title: Sir2 protein deacetylases: evidence for chemical intermediates and functions of a conserved histidine.
abstract: Sir2 NAD+-dependent protein deacetylases are implicated in a variety of cellular processes such as apoptosis, gene silencing, life-span regulation, and fatty acid metabolism. Despite this, there have been relatively few investigations into the detailed chemical mechanism. Sir2 proteins (sirtuins) catalyze the chemical conversion of NAD+ and acetylated lysine to nicotinamide, deacetylated lysine, and 2'-O-acetyl-ADP-ribose (OAADPr). In this study, Sir2-catalyzed reactions are shown to transfer an 18O label from the peptide acetyl group to the ribose 1'-position of OAADPr, providing direct evidence for the formation of a covalent alpha-1'-O-alkylamidate, whose existence is further supported by the observed methanolysis of the alpha-1'-O-alkylamidate intermediate to yield beta-1'-O-methyl-ADP-ribose in a Sir2 histidine-to-alanine mutant. This conserved histidine (His-135 in HST2) activates the ribose 2'-hydroxyl for attack on the alpha-1'-O-alkylamidate. The histidine mutant is stalled at the intermediate, allowing water and other alcohols to compete kinetically with the attacking 2'-hydroxyl. Measurement of the pH dependence of kcat and kcat/Km values for both wild-type and histidine-to-alanine mutant enzymes confirms roles of this residue in NAD+ binding and in general-base activation of the 2'-hydroxyl. Also, transfer of an 18O label from water to the carbonyl oxygen of the acetyl group in OAADPr is consistent with water addition to the proposed 1',2'-cyclic intermediate formed after 2'-hydroxyl attack on the alpha-1'-O-alkylamidate. The effect of pH and of solvent viscosity on the kcat values suggests that final product release is rate-limiting in the wild-type enzyme. Implications of this new evidence on the mechanisms of deacetylation and possible ADP-ribosylation catalyzed by Sir2 deacetylases are discussed.
authors.affiliation: Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
authors.forename: Brian C
authors.initials: BC
authors.lastname: Smith
authors.forename: John M
authors.initials: JM
authors.lastname: Denu
mesh_terms: D000107:Acetylation; D000246:Adenosine Diphosphate Ribose / Q000378:metabolism; D000409:Alanine / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D001665:Binding Sites; D002384:Catalysis; D006639:Histidine / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009154:Mutation; D009243:NAD / Q000378:metabolism; D011487:Protein Conformation; D037761:Sirtuins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D021241:Spectrometry, Mass, Electrospray Ionization; D013379:Substrate Specificity; D013816:Thermodynamics
chemical_list: D009243:NAD; D000246:Adenosine Diphosphate Ribose; D006639:Histidine; D037761:Sirtuins; D000409:Alanine