Sirtuin enzyme and p53. Computer artwork of a sirtuin (Sir2) enzyme (pink) bound to a p53 peptide (orange). Sir2 enzymes form a unique class of NAD(+)-dependent deacetylases that are required for diverse biological processes, including transcriptional silencing, regulation of apoptosis (programmed cell death), fat mobilisation, and lifespan regulation. NAD is shown as a stick model (centre). p53 is crucial in multicellular organisms, where it regulates the cell cycle (the means by which cells replicate) and prevents the proliferation of cells with damaged DNA.