Mechanisms Underlying Proton Release in CLC-type F/H Antiporters.

The CLC family of anion channels and transporters includes Cl/H exchangers (blocked by F) and F/H exchangers (or CLCs). CLCs contain a glutamate (E318) in the central anion-binding site that is absent in CLC Cl/H exchangers. The X-ray structure of the protein from (CLC-eca) shows that E318 tightly binds to F when the gating glutamate (E118; highly conserved in the CLC family) faces the extracellular medium. Here, we use classical and DFT-based QM/MM metadynamics simulations to investigate proton transfer and release by CLC-eca. After to movement of protonated E118, both glutamates combine with F to form a triad, from which protons and F anions are released as HF. Our results illustrate how glutamate insertion into the central anion-binding site of CLC-eca permits the release of H to the cytosol as HF, thus enabling a net 1:1 F/H stoichiometry.