3. The uncommon amino acid selenocysteine has an R group with the structure -CH₂-SeH (pK_a ≈ 5). In an aqueous solution, pH = 7.0, selenocysteine would:
(A) be a fully ionized zwitterion with no net charge.
(B)be found in proteins as D-selenocysteine.
(C) never be found in a protein.
(D)be nonionic.
(E)not be optically active.

1. The chirality of an amino acid results from the fact that its α carbon: (A) has no net charge. (B)is a carboxylic acid. (C) is bonded to four different chemical groups. (D)is in the L absolute configuration in naturally occurring proteins. (E)is symmetric.

複選題2. All of the amino acids that are found in proteins, except for proline, contain a(n): (A) amino group. (B)carbonyl group. (C) carboxyl group. (D)ester group. (E)thiol group.

4. The first step in two-dimensional gel electrophoresis generates a series of protein bands by isoelectric focusing. In a second step, a strip of this gel is turned 90 degrees, placed on another gel containing SDS, and an electric current is again applied. In this second step: (A) proteins with similar isoelectric points become further separated according to their molecular weights. (B)the individual bands become stained so that the isoelectric focus pattern can be visualized. (C) the individual bands become visualized by interacting with protein-specific antibodies in the second gel. (D)the individual bands undergo a second, more intense isoelectric focusing. (E)the proteins in the bands separate more completely because the second electric current is in the opposite polarity to the first current.

5. Which of the following describes the overall three-dimensional folding of a polypeptide? (A) Primary structure (B)Secondary structure (C) Tertiary structure (D)Quaternary structure (E)None of the above

6. In an aqueous solution, protein conformation is determined by two major factors. One is the formation of the maximum number of hydrogen bonds. The other is the: (A) formation of the maximum number of hydrophilic interactions. (B)maximization of ionic interactions. (C) minimization of entropy by the formation of a water solvent shell around the protein. (D)placement of hydrophobic amino acid residues within the interior of the protein. (E)placement of polar amino acid residues around the exterior of the protein.

7. In the α helix the hydrogen bonds: (A) are roughly parallel to the axis of the helix. (B)are roughly perpendicular to the axis of the helix. (C) occur mainly between electronegative atoms of the R groups. (D)occur only between some of the amino acids of the helix. (E)occur only near the amino and carboxyl termini of the helix.

8. The major reason that antiparallel β-stranded protein structures are more stable than parallel β-stranded structures is that the latter: (A) are in a slightly less extended configuration than antiparallel strands. (B)do not have as many disulfide crosslinks between adjacent strands. (C) do not stack in sheets as well as antiparallel strands. (D)have fewer lateral hydrogen bonds than antiparallel strands. (E)have weaker hydrogen bonds laterally between adjacent strands.

9. In the binding of oxygen to myoglobin, the relationship between the concentration of oxygen and the fraction of binding sites occupied can best be described as: (A) hyperbolic. (B)linear with a negative slope. (C) linear with a positive slope. (D)random. (E)sigmoidal.

10. In hemoglobin, the transition from T state to R state (low to high affinity) is triggered by: (A) Fe2+ binding. (B)heme binding. (C) oxygen binding. (D)subunit association. (E)subunit dissociation.

11. The fundamental cause of sickle-cell disease is a change in the structure of: (A) blood. (B)capillaries. (C) hemoglobin. (D)red cells. (E)the heart.

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