Kinetic Study of Yeast Hexokinase. 1. Steady-State Kinetics

Abstract

Thirty-one different models corresponding to all plausible mechanisms of glucose phosphorylation conditioned by yeast hexokinase have been analysed. Comparison of these models with the results given by kinetic study of the enzyme has enabled us to prove that the catalysis corresponds to an ordered mechanism in which hexokinase binds first glucose and then the chelate MgATP2−. The latter substrate is not able, to any great extent, to form directly the first complex with the phosphotransferase. The ionic species Mg2+ and ATP4− have only a negligible affinity for the enzyme, or the enzyme-glucose complex. Glucose-6-phosphate and ADP appear in the medium following the decomposition of the enzyme-glucose-magnesium-ATP complex. This decomposition is also controlled by an ordered mechanism, MgADP- being formed first, and glucose-6-phosphate afterwards. MgADP- is able not only to fix itself on the enzyme-glucose-6-phosphate complex but also on the enzyme-glucose complex. This mechanism is in contradiction with that of Fromm. On the other hand it agrees with that postulated but not proved by Hammes and Kochavi. The rate constant values in the proposed mechanism have been calculated. They are different from those calculated by Hammes and Kochavi.