Solutes Are Transported by Specific Carriers

By contrast, the kinetics of transport across a biological membrane usually does not exhibit such a linear dependence on AC. Instead, the rate approaches an asymptote at high concentration differences, much as the rate of an enzymatic reaction approaches a maximum rate at high substrate concentrations (fig. 17.24d). This behavior suggests that the transport protein has a specific binding site for the material that it transports. The overall rate of transport is limited by the number of these sites in the membrane, and thus fits an 

Just as in enzyme kinetics (see chapter 7), Km here is an algebraic function of the microscopic rate constants for binding, dissociation, and translocation of the substrate in either direction. 

The bacterial lactose-transport protein (lactose permease) transports j3-galactosides, such as lactose, o-nitrophenyl-jS-galactoside, and isopropyl-jS-thiogalactoside. It does not transport galactosides with an a-glycosidic linkage. 

Like conventional enzymes, transport proteins often are sensitive to specific inhibitors. For example, the anion transporter of the erythrocyte plasma membrane, which moves HC03- and Cl- ions across the membrane, is inhibited by 1,2-cyclohexadione or derivatives of stilbenedisul-fonate. Because these inhibitors react covalently with amino acid residues of the protein, they afford useful probes for the location and structure of the anion-binding site. 

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Most Solutes Are Transported by Specific Carriers materials inside the cell. But the plasma membrane is not... 

Biological membranes contain proteins that act as specific transporters of small molecules into and out of the cell. Most solutes are transported by specific carriers that are invariably proteins. 

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