Function and metabolism of sirtuin metabolite O-acetyl-ADP-ribose

Abstract

Sirtuins catalyze the NAD(+)-dependent deacetylation of target proteins, which are regulated by this reversible lysine modification. During deacetylation, the glycosidic bond of the nicotinamide ribose is cleaved to yield nicotinamide and the ribose accepts the acetyl group from substrate to produce O-acetyl-ADP-ribose (OAADPr), which exists as an approximately 50:50 mixture of 2' and 3' isomers at neutral pH. Discovery of this metabolite has fueled the idea that OAADPr may play an important role in the biology associated with sirtuins, acting as a signaling molecule and/or an important substrate for downstream enzymatic processes. Evidence for OAADPr-metabolizing enzymes indicates that at least three distinct activities exist that could modulate the cellular levels of this NAD(+)-derived metabolite. In Saccharomyces cerevisiae, NUDIX hydrolase Ysa1 cleaves OAADPr to AMP and 2- and 3-O-acetylribose-5-phosphate, lowering the cellular levels of OAADPr. A buildup of OAADPr and ADPr has been linked to a metabolic shift that lowers endogenous reactive oxygen species and diverts glucose towards preventing oxidative damage. In vitro, the mammalian enzyme ARH3 hydrolyzes OAADPr to acetate and ADPr. A third nuclear-localized activity appears to utilize OAADPr to transfer the acetyl-group to another small molecule, whose identity remains unknown. Recent studies suggest that OAADPr may regulate gene silencing by facilitating the assembly and loading of the Sir2-4 silencing complex onto nucleosomes. In mammalian cells, the Trpm2 cation channel is gated by both OAADPr and ADP-ribose. Binding is mediated by the NUDIX homology (NudT9H) domain found within the intracellular portion of the channel. OAADPr is capable of binding the Macro domain of splice variants from histone protein MacroH2A, which is highly enriched at heterochromatic regions. With recently developed tools, the pace of new discoveries of OAADPr-dependent processes should facilitate new molecular insight into the diverse biological processes modulated by sirtuins.

Figure 1. OAADPr production by Sir2/sirtuins deacetylation reaction

Sir2 and sirtuins catalyzes NAD⁺ dependent deacetylation of histone tails, or other non-histone acetylated proteins. The reaction transfers the acetyl group from acetylated lysine residues to the ADP-ribose moiety of NAD⁺, generating deacetylated histone tails, nicotinamide, and the novel metabolite OAADPr.

Figure 2. OAADPr metabolism by the ADPr pyrophosphatase, unknown esterase and acetyl transferase

The pyrophosphatase cleaves the pyrophosphate bond of OAADPr, generating acetyl-ribose-phosphate and AMP; the esterase removes the acetyl group of OAADPr; the unknown acetyl-transferase transfers the acetyl group from OAADPr to an unknown molecule X.

Figure 3. Model for the role of OAADPr in facilitating Sir complex assembly and loading onto the chromatin

OAADPr interaction with the Sir2-4 complex facilitates loading of the complex onto nucleosomes, probably through binding to Sir2/Sir3. OAADPr might bind Sir3 alone and promote Sir3 loading onto nucleosomes.

Figure 4

Left: model of mH2A 1.1 in complex with ADPR. Model generated by overlaying mH2A 1.1 structure (PDB ID: 2FXK) onto a macro domain-ADPR complex (PDB ID: 2BFQ). Two important glycine residues involved in pyrophosphate binding are colored magenta. F348 and D203 side chains are shown in stick representation. Right: the side chain of the Ile in the EIS insertion clashes with the adenine ring. mH2A 1.1 colored cyan, mH2A 1.2 colored yellow.

Figure 5. OAADPr and ADPr modulates calcium influx through TRPM2 channel

In mammalian cells, OAADPr produced by sirtuins binds the NudT9H domain of TRPM2 and induces calcium influx into the cells. ADPr, another metabolite of NAD⁺, is believed to gate TRPM2 in a similar mechanism as OAADPr.

Figure 6. OAADPr metabolism and proposed functions in S. cerevisiae

OAADPr is produced by sirtuins and can be metabolized by Ysa1, an unknown esterase, and unknown acetyl transferase. Cells lacking Ysa1 display accumulation of ADPr/OAADPr and a corresponding decrease in AMP. Increased ADPr/OAADPr levels might protect cells against ROS via inhibition of the ETC and generation of higher NADPH levels from increased flux through the pentose phosphate pathway.