Allosteric regulation of yeast inorganic pyrophosphatase by substrate

Kinetic and binding studies of yeast inorganic pyrophosphatase (EC 3.6.1.1) revealed a regulatory PPi-binding site. Rate vs substrate concentration dependencies were markedly nonhyperbolic in the range of 0.1-150 microM MgPPi at fixed Mg2+ levels of 0.05-10 mM provided that the enzyme had been preequilibrated with Mg2+. Imidodiphosphate, hydroxymethylenebisphosphonate, and phosphate eliminated the deviations from the Michaelis-Menten kinetics and inhibited PPi hydrolysis in a manner consistent with their binding at both active and regulatory sites. The results agreed with a model in which binding of uncomplexed PPi at the regulatory site markedly increases enzyme affinity for the activating Mg2+ ion. Ultrafiltration studies revealed the binding of at least 3 mol of the inhibitory hydroxymethylenebisphosphonate and of 2 mol of noninhibitory methylenebisphosphonate per mole of the dimeric enzyme.