Alternating Carrier Models and the Energy Conservation Laws

Abstract

Membrane proteins such as transporters, exchangers, and cotransporters, have traditionally been represented by different versions of the mobile carrier model as illustrated in Fig. 1 A. For a number of transporters, the apparent affinities for intracellular and extracellular substrates have been shown to be different. This has been accounted for in transport activity models by creating an asymmetry in the rate constants for the binding and debinding reactions on each side of the membrane. Care must be taken to adjust the rate constants of the other reactions in the transport cycle in such a way that the microscopic reversibility principle is respected. In an article (1) recently published in the Biophysical Journal, R. J. Naftalin argues that a model of the type shown in Fig. 1 A, presenting an asymmetry between the intracellular and extracellular binding constants, violates the energy conservation laws. We don't agree with this conclusion. Figure 1 Different kinetic representations of the transporter mechanism. Panel A represents the mobile carrier model that was implicitly used in the Biophysical Journal article on which we are commenting (1). Panel B represents the alternating access model, which ... The Carrier Model and the Alternating Access Mechanism In the carrier model, a transporter is pictured as a molecule whose binding site(s) is exposed to one side of the membrane at a time, where it can bind or release a substrate molecule. Even though some ionophores are believed to function according to this “ferry boat” mechanism, a multi-transmembrane segment protein is more likely to function with an alternating access mechanism as illustrated in Fig. 1 B, an assumption borne out by the recent crystallographic structures of transporters in different orientations (2–5). In this case, a binding site is alternatively exposed to each side of the membrane through a conformational change. In kinetic modeling, the two mechanisms are indistinguishable as they can be represented by the same number of states linked by the same number of rate constants (Fig. 1 C).