Equilibrium constants of phosphoryl transfer from C(1) to C(6) of α-d-glucose 1-phosphate and from glucose 6-phosphate to water

Abstract

Specific enzymic methods were used to estimate the equilibrium concentrations of ([alpha] + [beta])-D-glucose 6-phosphate and of [alpha]-D-glucose 1-phosphate in the products of the reaction catalyzed by muscle phosphoglucomutase at pH 7.0 and 25[degree] with 25 mM-Mg2+. The apparent equilibrium constant, [([alpha] + [beta])-D-glucose 6-phosphate]/[[alpha]-D-glucose 1-phosphate], was 17 [plus or minus] 2 and the apparent free energy of phosphoryl transfer (4 G'') was - 1.7 [plus or minus] 0.1 kcal./mole. Specific enzymic methods were also used to estimate the equilibrium concentrations of ([alpha] + [beta])-D-glucose and of ([alpha] + [beta])-D-glucose 6-phosphate in the products of hydrolysis of glucose 6-phosphate catalyzed by intestinal alkaline phosphatase. Ortho-phosphate was estimated as phosphomolybdate. At pH 7.0 with 5 mM-Mg2+ the apparent equilibrium constant [([alpha] +[beta])-D-glucose] [orthophosphate]/ ([alpha] + [beta])-D-glucose 6-phosphate] was 260 + 50, and the apparent free energy of hydrolysis was -3.3 [plus or minus] 0.1 kcal./mole. There was little change in the apparent equilibrium constant with increasing concentration of Mg2+ ions between 0 and 5 mM at pH 7.0. From these results, the apparent free energy of hydrolysis of [alpha]-D-glucose 1-phosphate is -5.0 kcal./mole at pH 7.0 in the presence of Mg2+ ions.