Solvent isotope effects on the hexokinase D reaction: evidence for the mnemonical interpretation of the kinetic co-operativity.

Hexokinase D ('glucokinase') from rat liver displays kinetic co-operativity with respect to glucose when studied in 1H2O, with a Hill coefficient of about 1.4 when the other substrate, MgATP, is present at a concentration of 4.3 mM. In 2H2O, however, this becomes negative co-operativity, with a Hill coefficient of about 0.6 under corresponding conditions. At high glucose concentrations there is a small normal isotope effect, but at low glucose concentrations there is a large inverse isotope effect. Graphical analysis shows that these results are consistent with the 'mnemonical' model for kinetic co-operativity, in which there are two forms of free enzyme with different affinities for glucose, if the principal effect of changing the solvent is to alter the relative magnitudes of the on rate constants for binding of glucose to the two forms of free enzyme.