Cytosolic nucleoside diphosphate kinases catalyze the reversible reaction of ribonucleoside and deoxyribonucleoside 5'-diphosphates with ATP to form the corresponding nucleoside 5'-triphosphates and ADP. These kinases are ubiquitously expressed enzymes with broad substrate specificities (Berg and Joklik 1954; Parks and Agarwal 1973). Three human cytosolic nucleoside diphosphate kinase proteins, NME1, 2, and 3, have been characterized biochemically (Gilles et al. 1991; Schaertl et al. 1998; Erent et al. 2001; Chen et al. 2003). All are catalytically active as hexamers: homohexamers of NME1, 2, and 3 have been described, as have heterohexamers containing all possible combinations of NME1 and 2 (Gilles et al. 1991; Erent et al. 2001).
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.