Hydrogen bonds stabilize Glu_ex in the central site. (A and B) Normalized time course of H⁺ transport (A) and mean normalized steady-state H⁺ uptake (B) mediated by the S107A mutant in varying symmetrical [Cl⁻]. (C and D) Normalized time course of H⁺ transport (C) and mean normalized steady-state H⁺ uptake (D) mediated by the Y445F mutant in varying symmetrical [Cl⁻]. (E and F) Normalized time course of H⁺ transport (E) and mean normalized steady-state H⁺ uptake (F) mediated by the S107A/Y445F double mutant in varying symmetrical [Cl⁻]. The asterisks denote the addition of valinomycin to initiate the experiment. Dashed lines represent exponential fits of the time courses. Bars, 10 s. The values are reported in Table 1; error bars are the SEM of the number of experiments indicated in Table 1.