Role of Bivalent Cations in the Phosphoglucomutase System

The metal ion requirement of the phosphoglucomutase reaction is satisfied by Mg 2 +, Ni 2 +, Co 2 +, Mn2+, Cd2+, or Zn 2 +. The relative activities elicited by these metals are 100, 60, 15, 5, 0.8, and 0.3, respectively, at pH 7.4 and saturating substrate; the relative activity of enzyme extensively dialyzed against EDTA type chelating agents is less than 0.001 under the same conditions but the expected activity is restored on addition of stoichiometric amounts of Co 2 +, Mn 2 + , or Zn2+ or by treatment with millimolar Mg 2 +. A differential effect of pH on the competitive binding of Mg2+, Mn 2 + , and Zn 2+ is observed; the differential effect may be caused by metal-dependent differences in pKa of two groups in the enzyme-metal complex which are not directly involved in metal binding. Two different types of pH effects on the activity of the various enzyme -metal complexes were observed: plots of log Vmax against pH for the Mg2+ enzyme were flat in the neutral pH range with down turns at extremes of pH; for the Co 2+ , Mn 2 +, and Zn 2+ enzymes, such curves were sigmoidal with inflections in the neutral pH range. It thus seems possible that the rate-controlling step differs in the two different cases.