Acetyl-lysine analog peptides as mechanistic probes of protein deacetylases

Class III histone deacetylases (Sir2 or sirtuins) catalyze the NAD+-dependent conversion of acetyl-lysine residues to nicotinamide, 2'-O-acetyl-ADP-ribose (OAADPr), and deacetylated lysine. Class I and II HDACs utilize a different deacetylation mechanism, utilizing an active site zinc to direct hydrolysis of acetyl-lysine residues to lysine and acetate. Here, using ten acetyl-lysine analog peptides, we have probed the substrate binding pockets of sirtuins and investigated the catalytic differences among sirtuins and class I and II deacetylases. For the sirtuin Hst2, acetyl-lysine analog peptide binding correlated with the hydrophobic substituent parameter pi with a slope of -0.35 from a plot of log Kd versus pi. Interestingly, propionyl- and butyryl-lysine peptides were found to bind tighter to Hst2 compared with acetyl-lysine peptide and showed measurable rates of catalysis with Hst2, Sirt1, Sirt2, and Sirt3, suggesting propionyl- and butyryl-lysine proteins may be sirtuin substrates in vivo. Unique among the acetyl-lysine analog peptides examined, homocitrulline peptide produced ADP-ribose instead of the corresponding OAADPr analog. The electron-withdrawing nature of each acetyl analog had a profound impact on the deacylation rate between deacetylase classes. The rate of catalysis with the acetyl-lysine analog peptides varied over five orders of magnitude with the class III deacetylase Hst2, revealing a linear free energy relationship with a slope of -1.57 when plotted versus the Taft constant, sigma*. HDAC8, a class I deacetylase, displayed the opposite trend with a slope of +0.79. These results are applicable toward the development of selective substrates and other mechanistic probes of protein deacetylases.     

The Sir 2 family of protein deacetylases

The importance of NAD(+)-dependent deacetylases (Sir 2 family or sirtuins) in cell survival, ageing and apoptosis has ignited a flurry of both chemical and cellular investigations aimed at understanding this unique class of enzymes. This review focuses on recent mechanistic advances that highlight structure, catalysis, substrate recognition and interactions with small-molecule effectors. Recent X-ray structures revealed binding sites for both NAD(+) and acetyl-peptide. Biochemical studies support a two-step chemical mechanism involving the initial formation of a 1'-O-alkylamidate adduct formed between the acetyl-group and the nicotinamide ribose of NAD(+). Acetyl transfer to the 2' ribose and addition of water yield deacetylated peptide and 2'-O-acetyl-ADP-ribose, a potential second messenger. Also, the molecular basis of nicotinamide inhibition was revealed, and sirtuin activators (resveratrol) and inhibitors (sirtinol and splitomicin) were identified through small-molecule library screening.     

Sir2 protein deacetylases: evidence for chemical intermediates and functions of a conserved histidine

Sir2 NAD+-dependent protein deacetylases are implicated in a variety of cellular processes such as apoptosis, gene silencing, life-span regulation, and fatty acid metabolism. Despite this, there have been relatively few investigations into the detailed chemical mechanism. Sir2 proteins (sirtuins) catalyze the chemical conversion of NAD+ and acetylated lysine to nicotinamide, deacetylated lysine, and 2'-O-acetyl-ADP-ribose (OAADPr). In this study, Sir2-catalyzed reactions are shown to transfer an 18O label from the peptide acetyl group to the ribose 1'-position of OAADPr, providing direct evidence for the formation of a covalent alpha-1'-O-alkylamidate, whose existence is further supported by the observed methanolysis of the alpha-1'-O-alkylamidate intermediate to yield beta-1'-O-methyl-ADP-ribose in a Sir2 histidine-to-alanine mutant. This conserved histidine (His-135 in HST2) activates the ribose 2'-hydroxyl for attack on the alpha-1'-O-alkylamidate. The histidine mutant is stalled at the intermediate, allowing water and other alcohols to compete kinetically with the attacking 2'-hydroxyl. Measurement of the pH dependence of kcat and kcat/Km values for both wild-type and histidine-to-alanine mutant enzymes confirms roles of this residue in NAD+ binding and in general-base activation of the 2'-hydroxyl. Also, transfer of an 18O label from water to the carbonyl oxygen of the acetyl group in OAADPr is consistent with water addition to the proposed 1',2'-cyclic intermediate formed after 2'-hydroxyl attack on the alpha-1'-O-alkylamidate. The effect of pH and of solvent viscosity on the kcat values suggests that final product release is rate-limiting in the wild-type enzyme. Implications of this new evidence on the mechanisms of deacetylation and possible ADP-ribosylation catalyzed by Sir2 deacetylases are discussed.     

Sirtuin-independent effects of nicotinamide on lifespan extension from calorie restriction in yeast

Two models have been proposed for how calorie restriction (CR) enhances replicative longevity in yeast: (i) suppression of rDNA recombination through activation of the sirtuin protein deacetylase Sir2 or (ii) decreased activity of the nutrient-responsive kinases Sch9 and TOR. We report here that CR increases lifespan independently of all Sir2-family proteins in yeast. Furthermore, we demonstrate that nicotinamide, an inhibitor of Sir2-mediated deacetylation, interferes with lifespan extension from CR, but does so independent of Sir2, Hst1, Hst2, and Hst4. We also find that 5 mm nicotinamide, a concentration sufficient to inhibit other sirtuins, does not phenocopy deletion of HST3. Thus, we propose that lifespan extension by CR is independent of sirtuins and that nicotinamide has sirtuin-independent effects on lifespan extension by CR.     

Insights into the sirtuin mechanism from ternary complexes containing NAD+ and acetylated peptide

Sirtuin proteins comprise a unique class of NAD+-dependent protein deacetylases. Although several structures of sirtuins have been determined, the mechanism by which NAD+ cleavage occurs has remained unclear. We report the structures of ternary complexes containing NAD+ and acetylated peptide bound to the bacterial sirtuin Sir2Tm and to a catalytic mutant (Sir2Tm(H116Y)). NAD+ in these structures binds in a conformation different from that seen in previous structures, exposing the alpha face of the nicotinamide ribose to the carbonyl oxygen of the acetyl lysine substrate. The NAD+ conformation is identical in both structures, suggesting that proper coenzyme orientation is not dependent on contacts with the catalytic histidine. We also present the structure of Sir2Tm(H116A) bound to deacteylated peptide and 3'-O-acetyl ADP ribose. Taken together, these structures suggest a mechanism for nicotinamide cleavage in which an invariant phenylalanine plays a central role in promoting formation of the O-alkylamidate reaction intermediate and preventing nicotinamide exchange.     

Small molecule regulation of Sir2 protein deacetylases

The Sir2 family of histone/protein deacetylases (sirtuins) is comprised of homologues found across all kingdoms of life. These enzymes catalyse a unique reaction in which NAD+ and acetylated substrate are converted into deacetylated product, nicotinamide, and a novel metabolite O-acetyl ADP-ribose. Although the catalytic mechanism is well conserved across Sir2 family members, sirtuins display differential specificity toward acetylated substrates, which translates into an expanding range of physiological functions. These roles include control of gene expression, cell cycle regulation, apoptosis, metabolism and ageing. The dependence of sirtuin activity on NAD+ has spearheaded investigations into how these enzymes respond to metabolic signals, such as caloric restriction. In addition, NAD+ metabolites and NAD+ salvage pathway enzymes regulate sirtuin activity, supporting a link between deacetylation of target proteins and metabolic pathways. Apart from physiological regulators, forward chemical genetics and high-throughput activity screening has been used to identify sirtuin inhibitors and activators. This review focuses on small molecule regulators that control the activity and functions of this unusual family of protein deacetylases.     

Sir2 regulation by nicotinamide results from switching between base exchange and deacetylation chemistry

Life span regulation and inhibition of gene silencing in yeast have been linked to nicotinamide effects on Sir2 enzymes. The Sir2 enzymes are NAD(+)-dependent protein deacetylases that influence gene expression by forming deacetylated proteins, nicotinamide and 2'-O-acetyl-ADPR. Nicotinamide is a base-exchange substrate as well as a biologically effective inhibitor. Characterization of the base-exchange reaction reveals that nicotinamide regulates sirtuins by switching between deacetylation and base exchange. Nicotinamide switching is quantitated for the Sir2s from Archeaglobus fulgidus (Sir2Af2), Saccharomyces cerevisiae (Sir2p), and mouse (Sir2alpha). Inhibition of deacetylation was most effective for mouse Sir2 alpha, suggesting species-dependent development of this regulatory mechanism. The Sir2s are proposed to form a relatively stable covalent intermediate between ADPR and the acetyl oxygen of the acetyllysine-protein substrate. During the lifetime of this intermediate, nicotinamide occupation of the catalytic site determines the fate of the covalent complex. Saturation of the nicotinamide site for mouse, yeast, and bacterial Sir2s causes 95, 65, and 21% of the intermediate, respectively, to return to acetylated protein. The fraction of the intermediate committed to deacetylation results from competition between the nicotinamide and the neighboring 2'-hydroxyl group at the opposite stereochemical face. Nicotinamide switching supports the previously proposed Sir2 catalytic mechanism and the existence of a 1'-O-peptidyl-ADPR.Sir2 intermediate. These findings suggest a strategy for increasing Sir2 enzyme catalytic activity in vivo by inhibition of chemical exchange but not deacetylation.     

Structural basis for nicotinamide inhibition and base exchange in Sir2 enzymes

The Sir2 family of proteins consists of broadly conserved NAD(+)-dependent deacetylases that are implicated in diverse biological processes, including DNA regulation, metabolism, and longevity. Sir2 proteins are regulated in part by the cellular concentrations of a noncompetitive inhibitor, nicotinamide, that reacts with a Sir2 reaction intermediate via a base-exchange reaction to reform NAD(+) at the expense of deacetylation. To gain a mechanistic understanding of nicotinamide inhibition in Sir2 enzymes, we captured the structure of nicotinamide bound to a Sir2 homolog, yeast Hst2, in complex with its acetyl-lysine 16 histone H4 substrate and a reaction intermediate analog, ADP-HPD. Together with related biochemical studies and structures, we identify a nicotinamide inhibition and base-exchange site that is distinct from the so-called "C pocket" binding site for the nicotinamide group of NAD(+). These results provide insights into the Sir2 mechanism of nicotinamide inhibition and have important implications for the development of Sir2-specific effectors.     

Structure of Sir2Tm bound to a propionylated peptide

Lysine propionylation is a recently identified post‐translational modification that has been observed in proteins such as p53 and histones and is thought to play a role similar to acetylation in modulating protein activity. Members of the sirtuin family of deacetylases have been shown to have depropionylation activity, although the way in which the sirtuin catalytic site accommodates the bulkier propionyl group is not clear. We have determined the 1.8 Å structure of a Thermotoga maritima sirtuin, Sir2Tm, bound to a propionylated peptide derived from p53. A comparison with the structure of Sir2Tm bound to an acetylated peptide shows that hydrophobic residues in the active site shift to accommodate the bulkier propionyl group. Isothermal titration calorimetry data show that Sir2Tm binds propionylated substrates more tightly than acetylated substrates, but kinetic assays reveal that the catalytic rate of Sir2Tm deacylation of propionyl‐lysine is slightly reduced to acetyl‐lysine. These results serve to broaden our understanding of the newly identified propionyl‐lysine modification and the ability of sirtuins to depropionylate, as well as deacetylate, substrates.     

Mitochondrial Metabolism,Sirtuins, and Aging

The sirtuins are a family of proteins that act predominantly as nicotinamide adenine dinucleotide (NAD)-dependent deacetylases. In mammals seven sirtuin family members exist, including three members, Sirt3, Sirt4, and Sirt5, that localize exclusively within the mitochondria. Although originally linked to life-span regulation in simple organisms, this family of proteins appears to have various and diverse functions in higher organisms. One particular property that is reviewed here is the regulation of mitochondrial number, turnover, and activity by various mitochondrial and nonmitochondrial sirtuins. An emerging consensus from these recent studies is that sirtuins may act as metabolic sensors, using intracellular metabolites such as NAD and short-chain carbon fragments such as acetyl coenzyme A to modulate mitochondrial function to match nutrient supply.     

Structural insights into intermediate steps in the Sir2 deacetylation reaction

Sirtuin enzymes comprise a unique class of NAD(+)-dependent protein deacetylases. Although structures of many sirtuin complexes have been determined, structural resolution of intermediate chemical steps are needed to understand the deacetylation mechanism. We report crystal structures of the bacterial sirtuin, Sir2Tm, in complex with an S-alkylamidate intermediate, analogous to the naturally occurring O-alkylamidate intermediate, and a Sir2Tm ternary complex containing a dissociated NAD(+) analog and acetylated peptide. The structures and biochemical studies reveal critical roles for the invariant active site histidine in positioning the reaction intermediate, and for a conserved phenylalanine residue in shielding reaction intermediates from base exchange with nicotinamide. The new structural and biochemical studies provide key mechanistic insight into intermediate steps of the Sir2 deacetylation reaction.     

Conserved metabolic regulatory functions of sirtuins

Silent information regulator 2 (Sir2) proteins, or sirtuins, are protein deacetylases/mono-ADP-ribosyltransferases found in organisms ranging from bacteria to humans. Their dependence on nicotinamide adenine dinucleotide (NAD+) links their activity to cellular metabolic status. In bacteria, the sirtuin CobB regulates the metabolic enzyme acetyl-coenzyme A (acetyl-CoA) synthetase. The earliest function of sirtuins therefore may have been regulation of cellular metabolism in response to nutrient availability. Recent findings support the idea that sirtuins play a pivotal role in metabolic control in higher organisms, including mammals. This review surveys evidence for an emerging role of sirtuins as regulators of metabolism in mammals.     

Metabolite of SIR2 reaction modulates TRPM2 ion channel

The transient receptor potential melastatin-related channel 2 (TRPM2) is a nonselective cation channel, whose prolonged activation by oxidative and nitrative agents leads to cell death. Here, we show that the drug puromycin selectively targets TRPM2-expressing cells, leading to cell death. Our data suggest that the silent information regulator 2 (Sir2 or sirtuin) family of enzymes mediates this susceptibility to cell death. Sirtuins are protein deacetylases that regulate gene expression, apoptosis, metabolism, and aging. These NAD+-dependent enzymes catalyze a reaction in which the acetyl group from substrate is transferred to the ADP-ribose portion of NAD+ to form deacetylated product, nicotinamide, and the metabolite OAADPr, whose functions remain elusive. Using cell-based assays and RNA interference, we show that puromycin-induced cell death is greatly diminished by nicotinamide (a potent sirtuin inhibitor), and by decreased expression of sirtuins SIRT2 and SIRT3. Furthermore, we demonstrate using channel current recordings and binding assays that OAADPr directly binds to the cytoplasmic domain of TRPM2 and activates the TRPM2 channel. ADP-ribose binds TRPM2 with similarly affinity, whereas NAD+ displays almost negligible binding. These studies provide the first evidence for the potential role of sirtuin-generated OAADPr in TRPM2 channel gating.     

Sirtuin Inhibition Induces Apoptosis-like Changes in Platelets and Thrombocytopenia

Sirtuins are evolutionarily conserved NAD⁺-dependent acetyl-lysine deacetylases that belong to class III type histone deacetylases. In humans, seven sirtuin isoforms (Sirt1 to Sirt7) have been identified. Sirtinol, a cell-permeable lactone ring derived from naphthol, is a dual Sirt1/Sirt2 inhibitor of low potency, whereas EX-527 is a potent and selective Sirt1 inhibitor. Here we demonstrate that Sirt1, Sirt2, and Sirt3 are expressed in enucleate platelets. Both sirtinol and EX-527 induced apoptosis-like changes in platelets, as revealed by enhanced annexin V binding, reactive oxygen species production, and drop in mitochondrial transmembrane potential. These changes were associated with increased phagocytic clearance of the platelets by macrophages. Expression of acetylated p53 and the conformationally active form of Bax were found to be significantly higher in both sirtinol- and EX-527-treated platelets, implicating the p53-Bax axis in apoptosis induced by sirtuin inhibitors. Administration of either sirtinol or EX-527 in mice led to a reduction in both platelet count and the number of reticulated platelets. Our results, for the first time, implicate sirtuins as a central player in the determination of platelet aging. Because sirtuin inhibitors are being evaluated for their antitumor activity, this study refocuses attention on the potential side effect of sirtuin inhibition in delimiting platelet life span and management of thrombosis.