title: Kinetic model for the action of the inorganic pyrophosphatase from Streptococcus faecalis.
abstract: Kinetic studies of the less active form of Streptococcus faecalis inorganic pyrophosphatase (EC 3.6.1.1), together with computational analysis, indicated that cooperativity in ligand binding contributes in a significant way to the behavior of this enzyme. The simplest model applicable to our data was a Monod-Wyman-Changeux-type, allosteric model, in which the enzyme is proposed to exist in two 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----T equilibrium in favor of the T form. Mg2+ stabilized the R state, thus activating the hydrolysis of MgPPi2- and inhibiting that of Mg2PPi.
mesh_terms: D013293:Enterococcus faecalis / Q000201:enzymology*; D043564:Inorganic Pyrophosphatase; D007700:Kinetics; D046911:Macromolecular Substances; D008433:Mathematics; D008954:Models, Biological; D011485:Protein Binding; D011755:Pyrophosphatases / Q000378:metabolism*
chemical_list: D046911:Macromolecular Substances; D011755:Pyrophosphatases; D043564:Inorganic Pyrophosphatase