The dephosphorylation of phosphoglucomutase by nucleophilic reagents

The phosphoryl group on the serine residue at the active site of phosphoglucomutase is presumed to undergo nucleophilic attack by the monophosphate substrates glucose 1- and glucose 6-phosphate to form glucose 1,6-diphosphate. Fluoride, hydroxylamine, and several thiol compounds have now been shown to serve as effective nucleophiles toward the active phosphate and result in the dephosphorylation of phosphoglucomutase. The more extensively studied nucleophiles, cysteine, hydroxylamine, and fluoride, are effective at a concentration as low as 1 mM with a relative reactivity of 40, 2, and 1, respectively. The reaction proceeds as long as the catalytic activity of the enzyme is maintained. Inactivation of the enzyme abolishes dephosphorylation by all nucleophilic reagents thus far studied. The dephosphorylation reaction shows optimal activity of pH 6.5. The rate of dephosphorylation exhibits saturation kinetics. With fluoride the Km is 534 mM. Dephosphorylation by fluoride is stimulated by some but not all bivalent cations. Cu+ and Co2+ are the most effective. Cu2+ not only augments the reaction with fluoride but also facilitates a nucleophilic attack by water, in the absence of the halogen, to yield inorganic phosphate. No augmentation of the rate of dephosphorylation by bivalent cations can be elicited with either cysteine or hydroxylamine. The products of the fluoride reaction are phosphorofluoridate, a small but variable amount of inorganic phosphate, and a fully active dephosphoenzyme. By constrast, cysteine and hydroxylamine yield inorganic phosphate and a partially inactive enzyme. The dephosphorylation rate varies with temperature. Arrhenius plots for the fluoride reaction reveal two distinct slopes. The heat of activation between 5-37 degrees was found to be 10.2 Cal per mol. Between 0-5 degrees, however, it was considerably greater amounting to 24.3 Cal per mol.