Symport carrier systems

The melibiose carrier MelB of E. coli is a well-studied sodium symport system. This carrier is of special interest, because it can also use protons or lithium ions for cotransport. The projection structure of MelB has been solved at 8 A resolution [107]. The 12 TM helices are arranged in an asymmetrical pattern similar to the previously solved structure of NhaA, which, however, follows an antiport mechanism (Na+ ions out of the cell and H+ into the cell). 

Transporters, particularly those carrying nonlipophilic species across biomembranes or model membranes, can be regarded as vectorial catalysts (and are also called carriers, translocators, permeases, pumps, and ports [e.g., symports and antiports]). Many specialized approaches and techniques have been developed to characterize such systems. This is reflected by the fact that there are currently twenty-three volumes in the Methods in Enzymology series (vols. 21,22,52-56,81,88,96-98,125-127,156-157, 171-174, and 191-192) devoted to biomembranes and their constituent proteins. Chapters in each of these volumes will be of interest to those investigating transport kinetics. Other volumes are devoted to ion channels (207), membrane fusion techniques (220 and 221), lipids (14, 35, 71, and 72), plant cell membranes (148), and a volume on the reconstitution of intracellular transport (219). See Ion Pumps... 

Figure 36. Uphill transport of K ions as a function of time with (1) double carrier membrane system and (2) a symport system (reprinted with permission from Anal. Chem. 1988, 60, 2302. Copyright 1993 American Chemical Society).

There are, however, various types of active transport systems, involving protein carriers and known as uniports, symports, and antiports as indicated in Figure 3.7. Thus, symports and antiports involve the transport of two different molecules in either the same or a different direction. Uniports are carrier proteins, which actively or passively (see section "Facilitated Diffusion") transport one molecule through the membrane. Active transport requires a source of energy, usually ATP, which is hydrolyzed by the carrier protein, or the cotransport of ions such as Na+ or H+ down their electrochemical gradients. The transport proteins usually seem to traverse the lipid bilayer and appear to function like membrane-bound enzymes. Thus, the protein carrier has a specific binding site for the solute or solutes to be transferred. For example, with the Na+/K+ ATPase antiport, the solute (Na+) binds to the carrier on one side of... 

Artificial analogues of the chloride transporter prodigiosin are effective symport HC1 carriers as exemplified in the model systems developed by the groups of Gale [39] and Davis [40], Biological inspiration is also behind another Cl- carrier. Cholic acid is a naturally occurring bile acid that functions as a surfactant in the intestine. Derivatives with three binding sites known as cholapods are able to transport isolated anions across lipid bilayers [41],... 

Transport of many compounds including drugs across cell membranes is mediated by membrane proteins called carrier proteins or channel proteins. Some of these proteins transport only one substrate molecule at a time across the membrane (uniport systems), while others act as cotransport systems (Figure 9.4). Depending on the direction of the second substrate, the proteins are also called symporters or antiporters, for example, Na /glucose cotransporter, H " /peptide cotransporter, or Na /K antiporter (—Na /K -ATPase). 

Mitchell [7] visualized three different systems for facilitated secondary transport. Uniport only one solute is translocated by the carrier protein. Symport two or more different solutes are translocated in the same direction by the carrier protein. Antiport two or more different solutes are translocated by one carrier in opposite directions. 

A carrier which transports a single molecule in one direction is called a uniport system. Alternatively, a carrier may carry two molecules simultaneously in the same direction, i.e. a symport system. TTiirdly, a carrier may exchange one molecule for another and therefore transport them in opposite directions, i.e an antiport system (Figure 9.4). 

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