CHEM 210 BIOCHEMISTRY MODULE 4 EXAM 2026/2027 |
Amino Acids, Proteins & Enzymes | Latest Q&A Verified |
Geneva College | Pass Guaranteed - A+ Graded
Q1: Which amino acid contains a side chain with a guanidinium group that is positively
charged at physiological pH (7.4)?
A. Lysine
B. Histidine
C. Arginine [CORRECT]
D. Asparagine
Correct Answer: C
Rationale: Arginine contains the guanidinium group, which has a pKa of approximately
12.5, making it permanently protonated and positively charged at physiological pH.
Lysine (A) has an amino group (pKa ~10.5) that is positively charged but not a
guanidinium. Histidine (B) has an imidazole ring with pKa ~6.0, making it partially
protonated at pH 7.4. Asparagine (D) has an uncharged polar side chain with an amide
group. The guanidinium group is one of the most basic biological functional groups due
to resonance stabilization of the protonated form.
Q2: The isoelectric point (pI) of an amino acid with two ionizable groups (α-COOH and
α-NH₃⁺) is calculated by:
A. Averaging the pKa values of the two ionizable groups [CORRECT]
B. Taking the difference between the two pKa values
C. Using the pKa of the side chain only
D. Adding the pKa values of all ionizable groups and dividing by three
,Correct Answer: A
Rationale: For amino acids with two ionizable groups (no ionizable side chain), pI =
(pKa₁ + pKa₂)/2, where pKa₁ is the α-carboxyl group (~2.3) and pKa₂ is the α-amino
group (~9.7). At the pI, the molecule has no net charge (zwitterion). Option B is
mathematically incorrect. Option C applies only to amino acids with ionizable side
chains (then pI = average of the two pKa values surrounding the neutral species). Option
D incorrectly includes a third pKa that doesn't exist for simple amino acids.
Q3: In a peptide bond formation between two amino acids, which functional groups are
involved and what is the byproduct?
A. α-amino group and α-carboxyl group; water [CORRECT]
B. Side chain groups; ammonia
C. α-amino group and side chain; carbon dioxide
D. α-carboxyl groups from both amino acids; hydrogen peroxide
Correct Answer: A
Rationale: Peptide bond formation is a condensation (dehydration) reaction between the
α-carboxyl group of one amino acid and the α-amino group of another, releasing one
water molecule. The resulting amide bond (peptide bond) has partial double-bond
character. Side chains (B) are not involved in standard peptide backbone formation.
Options C and D describe incorrect biochemistry—CO₂ release occurs in
decarboxylation reactions, not peptide synthesis.
Q4: Which structural feature of the peptide bond restricts rotation and contributes to the
planarity of the peptide backbone?
A. Complete single bond character between C and N
, B. Partial double bond character between carbonyl carbon and amide nitrogen
[CORRECT]
C. Hydrogen bonding between adjacent amino acids
D. Disulfide bridge formation
Correct Answer: B
Rationale: The peptide bond exhibits partial double bond character (40% double bond
character) due to resonance between the carbonyl oxygen and amide nitrogen. This
resonance creates a planar structure with restricted rotation around the C-N bond (ω
angle), with trans configuration favored (0.1% cis, usually with proline). Complete single
bond character (A) would allow free rotation. Hydrogen bonds (C) stabilize secondary
structure but don't create planarity. Disulfide bonds (D) are covalent cross-links in
tertiary structure, unrelated to peptide bond planarity.
Q5: Which of the following amino acids is classified as nonpolar and contains a sulfur
atom in its side chain?
A. Serine
B. Methionine [CORRECT]
C. Cysteine
D. Threonine
Correct Answer: B
Rationale: Methionine is a nonpolar, aliphatic amino acid containing a thioether (sulfide)
group (-CH₂-CH₂-S-CH₃). Cysteine (C) contains sulfur (thiol group) but is usually
classified as polar uncharged (though weakly so) due to the reactive -SH group. Serine
(A) and threonine (D) are polar uncharged hydroxyl-containing amino acids without
sulfur. The thioether in methionine is less reactive than cysteine's thiol, making
methionine more hydrophobic and stable.
Amino Acids, Proteins & Enzymes | Latest Q&A Verified |
Geneva College | Pass Guaranteed - A+ Graded
Q1: Which amino acid contains a side chain with a guanidinium group that is positively
charged at physiological pH (7.4)?
A. Lysine
B. Histidine
C. Arginine [CORRECT]
D. Asparagine
Correct Answer: C
Rationale: Arginine contains the guanidinium group, which has a pKa of approximately
12.5, making it permanently protonated and positively charged at physiological pH.
Lysine (A) has an amino group (pKa ~10.5) that is positively charged but not a
guanidinium. Histidine (B) has an imidazole ring with pKa ~6.0, making it partially
protonated at pH 7.4. Asparagine (D) has an uncharged polar side chain with an amide
group. The guanidinium group is one of the most basic biological functional groups due
to resonance stabilization of the protonated form.
Q2: The isoelectric point (pI) of an amino acid with two ionizable groups (α-COOH and
α-NH₃⁺) is calculated by:
A. Averaging the pKa values of the two ionizable groups [CORRECT]
B. Taking the difference between the two pKa values
C. Using the pKa of the side chain only
D. Adding the pKa values of all ionizable groups and dividing by three
,Correct Answer: A
Rationale: For amino acids with two ionizable groups (no ionizable side chain), pI =
(pKa₁ + pKa₂)/2, where pKa₁ is the α-carboxyl group (~2.3) and pKa₂ is the α-amino
group (~9.7). At the pI, the molecule has no net charge (zwitterion). Option B is
mathematically incorrect. Option C applies only to amino acids with ionizable side
chains (then pI = average of the two pKa values surrounding the neutral species). Option
D incorrectly includes a third pKa that doesn't exist for simple amino acids.
Q3: In a peptide bond formation between two amino acids, which functional groups are
involved and what is the byproduct?
A. α-amino group and α-carboxyl group; water [CORRECT]
B. Side chain groups; ammonia
C. α-amino group and side chain; carbon dioxide
D. α-carboxyl groups from both amino acids; hydrogen peroxide
Correct Answer: A
Rationale: Peptide bond formation is a condensation (dehydration) reaction between the
α-carboxyl group of one amino acid and the α-amino group of another, releasing one
water molecule. The resulting amide bond (peptide bond) has partial double-bond
character. Side chains (B) are not involved in standard peptide backbone formation.
Options C and D describe incorrect biochemistry—CO₂ release occurs in
decarboxylation reactions, not peptide synthesis.
Q4: Which structural feature of the peptide bond restricts rotation and contributes to the
planarity of the peptide backbone?
A. Complete single bond character between C and N
, B. Partial double bond character between carbonyl carbon and amide nitrogen
[CORRECT]
C. Hydrogen bonding between adjacent amino acids
D. Disulfide bridge formation
Correct Answer: B
Rationale: The peptide bond exhibits partial double bond character (40% double bond
character) due to resonance between the carbonyl oxygen and amide nitrogen. This
resonance creates a planar structure with restricted rotation around the C-N bond (ω
angle), with trans configuration favored (0.1% cis, usually with proline). Complete single
bond character (A) would allow free rotation. Hydrogen bonds (C) stabilize secondary
structure but don't create planarity. Disulfide bonds (D) are covalent cross-links in
tertiary structure, unrelated to peptide bond planarity.
Q5: Which of the following amino acids is classified as nonpolar and contains a sulfur
atom in its side chain?
A. Serine
B. Methionine [CORRECT]
C. Cysteine
D. Threonine
Correct Answer: B
Rationale: Methionine is a nonpolar, aliphatic amino acid containing a thioether (sulfide)
group (-CH₂-CH₂-S-CH₃). Cysteine (C) contains sulfur (thiol group) but is usually
classified as polar uncharged (though weakly so) due to the reactive -SH group. Serine
(A) and threonine (D) are polar uncharged hydroxyl-containing amino acids without
sulfur. The thioether in methionine is less reactive than cysteine's thiol, making
methionine more hydrophobic and stable.
