Anomeric Specificity of Human Liver and B-cell Glucokinase: Modulation by the Glucokinase Regulatory Protein

The anomeric specificity of the wild-type recombinant forms of human liver and B-cell glucokinase was investigated using radioactive anomers of d-glucose as tracers. With d-glucose at anomeric equilibrium and at 30°C, the maximal velocity, Hill number, and Ks amounted, respectively, to 16 μmol min−1 mg−1, 1.8 and 6.9 mM in the case of liver glucokinase, and 7.3 μmol min−1 mg−1, 2.0 and 7.1 mM in the case of B-cell glucokinase. Whether at 20–22 or 30°C, the maximal velocity, Hill number, and Km were significantly lower with α-d-glucose than with β-d-glucose in both liver and B-cell glucokinase. As a result of these differences, the reaction velocity was higher with α-d-glucose at low hexose concentrations, while the opposite situation prevailed at high hexose concentrations. In the presence of 0.2 mM d-fructose 6-phosphate, the glucokinase regulatory protein caused a concentration-related inhibition of d-glucose phosphorylation, such an effect fading out at high concentrations of either d-glucose or glucokinase relative to that of its regulatory protein. The phosphorylation of α-d-glucose by liver glucokinase appeared more resistant than that of β-d-glucose to the inhibitory action of d-fructose 6-phosphate, as mediated by the glucokinase regulatory protein. Such a phenomenon failed to achieve statistical significance in the case of the B-cell glucokinase. It is proposed that this information, especially the novel findings concerning the anomeric difference in both Hill number and sensitivity to the glucokinase regulatory protein, should be taken into account when considering the respective contributions of α- and β-d-glucose to the overall phosphorylation of equilibrated d-glucose by glucokinase.