Metabolic control by sirtuins and other enzymes that sense NAD+, NADH, or their ratio

Abstract

NAD⁺ is a dinucleotide cofactor with the potential to accept electrons in a variety of cellular reduction-oxidation (redox) reactions. In its reduced form, NADH is a ubiquitous cellular electron donor. NAD⁺, NADH, and the NAD⁺/NADH ratio have long been known to control the activity of several oxidoreductase enzymes. More recently, enzymes outside those participating directly in redox control have been identified that sense these dinucleotides, including the sirtuin family of NAD⁺-dependent protein deacylases. In this review, we highlight examples of non-redox enzymes that are controlled by NAD⁺, NADH, or NAD⁺/NADH. In particular, we focus on the sirtuin family and assess the current evidence that the sirtuin enzymes sense these dinucleotides and discuss the biological conditions under which this might occur; we conclude that sirtuins sense NAD⁺, but neither NADH nor the ratio. Finally, we identify future studies that might be informative to further interrogate physiological and pathophysiological changes in NAD⁺ and NADH, as well as enzymes like sirtuins that sense and respond to redox changes in the cell.

Keywords: Metabolism; Mitochondria; Nicotinamide adenine dinucleotide; Redox; Sirtuins.

Figure 1

Overview of NAD⁺ metabolism. The deamidated (blue) and amidated (green) pathways are two discrete routes to synthesize intracellular NAD⁺. QRPT: Quinolinate Phosphoribosyltransferase; NAPRT: Nicotinate Phosphoribosyltransferase; NAMPT: Nicotinamide Phosphoribosyltransferase; NRK: Nicotinamide Riboside Kinases; NMNAT: Nicotinamide (Mono)nucleotide Adenylyltransferase; NADS: glutamine-dependent NAD⁺ synthetase.

Figure 2

Summary sub-cellular dinucleotide pools and ratios. Based on published values, concentrations of NAD⁺ or NADH were converted to circle diagrams where the area of each shape accurately reflect the relative size of the nucleotide pool and the relationship between the sizes.

Figure 3

Schematic of sirtuin enzymatic activity. Mammalian sirtuins SIRT1-SIRT7 are NAD⁺-dependent protein deacylases on a wide range of cellular substrates. Using NAD⁺ as a co-substrate (black) the sirtuins (blue) catalyze a deacylation reaction (green) to form nicotinamide, O-acyl-ADP-ribose, and a deacylated protein.

Figure 4

Conformations of NAD⁺ vs. NADH. Top panel, dihedral angles: Glycosidic bond (green) and exocyclic amide (blue), pyramidalization of nitrogen (blue triangle); Middle panel, crystal structure of SIRT2 in complex with NAD⁺ and SirReal2–a SIRT2 selective inhibitor (pdb: 4RMG) showing almost parallel orientation of the planar pyrimidine moiety and the C–O bond in the ribose ring; Bottom panel, structure from molecular modeling of SIRT1 in complex with NADH and an acetylated substrate showing almost perpendicular orientation of the dihydropyridine ring, with significant pyramidalization of the nitrogen atom.