Roles of Magnesium and Cobalt in the Reaction of Glucose Isomerase from Streptomyces griseofuscus S-41

Abstract

The roles of magnesium and cobalt ions in the reaction of glucose isomerase from Streptomyces griseofuscus S-41 were kinetically investigated. Magnesium was superior to cobalt as an activator, but was inferior as a protector of the enzyme. The enzyme was highly stable against heat and acid inactivation in the presence of cobalt, but labile in the presence of magnesium. The cobalt-activated enzyme was also tolerant to the inhibitory effect of other metal ions.
The mechanism of metal activation of glucose isomerase seemed to be expressed by the random rapid equilibrium system. Dissociation constants of metals from binary, enzyme-metal complexes were estimated to be 1.2 × 10^-3M for magnesium and 1.5 × 10^-5M for cobalt, and those from ternary, enzyme-metal-substrate complexes were 4.8 × 10^-4M for magnesium and 9.0 × 10^-6M for cobalt. The dissociation constant of glucose from enzyme-substrate complex was estimated to be 6.4 × 10^-1M, and dissociation constants of glucose from enzyme-magnesium-substrate complex and enzyme-cobalt-substrate complex were 2.4 × 10^-1M and 4.3 × 10^-1M, respectively. Thus, the affinity of the substrate for enzyme was enhanced by combining a metal with the enzyme and similarly, the affinity of the metal for enzyme was enhanced by combining a substrate with the enzyme.
Magnesium and cobalt were found to behave competitively in the reaction of glucose isomerase, and the competition profile altered depending on the relative concentrations of the two metals. It was, therefore, suggested that both metals combine at the same site or at very close sites of the enzyme.