Histone acetyltransferase 1-2 Complex (HAT1/2): Difference between revisions

[https://proteopedia.org/wiki/index.php/Nucleosome Histones] are proteins found in the cell nucleus that are the key building blocks of [https://en.wikipedia.org/wiki/Chromatin chromatin], and are essential for proper DNA packaging and regulation of [https://en.wikipedia.org/wiki/Transcription_(biology) transcription]. In the first step of [https://www.hhmi.org/biointeractive/how-dna-packaged DNA packaging], two copies of the four core histone proteins ([https://en.wikipedia.org/wiki/Histone_H1 H1], [https://en.wikipedia.org/wiki/Histone_H2A H2A], [https://en.wikipedia.org/wiki/Histone_H3 H3], and [https://en.wikipedia.org/wiki/Histone_H4 H4]) form an [https://en.wikipedia.org/wiki/Histone_octamer octamer] that DNA wraps around, forming the [https://proteopedia.org/wiki/index.php/Nucleosome_structure nucleosome]. 20-24% of residues making up the histone octamer are arginine and lysine, causing a net positive charge, especially at the outer surfaces of the histone core where negatively charged DNA is bound (Figure 1). <ref> Watson, J D, et al. Molecular Biology of the Gene (Seventh Edition). (2014) Boston, MA: Benjamin-Cummings Publishing Company. </ref><ref name="Watanabe"> PMID: 20100606 </ref> The positively charged residues of the histone core tails are often subject to chemical modifications that can regulate the processes of DNA repair, replication, transcription, and heterochromatin maintenance.  

Histones can be reversibly modified in a variety of ways, including: methylation, acetylation, phosphorylation, and ubiquitination. These modifications all result in either the condensation or relaxation of DNA and consequently repressing or activating transcription. Histone acetylation is a histone modification that involves the transfer of an acetyl group from the co-factor Acetyl Coenzyme A ([https://en.wikipedia.org/wiki/Acetyl-CoA acetyl-CoA]) to the ε-amino group of a lysine residue in a histone protein. This reaction is catalyzed by various histone acetyltransferase ([https://en.wikipedia.org/wiki/Histone_acetyltransferase HAT]) enzymes. Lysine acetylation is an important [https://en.wikipedia.org/wiki/Epigenetics epigenetic] marker as it removes the positive charge of the lysine, thus reducing the strength of the interaction between DNA and hisotones. The net effect of acetylation is the relaxation of the DNA into [https://en.wikipedia.org/wiki/Euchromatin euchromatin] which is more transcriptionally active.

 

See [[Histone acetyltransferase]].

[[Image:Hat1.gif|400 px|left|thumb|Figure 2. Coenzyme A (CoA, green sticks) and the histone H4 peptide substrate (pink sticks) are shown in the binding groove of HAT1. The movie was made in PyMol using 4psw.pdb, then converted to gif format using EZgif.]] The CoA molecule is buried in a deep channel in the HAT1 subunit, where the free rotation of several bonds in the [https://en.wikipedia.org/wiki/Pantetheine pantetheine] group give the molecule a bent conformation (Figure 2). The conserved Arg/Gln-X-X-Gly-X-Gly/Ala <scene name='83/834210/Newcoa/11'>signature sequence</scene> (residues 227-232) of CoA containing proteins wraps around the 5'-diphosphate moiety of the CoA. This motif also positions the negatively charged β-phosphate group of CoA at the N-terminal dipole of the helix spanning residues 230-245, further stabilizing the interaction. The <scene name='83/834210/Hydrophobic_pocket/2'>β-mercaptoethylamine group</scene> of CoA rests in the hydrophobic pocket formed by the side chains of residues Ile-217,Pro-257 and Phe-261. In the HAT1-Acetyl coenzyme A structure determined without HAT2 and the H4 substrate (PDB: 1bob) the main chain atoms of <scene name='83/834210/Acetylcoa_structure/3'>Phe220</scene> were revealed to play a critical role in binding to acetyl-CoA. The carbonyl oxygen of Phe220 hydrogen bonds to the amine of the β-mercaptoethylamine group of the co-factor, while the Phe220 amide nitrogen hydrogen bonds to the acetyl group of acetyl-CoA.<ref name=”Dutnall”>PMID:10384314</ref> The latter interaction has important implications for the mechanism as described below.

An alternative mechanism (Figure 3) that is better supported by the HAT1-HAT2-histone H4 structure, proposes that the attacking lysine would be deprotonated upon entry into active site due the proximity of the acidic side chains of Glu255 and Asp256. Additionally, there are <scene name='83/834210/Mechanism_lys_carbonyls/3'>three carbonyl oxygens</scene> in the main chain of Ser218, Glu255, and Asp256 which are in hydrogen bonding distance with the ε-amine of the attacking lysine. They will act to withdraw positive charge from the attacking nitrogen improving its nucleophilic nature and perhaps better orient the lone pair electrons for nucleophillic attack. Additionally, the interaction of the Phe220 amide with the carbonyl oxygen of the acetyl group enhances the electrophilic nature the carbonyl carbon being attacked. In the second step of the reaction, the lone pair on the lysine attacks the carbonyl carbon of acetyl-CoA, forming an oxyanion containing tetrahedral transition state. The structure does not definitively reveal residues in an oxyanion hole that stabilize the transition state [[Image:HAT1_mechanism.png|400px|right|thumb|Figure 3: The proposed HAT1 Mechanism with the transferred acetyl group in red. (The carbonyl of Ser218 described in the text and <scene name='83/834210/Mechanism_lys_carbonyls/1'>green link</scene> is not shown activating the Lys12 nucleophile)]] but superposition of the HAT1-acetyl coenzyme A structure with the HAT1-HAT2-H4 substrate structure suggests <scene name='83/834210/Mechanism_final/5'>the main chain amide of Phe220</scene>, which binds the carbonyl oxygen of the acetyl group before attack, is a likely candidate. Finally, upon electron reorganization, the C-S scissile bond breaks leaving the H4Lys12 acetylated and Coenzyme A as products.