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Nucleoside diphosphate kinases (NDP kinases, NDPks): NDP kinases, responsible for the synthesis of nucleoside triphosphates (NTPs), are involved in numerous regulatory processes associated with proliferation, development, and differentiation. They are vital for DNA/RNA synthesis, cell division, macromolecular metabolism and growth. The enzymes generate NTPs or their deoxy derivatives by terminal (gamma) phosphotransfer from an NTP such as ATP or GTP to any nucleoside diphosphate (NDP) or its deoxy derivative. The sequence of NDPk has been highly conserved through evolution. There is a single histidine residue conserved in all known NDK isozymes, which is involved in the catalytic mechanism. The first confirmed metastasis suppressor gene was the NDP kinase protein encoded by the nm23 gene. Unicellular organisms generally possess only one gene encoding NDP kinase, while most multicellular organisms possess not only an ortholog that provides most of the NDP kinase enzymatic activity but also multiple divergent paralogous genes. The human genome codes for at least nine NDP kinases and can be classified into two groups, Groups I and II, according to their genomic architecture and distinct enzymatic activity. Group I isoforms (A-D) are well-conserved, catalytically active, and share 58-88% identity between each other, while Group II are more divergent, with only NDPk6 shown to be active. NDP kinases exist in two different quaternary structures; all known eukaryotic enzymes are hexamers, while some bacterial enzymes are tetramers, as in Myxococcus. The hexamer can be viewed as trimer of dimers, while tetramers are dimers of dimers, with the dimerization interface conserved.
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