Ion Binding Energies Determining Functional Transport of ClC Proteins

The ClC-type proteins, a large family of chloride transport proteins ubiquitously expressed in biological organisms, have been extensively studied for decades. Biological function of ClC proteins can be reflected by analyzing the binding situation of Cl^? ions. We investigate ion binding properties of ClC-ec1 protein with the atomic molecular dynamics simulation approach. The calculated electrostatic binding energy results indicate that Cl^? at the central binding site S_cen has more binding stability than the internal binding site S_int. Quantitative comparison between the latest experimental heat release data isothermal titration calorimetry (ITC) and our calculated results demonstrates that chloride ions prefer to bind at S_cen than S_int in the wild-type ClC-ec1 structure and prefer to bind at S_ext and S_cen than S_int in mutant E148A/E148Q structures. Even though the chloride ions make less contribution to heat release when binding to S_int and are relatively unstable in the Cl^? pathway, they are still part contributors for the Cl^? functional transport. This work provides a guide rule to estimate the importance of Cl^? at the binding sites and how chloride ions have influences on the function of ClC proteins.