[Calorimetric assay of yeast inorganic pyrophosphatase interaction with magnesium and phosphate ions]

The thermodynamic characteristics for the specific binding of one or two Mg2+ by the yeast inorganic pyrophosphatase and for the enzyme interaction with phosphate were determined. Saturation of the first binding site with Mg2+ causes structural rearrangements in the enzyme molecule without changing the temperature of protein denaturation. On the contrast, saturation of the second binding site results in stabilization of the system, i. e. a considerable fall in the entropy and a rise in the temperature of denaturation. Phosphorylation of the enzyme carboxylic group by inorganic phosphate requires saturation of the first binding site with Mg2+ and is not accompanied by changes in the enthalpy of the system. The pyrophosphate synthesis in the presence of the enzyme saturated with Mg2+ in both binding sites is associated with changes in the enthalpy and, possibly, in the entropy of the system.