[기출문제] 성균관대 생화학 2024-2 중간 기출문제 (정답 포함) 

1. 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 he steamed 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) Given our 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.

2.  Reversible (but Tight) Binding to an Antibody An antibody binds to an antigen with a Kd of
5 10 8 M. At what concentration of antigen will v be (a) 0.2, (b) 0.5, (c) 0.6, (d) 0.8?

3. Intracellular Concentration of Enzymes To approximate the actual concentration of enzymes in a
bacterial cell, assume that the cell contains equal concentrations of 1,000 different enzymes in solution
in the cytosol and that each protein has a molecular weight of 100,000. Assume also that the bacterial
cell is a cylinder (diameter 1.0 mm, height 2.0 mm), that the cytosol (specific gravity 1.20) is 20% solu-
ble protein by weight, and that the soluble protein consists entirely of enzymes. Calculate the average
molar concentration of each enzyme in this hypothetical cell.

1. (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 and stretching 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 and stretched 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 extended form. In silk, by contrast, the
polypeptide chains have a very stable b-pleated sheet structure, fully extended along
the axis of the fiber (see Fig. 4–6), and have small, closely packed amino acid side
chains (see Fig. 4–13). These characteristics make silk resistant to stretching and
shrinking.

2. (a) 1X10^(-8) M, (b) 5X10^(-8)M, (c) 8X10^(-8) M, (d) 2X10^(-7) M. These are calcu-
lated from a rearrangement of Equation 5–8 to give [L]=ΘKd/(1-Θ), and for this antigen-
antibody binding,  [L]=Θ(5X10^(-8) M)/(1 v). For example, for (a) [L]=0.2(5X10^(-8) M)/
(0.8)=1X10^(-8) M.