Phosphorylation by liver glucokinase of D-glucose anomers at anomeric equilibrium

The relative contribution of each anomer of D-glucose to the overall phosphorylation rate of the hexose tested at anomeric equilibrium was examined in rat liver postmicrosomal supernatants under conditions aimed at characterizing the activity of glucokinase, with negligible interference of either hexokinase, N-acetyl-D-glucosamine kinase or glucose-6-phosphatase (acting as a phosphotransferase). Both at 10 degrees and 30 degrees C, the relative contribution of each anomer was unaffected by the concentration of D-glucose. At both temperatures, the alpha/beta ratio for the contribution of each anomer was slightly, but significantly, lower than the alpha/beta ratio of anomer concentrations. These findings, which are consistent with the anomeric specificity of glucokinase in terms of affinity, cooperativity and maximal velocity, reveal that the preferred alpha-anomeric substrate for both glycogen synthesis and glycolysis is generated by glucokinase at a lower rate than is beta-D-glucose-6-phosphate.