Anomeric specificity of the native and mutant forms of human beta-cell glucokinase

The anomeric specificity of wild-type human beta-cell glucokinase and six of its mutant forms toward alpha- and beta-D-glucose was examined over 6-min incubation at 30 degrees C. When D-[U-14C]glucose at anomeric equilibrium was used as substrate, the wild-type form yielded a maximal velocity of 76 U/mg, a K(m) of 4-5 mM, and a Hill coefficient close to 1.2. The maximal velocity (2 to 89 U/mg) and K(m) (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 alpha-D-glucose but greater maximal velocity with beta-D-glucose. A variance, however, in four mutant forms, the maximal velocity was higher with alpha- than beta-D-glucose. These findings indicate that the higher insulinotropic efficiency of alpha- than beta-glucose cannot be ascribed to the intrinsic catalytic properties of human beta-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.