sentence: The simulation results agree with the experimental findings for E148A mutant , where PT is not observed , although Cl can passively transit through the protein.Collectively , our results suggest that the rate-limiting step for PT through ClC-ec1 requires the presence of Cl and depends on the direction of flow : for outward flux , the smallest calculated rate constant corresponds to E148 deprotonation to the extracellular solution , while for inward flux , the smallest rate constant comes from deprotonation of E148 to E203 in the central region .