Nicotinamide Riboside Kinase Structures Reveal New Pathways to NAD+

Abstract

The eukaryotic nicotinamide riboside kinase (Nrk) pathway, which is induced in response to nerve damage and promotes replicative life span in yeast, converts nicotinamide riboside to nicotinamide adenine dinucleotide (NAD+) by phosphorylation and adenylylation. Crystal structures of human Nrk1 bound to nucleoside and nucleotide substrates and products revealed an enzyme structurally similar to Rossmann fold metabolite kinases and allowed the identification of active site residues, which were shown to be essential for human Nrk1 and Nrk2 activity in vivo. Although the structures account for the 500-fold discrimination between nicotinamide riboside and pyrimidine nucleosides, no enzyme feature was identified to recognize the distinctive carboxamide group of nicotinamide riboside. Indeed, nicotinic acid riboside is a specific substrate of human Nrk enzymes and is utilized in yeast in a novel biosynthetic pathway that depends on Nrk and NAD+ synthetase. Additionally, nicotinic acid riboside is utilized in vivo by Urh1, Pnp1, and Preiss-Handler salvage. Thus, crystal structures of Nrk1 led to the identification of new pathways to NAD+.

Reduced Ssy1-Ptr3-Ssy5 (SPS) Signaling Extends Replicative Life Span by Enhancing NAD+ Homeostasis in Saccharomyces cerevisiae

“…To identify the source of elevated NR in ssy5⌬ cells (Fig. 2F), we examined the gene expression of pathways known to produce and salvage NAD ϩ and NR (8,10,(45)(46)(47). Expression levels of the genes in NAD ϩ synthetic pathways generally remained similar between WT and ssy5⌬ cells as we expected (Fig.…”

Microbial NAD Metabolism: Lessons from Comparative Genomics

“…been demonstrated in yeast (3,4,36). Finally, recent work indicates that nicotinic acid riboside can be converted to NaMN by nicotinamide riboside kinase and can also be utilized by the nucleosidase activity of Urh1, followed by nicotinic acid salvage (3,62). Nicotinamide riboside and nicotinic acid riboside salvage pathways are depicted in Fig.…”

“…The enzymatic basis for utilization of nicotinamide riboside by Haemophilus influenzae depends on a specific nicotinamide riboside kinase fused to a specific nicotinamide mononucleotide (NMN) adenylyltransferase encoded by the nadR gene (58). The eukaryotic nicotinamide riboside kinases (8) are also specific enzymes, possessing sequences different from those of bacterial nicotinamide riboside kinases, which phosphorylate nicotinamide riboside and nicotinic acid riboside (62). However, in addition to salvage of nicotinamide riboside by production of NMN, nicotinamide riboside salvage to nicotinamide, mediated by enzymes previously characterized as uridine hydrolase (Urh1) and purine nucleoside phosphorylase (Pnp1), has conversion to a dehydrogenated thioester with conversion of the coenzyme to NADH.…”

“…However, since eukaryotes have no NadR ortholog, a specific nicotinamide riboside kinase activity was not anticipated to exist in yeast or humans. Indeed, the yeast and human NRK enzymes (8) are structurally distinct from bacterial nicotinamide riboside kinases, though they are all members of the metabolite kinase superfamily (62).…”

NAD Biosynthesis in Humans - Enzymes, Metabolites and Therapeutic Aspects

“…In addition to QA, NA and Nam, two additional precursors were more recently discovered: the riboside forms of NA and NAM, nicotinic acid riboside (NAR) and nicotinamide riboside, respectively [118,119]. The riboside forms of NA and Nam are phosphorylated to NAMN and NMN, respectively, by nicotinamide ribose kinases (NRKs), where ATP serves as the phosphate donor.…”