Figure 1 | Scientific Reports

Figure 1

From: Low affinity uniporter carrier proteins can increase net substrate uptake rate by reducing efflux

Figure 1

Lower affinity can enhance uptake by reducing substrate efflux. (A) Illustration of the reduced-efflux hypothesis. Left panel: A high affinity of the transporter will cause both the inward facing and the outward facing binding sites of the transporter to be saturated with substrate. As a result, the efflux rate will be nearly as high as the influx rate, and the net uptake rate is very low. Right panel: Reducing the affinity of the transporter reduces the saturation of the transporter at the intracellular side. Provided the extracellular substrate concentration is high enough, the transporter will still be saturated at the extracellular side. The efflux will be reduced and the net uptake rate increases. (B) Model of nutrient transport by facilitated diffusion underlying Equation 1. The transporter switches between conformations with an inward-facing (e i ) and outward-facing (e e ) substrate binding site. When this conformation change takes places with an occupied substrate binding site (es e or es i ), this results in translocation of the substrate over the membrane. All steps are reversible and the state transitions rates are given by mass action kinetics. Throughout the main text, we make the assumption that binding is much faster than transport and that the carrier is symmetric. The former assumption allows us to use the quasi steady-state assumption for substrate to transporter binding, i.e. e x and es x are in equilibrium, with dissociation constant k D k1r/k1f = k3f/k3r. The latter assumption implies k2f = k2rk2 and k4f = k4rk4.