Vitamin B3 (niacin), NAD and NADP biosynthesis pathway (Zea mays)

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The active forms of niacin (a term for nicotinate and nicotinamide) are nicotinamide adenine dinucleotide (NAD) and its 2'-phosphate ester (NADP), which are coenzymes in hundreds of redox reactions. NAD is also the substrate for sirtuins, poly-(ADP-ribose) polymerases, and other enzymes that release the nicotinamide moiety. The de novo synthesis pathway begins in the plastid, where nicotinate mononucleotide (NaMN) is formed from aspartate, dihydroxyacetone phosphate, and a ribose phosphate moiety. NaMN then exits to the cytosol, where successive adenylation and amidation give nicotinate adenine dinucleotide (NaAD) and then NAD (Noctor et al., 2011). The adenylation, and likely amidation, steps also occur in mitochondria (Di Martino and Pallotta, 2011). NAD is converted to NADP in cytosol, plastids, peroxisomes, and probably mitochondria (Di Martino and Pallotta, 2011; Noctor et al., 2011). The nicotinamide released by NAD-consuming enzymes is salvaged by hydrolysis to nicotinate, which is converted to NaMN that re-enters the de novo pathway. NAD is cleaved by various enzymes to nicotinamide mononucleotide (NMN), which can be reconverted directly to NAD or hydrolysed to nicotinamide and recycled as above. These salvage reactions are all cytosolic; the NMN → NAD reaction is also likely mitochondrial (Di Martino and Pallotta, 2011; Noctor et al., 2011).