ACS Biochemistry Exam: Actual Practice Questions &
Answers | Official-Style Review for ACS Biochemistry
2026 | pdf
Table of Contents
Core Foundational Concepts
o Buffers, Amino Acids & Peptides
o Protein Structure
o Enzyme Kinetics & Mechanisms
o Nucleic Acids & DNA Technology
o Lipids & Membranes
o Carbohydrates
o Metabolism & Bioenergetics
Advanced Topics
Core Foundational Concepts
1. Buffers, Amino Acids & Peptides
Q1. What is the Henderson-Hasselbalch equation?
Answer: pH = pKa + log ([A⁻] / [HA])
This equation relates pH to pKa and the ratio of conjugate base to weak acid concentrations.
Q2. What is a zwitterion?
Answer: A dipolar ion with both positive and negative groups but an overall neutral charge that can act as
either an acid or base.
Q3. How is isoelectric point (pI) defined?
Answer: The pH at which a molecule carries no net electrical charge. For amino acids with two titratable
groups, pI = (pKa1 + pKa2)/2.
Q4. Which amino acids are always charged at physiological pH (~7.4)?
Answer: Glu (glutamate), Asp (aspartate), Lys (lysine), and Arg (arginine).
, Q5. What is the net charge at pH 7.0 of the peptide: Ala-Thr-Leu-Asp-Ala-Lys-Pro-Glu?
Answer: -1
Calculation: Asp (-1), Lys (+1), Glu (-1) = net -1.
Q6. What structural feature distinguishes proline from other standard amino acids?
Answer: Proline contains a secondary amine group, forming a cyclic side chain that links back to the
backbone nitrogen, making it an imino acid. This restricts conformational flexibility.
Q7. Which amino acids are classified as aromatic?
Answer: Phenylalanine (Phe), Tyrosine (Tyr), Tryptophan (Trp).
Q8. What are the pKa values of the ionizable side chains?
Answer: Arg (12.5), Asp (3.9), Cys (8.3), Glu (4.2), His (6.0), Lys (10.0), Tyr (10.1).
Q9. What is the structure of a peptide bond?
Answer: A C-N bond with partial double bond character due to resonance, which prevents rotation and
makes it planar.
Q10. What are phi (Φ) and psi (Ψ) angles?
Answer: Φ is the angle around the N-Cα bond; Ψ is the angle around the Cα-carbonyl carbon bond. These
define polypeptide backbone conformation.
2. Protein Structure
Q11. What stabilizes the alpha-helix secondary structure?
Answer: Intramolecular hydrogen bonds between the carbonyl oxygen of residue *i* and the amide
hydrogen of residue *i+4*.
Q12. How many amino acids per turn in an alpha-helix, and what is the rise between backbones?
Answer: 3.6 amino acids per turn; 5.4 Å between backbones.
Q13. Which amino acids are most commonly found in alpha-helices?
Answer: Alanine (Ala), Leucine (Leu), Methionine (Met), and Glutamic acid (Glu).
Q14. Which amino acids are most commonly found in beta-sheets?
Answer: Isoleucine (Ile), Valine (Val), and Phenylalanine (Phe).
Q15. What is the difference between parallel and antiparallel beta-sheets?
Answer: Antiparallel sheets have adjacent strands running in opposite directions with linear H-bonds.
Parallel sheets have strands running in the same direction with angled H-bonds.
Answers | Official-Style Review for ACS Biochemistry
2026 | pdf
Table of Contents
Core Foundational Concepts
o Buffers, Amino Acids & Peptides
o Protein Structure
o Enzyme Kinetics & Mechanisms
o Nucleic Acids & DNA Technology
o Lipids & Membranes
o Carbohydrates
o Metabolism & Bioenergetics
Advanced Topics
Core Foundational Concepts
1. Buffers, Amino Acids & Peptides
Q1. What is the Henderson-Hasselbalch equation?
Answer: pH = pKa + log ([A⁻] / [HA])
This equation relates pH to pKa and the ratio of conjugate base to weak acid concentrations.
Q2. What is a zwitterion?
Answer: A dipolar ion with both positive and negative groups but an overall neutral charge that can act as
either an acid or base.
Q3. How is isoelectric point (pI) defined?
Answer: The pH at which a molecule carries no net electrical charge. For amino acids with two titratable
groups, pI = (pKa1 + pKa2)/2.
Q4. Which amino acids are always charged at physiological pH (~7.4)?
Answer: Glu (glutamate), Asp (aspartate), Lys (lysine), and Arg (arginine).
, Q5. What is the net charge at pH 7.0 of the peptide: Ala-Thr-Leu-Asp-Ala-Lys-Pro-Glu?
Answer: -1
Calculation: Asp (-1), Lys (+1), Glu (-1) = net -1.
Q6. What structural feature distinguishes proline from other standard amino acids?
Answer: Proline contains a secondary amine group, forming a cyclic side chain that links back to the
backbone nitrogen, making it an imino acid. This restricts conformational flexibility.
Q7. Which amino acids are classified as aromatic?
Answer: Phenylalanine (Phe), Tyrosine (Tyr), Tryptophan (Trp).
Q8. What are the pKa values of the ionizable side chains?
Answer: Arg (12.5), Asp (3.9), Cys (8.3), Glu (4.2), His (6.0), Lys (10.0), Tyr (10.1).
Q9. What is the structure of a peptide bond?
Answer: A C-N bond with partial double bond character due to resonance, which prevents rotation and
makes it planar.
Q10. What are phi (Φ) and psi (Ψ) angles?
Answer: Φ is the angle around the N-Cα bond; Ψ is the angle around the Cα-carbonyl carbon bond. These
define polypeptide backbone conformation.
2. Protein Structure
Q11. What stabilizes the alpha-helix secondary structure?
Answer: Intramolecular hydrogen bonds between the carbonyl oxygen of residue *i* and the amide
hydrogen of residue *i+4*.
Q12. How many amino acids per turn in an alpha-helix, and what is the rise between backbones?
Answer: 3.6 amino acids per turn; 5.4 Å between backbones.
Q13. Which amino acids are most commonly found in alpha-helices?
Answer: Alanine (Ala), Leucine (Leu), Methionine (Met), and Glutamic acid (Glu).
Q14. Which amino acids are most commonly found in beta-sheets?
Answer: Isoleucine (Ile), Valine (Val), and Phenylalanine (Phe).
Q15. What is the difference between parallel and antiparallel beta-sheets?
Answer: Antiparallel sheets have adjacent strands running in opposite directions with linear H-bonds.
Parallel sheets have strands running in the same direction with angled H-bonds.
