Discrete-state model of coupled ion permeation and fast gating in ClC chloride channels

A simple discrete-state model of ion permeation through a channel protein is considered in which the flow of ions through the open channel is coupled to the opening/closing of a gate by virtue of configurational changes in a particular pore-lining amino acid residue. The model is designed so as to represent essential features of ClC chloride channels, in which a particular glutamate residue (E148 in bacterial ClC channels) is thought to switch from a conformation that is pinned back (away from the pore itself) to one where this side group blocks the channel at a Cl(-) binding site. Thus, competition between the gate residue and Cl(-) ions for this site leads to interesting kinetics, such as the saturation of the gate closing time with increasing concentration of internal Cl(-) concentration. Analysis of the model proposed here shows that it can account for many qualitative features of ion channel permeation and gate closing rates in ClC-type channels observed experimentally and in recent computer simulations of these processes.