Katrina L. Bogan, C. Brenner
2013
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Abstract
Nicotinamide adenine dinucleotide (NAD + ) and its phosphorylated form, nicotinamide adenine dinucleotide phosphate (NADP + ), are hydride-accepting coenzymes that play essential roles in substrate oxidation reactions in metabolism. The reduced forms, NADH and NADPH, are hydride-donating coenzymes in substrate reducing reactions. Structurally, the NAD + coenzyme can be viewed as a nicotinamide base in a β-glycosidic linkage with adenosine diphosphate (ADP)ribose. Hydride is transferred to and from the nicotinamide ring, such that the plus sign indicates a positive charge on the nitrogen ring. In contrast to flavin adenine dinucleotide co-enzymes, which are usually tightly bound to flavoproteins, NAD + and its equivalents are either dissociable from oxidoreductases or tightly bound to nicotinoprotein oxidoreductases. NAD + is also a substrate of enzymes unrelated to oxidoreductases. These NAD + -consuming enzymes, including poly(ADPribose) polymerases and sirtuins, produce an ADPribosyl product plus nicotinamide, thereby coupling signaling functions to NAD + turnover and necessitating regulated biosynthesis via salvage and de novo pathways. Because of the broad cellular and system functions of NAD + -dependent enzymes, NAD + and its equivalents have important roles in metabolism, regulation of gene expression, DNA repair, inflammation, intracellular trafficking, aging, and cell death.