Sandbox Reserved 774: Difference between revisions

Has a chain structure with 2.4 A resolution, and 2.9 A resolution with a <scene name='56/564050/Test/1' target=1>co-factor</scene> (acetyl-CoA).<ref name=Shiva/> The method used to determine the structure was [[X-ray crystallography]]. Sedimentation and crystal structure analysis clearly shows that Hpa2 is dimeric in solution and tetramerizes in the unit crystal. The crystal structure of the oligomer reveals that two Hpa2 dimers are held together by interaction between the bound acetyl-CoA molecules. The average B-factor value is 23.9 (<scene name='56/564050/Bakhbone_mainechain/1' target=0>main chain</scene>) with a 25.4 <scene name='56/564050/Sidechain/2' target=0>side chain</scene>. The R-factor is 0.19. <ref name=Shiva/> Core fold features include four conserved sequence motifs of the GNAT family and comprises a central highly curved five stranded <scene name='56/564050/Beta_sheets/1' target=0>Beta sheets</scene> (β1-β5) surrounded on both sides by helical segments (α1 and α3).<ref name=Shiva/>

==Co-factor==

Most of the secondary structure elements of the monomer contribute residues involved in dimer contacts. A large part of the interface is formed by two projections from the core part of the monomer structure. The first projection is formed by the C-terminal end of strand B3, turn B3-B4, and the N-terminal end of strand B4, while the second is formed by strand B7. Together with strands B5 and B6 they form a barrel-like structure containing ten strands in which the component strands of the barrel locked together.<ref name=Shiva> Most importantly, strand b7 from each monomer interacts between strands b5 and b6 of the opposite monomer, which also extends the central sheet structure by two strands. Also, the two projections interact with residues from helices a1 and a2, turn a1 a2, turn a2 b2, and helices a3 and a4 of the opposite monomer. There are eight beta-strands, four <scene name='56/564050/Alpha_helices/2'>Alpha-helices</scene>, and ten turns.  

=Mechanism=

The binding of substrates and release of products can be random, fully ordered, or a combination of both. It operates on a Bi-Bi mechanism. A study that employed product inhibitors CoA and acetylated (Lys14Ac) H3 peptide and dead-end inhibitor desulfo-CoA  in order to determine the order of substrate binding has yielded results consistent with a fully ordered Bi-Bi kinetic mechanism where AcCoA is the first substrate to bind, and CoA is the last product that is released. It is also important to note that the transcriptional co-activator GCN5 from yeast (yGCN5) is a histone acetyltransferase that is essential for the activation of target genes. Bi-substrate kinetic analysis using acetyl-coenzyme A and an H3 histone synthetic peptide indicated that both substrates must bind to form a ternary complex before catalysis. Product inhibition studies revealed that the product CoA was a competitive inhibitor as opposed to AcCoA. Desulfo-CoA, a dead end inhibitor, also demonstrated simple competitive inhibition versus AcCoA. Acetylated (Lys14Ac) H3 peptide displayed noncompetitive inhibition against both H3 peptide and AcCoA.<ref>Tanner, et al. "Kinetic Mechanism of the Histone Acetyltransferase GCN5 from Yeast." J. Biol. Chem. 275.29 (2000): 2-9. Web. 26 Nov. 2013.</ref>

==Chemical Mechanism==