Transport Proteins That Propagate Solute Gradients

If transport proteins migrated across membranes as carriers, solutes could conceivably drive them to one face of membranes with their total chemical potential gradients. In the case of, say, an antiporter, the protein could then follow its gradient back to the other face of the membrane carrying a different solute against its gradient. Transport proteins do not, however, migrate across membranes to catalyze transport. Rather, they undergo conformational changes while embedded in them. In the case of transport ATPases, such conformational changes are believed to be a part of the route of transfer of the free energy in phosphoric acid anhydride bonds to that of solute gradients. In contrast to some assessments, it is found in this chapter that the kinetics of antiport and symport may be at least as complex as the kinetics of primary active transport by P- and F-/V-type ATPases. Neither a ping-pong nor a two-site simultaneous model accounts well for the antiport catalyzed by anion exchange (AE) proteins. Moreover, this antiport may exhibit apparently hyperbolic kinetics as well as positive and negative cooperativity depending upon the conditions under which transport is measured. The function of AE proteins is further complicated in vivo owing to their also serving in some cases as channels for regulation of cellular volume. These transport functions may be regulated through interdependent associations of the proteins with other membrane proteins, the cytoskeleton, and cytosolic enzymes involved in intermediary metabolism.