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

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 Mg2+, Zn2+, mn2+ and Co2+ can fulfill both the activator and substrate roles. Several other metal ions inhibit the Mg2+-stimulated activity; the strong inhibition by Ca2+ (and probably Cd2+) is due to interference with both activator and substrate roles, while the weaker inhibition by Sr2+ (and possibly Cu2+ and Ni2+) is due to interference with only the substrate role. Rare earth ions strongly stimulate nonenzymic PPi hydrolysis but do not activate the enzyme. Despite its ability to fulfill both the activator and substrate roles. Zn2+ causes inactivation of the enzyme, probably by interference with the "structural" Mg2+. 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 greater than substrate greater than activator.