Sugar specificity of human beta-cell glucokinase: correlation of molecular models with kinetic measurements

Human beta-cell Glucokinase recognition and phosphorylation of different sugars was investigated by steady-state kinetic analysis, measurements of substrate-induced intrinsic fluorescence changes, and molecular modeling and calculation of interaction energies. Measurements of kcat/Km showed that Glucokinase phosphorylated the sugars in the order glucose = mannose > deoxyglucose > fructose = glucosamine. The mode of binding of these sugars to the open conformation of Glucokinase was predicted from molecular modeling. Glucokinase is predicted to form similar interactions with the 6-OH, 4-OH, and 1-OH groups of all these sugars. The interactions of the 2-OH and 3-OH groups differ and depend on the type of sugar and reflect differences in cooperative behavior. For example, glucose and deoxyglucose exhibited cooperative behavior with Hill coefficients of 1.8 and 1.5, respectively, while mannose and fructose demonstrated Michaelis-Menten behavior. Galactose, allose, and 2,5-anhydroglucitol were not substrates under the assay conditions used, and the alpha- and beta-anomers of methylglucose were poor substrates with Km's greater than 1000 mM. Glucokinase exhibited an ATPase activity which was 1/2000th that of the rate of the kinase reaction, and unlike yeast Hexokinase, it was not affected by the addition of lyxose. Glucosamine was a low affinity inhibitor as well as a substrate, while N-acetylglucosamine and mannoheptulose were high-affinity inhibitors. The change in intrinsic fluorescence that was induced by glucose, mannose, and mannoheptulose had the opposite sign for glucosamine, which implies a very different mode of binding from the other sugars. The calculated interaction energies of Glucokinase with glucose, mannose, deoxyglucose, and fructose agree very well with the measured values of kcat/Km, which indicates that these sugars are recognized by binding to the open conformation of Glucokinase.