ACS Biochemistry Exam: Practice Questions
& Answers
Official-Style Review for ACS Biochemistry
2026–2027
SECTION 1: AMINO ACIDS, PROTEIN STRUCTURE, AND FUNCTION
Question 1: Which amino acid is most commonly found in flexible regions of
proteins such as turns due to its minimal steric hindrance?
Answer: Glycine
Rationale: Glycine has only a hydrogen atom as its side chain, which minimizes
steric hindrance and allows greater conformational flexibility in peptide
backbones. This makes it ideal for tight turns and flexible regions where other
amino acids would be too bulky.
Question 2: What structural feature of proline makes it unique among the standard
amino acids and often disrupts alpha-helix formation?
Answer: Proline has a cyclic structure where its side chain bonds to the amino
nitrogen, creating a rigid ring
Rationale: The nitrogen in proline is part of a pyrrolidine ring, which restricts
rotation around the N-Cα bond and prevents the formation of the characteristic
hydrogen bonds needed for stable alpha-helix structure. Proline also cannot form
hydrogen bonds as a donor in helices.
Question 3: At physiological pH (~7.4), which amino acid side chains carry a net
positive charge?
,Answer: Lysine, arginine, and histidine
Rationale: Lysine (pKa ~10.5) and arginine (pKa ~12.5) are positively charged at
physiological pH because their pKa values are above 7.4, meaning they remain
protonated. Histidine has a pKa of ~6.0, so it exists in both protonated and
unprotonated forms at pH 7.4, giving it unique buffering capacity.
Question 4: Which amino acid has a pKa of approximately 8.3 for its side chain
and is important in enzyme active sites for nucleophilic catalysis?
Answer: Cysteine
Rationale: The thiol group of cysteine has a pKa around 8.3, meaning it can be
deprotonated to form a highly reactive thiolate anion at physiological pH. This
makes cysteine critical for catalysis in many enzymes and for forming disulfide
bonds that stabilize protein structure.
Question 5: What is the Henderson-Hasselbalch equation and how is it used in
biochemistry?
Answer: pH = pKa + log([A⁻]/[HA])
Rationale: This equation relates pH, pKa, and the ratio of deprotonated to
protonated forms of a weak acid. It is essential for calculating the charge state of
amino acid side chains at different pH values, designing buffer solutions, and
understanding enzyme activity as a function of pH.
Question 6: How does FMOC chemical synthesis work for peptide synthesis?
Answer: FMOC (9-fluorenylmethoxycarbonyl) is used as a protecting group on
the N-terminus during solid-phase peptide synthesis on polystyrene beads
Rationale: The FMOC group protects the amino group during chain extension and
can be selectively removed under mild basic conditions (piperidine) without
affecting other protecting groups. This allows for stepwise addition of amino acids
to build peptides from the C-terminus to N-terminus.
,Question 7: What is the isoelectric point (pI) of a protein and how is it calculated?
Answer: The pH at which a protein has no net charge; calculated as the average of
the pKa values flanking the zwitterionic form
Rationale: At the pI, the protein is least soluble and will precipitate. For a simple
amino acid, pI = (pKa1 + pKa2)/2. For proteins, the pI depends on the sum of all
ionizable groups and can be used in purification techniques like isoelectric
focusing.
Question 8: Describe the Ramachandran plot and its significance in protein
structure.
Answer: A Ramachandran plot shows allowed and disallowed combinations of phi
(φ) and psi (ψ) dihedral angles in protein backbones
Rationale: Steric clashes limit the possible conformations of the peptide backbone.
The plot reveals three main allowed regions corresponding to alpha-helices, beta-
sheets, and left-handed helices. Glycine has the most allowed conformations due to
its small side chain, while proline has the most restricted.
Question 9: What structural properties characterize an alpha-helix?
Answer: Right-handed helix with 3.6 residues per turn, rise of 1.5 Å per residue,
and hydrogen bonds between carbonyl O of residue i and amide H of residue i+4
Rationale: The alpha-helix is stabilized by backbone hydrogen bonds. Alanine is
common in alpha-helices, while glycine and proline are rare due to steric effects.
The helix has a dipole moment with the N-terminus being δ-positive and C-
terminus δ-negative.
Question 10: What are the key differences between parallel and antiparallel beta-
sheets?
, Answer: In antiparallel sheets, adjacent strands run in opposite directions with
k. k. k. k. k. k. k. k. k. k.
straight hydrogen bonds; in parallel sheets, strands run in same direction with
k. k. k. k. k. k. k. k. k. k. k. k.
angled hydrogen bonds
k. k. k.
Rationale: Antiparallel beta-sheets have stronger hydrogen bonding because the
k. k. k. k. k. k. k. k.
bonds are linear (optimal geometry). Parallel sheets have angled hydrogen bonds,
k. k. k. k. k. k. k. k. k. k. k.
making them slightly less stable. Beta-sheets are often twisted to increase stability.
k. k. k. k. k. k. k. k. k. k. k. k.
Question 11: What is the function of beta-turns in protein structure?
k. k. k. k. k. k. k. k. k. k.
Answer: Beta-turns allow the polypeptide chain to reverse direction, connecting
k. k. k. k. k. k. k. k. k.
adjacent strands in beta-sheets
k. k. k. k.
Rationale: Beta-turns typically involve four residues, with glycine often required
k. k. k. k. k. k. k. k. k.
at position 3 due to steric constraints. Proline can also be found at beta-turns
k. k. k. k. k. k. k. k. k. k. k. k. k. k.
because it can adopt a cis conformation at the peptide bond. These turns are critical
k. k. k. k. k. k. k. k. k. k. k. k. k. k. k.
for compact protein folding.
k. k. k. k.
Question 12: What is the role of disulfide bonds in protein structure and how are
k. k. k. k. k. k. k. k. k. k. k. k. k. k.
they formed?
k. k.
Answer: Disulfide bonds covalently link two cysteine residues through oxidation of
k. k. k. k. k. k. k. k. k. k.
thiol groups, stabilizing tertiary and quaternary structure
k. k. k. k. k. k. k.
Rationale: Two cysteine thiol groups (-SH) oxidize to form a covalent disulfide
k. k. k. k. k. k. k. k. k. k. k.
bond (-S-S-). These bonds are important for stabilizing protein structure,
k. k. k. k. k. k. k. k. k. k.
particularly in extracellular proteins. Beta-mercaptoethanol and DTT
k. k. k. k. k. k. k.
(dithiothreitol) can reduce (break) disulfide bonds.
k. k. k. k. k. k.
Question 13: Explain the process of salting out in protein purification.
k. k. k. k. k. k. k. k. k. k.
Answer: Salting out precipitates proteins by adding high concentrations of salt
k. k. k. k. k. k. k. k. k. k.
(typically ammonium sulfate) to reduce protein solubility
k. k. k. k. k. k. k.
Rationale: High salt concentrations compete with proteins for water molecules,
k. k. k. k. k. k. k. k. k.
reducing the hydration shell and causing protein-protein interactions to dominate.
k. k. k. k. k. k. k. k. k. k.
& Answers
Official-Style Review for ACS Biochemistry
2026–2027
SECTION 1: AMINO ACIDS, PROTEIN STRUCTURE, AND FUNCTION
Question 1: Which amino acid is most commonly found in flexible regions of
proteins such as turns due to its minimal steric hindrance?
Answer: Glycine
Rationale: Glycine has only a hydrogen atom as its side chain, which minimizes
steric hindrance and allows greater conformational flexibility in peptide
backbones. This makes it ideal for tight turns and flexible regions where other
amino acids would be too bulky.
Question 2: What structural feature of proline makes it unique among the standard
amino acids and often disrupts alpha-helix formation?
Answer: Proline has a cyclic structure where its side chain bonds to the amino
nitrogen, creating a rigid ring
Rationale: The nitrogen in proline is part of a pyrrolidine ring, which restricts
rotation around the N-Cα bond and prevents the formation of the characteristic
hydrogen bonds needed for stable alpha-helix structure. Proline also cannot form
hydrogen bonds as a donor in helices.
Question 3: At physiological pH (~7.4), which amino acid side chains carry a net
positive charge?
,Answer: Lysine, arginine, and histidine
Rationale: Lysine (pKa ~10.5) and arginine (pKa ~12.5) are positively charged at
physiological pH because their pKa values are above 7.4, meaning they remain
protonated. Histidine has a pKa of ~6.0, so it exists in both protonated and
unprotonated forms at pH 7.4, giving it unique buffering capacity.
Question 4: Which amino acid has a pKa of approximately 8.3 for its side chain
and is important in enzyme active sites for nucleophilic catalysis?
Answer: Cysteine
Rationale: The thiol group of cysteine has a pKa around 8.3, meaning it can be
deprotonated to form a highly reactive thiolate anion at physiological pH. This
makes cysteine critical for catalysis in many enzymes and for forming disulfide
bonds that stabilize protein structure.
Question 5: What is the Henderson-Hasselbalch equation and how is it used in
biochemistry?
Answer: pH = pKa + log([A⁻]/[HA])
Rationale: This equation relates pH, pKa, and the ratio of deprotonated to
protonated forms of a weak acid. It is essential for calculating the charge state of
amino acid side chains at different pH values, designing buffer solutions, and
understanding enzyme activity as a function of pH.
Question 6: How does FMOC chemical synthesis work for peptide synthesis?
Answer: FMOC (9-fluorenylmethoxycarbonyl) is used as a protecting group on
the N-terminus during solid-phase peptide synthesis on polystyrene beads
Rationale: The FMOC group protects the amino group during chain extension and
can be selectively removed under mild basic conditions (piperidine) without
affecting other protecting groups. This allows for stepwise addition of amino acids
to build peptides from the C-terminus to N-terminus.
,Question 7: What is the isoelectric point (pI) of a protein and how is it calculated?
Answer: The pH at which a protein has no net charge; calculated as the average of
the pKa values flanking the zwitterionic form
Rationale: At the pI, the protein is least soluble and will precipitate. For a simple
amino acid, pI = (pKa1 + pKa2)/2. For proteins, the pI depends on the sum of all
ionizable groups and can be used in purification techniques like isoelectric
focusing.
Question 8: Describe the Ramachandran plot and its significance in protein
structure.
Answer: A Ramachandran plot shows allowed and disallowed combinations of phi
(φ) and psi (ψ) dihedral angles in protein backbones
Rationale: Steric clashes limit the possible conformations of the peptide backbone.
The plot reveals three main allowed regions corresponding to alpha-helices, beta-
sheets, and left-handed helices. Glycine has the most allowed conformations due to
its small side chain, while proline has the most restricted.
Question 9: What structural properties characterize an alpha-helix?
Answer: Right-handed helix with 3.6 residues per turn, rise of 1.5 Å per residue,
and hydrogen bonds between carbonyl O of residue i and amide H of residue i+4
Rationale: The alpha-helix is stabilized by backbone hydrogen bonds. Alanine is
common in alpha-helices, while glycine and proline are rare due to steric effects.
The helix has a dipole moment with the N-terminus being δ-positive and C-
terminus δ-negative.
Question 10: What are the key differences between parallel and antiparallel beta-
sheets?
, Answer: In antiparallel sheets, adjacent strands run in opposite directions with
k. k. k. k. k. k. k. k. k. k.
straight hydrogen bonds; in parallel sheets, strands run in same direction with
k. k. k. k. k. k. k. k. k. k. k. k.
angled hydrogen bonds
k. k. k.
Rationale: Antiparallel beta-sheets have stronger hydrogen bonding because the
k. k. k. k. k. k. k. k.
bonds are linear (optimal geometry). Parallel sheets have angled hydrogen bonds,
k. k. k. k. k. k. k. k. k. k. k.
making them slightly less stable. Beta-sheets are often twisted to increase stability.
k. k. k. k. k. k. k. k. k. k. k. k.
Question 11: What is the function of beta-turns in protein structure?
k. k. k. k. k. k. k. k. k. k.
Answer: Beta-turns allow the polypeptide chain to reverse direction, connecting
k. k. k. k. k. k. k. k. k.
adjacent strands in beta-sheets
k. k. k. k.
Rationale: Beta-turns typically involve four residues, with glycine often required
k. k. k. k. k. k. k. k. k.
at position 3 due to steric constraints. Proline can also be found at beta-turns
k. k. k. k. k. k. k. k. k. k. k. k. k. k.
because it can adopt a cis conformation at the peptide bond. These turns are critical
k. k. k. k. k. k. k. k. k. k. k. k. k. k. k.
for compact protein folding.
k. k. k. k.
Question 12: What is the role of disulfide bonds in protein structure and how are
k. k. k. k. k. k. k. k. k. k. k. k. k. k.
they formed?
k. k.
Answer: Disulfide bonds covalently link two cysteine residues through oxidation of
k. k. k. k. k. k. k. k. k. k.
thiol groups, stabilizing tertiary and quaternary structure
k. k. k. k. k. k. k.
Rationale: Two cysteine thiol groups (-SH) oxidize to form a covalent disulfide
k. k. k. k. k. k. k. k. k. k. k.
bond (-S-S-). These bonds are important for stabilizing protein structure,
k. k. k. k. k. k. k. k. k. k.
particularly in extracellular proteins. Beta-mercaptoethanol and DTT
k. k. k. k. k. k. k.
(dithiothreitol) can reduce (break) disulfide bonds.
k. k. k. k. k. k.
Question 13: Explain the process of salting out in protein purification.
k. k. k. k. k. k. k. k. k. k.
Answer: Salting out precipitates proteins by adding high concentrations of salt
k. k. k. k. k. k. k. k. k. k.
(typically ammonium sulfate) to reduce protein solubility
k. k. k. k. k. k. k.
Rationale: High salt concentrations compete with proteins for water molecules,
k. k. k. k. k. k. k. k. k.
reducing the hydration shell and causing protein-protein interactions to dominate.
k. k. k. k. k. k. k. k. k. k.
