What is the Vmax in facilitated diffusion?
Facilitated carriers show enzyme-like saturation kinetics because transporter number is finite and each must cycle. At low substrate concentration occupancy low, rate rises near linearly as collisions produce binding, first-order regime. As concentration rises fractional occupancy climbs, rate limited by isomerization steps approaching plateau where all carriers cycle at maximal turnover kcat typically hundred to ten thousand per second for GLUTs. Maximal velocity Vmax reflects total functional transporters times turnover measured per minute. Beyond Vmax increasing gradient no longer raises flux because sites saturated. This yields specificity and competitive inhibition where glucose competes with analogs raising apparent Km. Simple diffusion never saturates. Vmax estimation proxies expression level, while Km reflects affinity. Regulatory hormone insulin recruitment of GLUT4 raises Vmax by increasing surface number without changing Km, enhancing uptake capacity after meals independent of affinity modulation, allowing muscle to clear glucose rapidly. Distinguishing Km and Vmax changes aids interpretation of mutations affecting binding versus trafficking defects in diabetes.
Ref: Widdas, Journal of Physiology 1952: Facilitated Diffusion – Vmax as Saturation of Carriers.