SIRT5 is a human member of a family of proteins called Sirtuins (Sir2-like proteins) and are present in prokaryotes and eukaryotes. All Sir2-like proteins have a sirtuin core domain, which contains a series of sequence motifs conserved in organisms ranging from bacteria to humans. Bacterial, yeast and mammalian sirtuins are able to metabolize NAD and possibly at as mono-ADP-ribosyltransferases. The enzymatic function of sirtuins is not yet completely understood but recent reports of histone-activated Sir2-mediated NAD metabolism and NAD-activated Sir2-mediated histone deacetylation suggest a possible coupled reciprocal activation mechanism involving interactions of Sir2 with NAD and the N epsilon-acetyl-lysine groups of acetylated histones.