1559T_ch26_454-46810/20/0515:39Pa9e46 ⊕ EQA Solutions to Problems.461 (a)Val-Ala-Cys:(b)Ser-Asp:(c)His-Thr-Pro-Lys:(d)Tyr-Gly-Gly-Phe-Leu R物e以e Amino acids (Problem 28):(a).(b).(d).(e)N.(f)A.(c).(g)C Peptides (Problem 35):(a)N.(b)A.(c)C.(d)N 38. -CH3,or- sheets.where only small groups will fit casily.The nonpolar nature of five of thesix groups isaso vers of compatible with their location,a relatively nonpolar region with few hydrogen-bonding groups in the vicinity. a-Helix Amino acid numbers a-Helix Amino acid numbers 3-18 58-77 20-35 8694 3642 G 100-118 51-57 H 125-148 istics of the five-memberedn 40.Fxce ssociated with the heme-hound iron atom all the r side chains are well positioned for hydrogen bonding with solvent molecules(water).In contrast.all the nonpolar side chains adopt interior positions,avoiding contact with polar solvent molecules. 41.(a)The shee side chains are exposed to the solvent.solubilizing the entire molcu (Problem 0).Similar effects 36. By convention, the short notation format always begins with the end of the peptide chain with the amino group (the “N-terminal” or “amino-terminal” end). (a) Val-Ala-Cys; (b) Ser-Asp; (c) His-Thr-Pro-Lys; (d) Tyr-Gly-Gly-Phe-Leu 37. Determine the net charge on the amino acid or peptide at pH 7 and then recall that negative species migrate to the anode (A), positive to the cathode (C), and neutrals do not migrate at all (N). Amino acids (Problem 28): (a), (b), (d), (e) N, (f) A, (c), (g) C Peptides (Problem 35): (a) N, (b) A, (c) C, (d) N 38. The side chains are all small (OH, OCH3, or OCH2OH), and mostly nonpolar. In the illustrations, especially Figure 26-3, note that the sheet structure packs the R groups into small channels between layers of sheets, where only small groups will fit easily. The nonpolar nature of five of the six groups is also compatible with their location, a relatively nonpolar region with few hydrogen-bonding groups in the vicinity. 39. The
-helical stretches are fairly noticeable by their spiral shape (compare Figure 26-4). Myoglobin in fact contains eight significant
-helical stretches, which are labeled by the letters A–H:
-Helix Amino acid numbers
-Helix Amino acid numbers A 3–18 E 58–77 B 20–35 F 86–94 C 36–42 G 100–118 D 51–57 H 125–148 In the figure, all but
-helix D (which is viewed on-end from this perspective) are fairly easy to pick out. The four prolines are located at or near the ends of
-helices and coincide with “kinks” in the overall tertiary structure of the molecule, a result of the conformational characteristics of the five-membered ring. 40. Except for the two histidines associated with the heme-bound iron atom, all the polar side chains are well positioned for hydrogen bonding with solvent molecules (water). In contrast, all the nonpolar side chains adopt interior positions, avoiding contact with polar solvent molecules. 41. (a) The sheet structure is favored by amino acids with small, nonpolar side chains and has very little ability to hydrogen bond to a polar solvent like water (Problem 38). (b) In globular proteins the polar side chains are exposed to the solvent, solubilizing the entire molecule (Problem 40). Similar effects are seen in micelles formed by soap molecules, in which polar groups are located on the surface, facilitating water solubility, whereas nonpolar groups are buried in the inside (Chemical Highlight 19-1). (c) If the tertiary structure of a globular protein is disrupted, its nonpolar amino acid side chains become exposed to the polar solvent, greatly reducing the overall solubility of the protein molecule. Solutions to Problems • 461 1559T_ch26_454-468 10/20/05 15:39 Page 461