The phosphate-pyrophosphate exchange and hydrolytic reactions of the membrane-bound pyrophosphatase ofRhodospirillum 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.

A membrane-bound pyrophosphatase from respiratory membranes of Rhodospirillum rubrum

“…1. The pH optimum for chromatophores was 6.5, as reported previously (Celis & Romero, 1987). Although the pyrophosphatase activity in respiratory membranes was lower than in chromatophores, the pH optimum was the same.…”

“…At high concentrations (> 1.0 mM), Zn2+ inhibited the activity completely and Mg2+ inhibited it partially. This was probably due to the presence of free Zn2+ and free Mg2+ at these high concentrations (Celis & Romero, 1987). The profile of divalent cation requirement obtained for respiratory-mem brane pyrop hosp hatase was the same as that obtained by Celis & Romero (1987) for the membrane-bound pyrophosphatase of chromatophores.…”

“…Mg2+ forms a Mg-PPi complex which is the true substrate for the enzyme (Randahl, 1979;Celis et al, 1985). Recently, we reported that the Zn-PPi complex at low concentrations is a better substrate than the Mg-PPi complex (Celis & Romero, 1987). Fig.…”

Characterization of the inorganic pyrophosphatase from the pathogenic bacterium Helicobacter pylori

“…Some PPases (Josse 1966;Butler 1971;Celis and Romero 1987) are activated by other divalent cations. Figure 2C shows the cation dependence of the H. pylori PPase.…”

Importance of Rhodospirillum rubrum H + -Pyrophosphatase under Low-Energy Conditions

“…These enzymes hydrolyze and synthesize PP i , generating or consuming an electrochemical proton gradient (1). The H ϩ PPase of Rhodospirillum rubrum is the most intensively studied enzyme of this kind (2,4,10,20,29), but its physiological role is not completely understood. Nyrén and Strid (19) hypothesized that the main function of the H ϩ PPase of R. rubrum is to maintain the proton motive force in lightgrown cells under conditions of low energy.…”