title: The Origin of Coupled Chloride and Proton Transport in a Cl-/H+Antiporter.
abstract: 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-cen), which displaces E148 and thereby facilitates its deprotonation. Conversely, in the absence of Cl-cenE148 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-cenis 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.
authors: Department of Chemistry, James Franck Institute, and Institute for Biophysical Dynamics, The University of Chicago , Chicago, Illinois 60637, United States.
authors: Sangyun
authors: S
authors: Lee
authors: Department of Chemistry, James Franck Institute, and Institute for Biophysical Dynamics, The University of Chicago , Chicago, Illinois 60637, United States.
authors: Heather B
authors: HB
authors: Mayes
authors: Department of Chemistry, James Franck Institute, and Institute for Biophysical Dynamics, The University of Chicago , Chicago, Illinois 60637, United States.
authors: Jessica M J
authors: JM
authors: Swanson
authors: Department of Chemistry, James Franck Institute, and Institute for Biophysical Dynamics, The University of Chicago , Chicago, Illinois 60637, United States.
authors: Gregory A
authors: GA
authors: Voth
mesh_terms: D017920:Antiporters / Q000737:chemistry / Q000378:metabolism*; D002712:Chlorides / Q000737:chemistry / Q000378:metabolism*; D004926:Escherichia coli / Q000737:chemistry / Q000378:metabolism*; D056004:Molecular Dynamics Simulation; D011522:Protons
chemical_list: D017920:Antiporters; D002712:Chlorides; D011522:Protons