Kinetic model for the action of the inorganic pyrophosphatase from Streptococcus faecalis

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Abstract
Kinetic studies of the less active form of S. faecalis inorganic pyrophosphatase (EC 3.6.1.1), together with computational analysis, indicated that cooperatively in ligand binding contributes in a significant way to the behavior of this enzyme. The simplest model applicable to the data was a Monod-Wyman-Changeux-type, allosteric model, in which the enzyme is proposed to exist in 2 states, referred to as R and T states, respectively. In the absence of ligands, 94% of the enzyme was in the T state. MgPPi2- was the only substrate for the enzyme in the R form. This substrate was bound equally well by both enzyme forms, but it was hydrolyzed 5 times more efficiently by the R form than it was by the T form. Mg2PPi was bound exclusively to the T state of the enzyme, and it was hydrolyzed 25% as rapidly as MgPPi2- by the T form. Mg2PPi inhibited the hydrolysis of the more efficient substrate, MgPPi2-, by competing with MgPPi2- for the enzyme in the T form and by shifting the R .dblarw. T equilibrium in favor of the T form, Mg2+ stabilized the R state, thus activating the hydrolysis of MgPPi2- and inhibiting that of Mg2PPi.