Guanosine pentaphosphate phosphohydrolase of Escherichia coli is a long-chain exopolyphosphatase

An exopolyphosphatase [exopoly(P)ase; EC 3.6.1.11] activity has recently been purified to homogeneity from a mutant strain of Escherichia coli which lacks the principal exopoly(P)ase. The second exopoly(P)ase has now been identified as guanosine pentaphosphate phosphohydrolase (GPP; EC 3.6.1.40) by three lines of evidence: (i) the sequences of five tryptic digestion fragments of the purified protein are found in the translated gppA gene, (ii) the size of the protein (100 kDa) agrees with published values for GPP, and (iii) the ratio of exopoly(P)ase activity to GPP activity remains constant throughout a 300-fold purification in the last steps of the procedure. The enzyme liberates orthophosphate by processive hydrolysis of the phosphoanyhydride bonds of polyphosphate [poly(P)] chains (1000 residues) or by hydrolysis of the 5'-gamma-phosphate of guanosine 5'-triphosphate 3'-diphosphate (pppGpp) to guanosine 5'-diphosphate 3'-diphosphate (ppGpp or "magic spot"). The Km for long-chain poly(P) as a substrate (approximately 0.5 nM) is far lower than that for pppGpp (0.13 mM); the kcat for the poly(P)ase activity is 1.1 s-1 and that for pppGpp hydrolase is 0.023 s-1. These and other findings direct attention to possible functions of poly(P) in the response of E. coli to stresses and deprivations.