Solvent isotope effects on the glucokinase reaction. Negative co-operativity and a large inverse isotope effect in 2H2O.

The solvent isotope effects on the reaction catalysed by rat-liver glucokinase have been studied. At low concentrations of glucose and high concentrations of MgATP2- there is an inverse solvent isotope effect of 3.5. At high glucose concentrations there is a normal solvent isotope effect of 1.3. In 1H2O there is positive co-operativity with respect to glucose [ Storer , A.C. and Cornish - Bowden , A. (1976) Biochem. J. 159, 7-14], but this is changed to negative co-operativity in 2H2O. The half-saturation points for both glucose and MgATP2- are decreased in 2H2O compared with those in 1H2O. Explanations of these effects in terms of the mnemonical model proposed by Storer and Cornish - Bowden [Biochem. J. 65, 61-69 (1977)] were considered in computer simulation. Two interpretations could account for the results, either a decrease in the rate of interconversion of the two forms of free enzyme postulated in the model, or an increase in the affinity for glucose of the enzyme form with the lower affinity in 1H2O. The results of a proton-inventory analysis were consistent with either of these interpretations. The solvent isotope effects thus provide additional evidence for the mnemonical model as an explanation of glucokinase co-operativity.