Effect of glycerol on glucokinase activity: loss of cooperative behavior with respect to glucose.

Glucose phosphorylation catalyzed by rat liver glucokinase measured at saturating concentrations of MgATP2- shows a cooperative response with respect to glucose in the concentration range 0.25-5 mM with a Hill coefficient of 1.6. In this range of glucose concentrations, the degree of cooperativity was dependent on the presence of glycerol in the assay mixture, and it decreased progressively and disappeared completely as the glycerol concentration reached about 20% (v/v) glycerol. If attention was confined to concentrations above 5 mM, no cooperativity could be detected either in the absence or in the presence of glycerol. The limiting velocity of the glucokinase reaction (measured at saturating concentrations of glucose and MgATP2-), and the half-saturation concentration for glucose and MgATP2- were all decreased by about 50-60% as the glycerol concentration was raised from zero to 30% (v/v). The presence of glycerol had no effect on the qualitative inhibition patterns of MgADP2-, glucose 6-phosphate, or N-acetylglucosamine, and only slight effects on the quantitative half-saturation values and inhibition constants. All of these effects caused by glycerol were fully reversible by decreasing the concentration of glycerol by dilution. Simulation studies based on the "mnemonical" model of glucokinase action proposed earlier [A. C. Storer and A. Cornish-Bowden (1977) Biochem. J. 165, 61-69] show that the effects of glycerol on glucokinase-catalyzed glucose phosphorylation can simply be explained assuming the glycerol favors the existence of the conformation of the enzyme with a higher affinity for glucose and thus supports the model.