Facilitated diffusion with consecutive reaction: optimal carrier affinity

The interplay between facilitated diffusion of a substrate through a membrane and a consecutive enzymic reaction, both of which follow Michaelis-Menten kinetics, has been theoretically investigated and the effect of the kinetic and transport parameters on the rate of substrate uptake is graphically illustrated. At steady state two characteristic features of the system have been identified. First, the substrate concentration at the internal enzymic side of the membrane cannot exceed a given value even at much higher external substrate concentrations. Second, the uptake rate is maximum at a given value of K_T, the kinetic parameter of the transport system that expresses the reciprocal carrier affinity of the substrate. The optimum value of K_T is approximately equal to the external substrate concentration. This particular dependence of the uptake rate on the carrier affinity is expected to play an important role in hormonal regulation.