In humans, the genesNADK[8] and MNADK[9] encode NAD+ kinases localized in cytosol[8] and mitochondria,[9] respectively. Similarly, yeast have both cytosolic and mitochondrial isoforms, and the yeast mitochondrial isoform accepts both NAD+ and NADH as substrates for phosphorylation.[10][11]
NADK phosphorylates NAD+ at the 2’ position of the ribose ring that carries the adenine moiety. It is highly selective for its substrates, NAD and ATP, and does not tolerate modifications either to the phosphoryl acceptor, NAD, or the pyridine moiety of the phosphoryl donor, ATP.[8] NADK also uses metal ions to coordinate the ATP in the active site. In vitro studies with various divalent metal ions have shown that zinc and manganese are preferred over magnesium, while copper and nickel are not accepted by the enzyme at all.[8] A proposed mechanism involves the 2' alcohol oxygen acting as a nucleophile to attack the gamma-phosphoryl of ATP, releasing ADP.
Proposed mechanism of action for NAD+ phosphorylation by NADK
NADK is highly regulated by the redox state of the cell. Whereas NAD is predominantly found in its oxidized state NAD+, the phosphorylated NADP is largely present in its reduced form, as NADPH.[12][13] Thus, NADK can modulate responses to oxidative stress by controlling NADP synthesis. Bacterial NADK is shown to be inhibited allosterically by both NADPH and NADH.[14] NADK is also reportedly stimulated by calcium/calmodulin binding in certain cell types, such as neutrophils.[15] NAD kinases in plants and sea urchin eggs have also been found to bind calmodulin.[16][17]
Due to the essential role of NADPH in lipid and DNA biosynthesis and the hyperproliferative nature of most cancers, NADK is an attractive target for cancer therapy. Furthermore, NADPH is required for the antioxidant activities of thioredoxin reductase and glutaredoxin.[18][19] Thionicotinamide and other nicotinamide analogs are potential inhibitors of NADK,[20] and studies show that treatment of colon cancer cells with thionicotinamide suppresses the cytosolic NADPH pool to increase oxidative stress and synergizes with chemotherapy.[21]
While the role of NADK in increasing the NADPH pool appears to offer protection against apoptosis, there are also cases where NADK activity appears to potentiate cell death. Genetic studies done in human haploid cell lines indicate that knocking out NADK may protect from certain non-apoptotic stimuli.[22]
^"Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^"Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^Magni G, Orsomando G, Raffaelli N (Jul 2006). "Structural and functional properties of NAD kinase, a key enzyme in NADP biosynthesis". Mini Reviews in Medicinal Chemistry. 6 (7): 739–46. doi:10.2174/138955706777698688. PMID16842123.
^ abcdLerner F, Niere M, Ludwig A, Ziegler M (Oct 2001). "Structural and functional characterization of human NAD kinase". Biochemical and Biophysical Research Communications. 288 (1): 69–74. doi:10.1006/bbrc.2001.5735. PMID11594753.
^ abZhang R (Aug 2015). "MNADK, a Long-Awaited Human Mitochondrion-Localized NAD Kinase". Journal of Cellular Physiology. 230 (8): 1697–701. doi:10.1002/jcp.24926. PMID25641397. S2CID11539186.
^Iwahashi Y, Hitoshio A, Tajima N, Nakamura T (Apr 1989). "Characterization of NADH kinase from Saccharomyces cerevisiae". Journal of Biochemistry. 105 (4): 588–93. doi:10.1093/oxfordjournals.jbchem.a122709. PMID2547755.
^Iwahashi Y, Nakamura T (Jun 1989). "Localization of the NADH kinase in the inner membrane of yeast mitochondria". Journal of Biochemistry. 105 (6): 916–21. doi:10.1093/oxfordjournals.jbchem.a122779. PMID2549021.
^Williams MB, Jones HP (Feb 1985). "Calmodulin-dependent NAD kinase of human neutrophils". Archives of Biochemistry and Biophysics. 237 (1): 80–7. doi:10.1016/0003-9861(85)90256-5. PMID2982330.
^Estrela JM, Ortega A, Obrador E (2006-01-01). "Glutathione in cancer biology and therapy". Critical Reviews in Clinical Laboratory Sciences. 43 (2): 143–81. doi:10.1080/10408360500523878. PMID16517421. S2CID8962293.
Stelzl U, Worm U, Lalowski M, Haenig C, Brembeck FH, Goehler H, Stroedicke M, Zenkner M, Schoenherr A, Koeppen S, Timm J, Mintzlaff S, Abraham C, Bock N, Kietzmann S, Goedde A, Toksöz E, Droege A, Krobitsch S, Korn B, Birchmeier W, Lehrach H, Wanker EE (Sep 2005). "A human protein-protein interaction network: a resource for annotating the proteome". Cell. 122 (6): 957–68. doi:10.1016/j.cell.2005.08.029. hdl:11858/00-001M-0000-0010-8592-0. PMID16169070. S2CID8235923.
