The thermodynamic and structural differences among the catalytically active complexes of phosphoglucomutase: metal ion effects.

When the identity of the metal ion activator, M, is changed within the series, Zn2+, Co2+, Mg2+, Ni2+, Mn2+, and Cd2+, the equilibrium distribution among the central complexes in the phosphoglucomutase system is markedly altered. (The central complexes are Ep-M-Glc-6-P, ED-M-Glc-1,6-P2, and Ep-M-Glc-1-P, where Ep and ED are the phospho and dephospho forms of the enzyme). This altered distribution is caused by a metal-specific change in the equilibrium constant for transfer of the enzymic PO3 group to bound glucose monophosphates: 65-fold as M is varied from Zn2+ to Cd2+. This change in equilibrium is related to metal-specific differences in chemical potential of the phosphate group in the Ep-M complex; these differences in chemical potential remain in the Ep-M-Glc-1-P and Ep-M-Glc-6-P complexes, but essentially disappear in the ED-M-Glc-1,6-P2 complex. If glucose monophosphates are considered as substrates, and glucose bisphosphate as the product, there is a direct relationship between the equilibrium concentration of enzyme-substrate and enzyme-product complexes (when these are varied by changing the identity of the bound metal ion) and the ultraviolet spectrum of the equilibrium mixture of complexes, as assessed by difference spectroscopy (Peck, E.J., Jr., and Ray, W.J., Jr. (1969), J. Biol, Chem. 244, 3754). These spectral changes apparently are caused by an alteration in the conformation of the enzyme during transfer of a PO3 group between the enzyme and the glucose phosphate moiety, or as the result of it. The extent to which conformational changes accompany group-transfer processes in other enzymic systems is not clear, but it is possible that analogous changes may help to account for the "half-of-the-sites reactivity" observed with a number of multimeric enzymes.