Catalytic C-terminal domain, second repeat, of Pseudomonas aeruginosa polyphosphate kinase 1 and similar proteins
Catalytic C-terminal domain, second repeat (C2 domain), of polyphosphate kinase (Poly P kinase 1 or PPK1, EC 2.7.4.1) from Pseudomonas aeruginosa (PaPPK1), Dictyostelium discoideum (DdPPK1), and other similar proteins. Inorganic polyphosphate (Poly P) plays an important role in bacterial stress responses and stationary-phase survival. PaPPK1 is the key enzyme responsible for the synthesis of Poly P in Pseudomonas aeruginosa. It can catalyze the reversible conversion of the terminal-phosphate of ATP to Poly P. PaPPK1 shows high sequence homolog to Escherichia coli polyphosphate kinase (EcPPK), which contains four structural domains per chain: the N-terminal (N) domain, the head (H) domain, and two closely related C-terminal (C1 and C2) domains. The N domain provides the upper binding interface for the adenine ring of the ATP. The H domain is involved in dimerization, while both the C1 and C2 domains contain residues crucial for catalytic activity. The intersection of the N, C1, and C2 domains forms a structural tunnel in which the PPK catalytic reactions are carried out. The polyphosphate kinase from Dictyostelium discoideum (DdPPK1) shares similar structural features with EcPPK1 in the ATP-binding pocket and poly P tunnel, but has a unique N-terminal extension that may be responsible for its enzymatic activity, cellular localization, and physiological functions. In spite of the lack of sequence homology, the C1 and C2 domains of the family members are structurally similar to the two repetitive catalytic domains of phospholipase D (PLD). Moreover, some residues in the HKD motif (H-x-K-x(4)-D, where x represents any amino acid residue) of the PLD superfamily are spatially conserved in the active site of PPK1. It is possible that the bacterial PPK1 family and the PLD family have a common ancestor and diverged early in evolution. In some bacteria, such as Pseudomonas aeruginosa, a second enzyme, PPK2, which is involved in the alternative pathway of polyphosphate synthesis, has been found. It can catalyze the synthesis of poly P from GTP or ATP, with a preference for Mn2+ over Mg2+. PPK2 shows no sequence similarity to PPK1 and belongs to a different superfamily.
Comment:Polyphosphate Kinase (PPK1) contains two closely related C-terminal catalytic domains (C1 and C2 domain) that, together with the N domain, form a structural tunnel, in which the PPK1 catalytic reactions are carried out.
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