The characterization of human adenylate kinases 7 and 8 demonstrates differences in kinetic parameters and structural organization among the family of adenylate kinase isoenzymes

Differences in expression profiles, substrate specificities, kinetic properties and subcellular localization among the adenylate kinase (AK) isoenzymes, have been shown to be important for maintaining a proper adenine nucleotide composition for many different cell functions. In the present study human AK7 was characterized and its substrate specificity, kinetic properties and subcellular localization determined. In addition, a novel member of the human adenylate kinase family, with two functional domains, was identified and characterized and it was assigned the name AK8. AK8 is the second known human AK with two complete and active AK domains within its polypeptide chain, a feature that previously was shown also for AK5. The full length AK8, as well as its two domains AK8p1 and AK8p2, all showed similar AK enzyme activity. AK7, full length AK8, AK8p1 and AK8p2 phosphorylated AMP, CMP, dAMP and dCMP with ATP as phosphate donor and also AMP, CMP and dCMP with GTP as phosphate donor. Both AK7 and the full length AK8 showed highest affinity for AMP with ATP as phosphate donor and proved to be more efficient in AMP phosphorylation as compared to the major cytosolic isoform AK1. Expression of the proteins fused with the green fluorescent protein demonstrated a cytosolic localization for both AK7 and AK8.