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Celis, H; Romero, I (1995)

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 RHODO

BIOCHIMIE 77(12):949-952
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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.