Biochemistry ACS Exam Study Guide 2025 |
American Chemical Society
Biochemistry Practice Questions and
Answers, Metabolism Pathways, Enzyme
Kinetics, Molecular Biology, Amino
Acids, Proteins, and Verified Exam
Preparation Materials for Top Scores
Question 1: Which amino acid contains a thiol group in its side chain that can form disulfide bonds
under oxidizing conditions?
A. Methionine
B. Cysteine
C. Serine
D. Threonine
CORRECT ANSWER: B. Cysteine
RATIONALE: Cysteine contains a sulfhydryl (thiol) group (-SH) in its side chain that can undergo
oxidation to form a covalent disulfide bond (-S-S-) with another cysteine residue. This reaction is critical
for stabilizing tertiary and quaternary protein structures. Methionine contains a thioether linkage that
cannot form disulfide bonds, while serine and threonine contain hydroxyl groups.
Question 2: At physiological pH (~7.4), what is the predominant ionic form of the α-amino and α-
carboxyl groups in a free amino acid?
A. Both groups are protonated
B. Both groups are deprotonated
C. α-Amino group protonated; α-carboxyl group deprotonated
D. α-Amino group deprotonated; α-carboxyl group protonated
CORRECT ANSWER: C. α-Amino group protonated; α-carboxyl group deprotonated
RATIONALE: At physiological pH, the α-carboxyl group (pKa ≈ 2.0–2.4) is deprotonated and carries a
negative charge, while the α-amino group (pKa ≈ 9.0–9.8) is protonated and carries a positive charge.
This zwitterionic form is the predominant species for free amino acids at neutral pH.
Question 3: Which level of protein structure is primarily stabilized by hydrogen bonds between
backbone amide and carbonyl groups?
A. Primary structure
B. Secondary structure
C. Tertiary structure
D. Quaternary structure
CORRECT ANSWER: B. Secondary structure
,RATIONALE: Secondary structure elements such as α-helices and β-sheets are stabilized by regular
patterns of hydrogen bonds between the backbone amide hydrogen and carbonyl oxygen atoms.
Primary structure refers to the covalent peptide bond sequence; tertiary and quaternary structures
involve side-chain interactions and subunit associations.
Question 4: The isoelectric point (pI) of a neutral amino acid with no ionizable side chain is best
calculated as:
A. The average of the pKa values of the α-carboxyl and α-amino groups
B. The pKa of the α-carboxyl group alone
C. The pKa of the α-amino group alone
D. The sum of the pKa values of both ionizable groups
CORRECT ANSWER: A. The average of the pKa values of the α-carboxyl and α-amino groups
RATIONALE: For amino acids without ionizable side chains (e.g., glycine, alanine), the pI is the pH at
which the molecule carries no net charge, occurring midway between the pKa values of the two
ionizable groups (α-carboxyl and α-amino). This is calculated as pI = (pKa₁ + pKa₂)/2.
Question 5: Which amino acid is classified as imino acid due to its cyclic side chain that bonds to the α-
amino group?
A. Glycine
B. Proline
C. Histidine
D. Arginine
CORRECT ANSWER: B. Proline
RATIONALE: Proline is unique among the standard amino acids because its side chain forms a cyclic
structure with the α-amino group, creating a secondary amine (imino group). This structural feature
restricts conformational flexibility and often introduces kinks in polypeptide chains.
Question 6: In an α-helix, how many amino acid residues are present per complete turn of the helix?
A. 2.5
B. 3.6
C. 4.2
D. 5.0
CORRECT ANSWER: B. 3.6
RATIONALE: The α-helix is a right-handed coiled conformation in which there are approximately 3.6
amino acid residues per turn. Hydrogen bonds form between the carbonyl oxygen of residue i and the
amide hydrogen of residue i+4, stabilizing the helical structure.
Question 7: Which technique is most appropriate for determining the three-dimensional atomic
structure of a purified protein in solution?
,A. X-ray crystallography
B. Nuclear magnetic resonance (NMR) spectroscopy
C. SDS-PAGE
D. Isoelectric focusing
CORRECT ANSWER: B. Nuclear magnetic resonance (NMR) spectroscopy
RATIONALE: NMR spectroscopy allows determination of protein structure in aqueous solution under
near-physiological conditions, providing dynamic information. X-ray crystallography requires crystallized
samples; SDS-PAGE and isoelectric focusing separate proteins but do not provide atomic-resolution
structural data.
Question 8: Which amino acid side chain is most likely to participate in catalysis as a general acid/base
at physiological pH?
A. Alanine
B. Histidine
C. Leucine
D. Valine
CORRECT ANSWER: B. Histidine
RATIONALE: Histidine has an imidazole side chain with a pKa near 6.0–7.0, allowing it to readily accept
or donate protons at physiological pH. This property makes histidine a frequent participant in enzyme
active sites as a general acid/base catalyst.
Question 9: What type of interaction primarily stabilizes the hydrophobic core of a globular protein?
A. Ionic bonds
B. Hydrogen bonds
C. Van der Waals interactions and hydrophobic effect
D. Disulfide bonds
CORRECT ANSWER: C. Van der Waals interactions and hydrophobic effect
RATIONALE: The hydrophobic effect drives nonpolar side chains to cluster in the protein interior,
minimizing contact with water. Van der Waals forces between closely packed hydrophobic residues
further stabilize this core. Ionic and hydrogen bonds are more common at the protein surface or in
specific structural motifs.
Question 10: Which statement correctly describes the effect of a competitive enzyme inhibitor?
A. It binds to the enzyme-substrate complex, decreasing Vmax
B. It binds to an allosteric site, altering enzyme conformation
C. It binds to the active site, increasing apparent Km without affecting Vmax
D. It irreversibly inactivates the enzyme by covalent modification
CORRECT ANSWER: C. It binds to the active site, increasing apparent Km without affecting Vmax
, RATIONALE: Competitive inhibitors resemble the substrate and bind reversibly to the active site,
competing with substrate binding. This increases the apparent Km (more substrate is needed to reach
half Vmax), but Vmax remains unchanged because sufficient substrate can outcompete the inhibitor.
Question 11: In Michaelis-Menten kinetics, what does the Km value represent?
A. The maximum reaction velocity
B. The substrate concentration at which reaction velocity is half of Vmax
C. The turnover number of the enzyme
D. The dissociation constant for the enzyme-product complex
CORRECT ANSWER: B. The substrate concentration at which reaction velocity is half of Vmax
RATIONALE: Km (Michaelis constant) is defined as the substrate concentration at which the reaction
velocity reaches half of Vmax. It provides a measure of the enzyme's affinity for its substrate; a lower
Km indicates higher affinity.
Question 12: Which plot is used to linearize Michaelis-Menten kinetic data for determination of Km
and Vmax?
A. Hill plot
B. Lineweaver-Burk plot
C. Scatchard plot
D. van't Hoff plot
CORRECT ANSWER: B. Lineweaver-Burk plot
RATIONALE: The Lineweaver-Burk plot (double-reciprocal plot of 1/v vs. 1/[S]) linearizes Michaelis-
Menten data, allowing graphical determination of Km (from the x-intercept) and Vmax (from the y-
intercept). Other plots serve different purposes: Hill plots assess cooperativity, Scatchard plots analyze
binding data.
Question 13: An enzyme exhibits sigmoidal kinetics when plotted as velocity versus substrate
concentration. This behavior is most consistent with:
A. Competitive inhibition
B. Allosteric regulation with cooperative substrate binding
C. Irreversible inhibition
D. Ping-pong mechanism
CORRECT ANSWER: B. Allosteric regulation with cooperative substrate binding
RATIONALE: Sigmoidal kinetics indicate positive cooperativity, where binding of substrate to one active
site increases the affinity of other sites. This is characteristic of allosteric enzymes with multiple
subunits, such as hemoglobin or aspartate transcarbamoylase.
Question 14: Which coenzyme is derived from vitamin B3 (niacin) and functions as an electron carrier
in redox reactions?
American Chemical Society
Biochemistry Practice Questions and
Answers, Metabolism Pathways, Enzyme
Kinetics, Molecular Biology, Amino
Acids, Proteins, and Verified Exam
Preparation Materials for Top Scores
Question 1: Which amino acid contains a thiol group in its side chain that can form disulfide bonds
under oxidizing conditions?
A. Methionine
B. Cysteine
C. Serine
D. Threonine
CORRECT ANSWER: B. Cysteine
RATIONALE: Cysteine contains a sulfhydryl (thiol) group (-SH) in its side chain that can undergo
oxidation to form a covalent disulfide bond (-S-S-) with another cysteine residue. This reaction is critical
for stabilizing tertiary and quaternary protein structures. Methionine contains a thioether linkage that
cannot form disulfide bonds, while serine and threonine contain hydroxyl groups.
Question 2: At physiological pH (~7.4), what is the predominant ionic form of the α-amino and α-
carboxyl groups in a free amino acid?
A. Both groups are protonated
B. Both groups are deprotonated
C. α-Amino group protonated; α-carboxyl group deprotonated
D. α-Amino group deprotonated; α-carboxyl group protonated
CORRECT ANSWER: C. α-Amino group protonated; α-carboxyl group deprotonated
RATIONALE: At physiological pH, the α-carboxyl group (pKa ≈ 2.0–2.4) is deprotonated and carries a
negative charge, while the α-amino group (pKa ≈ 9.0–9.8) is protonated and carries a positive charge.
This zwitterionic form is the predominant species for free amino acids at neutral pH.
Question 3: Which level of protein structure is primarily stabilized by hydrogen bonds between
backbone amide and carbonyl groups?
A. Primary structure
B. Secondary structure
C. Tertiary structure
D. Quaternary structure
CORRECT ANSWER: B. Secondary structure
,RATIONALE: Secondary structure elements such as α-helices and β-sheets are stabilized by regular
patterns of hydrogen bonds between the backbone amide hydrogen and carbonyl oxygen atoms.
Primary structure refers to the covalent peptide bond sequence; tertiary and quaternary structures
involve side-chain interactions and subunit associations.
Question 4: The isoelectric point (pI) of a neutral amino acid with no ionizable side chain is best
calculated as:
A. The average of the pKa values of the α-carboxyl and α-amino groups
B. The pKa of the α-carboxyl group alone
C. The pKa of the α-amino group alone
D. The sum of the pKa values of both ionizable groups
CORRECT ANSWER: A. The average of the pKa values of the α-carboxyl and α-amino groups
RATIONALE: For amino acids without ionizable side chains (e.g., glycine, alanine), the pI is the pH at
which the molecule carries no net charge, occurring midway between the pKa values of the two
ionizable groups (α-carboxyl and α-amino). This is calculated as pI = (pKa₁ + pKa₂)/2.
Question 5: Which amino acid is classified as imino acid due to its cyclic side chain that bonds to the α-
amino group?
A. Glycine
B. Proline
C. Histidine
D. Arginine
CORRECT ANSWER: B. Proline
RATIONALE: Proline is unique among the standard amino acids because its side chain forms a cyclic
structure with the α-amino group, creating a secondary amine (imino group). This structural feature
restricts conformational flexibility and often introduces kinks in polypeptide chains.
Question 6: In an α-helix, how many amino acid residues are present per complete turn of the helix?
A. 2.5
B. 3.6
C. 4.2
D. 5.0
CORRECT ANSWER: B. 3.6
RATIONALE: The α-helix is a right-handed coiled conformation in which there are approximately 3.6
amino acid residues per turn. Hydrogen bonds form between the carbonyl oxygen of residue i and the
amide hydrogen of residue i+4, stabilizing the helical structure.
Question 7: Which technique is most appropriate for determining the three-dimensional atomic
structure of a purified protein in solution?
,A. X-ray crystallography
B. Nuclear magnetic resonance (NMR) spectroscopy
C. SDS-PAGE
D. Isoelectric focusing
CORRECT ANSWER: B. Nuclear magnetic resonance (NMR) spectroscopy
RATIONALE: NMR spectroscopy allows determination of protein structure in aqueous solution under
near-physiological conditions, providing dynamic information. X-ray crystallography requires crystallized
samples; SDS-PAGE and isoelectric focusing separate proteins but do not provide atomic-resolution
structural data.
Question 8: Which amino acid side chain is most likely to participate in catalysis as a general acid/base
at physiological pH?
A. Alanine
B. Histidine
C. Leucine
D. Valine
CORRECT ANSWER: B. Histidine
RATIONALE: Histidine has an imidazole side chain with a pKa near 6.0–7.0, allowing it to readily accept
or donate protons at physiological pH. This property makes histidine a frequent participant in enzyme
active sites as a general acid/base catalyst.
Question 9: What type of interaction primarily stabilizes the hydrophobic core of a globular protein?
A. Ionic bonds
B. Hydrogen bonds
C. Van der Waals interactions and hydrophobic effect
D. Disulfide bonds
CORRECT ANSWER: C. Van der Waals interactions and hydrophobic effect
RATIONALE: The hydrophobic effect drives nonpolar side chains to cluster in the protein interior,
minimizing contact with water. Van der Waals forces between closely packed hydrophobic residues
further stabilize this core. Ionic and hydrogen bonds are more common at the protein surface or in
specific structural motifs.
Question 10: Which statement correctly describes the effect of a competitive enzyme inhibitor?
A. It binds to the enzyme-substrate complex, decreasing Vmax
B. It binds to an allosteric site, altering enzyme conformation
C. It binds to the active site, increasing apparent Km without affecting Vmax
D. It irreversibly inactivates the enzyme by covalent modification
CORRECT ANSWER: C. It binds to the active site, increasing apparent Km without affecting Vmax
, RATIONALE: Competitive inhibitors resemble the substrate and bind reversibly to the active site,
competing with substrate binding. This increases the apparent Km (more substrate is needed to reach
half Vmax), but Vmax remains unchanged because sufficient substrate can outcompete the inhibitor.
Question 11: In Michaelis-Menten kinetics, what does the Km value represent?
A. The maximum reaction velocity
B. The substrate concentration at which reaction velocity is half of Vmax
C. The turnover number of the enzyme
D. The dissociation constant for the enzyme-product complex
CORRECT ANSWER: B. The substrate concentration at which reaction velocity is half of Vmax
RATIONALE: Km (Michaelis constant) is defined as the substrate concentration at which the reaction
velocity reaches half of Vmax. It provides a measure of the enzyme's affinity for its substrate; a lower
Km indicates higher affinity.
Question 12: Which plot is used to linearize Michaelis-Menten kinetic data for determination of Km
and Vmax?
A. Hill plot
B. Lineweaver-Burk plot
C. Scatchard plot
D. van't Hoff plot
CORRECT ANSWER: B. Lineweaver-Burk plot
RATIONALE: The Lineweaver-Burk plot (double-reciprocal plot of 1/v vs. 1/[S]) linearizes Michaelis-
Menten data, allowing graphical determination of Km (from the x-intercept) and Vmax (from the y-
intercept). Other plots serve different purposes: Hill plots assess cooperativity, Scatchard plots analyze
binding data.
Question 13: An enzyme exhibits sigmoidal kinetics when plotted as velocity versus substrate
concentration. This behavior is most consistent with:
A. Competitive inhibition
B. Allosteric regulation with cooperative substrate binding
C. Irreversible inhibition
D. Ping-pong mechanism
CORRECT ANSWER: B. Allosteric regulation with cooperative substrate binding
RATIONALE: Sigmoidal kinetics indicate positive cooperativity, where binding of substrate to one active
site increases the affinity of other sites. This is characteristic of allosteric enzymes with multiple
subunits, such as hemoglobin or aspartate transcarbamoylase.
Question 14: Which coenzyme is derived from vitamin B3 (niacin) and functions as an electron carrier
in redox reactions?
