene> are shown in grey, while hydrophilic amino acids are shown in purple.

<scene name='78/784820/Doxepin_ball_stick/1'>Doxepin</scene> was originally made as a tricyclic antidepressant, but it also is a potent antihistamine <ref>PMID: 39202</ref> binds among the transmembrane alpha helices. Binding is stabilized by a number of <scene name='78/784820/Interacting_amino_acids/4'>interactions with amino acids</scene>. Like many G protein coupled receptors, the bottom of the binding pocket contains a conserved <scene name='78/784820/Trp_428/1'>tryptophan</scene> residue. Interestingly, second generation antihistamines take advantage of an anion binding site formed by <scene name='78/784820/Lys/2'>two lysine residues</scene>; in this structure, they interact with a phosphate.

Like other [[G protein-coupled receptor]]s, the Histamine H1 Receptor contains a <scene name='78/784820/Dry_motif/1'>conserved DRY</scene> (aspartate (D), arginine (R), tyrosine (Y)) motif in the seven helix transmembrane surface near <scene name='78/784820/Dry_motif/4'>the cytosolic face</scene>. In some G protein receptors, an "ionic lock" interaction between the asparate and arginine in this motif stabilizes the inactive state<ref>PMID:17192495</ref>; however, in the Histamine H1 receptor, Arginine 125 forms a hydrogen bond with <scene name='78/784820/Arg125_gln_416_salt_bridge/1'>glutamine 416</scene>, which stabilizes the inactive state.

A cryo-EM structure of the <scene name='78/784820/G_protein_receptor_complex/1'>histamine-bound H1 receptor association with the Gq protein</scene> has been published. Histamine activates receptor via interacting with the key <scene name='78/784820/Histamine_interactions/1'>residues</scene> of both transmembrane domain 3 (TM3) and TM6 to squash the binding pocket on the extracellular side and to open the cavity on the intracellular side for Gq engagement. This can be seen by the farther distance</scene> between arginine 125 and Gln 416; they are now 2 angstroms (0.2 nm) farther apart.
See also:
* [[G protein-coupled receptor]]
*[[Receptor]]
*[[Transmembrane (cell surface) receptors]]
*[[Neurotransmitters]]
</StructureSection>
==3D structures of histamine H1 receptor==

Updated on {{REVISIONDAY2}}-{{MONTHNAME|{{REVISIONMONTH}}}}-{{REVISIONYEAR}}

[[3rze]] - hHHR + doxepin + lipid + phosphate - human 
[[7dfl]], [[8yn2]], [[9lrb]], [[9lrd]] - hHHR + guanine nucleotide-binding protein + scFv + histamine - Cryo EM 
[[8x5x]], [[8x5y]] – hHHR/B562- Cryo EM 
[[8x63]] – hHHR/B562 + mepyramine - Cryo EM 
[[8x64]] – hHHR/B562 + desloratadine - Cryo EM 

== References ==
<references/>
[[Category:Topic Page]]

Basics of Protein Structure

2026-02-10T05:08:02Z

Ann Taylor:

<StructureSection load='3I40' size='350' side='right' caption='Structure of insulin (PDB entry [[3I40]])' scene='' pspeed='8'>

''This tutorial illustrates some basic properties of protein structure for a general audience. For a more in depth discussion, please visit [[Introduction to protein structure]]. Words shown in green change the protein view in the box to the right; blue words are links to other pages.''

Proteins perform many important functions in living organisms, including movement, immune responses, sensing the environment, energy acquisition, and catalyzing reactions. The protein shown to the right is insulin; when insulin isn't properly synthesized or recognized, diabetes occurs.

Proteins are long [[chains]] of [[Amino Acids|amino acids]], and are synthesized by the [[ribosome]], using messenger [[RNA]] as a template. There are 20 amino acids commonly found in proteins. <scene name='60/604417/Ala/2'>Amino acids</scene> contain an <scene name='60/604417/Ala_amino/1'>amino group</scene>, a central carbon atom called the <scene name='60/604417/Ala_alpha/1'>alpha carbon</scene>, and a <scene name='60/604417/Ala_cooh/1'>carboxylic acid</scene>. The 20 amino acids differ by what is attached to the central atom; is variable portion is referred to as the <scene name='60/604417/Ala_side_chain/1'>side chain</scene>. The amino acid shown is alanine; its side chain is a methyl (-CH3) group. The atoms are displayed using the [[CPK|coloring convention]] '''Carbon,  Hydrogen , Oxygen, Nitrogen''': {{Template:ColorKey_Element_C}}, {{Template:ColorKey_Element_H}}, {{Template:ColorKey_Element_O}}, {{Template:ColorKey_Element_N}}.

Proteins are sometimes compared to <scene name='60/604417/Ins_bead_backbone/3'>beads on a string</scene>, where each amino acid residue is a bead. These long chains form complicated structures that allow them to perform their function. Even small alterations in any level of the structure can change how the protein does its job, and can lead to diseases.

== Ways of representing protein structure ==
Protein structures can be displayed in many different ways. In <scene name='60/604417/Spacefill/1'>spacefilling</scene> models, all of the non-hydrogen atoms are shown as spheres with their van der Waals radii. This view is the easiest to use to see holes, clefts or other large scale features, but it is hard to identify individual amino acids or finer structural details. In the <scene name='60/604417/Ball_and_stick/1'>ball and stick</scene> model, the atoms are shown as smaller balls, connected by sticks; this is further simplified in the <scene name='60/604417/Stick/1'>stick</scene> model, which only shows the bonds between atoms. <scene name='60/604417/Backbone/1'>Backbone</scene> representation shows only the N-Calpha-C=O repeating unit; the side chains are omitted. The <scene name='60/604417/Cartoon/1'>cartoon</scene> representation is based upon the backbone, but highlights specific secondary structures (more on that later!).

== Levels of Protein Structure ==
There are [[Four levels of protein structure|four different levels of protein structure]]. The <scene name='60/604417/Ins_bead_backbone_labels/1'>primary structure</scene> is the amino acid sequence. The amino acids are connected by an amide bond, made from the amino group (NH2) of one amino acid, and the carboxylic acid (C=O) from another amino acid. In the process of making the bond, a water molecule is removed. The amino acids are linked in a repeating pattern. The [[Backbone representations|backbone]] of the protein is the repeating <scene name='60/604417/N_calpha_co/2'>N-C-C=O</scene> pattern, with the <scene name='60/604417/Side_chains/1'>side chains</scene> projecting out from the backbone. The end with the free -NH2 group is called the Amino or <scene name='60/604417/N_terminus/1'>N terminus</scene>, while the end with a free carboxylic acid is called the <scene name='60/604417/C_terminus/1'>C terminus</scene>. Notice that most protein structure representations do not show the hydrogens. The sequence of amino acids is written and numbered from the N terminus (where protein synthesis begins) to the C terminus (where amino acids are added during protein synthesis), so for <scene name='60/604417/N_to_c/1'>the segment shown</scene>, the sequence would be Val-Asn-Gln, or VNQ, if one letter abbreviations are used for the amino acids. For more practice identifying peptide bonds between amino acids, please try [[User:Stephen Mills/Peptide tutorial 1|Peptide tutorial 1 part 1]] and [[User:Stephen Mills/Peptide tutorial 2|Peptide tutorial 1 part 2]].

The second level of structure is called secondary structure, and is the shapes (conformations) formed by short sequences of amino acids. This level of structure is stabilized by <scene name='60/604417/H_bonds/2'>hydrogen bonds</scene> along the backbone. Hydrogen bonds are attractions between an N, O or F and a hydrogen attached to an N, O or F (More about [[hydrogen bonds]].) The two most common shapes are [[Helices in Proteins|alpha helices]] and [[Sheets in Proteins|beta strands]]. These are favored simply because [[Tutorial:Ramachandran principle and phi psi angles|two atoms cannot occupy the same space]] (steric collisions). Insulin only contains <scene name='60/604417/Secondary_structure/1'>alpha helices</scene>; they are shown in pink.

The third level of structure, or tertiary structure, is how the secondary structures pack together to form the overall form of the entire peptide chain. Side chains play an important role in tertiary structure formation, especially the burying of hydrophobic ("water fearing") amino acids in the middle of the structure. In <scene name='60/604417/Hexamer_hydrophobicity/1'>this view</scene>, {{Template:ColorKey_Hydrophobic}} residues are grey and {{Template:ColorKey_Polar}} atoms are shown in light purple. Water molecules are shown with red balls; notice that they tend to be close to the hydrophilic (water loving) groups. Some proteins, like insulin, are also stabilized by<scene name='60/604417/Disulfide_bonds/1'> covalent bonds between the sulfur atoms</scene> (shown in yellow) called disulfide bonds.

Not all proteins have the fourth level of structure, quaternary structure. Quaternary structure is the association of more than one chain to form a larger structure. Insulin forms a <scene name='60/604417/Hexamer/2'>hexamer</scene>. In this view, each insulin monomer is shown in a different color. Quaternary structure can be very important in how the protein functions. Minor changes in insulin's sequence leads to tighter or weaker association between the chains, and is the difference between long lasting and quick acting insulin. For a more in depth discussion about insulin's structure and function, please visit the [[Insulin]] page.

==Protein Structure Data==
The [[Protein Data Bank|World Wide Protein Data Bank]] (WWPDB) is where all experimentally-determined published protein structures are made freely available. Each model has a unique accession code, called a [[PDB code]]. One model of human insulin, shown at right has the PDB code [[3i40]]. Many examples are illustrated in the [http://atlas.molviz.org Atlas of Macromolecules]. Looking for a model of a specific protein? See [[Practical_Guide_to_Homology_Modeling#Is_there_an_empirical_model.3F|Is there an empirical model?]] After you find a PDB code of interest, see [[Introduction to molecular visualization]].

==Further Reading==

* [[The Building Blocks]]
** [[Amino Acids]]
** [[Peptide]]

** [[Phi and Psi Angles]]
** [[Ramachandran Plots]]

* [[Four levels of protein structure]] -- [[Four levels of protein structure (Spanish)|Los cuatro niveles estructurales de las proteínas]]
* [[Secondary structure]]
** [[Helices in Proteins]]
*** [[User:Stephen Mills/Secondary Structure: Helices|Secondary Structure tutorial - Helices]]
** [[Sheets in Proteins]]
*** [[User:Stephen Mills/Secondary Structure: Sheets|Secondary Structure tutorial - Sheets]]
** [[Tutorial:Ramachandran principle and phi psi angles]] also available as a [http://tinyurl.com/RamachandranPrincipleYouTube YouTube Video]
* [[Quaternary structure]]

* [[Highest impact structures]]
* [[Molecular sculpture]]
* [[Introduction to molecular visualization]]
</StructureSection>
__NOTOC__
__NOEDITSECTION__

2024-10-30T03:32:54Z

2023-12-05T01:52:45Z

Ann Taylor:

[[Cas9]] is the RNA-guided [[DNA]] [[endonuclease]] used by the CRISPR (clustered regularly interspaced short palindromic repeats)-associated systems to generate double-strand DNA breaks in the invading DNA during an adaptive bacterial immune response.

See also [[Cas9 (hebrew)]].

The CRISPR-associated endonuclease [[Cas9]] has been exploited for use in genome editing systems. In such systems, an engineered single-guide RNA (sgRNA) is used to target double-stranded breaks in genomic DNA. Depending on what repair pathway is triggered, often dictated by the inclusion of additional engineered components, the targeted site either is disrupted or incorporates additional genetic sequences.

[[Image:Layout for schematic and structure with structure.png|660px]]

<center><html5media height=“315” width=“560” frameborder="0" allowfullscreen>https://www.youtube.com/embed/TdBAHexVYzc</html5media></center>
<center>Geneticist and 2020 Nobel laureate [http://rna.berkeley.edu/ Jennifer Doudna], from UC Berkeley, is one of the co-inventors of the groundbreaking </center>
<center>new technology for editing genes, called CRISPR-Cas9. The tool allows scientists</center>
<center>to make precise edits to DNA strands, which could lead to treatments for genetic diseases … </center>
<center>but could also be used to create so-called "designer babies." </center>
<center>Doudna reviews how CRISPR-Cas9 works — and asks the scientific community </center>
<center>to pause and discuss the ethics of this new tool.</center>

<center> Microbiologist and 2020 Nobel laureate [https://www.emmanuelle-charpentier-lab.org/ Emanuelle Charpentier], from Max Planck Institute for Infection Biology in Berlin, is one of the co-inventors of the groundbreaking </center>
<center>new technology for editing genes, called CRISPR-Cas9. The tool allows scientists</center>
<center>to make precise edits to DNA strands, which could lead to treatments for genetic diseases … </center>
<center>but could also be used to create so-called "designer babies." </center>
<center>Charpentier reviews how CRISPR-Cas9 works in [https://www.cnn.com/videos/tech/2016/04/27/crispr-cas9-explainer-natpkg.cnn/video/playlists/fertility-health/ this 2016 talk]. </center>

<center><html5media height=“315” width=“560” frameborder="0" allowfullscreen>https://www.youtube.com/embed/MnYppmstxIs</html5media>
 This video, by Paul Andersen, explains how the CRISPR/Cas immune system was identified in bacteria and how the CRISPR/Cas9 system was developed to edit genomes.</center>
 

<center><html5media height=“315” width=“560” frameborder="0" allowfullscreen>https://www.youtube.com/embed/2pp17E4E-O8</html5media></center>
<center>Movie from the [http://mcgovern.mit.edu/ McGovern Institute for Brain Research at MIT]</center>
<center>This animation depicts the CRISPR-Cas9 method for genome editing: a powerful new technology with many applications in biomedical research, including the potential to treat human genetic disease.</center>

Articles in Proteopedia concerning Cas9 include:
* [[Cas9 Sandbox|Brett Thumm's Student Project page on Cas9]] 
*[[Cas9 (hebrew)]].

==3D structures of Cas9==

See [[Endonuclease 3D structures]].

[[Image:4un3 labeled.png|right|390px]]

 

[[Category: Crispr]]
[[Category: Crispr-associated]]
[[Category: endonuclease]]

==STRUCTURE OF Cas9 IN STAPHYLOCOCCUS AUREUS IN COMPLEX WITH sgRNA==
<StructureSection load='5axw' size='340' side='right'caption='[[5axw]], [[Resolution|resolution]] 2.70Å' scene=''>
== Cas9 Overview ==
CRISPR is a bacterial immune response to bacteriophages to prevent subsequent infections. CRISPR is a form of acquired immunity used by bacteria. CRISPR stands for clustered regularly interspaced short palindromic repeats because the bacterial genome includes genetic sequences clustered together from bacteriophages of previous infections that are used by Cas9 to cut viral DNA. Within the CRISPR system, Cas9 is a protein responsible for cutting the viral DNA, rendering it inert. <scene name='92/925538/Cas9_overview/4'>Cas9</scene> structure in Staphylococcus aureus (SaCas9) utilizes a single-stranded guide RNA (sgRNA) to complimentarily bind the target DNA that will create a double stranded DNA cut in the proper location. The target DNA must also have a PAM sequence to bind for Cas9 to cut target DNA. The PAM sequence stands for protospacer adjacent motif and is downstream from the cut site of the nuclease. The PAM sequence acts as a two-factor authentication in junction with the sgRNA that tells the Cas9 to cut this portion of DNA.

The main domains in the <scene name='92/925538/Lobes_and_linkers/4'>Cas9</scene> are the <scene name='92/925538/Lobes_and_linkers/16'>REC lobe</scene> (residues 41–425) and <scene name='92/925538/Lobes_and_linkers/15'>NUC lobe</scene> (residues 1–40 and 435–1053). The REC lobe stands for the recognition lobe is responsible for recognizing the target DNA. The NUC (nuclease) lobe contains RuvC, HNH, WED, and PI domains <ref name="Cas9">PMID:26317473</ref>; each of these domains are involved in how Cas9 cuts the target DNA <ref>PMID:15596446</ref>,<ref>PMID:24634220</ref>,<ref>PMID:24529477</ref>. These lobes are connected by an arginine rich bridge helix (residues 41–73) and a linker loop (residues 426–434). Cas9 has four main mechanisms that are important for successful cleavage, including recognition of the sgRNA-target heteroduplex, recognition of the PAM sequence, recognition of the sgRNA scaffold, and endonuclease activity by HNH and RuvC.

== Recognition of the sgRNA-target heteroduplex ==
The recognition of the sgRNA-target heteroduplex in Cas9 begins by inserting the heteroduplex into the central channel between the REC and NUC lobes. A heteroduplex is the binding of the complimentary strands of the sgRNA and target DNA. The REC lobe and bridge helix interacts with the seed region of the <scene name='92/925538/Lobes_and_linkers/17'>sgRNA</scene> (C13-C20). The positive charged residues on the bridge helix (Asn44, Arg48, Arg51, Arg55, Arg59, and Arg60) and REC lobe (Arg116, Arg165, Asn169, and Arg209) interact with the negative phosphate backbone. The seed region is in the <scene name='92/925538/Lobes_and_linkers/8'>A-form conformation</scene>, so it can bind the target DNA. Only the REC lobe interacts with the PAM distal region pf the sgRNA (A3-U6) through the <scene name='92/925538/Lobes_and_linkers/21'>sugar-phosphate backbone</scene> (the hydrogen bonds are shown as black dashes). The target DNA binds to the <scene name='92/925538/Lobes_and_linkers/9'>REC lobe and RuvC domain</scene> for the proper conformation for base paring between the target DNA and sgRNA<ref name="Cas9" />.

== Recognition of the PAM sequence ==
For the recognition of the <scene name='92/925538/Lobes_and_linkers/22'>PAM sequence</scene>, the target DNA with the PAM sequence (5’-NNGRRN-3’) is bound to SaCas9 through hydrogen bonds as well as direct and water mediated hydrogen bonds through the major groove in the PI domain. This PAM sequence is differnt that other PAM sequences like the one found in SpCas9 (5'-NGG-3'). The WED domain recognizes the minor groove phosphate backbone of the duplex <ref name="Cas9" />.

== Recognition of the sgRNA scaffold ==
The SaCas9 recognizes the sgRNA scaffold within the <scene name='92/925538/Lobes_and_linkers/20'>REC lobe and WED domain</scene>. The WED contains five stranded beta sheets flanked with four alpha helices to allow binding of the repeat: anti-repeat duplex. REC lob binds the scaffold and secures it into the SaCas9 <ref name="Cas9" />.

== Endonuclease Activity of Cas9 ==
Finally, <scene name='92/925538/Lobes_and_linkers/12'>RuvC and HNH</scene> are involved in endonuclease activity. Binding to the target DNA causes a conformational change in the [[H-N-H motif]]<ref>PMID:30555184</ref>, a conserved endonuclease structure, named for its characteristic histidine-asparagine-histidine conserved residues. RuvC uses two manganese ions to cleave the non-target DNA through manganese coordinating with the phosphate backbone and aspartic acid residues. These phosphate oxygens coordinated with the manganese makes the phosphate a greater target for nucleophillic attack. A histidine then acts as a base to create a hydroxide nucleophile that attacks the phosphate bond and cleaves the non-target DNA. The binding of the RuvC to the target DNA changes the conformation of a linker protein region between the RuvC domain and the HNH domain. The conformational change of the linker brings the HNH domain close enough to the target DNA to cut the DNA. This linker conformational change is not present in the crystal structure, therefore the HNH appears to be far from the target DNA and in an inactive state. The HNH follows a similar mechanism as to RuvC using a histidine base to create a hydroxide ion nucleophile that attacks the phosphate bond using one manganese ion instead of two. This is modeled as manganese however, it magnesium is used in cells <ref name="Cas9" />.
</structuresection>
==References==

<references />