Comparison of the hydrolysis of Zn-PPi(2-) and the MgPPi(2-) 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-PPi(2-) as substrate was studied and compared with Mg-PPi(2-) hydrolysis. The pH profile of Zn-PPi(2-) hydrolysis is a bell shaped curve with an optimum at 5.25. This behavior is different from the sigmoidal profile obtained for Mg-PPi(2-) hydrolysis, which has a plateau from pH 6.5 to 9.0. Zn-PPi(2-) 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-PPi(2-)) and therefore free Zn2+ is an essential activator of Zn-PPi(2-) hydrolytic activity. Free Mg2+, on the contrary, acts as an inhibitor of Zn-PPi(2-) hydrolysis. The dependence of the reaction rate on the Zn-PPi(2-) concentration is sigmoidal.