Polar groups in the peptide bonds are not involved in

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polar groups in the peptide bonds are not involved in bonding to one another. Such disordered segments can exist in proteins because hydrogen bonds can be made with water molecules or to other polar groups in the protein. In a membrane, however, the hydrophobic hydrocarbon chains of the bilayer provide no hydrogen-bonding partners. As a result, a disordered peptide chain in a membrane is energetically very unfavorable. 10–55 Transmembrane a helices are thought to be more common than transmem- brane b barrels because they provide a more flexible arrangement of trans- membrane segments. Because a helices can slide against one another, they allow the protein to undergo conformational changes that can be exploited to gate ion channels, transport solutes, or transmit information across the bilayer. By contrast, the b strands in a b barrel are rigidly fixed to their neigh- bors by hydrogen bonds that lock the protein into a single conformation. 10–56 Your friend’s suggestion is based on an important difference between inside- out and right-side-out vesicles. The contaminating right-side-out vesicles will carry carbohydrate on their exposed surface and should therefore be MEMBRANE PROTEINS A229 hydrophobic face hydrophilic face hydrophilic pore lipid bilayer Figure 10–14 Proposed structure for a hydrophilic pore formed by five membrane-spanning a helices ( Answer 10–52 ).
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retained on a lectin affinity column. Inside-out vesicles, by contrast, will lack carbohydrate on their exposed surface and should therefore pass through the column. 10–57 The environment in the cytosol is sufficiently reducing that disulfide bonds rarely form, whereas outside the cell (and in internal lumens) the environ- ment is sufficiently oxidizing that disulfide bonds form readily. 10–58 The sulfate group in SDS is charged and therefore hydrophilic. The OH group and the C–O–C groups in Triton X-100 are polar; they can form hydro- gen bonds with water and are therefore hydrophilic. The gray portions of these detergents are either hydrocarbon chains or aromatic rings, neither of which have polar groups that can hydrogen bond to water molecules; they are therefore hydrophobic. 10–59 Membrane proteins anchor the lipid bilayer to the cytoskeleton, which strengthens the plasma membrane so that it can withstand the shear forces red blood cells are subjected to when they are pumped through small blood vessels. Membrane proteins also transport nutrients and ions across the plasma membrane. 10–60 Normally the cytosol is sufficiently reducing that it contains no disulfide bonds, even in G6PD-deficient individuals. However, in G6PD-deficient individuals who eat fava beans, the cytosol of red cells can become su- fficiently oxidizing that disulfide bonds form. Since all such bonds are inappropriate in the cytosol, they link proteins in ways that were never in- tended, leading to clumps and aggregates that stick to the cell membrane.
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