1048 the principles are the same for both exposure of

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10–48 The principles are the same for both. Exposure of hydrophobic amino acid side chains to water is energetically unfavorable. There are two ways that such side chains can be sequestered away from water to achieve an energeti- cally more favorable state. One, they can form a -helical transmembrane segments that span a lipid bilayer, so that the hydrophobic amino acid side chains can interact with the hydrophobic lipids in the membrane. Two, the hydrophobic amino acid side chains can be sequestered in the interior of the folded polypeptide chain, where they can interact with each other. A228 Chapter 10: Membrane Structure
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10–49 D. The mass ratio depends on the membrane. In the myelin membrane around nerve-cell axons, proteins account for less than 25% of the mass. In mito- chondrial cytoplasmic membranes, proteins account for about 75% of the total mass. In typical plasma membranes the masses of proteins and lipids are about the same. 10–50 Fatty acid chains, prenyl groups, and glycosylphosphatidylinositol (GPI) anchors are the three common lipid anchors for membrane proteins. 10–51 A. Sequence A is the actual membrane-spanning a -helical segment of gly- cophorin, a transmembrane protein from red blood cells. It is composed predominantly of hydrophobic amino acids, although it does contain the uncharged polar amino acids threonine (T) and serine (S), which are not uncommon in membrane-spanning a helices. Sequence B is unlikely to be a membrane-spanning segment because it contains three prolines (P), which would disrupt an a helix and thereby expose polar groups to the hydrophobic environment of the lipid bilayer. Sequence C is also unlikely to be a transmembrane segment because it contains three charged amino acids, glutamic acid (E), arginine (R), and aspartic acid (D), whose presence in the hydrophobic lipid bilayer would be energetically unfavorable. 10–52 The hydrophilic faces of the five membrane-spanning a helices, each con- tributed by a different subunit, can come together to form a hydrophilic pore across the lipid bilayer that is lined with the hydrophilic amino acid side chains (Figure 10–14). The hydrophobic side chains on the opposite sides of the a helices can then interact with the hydrophobic lipid tails in the bilayer. 10–53 A. In a b strand, adjacent amino acid side chains protrude from opposite sides of the strand; thus, every other amino acid side chain will face the same side of the strand. If a b strand is part of a b -barrel pore, its amino acid side chains will alternate between hydrophobic and hydrophilic, so that one side of the strand will be hydrophobic and the other side will be hydrophilic. Only choice A has alternating hydrophobic and hydrophilic amino acids. 10–54 In both an a helix and a b barrel the polar hydrogen-bonding groups in the peptide bond are fully satisfied by internal hydrogen bonds with groups in other peptide bonds. These internal hydrogen bonds dictate the secondary structures known as a helices and b sheets (or b barrels when the edges of a sheet pair to complete the cylinder). By contrast, in a disordered chain the
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