Portage Learning CHEM 210 Biochemistry
Module 6 Exam | Actual Amino Acids
Proteins and Enzymes Assessment with
Answers
Domain: Amino Acid Structure & Classification
Q1: Which of the following amino acids has a side chain that contains a sulfhydryl (-SH)
group and can form disulfide bonds?
A. Methionine
B. Cysteine
C. Serine
D. Threonine
Correct Answer: B
Rationale: B. [CORRECT] Cysteine contains a sulfhydryl (-SH) group in its side chain.
Two cysteine residues can form a disulfide bond (-S-S-), which stabilizes protein tertiary
and quaternary structure. A is incorrect; methionine contains a thioether group (-S-CH3),
not a sulfhydryl. C and D are incorrect; serine and threonine contain hydroxyl (-OH)
groups, not sulfhydryls.
Q2: Which of the following correctly classifies the amino acid lysine?
A. Nonpolar, hydrophobic
B. Polar uncharged
C. Acidic (negatively charged)
D. Basic (positively charged)
Correct Answer: D
Rationale: D. [CORRECT] Lysine has a side chain with an amino group (pKa ~10.5) that
is positively charged at physiological pH, classifying it as a basic (positively charged)
,amino acid. A is incorrect; lysine is polar and charged, not hydrophobic. B is incorrect;
lysine carries a net positive charge. C is incorrect; acidic amino acids (aspartate,
glutamate) have carboxyl side chains and are negatively charged.
Q3: Which of the following pairs correctly matches the one-letter abbreviation with the
three-letter abbreviation for an amino acid?
A. A — Arg
B. R — Ala
C. K — Lys
D. E — Gln
Correct Answer: C
Rationale: C. [CORRECT] K is the one-letter abbreviation for lysine (Lys), derived from its
German name "Lysin" to avoid confusion with leucine (L). A is incorrect; A is alanine
(Ala), while arginine is Arg (R). B is incorrect; R is arginine, not alanine. D is incorrect; E
is glutamic acid (Glu), while glutamine is Gln (Q).
Q4: At physiological pH (approximately 7.4), which of the following amino acids carries
a net negative charge?
A. Lysine
B. Arginine
C. Glutamate
D. Histidine
Correct Answer: C
Rationale: C. [CORRECT] Glutamate has a side chain carboxyl group with a pKa around
4.1, which is deprotonated (negatively charged) at physiological pH. A and B are
incorrect; lysine (side chain pKa ~10.5) and arginine (pKa ~12.5) are positively charged
at pH 7.4. D is incorrect; histidine has a side chain pKa around 6.0 and is largely
neutral/protonated at pH 7.4.
Q5: Which of the following is classified as an essential amino acid in humans?
A. Alanine
B. Glutamate
C. Tryptophan
D. Asparagine
Correct Answer: C
, Rationale: C. [CORRECT] Tryptophan is an essential amino acid that cannot be
synthesized by humans and must be obtained from the diet. A, B, and D are incorrect;
alanine, glutamate, and asparagine are nonessential amino acids that can be
synthesized from metabolic intermediates.
Q6: Which amino acid contains an indole ring in its side chain?
A. Phenylalanine
B. Tyrosine
C. Tryptophan
D. Histidine
Correct Answer: C
Rationale: C. [CORRECT] Tryptophan contains a bicyclic indole ring system attached to
its side chain. A is incorrect; phenylalanine contains a benzyl group (phenyl ring). B is
incorrect; tyrosine contains a phenolic ring (hydroxylated phenyl). D is incorrect;
histidine contains an imidazole ring.
Q7: The isoelectric point (pI) of an amino acid with a non-ionizable side chain is
calculated by:
A. Averaging the pKa values of the α-carboxyl and α-amino groups
B. Taking the pKa of the side chain only
C. Averaging all three pKa values (if present)
D. Subtracting the pKa of the α-carboxyl from the pKa of the α-amino group
Correct Answer: A
Rationale: A. [CORRECT] For amino acids with non-ionizable side chains (e.g., glycine,
alanine), the pI is the arithmetic mean of the pKa of the α-carboxyl group (~2.3) and the
α-amino group (~9.7), resulting in a pI near 6.0. B is incorrect; side chains of these
amino acids have no ionizable group. C applies only to amino acids with ionizable side
chains (like aspartate or lysine). D is mathematically incorrect.
Q8: Structure-Based Question: Which amino acid has a side chain consisting of a single
methylene group (-CH2-) attached to an additional carboxylate group, making it acidic?
A. Aspartate
B. Glutamate
C. Serine
Module 6 Exam | Actual Amino Acids
Proteins and Enzymes Assessment with
Answers
Domain: Amino Acid Structure & Classification
Q1: Which of the following amino acids has a side chain that contains a sulfhydryl (-SH)
group and can form disulfide bonds?
A. Methionine
B. Cysteine
C. Serine
D. Threonine
Correct Answer: B
Rationale: B. [CORRECT] Cysteine contains a sulfhydryl (-SH) group in its side chain.
Two cysteine residues can form a disulfide bond (-S-S-), which stabilizes protein tertiary
and quaternary structure. A is incorrect; methionine contains a thioether group (-S-CH3),
not a sulfhydryl. C and D are incorrect; serine and threonine contain hydroxyl (-OH)
groups, not sulfhydryls.
Q2: Which of the following correctly classifies the amino acid lysine?
A. Nonpolar, hydrophobic
B. Polar uncharged
C. Acidic (negatively charged)
D. Basic (positively charged)
Correct Answer: D
Rationale: D. [CORRECT] Lysine has a side chain with an amino group (pKa ~10.5) that
is positively charged at physiological pH, classifying it as a basic (positively charged)
,amino acid. A is incorrect; lysine is polar and charged, not hydrophobic. B is incorrect;
lysine carries a net positive charge. C is incorrect; acidic amino acids (aspartate,
glutamate) have carboxyl side chains and are negatively charged.
Q3: Which of the following pairs correctly matches the one-letter abbreviation with the
three-letter abbreviation for an amino acid?
A. A — Arg
B. R — Ala
C. K — Lys
D. E — Gln
Correct Answer: C
Rationale: C. [CORRECT] K is the one-letter abbreviation for lysine (Lys), derived from its
German name "Lysin" to avoid confusion with leucine (L). A is incorrect; A is alanine
(Ala), while arginine is Arg (R). B is incorrect; R is arginine, not alanine. D is incorrect; E
is glutamic acid (Glu), while glutamine is Gln (Q).
Q4: At physiological pH (approximately 7.4), which of the following amino acids carries
a net negative charge?
A. Lysine
B. Arginine
C. Glutamate
D. Histidine
Correct Answer: C
Rationale: C. [CORRECT] Glutamate has a side chain carboxyl group with a pKa around
4.1, which is deprotonated (negatively charged) at physiological pH. A and B are
incorrect; lysine (side chain pKa ~10.5) and arginine (pKa ~12.5) are positively charged
at pH 7.4. D is incorrect; histidine has a side chain pKa around 6.0 and is largely
neutral/protonated at pH 7.4.
Q5: Which of the following is classified as an essential amino acid in humans?
A. Alanine
B. Glutamate
C. Tryptophan
D. Asparagine
Correct Answer: C
, Rationale: C. [CORRECT] Tryptophan is an essential amino acid that cannot be
synthesized by humans and must be obtained from the diet. A, B, and D are incorrect;
alanine, glutamate, and asparagine are nonessential amino acids that can be
synthesized from metabolic intermediates.
Q6: Which amino acid contains an indole ring in its side chain?
A. Phenylalanine
B. Tyrosine
C. Tryptophan
D. Histidine
Correct Answer: C
Rationale: C. [CORRECT] Tryptophan contains a bicyclic indole ring system attached to
its side chain. A is incorrect; phenylalanine contains a benzyl group (phenyl ring). B is
incorrect; tyrosine contains a phenolic ring (hydroxylated phenyl). D is incorrect;
histidine contains an imidazole ring.
Q7: The isoelectric point (pI) of an amino acid with a non-ionizable side chain is
calculated by:
A. Averaging the pKa values of the α-carboxyl and α-amino groups
B. Taking the pKa of the side chain only
C. Averaging all three pKa values (if present)
D. Subtracting the pKa of the α-carboxyl from the pKa of the α-amino group
Correct Answer: A
Rationale: A. [CORRECT] For amino acids with non-ionizable side chains (e.g., glycine,
alanine), the pI is the arithmetic mean of the pKa of the α-carboxyl group (~2.3) and the
α-amino group (~9.7), resulting in a pI near 6.0. B is incorrect; side chains of these
amino acids have no ionizable group. C applies only to amino acids with ionizable side
chains (like aspartate or lysine). D is mathematically incorrect.
Q8: Structure-Based Question: Which amino acid has a side chain consisting of a single
methylene group (-CH2-) attached to an additional carboxylate group, making it acidic?
A. Aspartate
B. Glutamate
C. Serine
