Ch4-6 ans

Ch4-6 ans - Chapter 4 The Three-Dimensional Structure of...

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Chapter 4 The Three-Dimensional Structure of Proteins 1. Properties of the Peptide Bond Answer (a) The higher the bond order (double or triple vs. single), the shorter and stronger are the bonds. Thus bond length is an indication of bond order. For example, the C=N bond is shorter (1.27 D ) and has a higher order ( n = 2.0) than a typical C–N bond (length = 1.49 D , n = 1.0). The length of the C–N linkage in the peptide bond (1.32 D ) indicated that it is intermediate in strength and bond order between a single and double bond. (b) Rotation about a double bond is generally impossible at physiological temperatures, and the steric relationship of the groups attached to the two atoms involved in the double bond is spatially “fixed.” Since the peptide bond has considerable double-bond character, there is essentially no rotation, and the C=O and N–H groups are fixed in the trans configuration. 2. Structural and Functional Relationships in Fibrous Proteins Answer (a) The principal structural units in the wool fiber polypeptides are successive turns of the " helix, which are spaced at 5.4 D intervals. The intrinsic stability of the helix (and thus the fiber) results from intra chain hydrogen bonds (see Fig. 4-4b). Steaming and stretching the fiber yields and extended polypeptide chain with the $ conformation, in which the distance between adjacent R groups is about 7.0 D . Upon resteaming, the polypeptide chains assume the less-extended " helix conformation, and the fiber shrinks. (b) Wool freshly sheared from a sheep is primarily in its " -keratin ( " -helical coiled coil) form (see Fig. 4-11). Because raw wool is crimped or curly, it is combed and stretched to straighten it before it is spun into fibers for clothing. This processing converts the wool from its native " -helical conformation to a more extended $ form. Moist heat triggers a conformational change back to the native " -helical structure, which shrinks both the fiber and the clothing. Under conditions of mechanical tension and moist heat, wool can be stretched back to a fully extended form. In contrast, the polypeptide chains of silk have a very stable $ -pleated sheet structure, fully extended along the axis of the fiber (see Fig. 4-7), and have small closely packed amino acid side chains (see Fig. 4-14). These characteristics make silk resistant to stretching or shrinking. 3. Rate of Synthesis of Hair " -Keratin Answer Since there are 3.6 amino acids (Aas) per turn and the rise is 5.4 D /turn, the length per AA of the " helix is 5.4 D /turn = 1.5 D /AA = 1.5 x 10 -10 m/AA 3.6 AA/turn A growth rate of 20 cm/yr is equivalent to
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20 cm/year = 6.4 x 10 -7 cm/s = 6.4 x . 10 -9 m/s (365 days/yr) (24 h/day) (60 min/h) (60 s/min) Thus the rate at which AAs are added is 6.4 X 10 -9 m/s . 43 AA/s . 43 peptide bonds per second 1.5 X 10 -10 m/AA 4. The Effect of pH on the Conformatin of " -Helical Secondary Structures Answer At pH values above 6, deprotonation of the carboxylate side chains of poly(Glu) leads
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This note was uploaded on 06/06/2008 for the course BIO bch 3033 taught by Professor Makemson during the Spring '08 term at FIU.

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Ch4-6 ans - Chapter 4 The Three-Dimensional Structure of...

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