Pyrophosphate of high and low energy. Contributions of pH, Ca2+, Mg2+, and water to free energy of hydrolysis.

L De Meis

doi:10.1016/s0021-9258(20)82109-7

Abstract

The equilibrium between inorganic pyrophosphate and inorganic orthophosphate was determined at pH values varying between 6.0 and 8.0, in the presence of different concentrations of MgCl2, mixtures of MgCl2 and CaCl2, and different organic solvents. The reactions were catalyzed by yeast inorganic pyrophosphatase. It was found that at 35 degrees C, depending on the conditions used, the observed equilibrium constant of pyrophosphate hydrolysis vary from a value higher than 4 X 10(3) M (delta Goobs more negative than -5.1 kcal/mol) to a value as low as 3 M (delta Goobs -0.7 kcal/mol). The experimental data were used to compute the equilibrium constants of the reactions involving different ionic species. The data presented are interpreted according to the concept that the Keq of hydrolysis of a high energy compound depends on the difference in solvation energy of reactants and products.

Highlights

The equilibrium between inorganic pyrophosphate and inorganic orthophosphate was determined at pH values varying between 6.0 and 8.0, in the presence of different concentrations of MgC12, mixtures of M&lz and CaC12, and different organic solvents
The reacmedium is decreased by the addition of different organic solvents, the Kob for pyrophosphate hydrolysis in solution decreases to values similar to thatdetermined for the tightly bound pyrophosphate
The K0b of pyrophosphate hydrolysis may vary from a value higher than 4000 M to 60 M depending on the pH, Mg+, and Piconcentration in themedium (Figs. 1 to 4)