Fine print in isotope effects: The glucose anomeric equilibrium and binding of glucose to human brain hexokinase

Binding isotope effects are a sensitive measure of changes in molecular vibrational character that occur during ligand-receptor binding. In this study, we have measured isotope effects on the binding of glucose to human brain hexokinase using the ultrafiltration method, with the following results: 0.991±0.001, 0.908±0.003, 1.010±0.001, 0.974±0.002, 1.022±0.002 for [¹⁴C]-glucose mixed with [1-³H]-, [2-³H]-, [3-³H]-, [5-³H]-, [6,6-³H₂]-glucose, respectively. Comparing the observed data with isotope effects on the anomeric equilibrium in glucose reported previously [3] proves the existence of binding isotope effects in this system. Preliminary computational results are presented to explain the observed binding isotope effects in terms of hydrogen bond patterns and molecular crowding found in the binary complex of sugar and enzyme.