A permeation theory for single-file ion channels: Corresponding occupancy states produce Michaelis-Menten behavior

A kinetic theory is proposed for permeation in open ion channels. Within the model, the selectivity filter of the ion channel contains either n or (n - 1) single-file ions. Association and/or dissociation of the nth ion from the selectivity filter is the rate limiting step. Ion translocation occurs via concerted single-file motion of the (n - 1) ions and any water molecules contained within the selectivity filter. This concerted motion is assumed to be sufficiently rapid that translocation is not rate limiting. Ion permeation is thus sorption-limited, and the reduced permeation rate j* is a universal function of the reduced concentration c*, taking the form of a Langmuir adsorption isotherm. A corresponding occupancy-states explanation is thus provided for the Michaelis-Menten kinetics observed experimentally for many ion channels. Published experimental data for cation conduction in the open pores of the KcsA and gramicidin A channels are shown to exhibit the universal saturating behavior predicted by the theory. Additional validation of the theory by asymmetric-solution and tracer counterpermeation experiments is also discussed. (C) 2002 American Institute of Physics.