Active Transport Driven by Na Gradients

Many transmembrane transporter proteins, termed secondary transporters, use the discharge of an ionic gradient to power the uphill translocation of a solute molecule across membranes. Couphng solute movement to ion transport enables these secondary transporters to concentrate solutes by a factor of 10 with a solute flux 10 faster than by simple diffusion. We have already encountered the co-transport of leucine and Na+ by LeuT, but there are many other examples. Sugars and amino acids can be transported into cells by Na+-dependent symports. Dietary glucose is concentrated in the epithelial cells of the small intestine by a Na -dependent symport, and is then 

FIGURE 9.12 Epithelial brush border cells of the small intestine concentrate glucose from the intestinal lumen in symport with Na this is driven by the Na -K -ATPase located on the capillary side of the cell. The glucose is then exported out of the brush border cell by a passive uniport system. Adapted from Voet Voet, 2004.) 

FIGURE 9.13 Transport cycle and stoichiometry of excitatory amino acid transporters (EAATs). Simplified state diagram of the EAAT transport cycle. After glutamate and coupled ions (step 1) bind to the transporter (T), they are translocated (step 2) and released into the cell cytosol (step 3). Next binds from the intracellular side (step 4) and reorients the substrate-fiEe transporter (step 5). is released outside the cell (step 6). (From Jiang Amara, 2011. Reproduced with permission from Elsevier.) 

After glutamate and coupled ions bind to the transporter, they are translocated and released into the cell cytosol. Next, K binds from the intracellular side and reorients the substrate-free transporter, and finally K is released outside the cell. 

FIGURE 9.14 (D) Cartoon representation of the Glt/ /, protomer viewed in the plane of the membrane. Transmembrane domain sl-6 coloured 

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