Comparison of the kinetics and thermodynamics of the carrier systems for glucose and leucine in human red blood cells.

Kinetic data for the transport of glucose and leucine in human red blood cells are fitted to the conventional carrier model and the thermodynamics of the two carrier mechanisms are compared. In the absence of the carried molecule both carriers exist mainly in the inward-facing conformation at low temperatures and the outward-facing conformation at physiological or supra-physiological temperatures, this finding reflecting the strongly endothermic process involved in changing from the inward- to outward-facing forms. Reorientations from inward- to outward-conformations also involve substantial increases in entropy for both carriers. In contrast, substrate binding to the glucose carrier involves little change in enthalpy and an increase in entropy, while leucine binding is strongly exothermic and associated with a decrease in entropy. Application of transition state theory to glucose carrier kinetics reveals that the entropy of formation of the transition state of the carrier is much greater than that for the transition state of the carrier-glucose complex.