Molecular Insights into Fluoride Ion Uptake and Selectivity in the CLCF Fluoride/Proton Antiporter

In this study, we investigated the effect of the protonation state of glutamate E118 (Gluex) and glutamate E318 (Gluin) on fluoride ion uptake and selectivity in the CLCF F^-/H⁺ antiporter using molecular dynamics simulations. Analyses of pore size and the potential of mean force (PMF) revealed that fluoride uptake is facilitated under the deprotonated E118 and protonated E318 state, consistent with the fluoride uptake state proposed in the original windmill mechanism. In this state, an increased pore size reduces the energy barrier, promoting fluoride transport from the intracellular solution to the intracellular binding site (S_cen). Interestingly, we also observed a helix-to-coil transition (residues 74-87) in the presence of chloride at S_cen, which enhances chloride dehydration and stabilizes its interaction with the coil structure. This conformational change likely impedes chloride transport, contributing to fluoride ion selectivity. Our findings confirm that fluoride ion selectivity is enhanced in the E118_E318p state, reinforcing its role in the original windmill mechanism. Additionally, we propose that refining the fluoride uptake process in the modified windmill mechanism could lead to a comparable selectivity mechanism, ultimately converging on a unified fluoride-selective uptake mechanism that integrates key aspects of both pathways.