Sirtuin Catalysis and Regulation

Abstract

Sirtuins are a family of NAD؉ -dependent protein deacetylases/deacylases that dynamically regulate transcription, metabolism, and cellular stress response. Their general positive link with improved health span in mammals, potential regulation of pathways mediated by caloric restriction, and growing links to human disease have spurred interest in therapeutics that target their functions. Here, we review the current understanding of the chemistry of catalysis, biological targets, and endogenous regulation of sirtuin activity. We discuss recent efforts to generate small-molecule regulators of sirtuin activity.Accumulating data indicate that lysine acetylation is a prevalent regulatory mechanism of protein function, with thousands of acetylated proteins identified by mass spectrometry (1-3). Sir2 (silent information regulator 2 or sirtuin) protein deacetylases are a class of evolutionarily conserved enzymes that function in critical cellular processes such as transcription, DNA repair, metabolism, and stress resistance (4). Among the major classes of lysine deacetylases, the sirtuins utilize a unique catalytic mechanism that consumes NAD ϩ , providing a direct connection between protein deacetylation and central metabolic pathways. There are seven human sirtuins (SIRT1-7), each with diverse subcellular localization and protein substrates (5). SIRT1-3 display robust deacetylation activity, whereas recent reports implicate SIRT5 as a protein desuccinylase and demalonylase (6). Thus, sirtuins can be considered deacylases. The activities of several other human sirtuins are unsettled. SIRT6 and SIRT7 display weak deacetylase activity in vitro, and SIRT4 was reported to harbor ADP-ribosyltransferase activity (7,8). Structural analysis of the sirtuin family members reveals a conserved catalytic core composed of two subdomains, a Rossmann fold domain at one end and a smaller, more variable zinc-binding domain at the opposite end (Fig. 1). The two domains are connected by several loops that form a binding cleft for the nicotinamide and ribose moieties of NAD ϩ and the acyllysine substrate. Several invariant amino acids are located in the cleft and are responsible for substrate binding and catalysis. The varying hydrophobicity and charge distribution of the acyl-substrate binding cleft allow for varied substrate selectivity among the different human sirtuins (6, 9). Given their regulatory role in transcription, metabolism, and genome maintenance, sirtuins are desirable targets for therapeutic development. In this minireview, we highlight the current molecular understanding of the chemical mechanism, regulation, and substrate selectivity of sirtuins. Unique ChemistrySirtuins catalyze NAD ϩ -dependent deacetylation of acetyllysine, resulting in the production of deacetylated lysine, nicotinamide, and 2Ј-O-acetyl-ADP-ribose (OAADPr) 3 (Fig. 2) (10). Kinetic and biochemical studies revealed that the enzyme binds the acetyllysine substrate prior to NAD ϩ . Nicotinamide is cleaved from NAD ϩ and is the first product released, follo...

Mitochondrial Sirtuins and Their Relationships with Metabolic Disease and Cancer

“…Tan et al described lysine crotonylation as novel histone PTM, which is specifically enriched at active gene promoters and potential enhancers in mammalian genome (155). A very recent study by Bao et al reported that SIRT3 possesses decrotonylase activity, using a similar catalytic mechanism as for acetyl lysine hydrolysis (10,36). siRNA-mediated SIRT3 FIG.…”

“…However, certain sirtuins are now known to possess alternative catalytic functions-for example, deacylase, decrotonylase, desuccinylase, demalonylase, deglutarylase, and ADP-ribosyltransferase activities (Fig. 1)-that play crucial roles in the regulation of diverse cellular processes (2,10,32,34,36,48,65,72,121,156).…”

The Arabidopsis Class II Sirtuin Is a Lysine Deacetylase and Interacts with Mitochondrial Energy Metabolism

“…S4). Because the true enzymatic function of the mammalian SIRT4 is still under debate (Feldman et al, 2012;Newman et al, 2012), we purified the two mature SRT2A and SRT2B proteins as N-terminally His-tagged proteins upon heterologous expression in E. coli. Strikingly, SRT2A and, to a slightly lesser extent, SRT2B were active as protein Lys deacetylases (Fig.…”

Activation of the Protein Deacetylase SIRT6 by Long-chain Fatty Acids and Widespread Deacylation by Mammalian Sirtuins

“…SIRT1-3 display robust protein deacetylase activity, and numerous studies have identified in vivo substrates (35,36). The recent observation that SIRT5 prefers succinylated/malonylated substrates helps explain the observed low activity on acetylated substrates (8).…”