Metal Ion Transfer in Phosphoglucomutase-Chelate Systems

The rate at which zinc dissociates from the inactive zinc complex of phosphoglucomutase is measured under a variety of conditions. The zinc dissociation rate is reduced about 50-fold by bound substrate, whereas added histidine increases the dissociation rate in a concentration-dependent manner; however, neither substrate nor histidine appreciably alters zinc binding by the enzyme. Histidine and other ligands probably participate in metal transfer in a manner formally analogous to the participation of buffer components in hydrogen ion transfer, whereas the substrate effect appears to be steric in nature. Possible applications of these observations to the general problem of removing metal ions from proteins are discussed.