Cytosolic nucleoside diphosphate kinases catalyze the reversible reaction of ribonucleoside and deoxyribonucleoside 5'-diphosphates with ADP to form the corresponding nucleoside 5'-diphosphates and ATP. 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 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.