Kinetic studies of yeast hexokinase.

The mechanism of calf brain hexokinase has recently been studied in our laboratory (1). It was concluded from kinetic experiments that adenosine diphosphate (ADP) dissociates from the enzyme before addition of substrate glucose. This hypothesis was supported by kinetic studies of n-mannose, ADP, and glucose g-phosphate inhibition. The experiments appear to lend credence to theories in which an enzyme-phosphate (2) or enzyme-glucose (3) intermediate is postulated. The experiments of Trayser and Colowick (4, 5) on yeast hexokinase appear to be at variance with the aforementioned hypotheses. These authors have suggested that substrates glucose and ATP react simultaneously in the presence of the enzyme. Their conclusions are consistent with observations on the lack of enzymatic ADP-ATP exchange in the absence of glucose (6) and inability of yeast hexokinase to catalyze a glucose-glucose 6-phosphate exchange reaction in systems in which the nucleotide substrates are absent (7). In view of the obvious inconsistencies regarding the mechanisms postulated for yeast hexokinase, we undertook studies of the reaction with a kinetic approach. It will be shown that the results obtained are in harmony with the mechanism suggested by Trayser and Colowick (5). The enzyme-phosphate (2) and enzyme-glucose (3) hypotheses relative to yeast hexokinase appear to be untenable in light of the kinetic findings.