Polyphosphate kinase (Ppk) catalyses the formation of polyphosphate from ATP, with chain lengths of up to a thousand or more orthophosphate molecules. This C2-terminal domain has a structure similar to phospholipase D. It is one of two closely related carboxy-terminal domains (C1 and C2 domains). Both the C1 and C2 domains (residues 322-502 and 503-687, respectively) consist of a sevenstranded mixed beta-sheet flanked by five alpha-helices. However, the structural topology and relative orientations of the helices to the beta-sheet in these two domains are different. The C1 and C2 domains are highly conserved in the PPK family. Some of the residues previously shown to be crucial for the enzyme catalytic activity are located in these two domains [1].

1: Crystal structure of a polyphosphate kinase and its implications for polyphosphate synthesis. Zhu Y, Huang W, Lee SS, Xu W; EMBO Rep. 2005;6:681-687. PMID:15947782