Simple model for the chemical potential change of a transported ion in active transport.

Abstract

The mechanism for active transport of ions across a membrane probably involves 2 distinct conformational states of the transport protein, in which the binding sites for the transported ion face opposite sides of the membrane. The binding affinity for the ion probably changes in synchrony with the change in site orientation, such that the affinity is high on the uptake side of the membrane and low on the discharge side. A structural model is proposed for the transmembrane portion of such a protein, based on the known multihelical structure of bacteriorhodopsin. This structure is well adapted to a cyclical alternation between 2 conformations that differ simultaneously in orientation and binding affinity. No unfolding of the helices or other significant alterations in secondary structure is required. The model is explicitly intended as a hypothetical representation of the E1 and E2 states of ATP-driven Na+, K+ and Ca2+ pumps.