The Origin of Coupled Chloride and Proton Transport in a Cl/H Antiporter

The ClC family of transmembrane proteins functions throughout nature to control the transport of Cl ions across biological membranes. ClC-ec1 from Escherichia coli is an antiporter, coupling the transport of Cl and H ions in opposite directions and driven by the concentration gradients of the ions. Despite keen interest in this protein, the molecular mechanism of the Cl/H coupling has not been fully elucidated. Here, we have used multiscale simulation to help identify the essential mechanism of the Cl/H coupling. We find that the highest barrier for proton transport (PT) from the intra- to extracellular solution is attributable to a chemical reaction, the deprotonation of glutamic acid 148 (E148). This barrier is significantly reduced by the binding of Cl in the "central" site (Cl), which displaces E148 and thereby facilitates its deprotonation. Conversely, in the absence of Cl E148 favors the "down" conformation, which results in a much higher cumulative rotation and deprotonation barrier that effectively blocks PT to the extracellular solution. Thus, the rotation of E148 plays a critical role in defining the Cl/H coupling. As a control, we have also simulated PT in the ClC-ec1 E148A mutant to further understand the role of this residue. Replacement with a non-protonatable residue greatly increases the free energy barrier for PT from E203 to the extracellular solution, explaining the experimental result that PT in E148A is blocked whether or not Cl is present. The results presented here suggest both how a chemical reaction can control the rate of PT and also how it can provide a mechanism for a coupling of the two ion transport processes.