How does the chemical potential of the substrate drive a uniporter?

Abstract

Uniporters are a large class of transporters mediating facilitated diffusion of substrates along the direction of the substrate concentration gradient. Recently, structures of several important uniporters have been reported; however, the precise mechanisms of uniporter function remain subject of debate. Here, we present a series of general thermodynamic descriptions of uniporters, aimed at understanding the structure-function relationship of uniporters, and in particular to reconcile biochemical phenomena of uniporters with our previously proposed thermodynamic model of general transporters.Keywords: uniporters; differential binding energy; elastic conformational energy; transition-state energy barrier; rate-limiting step; kinetic asymmetry UniporterThe cellular membrane presents a major energy barrier to hydrophilic small molecules to move across. This barrier is usually so high that translocation of these small molecules across the membrane along their concentration gradients is practically halted, even in the presence of a thermodynamically favorable chemical potential. The function of uniporters is to lower this energy barrier for selected substrates, so that the substrates can be transported along their own concentration gradients, a process called facilitated diffusion. A uniporter is a transporter that does not require energy input other than the substrate chemical potential. Like all transporters, a uniporter differs from a channel in that the former uses the alternating access mechanism 1 while the latter does not. Using this mechanism, a uniporter switches between two major conformations, namely inward-facing (C In ) and outwardfacing (C Out ), allowing its substrate-binding site to be accessible alternatingly to both sides of the membrane. The energy barrier for a hydrophilic molecule to transport passively across the cellular membrane is analogous to the transition-state energy barrier in a chemical reaction. According to the transitionstate theory of enzymology, an enzyme stabilizes the transition state of the reactants, thus effectively lowering the transition-state energy barrier. However, the reaction is driven by the chemical energy Abbreviations: C In , inward-facing conformation; C Out , outwardfacing conformation.

Uniporter substrate binding and transport: reformulating mechanistic questions

“…This phenomenon is called asymmetric transport. The logic presented here to interpret asymmetric transport, which was also employed in earlier work by others (Lowe and Walmsley 1986), is conceptually more straight forward than a model proposed recently (Zhang and Han 2016). For GLUT1, it was estimated that at 0 °C V efflux is more than 10 times faster than V influx (Lowe and Walmsley 1986).…”

“…The small value of ∆ G D suggests that K d,in and K d,out are nearly identical and that differential binding energy contributes almost zero to the driving force for GLUT1. Thus, what actually drives the transport process in this case can only originate from the ∆ G L and ∆ G R terms, by favoring the forward movement and/or preventing the backward movement (Zhang and Han 2016). In addition, in both the absence and presence of substrate, GLUT1 stays predominantly in the C in state (with f (0) = 0.06 and f (∞) = 0.07), which is in agreement with the above-mentioned positive value of ∆ G E .…”

“…3) is a useful tool to visually represent the transport process, for instance whether a step is thermodynamically favorable (Zhang et al 2015; Zhang and Han 2016). While the vertical dimension of the plot represents Gibbs free energy, the horizontal dimension can be considered as an alternative expression of the King–Altman plot.…”

A critique of the alternating access transporter model of uniport glucose transport

“…Instead Mueckler and Thorens contend that the passive glucose transport system is symmetrical and its apparent asymmetries are due to the experimental error obtained owing to very rapid glucose fluxes at or above room temperatures. Zhang and Han (2016a, b) also claim that dissociation constants for glucose at the inside and outside of the transporter namely K D in and K D out are nearly identical.…”

Thermodynamic secrets of multidrug resistance: A new take on transport mechanisms of secondary active antiporters

“…Furthermore, unlike an inhibitor which drastically increases the energy barrier of the conformational transition, an ideal substrate significantly reduces the barrier, thus kinetically accelerating the transport process. 36 Such a property of a substrate is often mistakenly considered as evidence that substrate itself drives the transport process even against its own concentration gradient; this misconception would be equivalent to a perpetual motion machine at the molecule level. Importantly, the number of substrates that satisfy the above requirements for a given MDR transporter is potentially large.…”