Interconversion of nucleotide di- and triphosphates (Homo sapiens)
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Nucleoside monophosphate kinases catalyze the reversible phosphorylation of nucleoside and deoxynucleoside 5'-monophosphates to form the corresponding nucleoside 5'-diphosphates. Most appear to have restricted specificities for nucleoside monophosphates, and to use ATP preferentially (Van Rompay et al. 2000; Anderson 1973; Noda 1973). The total number of human enzymes that catalyze these reactions in vivo is not clear. In six cases, a well-defined biochemical activity has been associated with a purified protein, and these are annotated here. However, additional nucleoside monophosphate kinase-like human proteins have been identified in molecular cloning studies whose enzymatic activities are unknown, and several distinctive nucleoside monophosphate kinase activities detected in cell extracts, e.g., a GTP-requiring adenylate kinase activity (Wilson et al. 1976) and one or more guanylate kinase activities (Jamil et al. 1975) have not been unambiguously associated with specific human proteins.<P>The nucleoside monophosphates against which each of the six well-characterized enzymes is active is shown in the table (Van Rompay et al. 2000). All six efficiently use ATP as a phosphate donor, but have some activity with other nucleoside triphosphates as well in vitro. The high concentrations of ATP relative to other nucleoside triphosphates in vivo makes it the likely major phosphate donor in these reactions under most conditions.<P>All of these phosphorylation reactions are freely reversible in vitro when carried out with purified enzymes and substrates, having equilibrium constants near 1. In vivo, high ratios of ATP to ADP are likely to favor the forward direction of these reactions, i.e., the conversion of (d)NMP and ATP to (d)NDP and ADP. At the same time, the reversibility of the reactions and the overlapping substrate specificities of the enzymes raises the possibility that this group of reactions can buffer the intracellular nucleotide pool and regulate the relative concentrations of individual nucleotides in the pool: if any one molecule builds up to unusually high levels, multiple routes appear to be open not only to dispose of it but to use it to increase the supply of less abundant nucleotides.<p>Ribonucleotide reductase catalyzes the synthesis of deoxyribonucleotide diphosphates from ribonucleotide diphosphates. View original pathway at:Reactome.</div>
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The reducing equivalents needed for ribonucleotide reductase activity can be provided by either of two small proteins, glutaredoxin or thioredoxin (Holmgren 1989; Sun et al. 1998; Zahedi Avval & Holmgren 2009). Both are re-reduced with NADPH as the donor of reducing equivalents. The relative contributions of glutaredoxin and thioredoxin in vivo are unknown.
The reducing equivalents needed for ribonucleotide reductase activity can be provided by either of two small proteins, glutaredoxin or thioredoxin (Holmgren 1989; Sun et al. 1998; Zahedi Avval & Holmgren 2009). Both are re-reduced with NADPH as the donor of reducing equivalents. The relative contributions of glutaredoxin and thioredoxin in vivo are unknown.
The reducing equivalents needed for ribonucleotide reductase activity can be provided by either of two small proteins, glutaredoxin or thioredoxin (Holmgren 1989; Sun et al. 1998; Zahedi Avval & Holmgren 2009). Both are re-reduced with NADPH as the donor of reducing equivalents. The relative contributions of glutaredoxin and thioredoxin in vivo are unknown.
While cytosolic nucleoside diphosphate kinases can efficiently use several nucleotide triphosphates as a phosphate donor, the high concentrations of ATP relative to other nucleoside triphosphates in vivo makes it the likely major phosphate donor in these reactions and only reactions with ATP as the phosphate donor are annotated. All of these phosphorylation reactions are freely reversible in vitro (Parks and Agarwal 1973; Schaertl et al. 1998), but the high ratio of ATP to ADP concentrations in the cytosol should favor the conversion of (d)NDP and ATP to (d)NTP and ADP.
While the high ratio of ATP to ADP concentrations in the cytosol normally favors the conversion of (d)NDP and ATP to (d)NTP and ADP, the reversibility of the reactions and the overlapping substrate specificities of the enzymes suggest that this group of reverse reactions can buffer the intracellular nucleotide pool and regulate the relative concentrations of individual nucleoside di- and tri-phosphates in the pool.
The reducing equivalents needed for ribonucleotide reductase activity can be provided by either of two small proteins, glutaredoxin or thioredoxin (Holmgren 1989; Sun et al. 1998; Zahedi Avval & Holmgren 2009). Both are re-reduced with NADPH as the donor of reducing equivalents. The relative contributions of glutaredoxin and thioredoxin in vivo are unknown.