High Quality Content by WIKIPEDIA articles! Silent Information Regulator Two (Sir2) proteins, or sirtuins, are a class of proteins which possess either histone deacetylase or mono-ribosyltransferase activity and are found in organisms ranging from bacteria to humans. Named after the yeast silent mating type information regulation two,[4] the gene responsible for cellular regulation in yeast, sirtuins regulate important biological pathways in eubacteria, archaea and eukaryotes. Yeast Sir2 and some, but not all, sirtuins are protein deacetylases. Unlike other known protein deacetylases, which simply hydrolyze acetyl-lysine residues, the sirtuin-mediated deacetylation reaction couples lysine deacetylation to NAD hydrolysis.