Structural similarities and differences among metal ion complexes of phosphoglucomutase by solvent perturbation and...

Although the binding of bivalent metal-ion activators to phosphoglucomutase produces substantial changes in the near ultraviolet spectrum of the enzyme, the extent to which aromatic residues are exposed to the aqueous environment, as assessed by means of solvent perturbation spectroscopy (using D2O), does not appear to be significantly altered by the binding process. Other ways in which the spectral effects induced by activation might arise are considered by making comparisons with those changes induced by various nonactivating monovalent and bivalent cations. The observed differences are most easily interpreted in terms of an electrostatic perturbation of (at least) two different tryptophan residues. This interpretation is supported by using cationic vs, neutral (zwitterionic) tryptophan in various solvent systems to generate difference spectra that are similar either to the observed metal-ion induced spectral differences or to the differences in the spectral changes produced by various pairs of metal ions. Although a rationale for the striking similarity in the spectral changes produced by Mg2+ and by Li+ (which elicits less than 2 X 10(-8) of the enzymic activity induced by Mg2+) cannot be ascribed to a simple electrostatic effect, alone, the involvement of an additional, negatively charged group in the binding of Mg2+ (but not Li+) could reduce the effective charge of bound Mg2+ to a value close to that of bound Li+.