When mixed with membranes, the hydrophobic ends of detergents bind to the hydrophobic regions of the membrane proteins, where they displace lipid molecules with a collar of detergent molecules. Since the other end of the detergent
Figure 10–25 The carbohydrate layer on the cell surface. (A) this electron micrograph of the surface of a lymphocyte stained with ruthenium red emphasizes the thick carbohydrate-rich layer surrounding the cell. (B) the carbohydrate layer is made up of the oligosaccharide side chains of membrane glycolipids and membrane glycoproteins and the polysaccharide chains on membrane proteoglycans. In addition, adsorbed glycoproteins, and adsorbed proteoglycans (not shown), contribute to the carbohydrate layer in many cells. note that all of the carbohydrate is on the extracellular surface of the membrane. (A, courtesy of Audrey
M. glauert and g.M.W. cook.)
Figure 10–26 The structure and function of detergents. (A) three commonly used detergents are sodium dodecyl sulfate (SDS), an anionic detergent, and triton X-100 and β-octylglucoside, two nonionic detergents. triton X-100 is a mixture of compounds in which the region in brackets is repeated between 9 and 10 times. the hydrophobic portion of each detergent is shown in yellow, and the hydrophilic portion is shown in orange. (B) At low concentration, detergent molecules are monomeric in solution. As their concentration is increased beyond the critical micelle concentration (cMc), some of the detergent molecules form micelles. note that the concentration of detergent monomer stays constant above the cMc. (c) Because they have both polar and nonpolar ends, detergent molecules are amphiphilic; and because they are cone-shaped, they form micelles rather than bilayers (see Figure 10–7). Detergent micelles are thought to have irregular shapes, and, due to packing constraints, the hydrophobic tails are partially exposed to water. (D) the space-filling model shows the structure of a micelle composed of 20 β-octylglucoside molecules, predicted by molecular dynamics calculations. the head groups are shown in red and the hydrophobic tails in gray. (B, adapted from g. gunnarsson, B. Jönsson and h. Wennerström, J. Phys. Chem. 84:3114–3121, 1980; c, from S. Bogusz, r.M. Venable and r.W. pastor, J. Phys. Chem. B 104:5462–5470, 2000.) molecule is polar, this binding tends to bring the membrane proteins into solution as detergent–protein complexes (Figure 10–27). Usually, some lipid molecules also remain attached to the protein.
Strong ionic detergents, such as SDS, can solubilize even the most hydrophobic membrane proteins. This allows the proteins to be analyzed by SDS polyacrylamide-gel electrophoresis (discussed in Chapter 8), a procedure that has revolutionized the study of proteins. Such strong detergents, however, unfold (denature) proteins by binding to their internal “hydrophobic cores,” thereby rendering the proteins inactive and unusable for functional studies. Nonetheless, proteins can be readily separated and purified in their SDS-denatured form. In some cases, removal of the SDS allows the purified protein to renature, with recovery of functional activity.