title: The phosphate-pyrophosphate exchange and hydrolytic reactions of the membrane-bound pyrophosphatase of Rhodospirillum rubrum: effects of pH and divalent cations.
abstract: The relation that exist 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 pH. The optimal pH for hydrolysis was 6.5 Mg2+ or Pi for the enzyme; Mn2+ and Co2+ support the Pi-PPi exchange reaction partially (50%), but the reaction is slower than with Mg2+; other divalent cations like Zn2+ or Ca2+ do not support the exchange reaction. In the hydrolytic reaction, Zn2+, at low concentration, substitutes for Mg2+ as substrate, and Co2+ also substitutes in limited amount (50%). Other cations (Ca2+, Cu2+, Fe2+, etc.) do not act as substrates in complex with PPi. The Zn2+ at high concentrations inhibited the hydrolytic reaction, probably due to uncomplexed free Zn2+. In the presence of high concentration of substrate for the hydrolysis (Mg-PPi) the divalent cations are inhibitory in the following order: Zn2+ greater than Mn2+ greater than Ca2+ greater than or equal to Co2+ greater than Fe2+ greater than Cu2+ greater than Mg2+. The data in this work suggest that H+ and divalent cations in their free form induced changes in the kinetic properties of the enzyme.
mesh_terms: D002413:Cations, Divalent; D011756:Diphosphates / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007700:Kinetics; D010710:Phosphates / Q000378:metabolism; D011755:Pyrophosphatases / Q000378:metabolism*; D012247:Rhodospirillum rubrum / Q000378:metabolism*
chemical_list: D002413:Cations, Divalent; D011756:Diphosphates; D010710:Phosphates; D011755:Pyrophosphatases