Comparison of the hydrolysis of Zn-PPi and the MgPPi as substrates and the effect of free cations upon membrane-bound pyrophosphatase of Rhodospirillum rubrum

The hydrolytic activity of chromatophore membrane-bound pyrophosphatase with Zn-PPi²⁻ as substrate was studied and compared with Mg-PPi²⁻ hydrolysis. The pH profile of Zn-PPi²⁻ hydrolysis is a bell shaped curve with an optimum at 5.25. This behavior is different from the sigmoidal profile obtained for Mg-PPi²⁻ hydrolysis, which has a plateau from pH 6.5 to 9.0. Zn-PPi²⁻ hydrolytic activity is inhibited by 1-butanol and methylene-diphosphate but not by NaF. The enzyme has no activity when free Zn²⁺ concentration is lower than 7.5 pM (at 0.9–1.2 mm Zn-PPi²⁻ and therefore free Zn²⁺ is an essential activator of Zn-PPi²⁻ hydrolytic activity. Free Mg²⁺, on the contrary, acts as an inhibitor of Zn-PPi²⁻ hydrolysis. The dependence of the reaction rate on the Zn-PPi²⁻ concentration is sigmoidal.