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Multiple Choice Questions on Amino acids

1.The chirality of an amino acid results from the fact that its a carbon:
Amino acid general structure
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.

2. Of the 20 standard amino acids, only ----- is not optically active. The reason is that its side chain -----.
a) alanine; is a simple methyl group
b) glycine; is a hydrogen atom
c) glycine; is unbranched
d) lysine; contains only nitrogen
e) proline; forms a covalent bond with the amino group

3.  Two amino acids of the standard 20 contain sulphur atoms. They are:
a) cysteine and serine.
b) cysteine and threonine.
c) methionine and cysteine
d) methionine and serine
e) threonine and serine.

4. 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.

5. Which of the following statements about aromatic amino acids is correct?
a) All are strongly hydrophilic.
b) Histidine’s ring structure results in its being categorized as aromatic or basic, depending on pH.
c) On a molar basis, tryptophan absorbs more ultraviolet light than tyrosine.
d) The major contribution to the characteristic absorption of light at 280 nm by proteins is the phenylalanine R group.
e) The presence of a ring structure in its R group determines whether or not an amino acid is aromatic.

6. Which of the following statements about cystine is correct?
a) Cystine forms when the —CH2—SH R group is oxidized to form a —CH2—S—S—CH2— disulfide bridge between two cysteines.
b) Cystine is an example of a nonstandard amino acid, derived by linking two standard amino acids.
c) Cystine is formed by the oxidation of the carboxylic acid group on cysteine.
d) Cystine is formed through a peptide linkage between two cysteines.
e) Two cystines are released when a —CH2—S—S—CH2— disulfide bridge is reduced to —CH2—SH.

7. The uncommon amino acid selenocysteine has an R group with the structure —CH2—SeH (pKa » 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.

8. Amino acids are ampholytes because they can function as either a(n):
a) acid or a base.
b) neutral molecule or an ion.
c) polar or a nonpolar molecule.
d) standard or a nonstandard monomer in proteins.
e) transparent or a light-absorbing compound.

9.Titration of valine by a strong base, for example NaOH, reveals two pK’s. The titration reaction occurring at pK2 (pK2 = 9.62) is:
a) —COOH + OH- -> —COO- + H2O.
b) —COOH + —NH-> —COO- + —NH2+.
c) —COO- + —NH2-> —COOH + —NH2.
d) —NH3+ + OH- -> —NH2 + H2O.
e) —NH2 + OH- ->  —NH- + H2O.

10.In a highly basic solution, pH = 13, the dominant form of glycine is:
a) NH2—CH2—COOH.
b) NH2—CH2—COO-.
c) NH2—CH3+—COO-.
d) NH3+—CH2—COOH.
e) NH3+—CH2—COO-.

11. For amino acids with neutral R groups, at any pH below the pI of the amino acid, the population of amino acids in solution will have:
a) a net negative charge.
b) a net positive charge.
c) no charged groups.
d) no net charge.
e) positive and negative charges in equal concentration.

12. What is the approximate charge difference between glutamic acid and a-ketoglutarate at pH 9.5?
a) 0
b) ½
c) 1
d) 1½
e) 2
13. The element found in all amino acids that is not found in carbohydrates is
(a) Sulphur (b) Carbon (c) Oxygen (d) Hydrogen (e) Nitrogen
Reference: Lehninger Principles of Biochemistry Fourth Edition (Nelson and Cox)

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Answers:
1. c) is bonded to four different chemical groups.
2. b) glycine; is a hydrogen atom
3. c) methionine and cysteine
4. a) amino group.
5. c) On a molar basis, tryptophan absorbs more ultraviolet light than tyrosine.
6. a) Cystine forms when the —CH2—SH R group is oxidized to form a —CH2—S—S—CH2— disulfide bridge between two cysteines.
7. a) be a fully ionized zwitterion with no net charge.
8. a) acid or a base.
9. d) —NH3+ + OH- ® —NH2 + H2O.
10. c) NH2—CH3+—COO-.
11. b) a net positive charge.
12. b) ½
13. (e) Nitrogen

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