Solucionario De Bioquimica Proteinas

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chapter

4

The Three-Dimensional
Structure of Proteins

1. Properties of the Peptide Bond In x-ray studies of crystalline peptides, Linus Pauling and Robert
Corey found that the CON bond in the peptide link is intermediate in length (1.32 Å) between a typical
CON single bond (1.49 Å) and aCPN double bond (1.27 Å). They also found that the peptide bond is
planar (all four atoms attached to the C—N group are located in the same plane) and that the two
a-carbon atoms attached to the CON are always trans to each other (on opposite sides of the peptide
bond).
(a) What does the length of the CON bond in the peptide linkage indicate about its strength and its
bond order (i.e.,whether it is single, double, or triple)?
(b) What do the observations of Pauling and Corey tell us about the ease of rotation about the CON
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 CPN
bond is shorter (1.27 Å) and has a higher order (n 2.0) than atypical CON bond
(length 1.49 Å, n 1.0). The length of the CON bond of the peptide link (1.32 Å)
indicates 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 OCPO and ONOH groups are fixed in the
trans configuration.
2. Structural and Functional Relationships in Fibrous Proteins William Astbury discovered that
the x-ray diffraction pattern of wool shows a repeating structural unit spaced about 5.2 Å along the
length of the wool fiber. When hesteamed and stretched the wool, the x-ray pattern showed a new
repeating structural unit at a spacing of 7.0 Å. Steaming and stretching the wool and then letting it
shrink gave an x-ray pattern consistent with the original spacing of about 5.2 Å. Although these
observations provided important clues to the molecular structure of wool, Astbury was unable to
interpret them at the time.
(a) Givenour current understanding of the structure of wool, interpret Astbury’s observations.
(b) When wool sweaters or socks are washed in hot water or heated in a dryer, they shrink. Silk, on
the other hand, does not shrink under the same conditions. Explain.

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Chapter 4 The Three-DimensionalStructure of Proteins

Answer
(a) The principal structural units in the wool fiber polypeptide, a-keratin, are successive
turns of the a helix, which are spaced at 5.4 Å intervals; two a-keratin strands
twisted into a coiled coil produce the 5.2 Å spacing. The intrinsic stability of the helix
(and thus the fiber) results from intra chain hydrogen bonds (see Fig. 4–4a).
Steaming andstretching the fiber yields an extended polypeptide chain with the
b conformation, in which the distance between adjacent R groups is about 7.0 Å.
Upon resteaming, the polypeptide chains again assume the less-extended a-helix
conformation.
(b) Freshly sheared wool is primarily in its a-keratin (a-helical coiled coil) form (see
Fig. 4–10). Because raw wool is crimped or curly, it is combed andstretched to
straighten it before being spun into fibers for clothing. This processing converts the
wool from its native a-helical conformation to a more extended b form. Moist heat
triggers a conformational change back to the native a-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...