The phosphate-pyrophosphate exchange and hydrolytic reactions of the membrane-bound pyrophosphatase of Rhodospirillum rubrum: Effects of Mg 2+, phosphate, and pyrophosphate

Abstract

The relation that exists between the P i- PP i 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 Mg 2+. Optimal rates of hydrolysis were attained at 1 m m Mg 2+ with 0.67 m m pyrophosphate; the rate of hydrolysis correlated with the concentration of Mg-pyrophosphate, which indicated that the latter was the substrate for hydrolysis. The P i- PP i exchange reaction rate was low at concentrations of added Mg 2+ below 1 m m (0.67 m m pyrophosphate), but increased as the concentration of Mg 2+ in the medium was increased. The P i- PP i exchange reaction depends on the concentration of MgHPO 4, which suggests that this is the substrate in the exchange reaction. However, it is likely that free Mg 2+ also exerts a favorable effect on the P i- PP i exchange reaction. The optimal concentration for the P i- PP i exchange reaction was approx 240 μ m, which suggests that the concentration of the hydrolyzable substrates modulates the kinetic characteristics of the enzyme.