Silent information regulator 2 (Sir2) proteins, also known as sirtuins, are a family of nicotine adenine dinucleotide (NAD) dependent protein deacetylases in organisms ranging from bacteria to humans that are characterized by the presence of a unique and highly conserved catalytic domain of approximately 260 amino acids. Sirtuins are divided into 5 classes, including Class I (subclasses Ia, Ib and Ic), Class II, Class III, Class IV (subclasses IVa and IVb) and Class U (Gram-positive bacteria specific). The catalytic domain of sirtuins is comprised of two bilobed globular domains, the large domain containing the NAD-binding pocket and the small domain binding the acetyl-lysine substrate.