The phosphate-pyrophosphate exchange and hydrolytic reactions of the membrane-bound pyrophosphatase of Rhodospirillum rubrum: effects of Mg2+, phosphate, and pyrophosphate.

The relation that exists between the Pi-PPi exchange reaction and pyrophosphate hydrolysis by the membrane-bound pyrophosphatase of chromatophores of Rhodospirillum rubrum was studied. The two reactions have a markedly different requirement for added Mg2+. Optimal rates of hydrolysis were attained at 1 mM Mg2+ with 0.67 mM pyrophosphate; the rate od hydrolysis correlated with the concentration of Mg-pyrophosphate, which indicated that the latter was the substrate for hydrolysis. The Pi-PPi exchange reaction rate was low at concentrations of added Mg2+ below 1 mM (0.67 mM pyrophosphate), but increased as the concentration of Mg2+ in the medium was increased. The Pi-PPi exchange reaction depends on the concentration of MgHPO4, which suggests that this is the substrate in the exchange reaction. However, it is likely that free Mg2+ also exerts a favorable effect on the Pi-PPi exchange reaction. The optimal concentration for the Pi-PPI exchange reaction was approx 240 microM, which suggests that the concentration of the hydrolyzable substrates modulates the kinetic characteristics of the enzyme.