Activation and Inhibition in the Phosphoglucomutase System

The activation of phosphoglucomutase prior to the enzymic assay is interpreted in terms of the direct removal of inhibiting metals or their replacement by Mg2+ which occurs readily only in the absence or in the presence of very low concentrations of substrate. Inhibition of the enzyme by high concentrations of Mg2+ is caused by the formation of inactive magnesium complexes of glucose 1-phosphate and glucose 1,6-diphosphate and is complicated by dead-end substrate inhibition. Inhibition by chloride, acetate, bromide, nitrate, iodide, and perchlorate increases in the order listed. The pKaa3 and pKaa4 of glucose 1,6-diphosphoric acid are 6.0 and 6.8, respectively, at ionic strength of 0.024, thus ruling out the possibility of a stable hydrogen bond between the two phosphate groups in the trianion. Comparison of the binding of Mg2+ by glucose-1-P and glucose-1,6-di-P indicates that bidentate binding effects involving the two phosphate groups of glucose-1,6-di-P are unimportant.