Regulation of divalent cations of the membrane-bound pyrophosphatase of Rhodospirillum rubrum, as shown by the hydrolysis of tripositive-pyrophosphate complexes

Abstract

Tripositive-pyrophosphate [M(III)-PPi] complexes were used to investigate the role of free divalent cations on the membrane-bound pyrophosphatase. Divalent cations remain free and the M(III)-PPi complexes were employed as substrates. Formation of a La-PPi complex was studied by fluorescence, and the fact that Zn2+ and Mg2+ remain free in the solution was validated. Hydrolysis of La-PPi is stimulated by the presence of fixed concentrations of free Mg2+ or Zn2+ and this stimulation depends on the concentration of the cations when the La-PPi complex is fixed. The divalent cation stimulation order is Zn2+ > Co2+ > Mg2+ > Mn2+ > Ca2+ (at 0.5 mm of free cation). With different M(III)-PPi complexes, Zn2+ produces the same Km, for all the complexes and Mg2+ stimulates with a different Km. The results suggest that both Mg2+ and Zn2+ activate the membrane-bound pyrophosphatase but through different mechanisms.