Proton Transport Pathway in the CIC Cl-/H+ Antiporter

A fundamental question concerning the CIC Cl-/H+ antiporters is the nature of their proton transport (PT) pathway, We addressed this issue by using a novel computational methodology capable of describing the explicit PT dynamics in the CIC-ec1 protein. The main result is that the Glu(203) residue delivers a proton from the intracellular solution to the core of CIC-ec1 via a rotation of its side chain and subsequent acid dissociation. After reorientation of the Glu(203) side chain, a transient water-mediated PT pathway between Glu(203) and Glu(148) is established that is able to receive and translocate the proton via Grotthuss shuttling after deprotonation of Glu(203). A molecular-dynamics simulation of an explicit hydrated excess proton in this pathway suggests that a negatively charged Glu(148) and the central Cl- ion act together to drive H+ to the extracellular side of the membrane. This finding is consistent with the experimental result that Cl- binding to the central site facilitates the proton movement. A calculation of the PT free-energy barrier for the CIC-ec1 E203V mutant also supports the proposal that a dissociable residue is required at this position for efficient delivery of H+ to the protein interior, in agreement with recent experimental results.