Regulatory Properties of an Inorganic Pyrophosphatase from the Photosynthic Bacterium Rhodospirillum Rubrum

In Rhodospirillum rubrum, inorganic pyrophosphatase activity is observed in both the cytoplasmic and membrane fractions. The soluble enzyme accounts for about 80% of the total activity in crude extracts, and is the subject of this report. Zn(2+) is required for both activity and stability of the enzyme, which has a molecular weight of approximately 90,000 (gel-filtration determinations). The substrate is MgP(2)O(7) (2-), and free pyrophosphate (P(2)O(7) (4-)) is a strong inhibitor. Kinetic experiments indicate homotropic interactions between substrate-binding sites; these interactions are influenced by Mg(2+), which is an activator. At low concentrations of Zn(2+), the pyrophosphatase is inhibited by NADH, NADPH, and MgATP; 50% inhibition occurs at 0.4-0.7 mM. These effects are reversed by high concentrations of Zn(2+) (10(-4)-10(-3) M). The nucleotides appear to inhibit activity of the "native" enzyme through an effect on Zn(2+) binding. The R. rubrum enzyme seems to be the first known example of a bacterial inorganic pyrophosphatase subject to allosteric regulation.