NAD salvage pathway II (Escherichia coli)
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Description
Since NAD+, which is obtained from extracellular sources, is highly polar, it has to be hydrolyzed before it can be transported across the cytoplasmic membrane for final uptake. A key enzyme in this pathway is a periplasmic protein that hydrolyzes NAD+ to nicotinamide mononucleotide (NMN), and hydrolyzes NMN to nicotinamide riboside (NR) [Kemmer01]. The nicotinamide riboside thus formed is transported across the inner membrane into the cytoplasm, where it is it is converted back to NMN, and eventually, to NAD+.
The identity of the NR transporter has not been confirmed experimentally, but it is suggested that it is encoded by the pnuC gene, although earlier work with the PnuC protein of Salmonella enterica serovar Typhimurium suggested that it encodes the transport of NMN [Liu82, Kemmer01].
NADP+ is recycled in the same manner, following dephosphorylation to NAD+. In H. influenzae, this dephosphorylation is performed by the outer membrane glycoprotein e (P4), encoded by the hel gene.
While the pathway is predicted to be present in Enterobacteriaceae, it probably serves a minor role compared to de novo biosynthesis and the main NAD salvage cycle (NAD salvage pathway I). Some of the enzymatic activities associated with this pathway have been described in Enterobacteriaceae as early as 1951 [Rowen51], but it was only recently that the genes responsible for them were identified. Surprisingly, several activities were found to be functions of the multifunctional protein NadR, which was initially believed to have a role in regulation only [Raffaelli99, Kurnasov02].
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