Chapter 5 – Coupling of Flows of Substrates: Antiporters and Symporters

This chapter deals with secondary active transport in which coupling of flows of different substrates takes place. Here, flow of a substrate that is being pumped up its electrochemical gradient is coupled to flow of a second substrate down its electrochemical gradient. The gradient of the second substrate is maintained by a primary active transport system (to be discussed in Chapter 6). A vast array of primary and secondary transport systems operate across the outer plasma membrane and the membranes within the living cell that bound the intracellular organelles, producing and consuming electrochemical gradients for a wide variety of substrates. The two classes of secondary active transporters are, first, the antiporters where one substrate is exchanged for another, second, the cotransporters or symporters where driving and driven substrates travel together in coupled movement across the cell membrane. The transport kinetics of both classes is discussed in detail, examples of each being given. The molecular structure of several transporters of both classes is depicted and the relation between structure and function discussed. In particular, studies on antiporters EmrE and NhaA, and on symporters LacY and SGLT are discussed in detail.