Multiple roles of metal ions in the reaction catalyzed by yeast inorganic pyrophosphatase

Abstract

Yeast inorganic pyrophosphatase has three roles for metal ions in its reaction: activator, substrate and structural. Out of a wide variety of metal ions tested, only Mg 2+, Zn 2+, Mn 2+ and Co 2+ can fulfill both the activator and substrate roles. Several other metal ions inhibit the Mg 2+-stimulated activity; the strong inhibition by Ca 2+ (and probably Cd 2+) is due to interference with both activator and substrate roles, while the weaker inhibition by Sr 2+ (and possibly Cu 2+ and Ni 2+) is due to interference with only the substrate role. Rare earth ions strongly stimulate nonenzymic PP i hydrolysis but do not activate the enzyme. Despite its ability to fulfill both the activator and substrate roles, Zn 2+ causes inactivation of the enzyme, probably by interference with the “structural” Mg 2+. The results suggest that the three roles for metal ions are independent (an individual metal ion can satisfy only one at a time) and that the metal ion specificity for the three roles declines in the order: structural > substrate > activator.