Active transporters as enzymes: an energetic framework applied to major facilitator superfamily and ABC importer systems.

Active membrane transporters are dynamic molecular machines that catalyse transport across a membrane by coupling solute movement to a source of energy such as ATP or a secondary ion gradient. A central question for many active transporters concerns the mechanism by which transport is coupled to a source of energy. The transport process and associated energetic coupling involve conformational changes in the transporter. For efficient transport, the conformational changes must be tightly regulated and they must link energy use to movement of the substrate across the membrane. The present review discusses active transport using the well-established energetic framework for enzyme-mediated catalysis. In particular, membrane transport systems can be viewed as ensembles consisting of low-energy and high-energy conformations. The transport process involves binding interactions that selectively stabilize the higher energy conformations, and in this way promote conformational changes in the system that are coupled to decreases in free energy and substrate translocation. The major facilitator superfamily of secondary active transporters is used to illustrate these ideas, which are then be expanded to primary active transport mediated by ABC (ATP-binding cassette) import systems, with a focus on the well-studied maltose transporter.