A Pre-Steady State Analysis of Ligand Binding to Human Glucokinase: Evidence for a Preexisting Equilibrium

Cooperativity with glucose is a key feature of human glucokinase (GK), allowing its crucial role as a glucose sensor in hepatic and pancreatic cells. We studied the changes in enzyme intrinsic tryptophan fluorescence induced by binding of different ligands to this monomeric enzyme using stopped-flow and equilibrium binding methods. Glucose binding data under pre-steady state conditions suggest that the free enzyme in solution is in a preexisting equilibrium between at least two conformers (super-open and open) which differ in their affinity for glucose (Kd* = 0.17 ± 0.02 mM and Kd = 73 ± 18 mM). Increasing the glucose concentration changes the ratio of the two conformers, thus yielding an apparent Kd of 3 mM (different from a Km of 7−10 mM). The rates of conformational transitions of free and GK complexed with sugar are slow and during catalysis are most likely affected by ATP binding, phosphate transfer, and product release steps to allow the kcat to be 60 s-1. The ATP analogue PNP-AMP binds to free GK (...