Quaternary structure of nucleoside diphosphate kinases

Nucleoside (NDP) diphosphate kinases are oligomeric enzymes. Most are hexam eric, but some bacterial enzymes are tetrameric. Hexamers and tetramers are constructed by assembling identical dimers. The hexameric structure is imp ortant for protein stability, as demonstrated by studies with natural mutan ts (the Killer-of-prune mutant of Drosophila NDP kinase and the S120G mutan t of the human NDP kinase A in neuroblastomas) and with mutants obtained by site-directed mutagenesis. It is also essential for enzymic activity. The function of the tetrameric structure is unclear.