The Addition and Release of Magnesium in the Phosphoglucomutase Reaction

The order of addition of magnesium and glucose 1-phosphate to the phosphoenzyme (monophosphate phase) and of magnesium and glucose 1,6-diphosphate to the dephosphoenzyme (diphosphate phase) was investigated via substrate-velocity and equilibrium-isotope exchange experiments. All pathways ordered in both phases of the reaction were ruled out leaving only pathways random in one or both phases of the reaction; indicative evidence suggests that the pathway is actually random in both phases. The variations in maximum velocity and in the Michaelis constant for magnesium are described as a function of pH; the maximum velocity in the forward reaction is independent of pH from 6.5 to 8.3 while the Michaelis constant for magnesium depends on the basic form of two groups with pKa values in the range of 7.0 to 8.5.