CHEM 210 MODULE 1 EXAM 2026/2027 | Biochemistry
Foundational Concepts | Portage Learning | Verified
Questions & Answers | Pass Guaranteed - A+ Graded
Q1: Which of the following amino acids contains a sulfur atom in its side chain?
A. Serine
B. Methionine
C. Asparagine
D. Valine
Correct Answer: B
Rationale: Methionine contains a thioether group (-CH2-CH2-S-CH3) in its side chain,
making it one of two sulfur-containing amino acids (the other being cysteine). Serine (A)
contains a hydroxyl group; asparagine (C) contains an amide group; valine (D) is a
branched-chain nonpolar amino acid with only carbon and hydrogen in its side chain.
The sulfur in methionine is important for initiating protein synthesis (AUG codon) and
for methylation reactions.
Q2: At pH 7.0, which amino acid has a net positive charge?
A. Glutamate
B. Lysine
C. Serine
D. Alanine
Correct Answer: B
Rationale: Lysine has a side chain pKa of approximately 10.5. At pH 7.0, which is well
below this pKa, the ε-amino group of the side chain remains protonated (-NH3+).
Combined with the protonated α-amino group, this gives lysine a net +1 charge (two
positive charges from amino groups, one negative from the carboxylate). Glutamate (A)
has a side chain pKa of ~4.1, so at pH 7.0 its side chain is deprotonated (-COO-), giving
, it a net -1 charge. Serine (C) and alanine (D) have no ionizable side chains, so they exist
as zwitterions with net zero charge at pH 7.0.
Q3: The peptide bond that connects amino acids in a protein is best described as:
A. A flexible single bond with free rotation
B. A rigid planar structure with partial double-bond character
C. An ionic interaction between charged groups
D. A hydrophobic interaction between side chains
Correct Answer: B
Rationale: The peptide bond exhibits partial double-bond character due to resonance
between the carbonyl oxygen and amide nitrogen. This creates a rigid, planar structure
with restricted rotation (ω angle), forcing the six atoms involved (Cα, C, O, N, H, Cα) into
a planar arrangement. This planarity is essential for protein secondary structure
formation. Option A is incorrect because the C-N bond has ~40% double-bond character,
preventing free rotation; C describes ionic interactions (tertiary structure); D describes
hydrophobic effects (also tertiary structure), not covalent peptide bonds.
Q4: Which level of protein structure describes the sequence of amino acids connected
by peptide bonds?
A. Secondary structure
B. Tertiary structure
C. Primary structure
D. Quaternary structure
Correct Answer: C
Rationale: Primary structure is defined as the linear sequence of amino acids linked by
covalent peptide bonds, read from N-terminus to C-terminus. This sequence is encoded
by DNA and determines all higher-order structure. Secondary structure (A) refers to
local folding patterns (α-helices, β-sheets) stabilized by hydrogen bonds; tertiary
structure (B) is the overall 3D shape from interactions among side chains; quaternary
structure (D) describes assembly of multiple polypeptide subunits. The primary
structure is the foundation—changes here can alter all other levels.
Foundational Concepts | Portage Learning | Verified
Questions & Answers | Pass Guaranteed - A+ Graded
Q1: Which of the following amino acids contains a sulfur atom in its side chain?
A. Serine
B. Methionine
C. Asparagine
D. Valine
Correct Answer: B
Rationale: Methionine contains a thioether group (-CH2-CH2-S-CH3) in its side chain,
making it one of two sulfur-containing amino acids (the other being cysteine). Serine (A)
contains a hydroxyl group; asparagine (C) contains an amide group; valine (D) is a
branched-chain nonpolar amino acid with only carbon and hydrogen in its side chain.
The sulfur in methionine is important for initiating protein synthesis (AUG codon) and
for methylation reactions.
Q2: At pH 7.0, which amino acid has a net positive charge?
A. Glutamate
B. Lysine
C. Serine
D. Alanine
Correct Answer: B
Rationale: Lysine has a side chain pKa of approximately 10.5. At pH 7.0, which is well
below this pKa, the ε-amino group of the side chain remains protonated (-NH3+).
Combined with the protonated α-amino group, this gives lysine a net +1 charge (two
positive charges from amino groups, one negative from the carboxylate). Glutamate (A)
has a side chain pKa of ~4.1, so at pH 7.0 its side chain is deprotonated (-COO-), giving
, it a net -1 charge. Serine (C) and alanine (D) have no ionizable side chains, so they exist
as zwitterions with net zero charge at pH 7.0.
Q3: The peptide bond that connects amino acids in a protein is best described as:
A. A flexible single bond with free rotation
B. A rigid planar structure with partial double-bond character
C. An ionic interaction between charged groups
D. A hydrophobic interaction between side chains
Correct Answer: B
Rationale: The peptide bond exhibits partial double-bond character due to resonance
between the carbonyl oxygen and amide nitrogen. This creates a rigid, planar structure
with restricted rotation (ω angle), forcing the six atoms involved (Cα, C, O, N, H, Cα) into
a planar arrangement. This planarity is essential for protein secondary structure
formation. Option A is incorrect because the C-N bond has ~40% double-bond character,
preventing free rotation; C describes ionic interactions (tertiary structure); D describes
hydrophobic effects (also tertiary structure), not covalent peptide bonds.
Q4: Which level of protein structure describes the sequence of amino acids connected
by peptide bonds?
A. Secondary structure
B. Tertiary structure
C. Primary structure
D. Quaternary structure
Correct Answer: C
Rationale: Primary structure is defined as the linear sequence of amino acids linked by
covalent peptide bonds, read from N-terminus to C-terminus. This sequence is encoded
by DNA and determines all higher-order structure. Secondary structure (A) refers to
local folding patterns (α-helices, β-sheets) stabilized by hydrogen bonds; tertiary
structure (B) is the overall 3D shape from interactions among side chains; quaternary
structure (D) describes assembly of multiple polypeptide subunits. The primary
structure is the foundation—changes here can alter all other levels.
