The Kinetics of Yeast Hexokinase in the Light of the Induced Fit Involved in the Binding of its Sugar Substrate

The kinetics of the initial reaction rates of yeast hexokinase, both in the forward and in the backward direction, have been re‐examined in the light of recent findings concerning the properties of the ATPase activity of hexokinase and its modification by certain non‐phosphorylable analogues of the sugar substrates. A single pattern has been found in the double reciprocal plots even if the concentrations of both the acceptor and the donor substrates are varied over a wide range, with ITP as well as with ATP as donor substrates. A similar pattern has been found for the backward reaction. These results are interpreted as indicative that in the forward as well as in the backward reaction, the interconversion of the ternary complex of the enzyme with both substrates is the limiting step. This fact and the complex effect of glucose 6‐phosphate as an initially‐added product, prevents in principle the distinction between the possible kinetic mechanisms leading to the ternary complex. Nevertheless, taking into account the change in affinities for the nucleotide substrates specifically induced by certain sugars, it becomes possible to formulate a plausible hypothesis. It is postulated that binding of a sugar to an enzyme‐nucleotide complex results in an abortive ternary complex. Accordingly, formation of active ternary complexes in the hexokinase reaction requires the sequential addition of a nucleotide substrate to a binary complex of the enzyme with a sugar substrate.