During the catabolism of nucleic acids, nucleoside mono- and diphosphates are released. The nucleosides do not accumulate to any significant degree, owing to the action of nucleoside kinases. These include both nucleoside monophosphate (NMP) kinases and nucleoside diphosphate (NDP) kinases. The NMP kinases catalyze ATP-dependent reactions of the type:
(d)NMP + ATP <——> (d)NDP + ADP
There are four classes of NMP kinases that catalyze, respectively, the phosphorylation of:
1. AMP and dAMP; this kinase is known as adenylate kinase.
2. GMP and dGMP.
3. CMP, UMP and dCMP.
The enzyme adenylate kinase is important for ensuring adequate levels of energy in cells such as liver and muscle. The predominant reaction catalyzed by adenylate kinase is:
2ADP <——> AMP + ATP
The NDP kinases catalyze reaction of the type:
N1TP + N2DP <——> N1DP + N2TP
N1 can represent a purine ribo– or deoxyribonucleotide; N2 a pyrimidine ribo– or deoxyribonucleotide. The activity of the NDP kinases can range from 10 to 100 times higher than that of the NMP kinases. This difference in activity maintains a relatively high intracellular level of (d)NTPs relative to that of (d)NDPs. Unlike the substrate specificity seen for the NMP kinases, the NDP kinases recognize a wide spectrum of (d)NDPs and (d)NTPs.