Ligand-induced conformations of rat brain hexokinase: effects of glucose-6-phosphate and inorganic phosphate

Binding of glucose-6-P induces conformational change in rat brain hexokinase (ATP:d-hexose 6-phosphotransferase, EC 2.7.1.1) as indicated by decreased susceptibility to digestion by chymotrypsin and an increased sedimentation coefficient on sucrose density gradients. These effects are competitively reversed by P_i, as are solubilization (of the mitochondrial form of hexokinase) and inhibition by glucose-6-P. Thus, the observed conformational changes are likely to be directly related to the effect of these ligands on catalytic activity and the interaction of the hexokinase with the mitochondrial membrane.Both glucose-6-P and P_i stabilize the enzyme against heat inactivation; this effect, as well as the effect of glucose-6-P on inactivation by chymotrypsin, have been used to estimate the dissociation constants for the complexes of hexokinase with glucose-6-P and P_i; the values are 7–8 μm, and 0.25 mm, respectively.These observations are consistent with a model in which brain hexokinase may exist in two distinct conformations, rapidly and reversibly interconvertible. The effect of glucose-6-P and P_i are explained by highly preferential binding to one or the other of these conformations.