Anomeric Specificity of the Native and Mutant Forms of Human β-Cell Glucokinase

The anomeric specificity of wild-type human β-cell glucokinase and six of its mutant forms toward α- and β-D-glucose was examined over 6-min incubation at 30°C. WhenD-[U-14C]glucose at anomeric equilibrium was used as substrate, the wild-type form yielded a maximal velocity of 76 U/mg, aKmof 4–5 mM,and a Hill coefficient close to 1.2. The maximal velocity (2 to 89 U/mg) andKm(2.4 to 209.8 mM) of the mutant forms both covered a range of about two orders to magnitude. Wild-type glucokinase displayed a higher affinity for α-D-glucose but greater maximal velocity with β-D-glucose. At variance, however, in four mutant forms, the maximal velocity was higher with α- than β-D-glucose. These findings indicate that the higher insulinotropic efficiency of α- than β-glucose cannot be ascribed to the intrinsic catalytic properties of human β-cell glucokinase. They also suggest that the perturbation of the anomeric specificity of glucose-stimulated insulin release in type-2 diabetes could conceivably be attributable, on occasion and at least in part, to a mutation of the glucokinase gene.