MID TERM EXAM FOR BIOL 204 -BIOCHEMISTRY(2019) -ALL ANSWERS CORRECT-GRADED A

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1. If the thickness of the lipid bilayer of a typical biological membrane is 30 Å, how many amino acid residues would be required in a polypeptide if it were to project just far enough to span the ... bilayer in its entirety if all of the residues in question were conformed as an a-helix? Assume that the a-helix is perpendicular to the plane of the bilayer. Show your reasoning. 4 pts The pitch of an a-helix is 5.4 Å, meaning that for every complete turn of the helix its long axis projects by 5.4 Å. What this implies is that a helix capable of projecting 30 Å would require a minimum of 30/5.4 = 5.56 turns. Since each complete turn of an a-helix contains 3.6 residues, 5.56 turns is equivalent to 5.56 x 3.6 = 20 amino acid residues. Which of the amino acids shown below in ‘old school’ notation (A, B, C, D, E, F, and/or G) would (generally) be expected to be most abundant in transmembrane a-helices that are capable of spanning membranes without interacting with any other components other than the hydrocarbon tails of the phospholipids in the lipid bilayer? Explain your reasoning in terms of the orientation of the side-chains of the amino acid residues in ahelices and the types of interactions in which transmembrane a-helices have to engage when they span the hydrocarbon phase of a lipid bilayer. 4 pts [Show More]

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