Polyphosphate (
polyP) is a ubiquitous
biopolymer whose function and metabolism are incompletely understood. The
polyphosphate kinase (PPK) of Acinetobacter sp. strain ADP1, an organism that accumulates large amounts of
polyP, was purified to homogeneity and characterized. This
enzyme, which adds the terminal
phosphate from
ATP to a growing chain of
polyP, is a 79-kDa monomer. PPK is sensitive to
magnesium concentrations, and optimum activity occurs in the presence of 3 mM MgCl(2). The optimum pH was between pH 7 and 8, and significant reductions in activity occurred at lower pH values. The greatest activity occurred at 40 degrees C. The half-saturation
ATP concentration for PPK was 1 mM, and the maximum PPK activity was 28 nmol of
polyP monomers per microg of
protein per min. PPK was the primary, although not the sole,
enzyme responsible for the production of
polyP in Acinetobacter sp. strain ADP1. Under low-
phosphate (P(i)) conditions, despite strong induction of the ppk gene, there was a decline in net
polyP synthesis activity and there were near-zero levels of
polyP in Acinetobacter sp. strain ADP1. Once excess
phosphate was added to the P(i)-starved culture, both the
polyP synthesis activity and the levels of
polyP rose sharply. Increases in
polyP-degrading activity, which appeared to be mainly due to a
polyphosphatase and not to PPK working in reverse, were detected in cultures grown under low-P(i) conditions. This activity declined when
phosphate was added.