The coupled proton transport in the ClC-ec1 Cl(-)/H(+) antiporter

Using a reactive molecular dynamics simulation methodology, the free energy barrier for water-mediated proton transport between the two proton gating residues Glu²⁰³ and Glu¹⁴⁸ in the ClC-ec1 antiporter, including the Grotthuss mechanism of proton hopping, was calculated. Three different chloride-binding states, with 1), both the central and internal Cl⁻, 2), the central Cl⁻ only, and 3), the internal Cl⁻ only, were considered and the coupling to the H⁺ transport studied. The results show that both the central and internal Cl⁻ are essential for the proton transport from Glu²⁰³ to Glu¹⁴⁸ to have a favorite free energy driving force. The rotation of the Glu¹⁴⁸ side chain was also found to be independent of the internal chloride binding state. These results emphasize the importance of the 2:1 stoichiometry of this well-studied Cl⁻/H⁺ antiporter.