Modulating the Chemical Transport Properties of a Transmembrane Antiporter via Alternative Anion Flux
Z Wang and JMJ Swanson and GA Voth, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 140, 16535-16543 (2018).
ClC-ecl is a prototype of the ClC antiporters, proteins that stoichiometrically exchange Cl- and H+ ions in opposite directions across a membrane. It has been shown that other polyatomic anions, such as NO3- and SCN-, can also be transported by ClC-ecl, but with partially or completely uncoupled proton flux. Herein, with the help of multiscale computer simulations in which the Grotthuss mechanism of proton transport (PT) is treated explicitly, we demonstrate how the chemical nature of these anions alters the coupling mechanism and qualitatively explain the shifts in the ion stoichiometry. Multidimensional free energy profiles for PT and the coupled changes in hydration are presented for NO3- and SCN-. The calculated proton conductances agree with experiment, showing reduced or abolished proton flux. Surprisingly, the proton affinity of the anion is less influential on the PT, while its size and interactions with the protein significantly alter hydration and shift its influence on PT from facilitating to inhibiting. We find that the hydration of the cavity below the anion is relatively fast, but connecting the water network past the steric hindrance of these polyatomic anions is quite slow. Hence, the most relevant coordinate to the PT free energy barrier is the water connectivity along the PT pathway, but importantly only in the presence of the excess proton, and this coordinate is significantly affected by the nature of the bound anion. This work again demonstrates how PT is intrinsically coupled with protein cavity hydration changes as well as influenced by the protein environment. It additionally suggests ways in which ion exchange can be modulated and exchange stoichiometries altered.
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