WGU C785 BIOCHEMISTRY UNIT EXAM 2026/2027 | Updated
Edition | Complete Solution | Competency-Based
Assessment | Pass Guaranteed - A+ Graded
Section 1: Biomolecular Structure & Function (25 Questions)
Q1: Which level of protein structure is stabilized primarily by hydrogen bonds between
backbone amide and carbonyl groups?
● A. Primary
● B. Secondary
● C. Tertiary
● D. Quaternary
Correct Answer: B
Rationale: Secondary structure (α-helices and β-sheets) is stabilized by hydrogen bonds
between the carbonyl oxygen and amide hydrogen of the polypeptide backbone. Primary
structure is the amino acid sequence held by covalent peptide bonds. Tertiary structure
involves side-chain interactions (hydrophobic, ionic, hydrogen bonds). Quaternary
structure involves interactions between multiple polypeptide subunits. [CORRECT]
Q2: A patient presents with fatigue, muscle weakness, and lactic acidosis after mild
exercise. Laboratory testing reveals a deficiency in an enzyme that converts pyruvate to
acetyl-CoA. Which enzyme is most likely deficient?
● A. Lactate dehydrogenase
● B. Pyruvate carboxylase
● C. Pyruvate dehydrogenase
● D. Phosphofructokinase-1
Correct Answer: C
,Rationale: Pyruvate dehydrogenase (PDH) complex converts pyruvate to acetyl-CoA for
entry into the citric acid cycle. PDH deficiency leads to pyruvate accumulation, which is
converted to lactate, causing lactic acidosis—especially during anaerobic conditions like
exercise. Lactate dehydrogenase converts pyruvate to lactate (not deficient here).
Pyruvate carboxylase converts pyruvate to oxaloacetate (gluconeogenesis).
Phosphofructokinase-1 is a glycolytic enzyme. [CORRECT]
Q3: Which amino acid classification includes tyrosine, phenylalanine, and tryptophan?
● A. Polar uncharged
● B. Nonpolar aliphatic
● C. Aromatic
● D. Negatively charged
Correct Answer: C
Rationale: Tyrosine, phenylalanine, and tryptophan are aromatic amino acids
characterized by aromatic ring structures. Tyrosine has a phenol ring, phenylalanine has
a benzyl ring, and tryptophan has an indole ring. These rings absorb UV light at 280nm,
which is used to quantify proteins. Polar uncharged amino acids include serine,
threonine, asparagine, and glutamine. Nonpolar aliphatic amino acids include glycine,
alanine, valine, leucine, isoleucine, methionine, and proline. Negatively charged (acidic)
amino acids are aspartate and glutamate. [CORRECT]
Q4: In sickle cell anemia, the substitution of valine for glutamate at position 6 of the
beta-globin chain affects which level of protein structure?
● A. Primary structure only
● B. Secondary structure only
● C. Tertiary structure only
● D. Primary, secondary, tertiary, and quaternary structures
Correct Answer: D
,Rationale: The single amino acid substitution changes the primary structure (amino acid
sequence). This alteration creates a hydrophobic "sticky patch" that affects how
hemoglobin molecules interact, disrupting quaternary structure (tetramer assembly).
The mutation also alters the overall 3D folding (tertiary structure) and can affect local
secondary structure patterns. This demonstrates how a primary structure change can
propagate through all levels of protein organization, leading to pathological
polymerization and sickling of red blood cells under low oxygen conditions. [CORRECT]
Q5: Which interaction is most responsible for stabilizing the tertiary structure of
globular proteins in aqueous environments?
● A. Hydrogen bonds between backbone atoms
● B. Disulfide bridges between cysteine residues
● C. Hydrophobic interactions among nonpolar side chains
● D. Ionic bonds between charged side chains
Correct Answer: C
Rationale: Hydrophobic interactions are the primary driving force for protein folding and
tertiary structure stabilization. Nonpolar amino acid side chains cluster in the protein
interior, away from water, minimizing unfavorable interactions with the aqueous
environment. While hydrogen bonds between backbone atoms stabilize secondary
structure, and disulfide bridges/ionic bonds contribute to tertiary stability, the
hydrophobic effect provides the major thermodynamic driving force for the compact,
globular folding pattern. In hemoglobin and myoglobin, hydrophobic pockets cradle the
heme groups. [CORRECT]
Q6: Myoglobin and hemoglobin both bind oxygen, but myoglobin has a hyperbolic
oxygen dissociation curve while hemoglobin has a sigmoidal curve. What structural
feature explains this difference?
, ● A. Myoglobin contains heme while hemoglobin does not
● B. Hemoglobin exhibits cooperative binding between subunits
● C. Myoglobin has a higher molecular weight than hemoglobin
● D. Hemoglobin binds oxygen more tightly than myoglobin
Correct Answer: B
Rationale: Hemoglobin is a tetramer with four oxygen-binding sites that exhibit
cooperative binding—binding of oxygen to one subunit increases affinity in remaining
subunits, producing the sigmoidal curve. This cooperativity enables efficient oxygen
loading in lungs and unloading in tissues. Myoglobin is a monomer with one binding
site, showing simple hyperbolic kinetics without cooperativity. Both contain heme
groups (A incorrect). Hemoglobin (64 kDa) is larger than myoglobin (17 kDa) (C
incorrect). Myoglobin actually binds oxygen more tightly (lower P50) than hemoglobin
(D incorrect). [CORRECT]
Q7: Which of the following best describes the quaternary structure of adult hemoglobin
(HbA)?
● A. Two identical monomers associated through disulfide bonds
● B. Two alpha chains and two beta chains forming a tetramer
● C. Four identical beta chains in a square arrangement
● D. One alpha chain and three beta chains
Correct Answer: B
Rationale: Adult hemoglobin (HbA) has a quaternary structure consisting of two
alpha-globin chains (141 amino acids each) and two beta-globin chains (146 amino
acids each), forming an α₂β₂ tetramer. Each subunit contains one heme prosthetic
group, giving four oxygen-binding sites total. The subunits associate through
non-covalent interactions (hydrophobic, ionic, hydrogen bonds), not primarily disulfide
bonds. Fetal hemoglobin (HbF) is α₂γ₂. The tetrameric structure enables cooperative
binding essential for oxygen transport physiology. [CORRECT]
Edition | Complete Solution | Competency-Based
Assessment | Pass Guaranteed - A+ Graded
Section 1: Biomolecular Structure & Function (25 Questions)
Q1: Which level of protein structure is stabilized primarily by hydrogen bonds between
backbone amide and carbonyl groups?
● A. Primary
● B. Secondary
● C. Tertiary
● D. Quaternary
Correct Answer: B
Rationale: Secondary structure (α-helices and β-sheets) is stabilized by hydrogen bonds
between the carbonyl oxygen and amide hydrogen of the polypeptide backbone. Primary
structure is the amino acid sequence held by covalent peptide bonds. Tertiary structure
involves side-chain interactions (hydrophobic, ionic, hydrogen bonds). Quaternary
structure involves interactions between multiple polypeptide subunits. [CORRECT]
Q2: A patient presents with fatigue, muscle weakness, and lactic acidosis after mild
exercise. Laboratory testing reveals a deficiency in an enzyme that converts pyruvate to
acetyl-CoA. Which enzyme is most likely deficient?
● A. Lactate dehydrogenase
● B. Pyruvate carboxylase
● C. Pyruvate dehydrogenase
● D. Phosphofructokinase-1
Correct Answer: C
,Rationale: Pyruvate dehydrogenase (PDH) complex converts pyruvate to acetyl-CoA for
entry into the citric acid cycle. PDH deficiency leads to pyruvate accumulation, which is
converted to lactate, causing lactic acidosis—especially during anaerobic conditions like
exercise. Lactate dehydrogenase converts pyruvate to lactate (not deficient here).
Pyruvate carboxylase converts pyruvate to oxaloacetate (gluconeogenesis).
Phosphofructokinase-1 is a glycolytic enzyme. [CORRECT]
Q3: Which amino acid classification includes tyrosine, phenylalanine, and tryptophan?
● A. Polar uncharged
● B. Nonpolar aliphatic
● C. Aromatic
● D. Negatively charged
Correct Answer: C
Rationale: Tyrosine, phenylalanine, and tryptophan are aromatic amino acids
characterized by aromatic ring structures. Tyrosine has a phenol ring, phenylalanine has
a benzyl ring, and tryptophan has an indole ring. These rings absorb UV light at 280nm,
which is used to quantify proteins. Polar uncharged amino acids include serine,
threonine, asparagine, and glutamine. Nonpolar aliphatic amino acids include glycine,
alanine, valine, leucine, isoleucine, methionine, and proline. Negatively charged (acidic)
amino acids are aspartate and glutamate. [CORRECT]
Q4: In sickle cell anemia, the substitution of valine for glutamate at position 6 of the
beta-globin chain affects which level of protein structure?
● A. Primary structure only
● B. Secondary structure only
● C. Tertiary structure only
● D. Primary, secondary, tertiary, and quaternary structures
Correct Answer: D
,Rationale: The single amino acid substitution changes the primary structure (amino acid
sequence). This alteration creates a hydrophobic "sticky patch" that affects how
hemoglobin molecules interact, disrupting quaternary structure (tetramer assembly).
The mutation also alters the overall 3D folding (tertiary structure) and can affect local
secondary structure patterns. This demonstrates how a primary structure change can
propagate through all levels of protein organization, leading to pathological
polymerization and sickling of red blood cells under low oxygen conditions. [CORRECT]
Q5: Which interaction is most responsible for stabilizing the tertiary structure of
globular proteins in aqueous environments?
● A. Hydrogen bonds between backbone atoms
● B. Disulfide bridges between cysteine residues
● C. Hydrophobic interactions among nonpolar side chains
● D. Ionic bonds between charged side chains
Correct Answer: C
Rationale: Hydrophobic interactions are the primary driving force for protein folding and
tertiary structure stabilization. Nonpolar amino acid side chains cluster in the protein
interior, away from water, minimizing unfavorable interactions with the aqueous
environment. While hydrogen bonds between backbone atoms stabilize secondary
structure, and disulfide bridges/ionic bonds contribute to tertiary stability, the
hydrophobic effect provides the major thermodynamic driving force for the compact,
globular folding pattern. In hemoglobin and myoglobin, hydrophobic pockets cradle the
heme groups. [CORRECT]
Q6: Myoglobin and hemoglobin both bind oxygen, but myoglobin has a hyperbolic
oxygen dissociation curve while hemoglobin has a sigmoidal curve. What structural
feature explains this difference?
, ● A. Myoglobin contains heme while hemoglobin does not
● B. Hemoglobin exhibits cooperative binding between subunits
● C. Myoglobin has a higher molecular weight than hemoglobin
● D. Hemoglobin binds oxygen more tightly than myoglobin
Correct Answer: B
Rationale: Hemoglobin is a tetramer with four oxygen-binding sites that exhibit
cooperative binding—binding of oxygen to one subunit increases affinity in remaining
subunits, producing the sigmoidal curve. This cooperativity enables efficient oxygen
loading in lungs and unloading in tissues. Myoglobin is a monomer with one binding
site, showing simple hyperbolic kinetics without cooperativity. Both contain heme
groups (A incorrect). Hemoglobin (64 kDa) is larger than myoglobin (17 kDa) (C
incorrect). Myoglobin actually binds oxygen more tightly (lower P50) than hemoglobin
(D incorrect). [CORRECT]
Q7: Which of the following best describes the quaternary structure of adult hemoglobin
(HbA)?
● A. Two identical monomers associated through disulfide bonds
● B. Two alpha chains and two beta chains forming a tetramer
● C. Four identical beta chains in a square arrangement
● D. One alpha chain and three beta chains
Correct Answer: B
Rationale: Adult hemoglobin (HbA) has a quaternary structure consisting of two
alpha-globin chains (141 amino acids each) and two beta-globin chains (146 amino
acids each), forming an α₂β₂ tetramer. Each subunit contains one heme prosthetic
group, giving four oxygen-binding sites total. The subunits associate through
non-covalent interactions (hydrophobic, ionic, hydrogen bonds), not primarily disulfide
bonds. Fetal hemoglobin (HbF) is α₂γ₂. The tetrameric structure enables cooperative
binding essential for oxygen transport physiology. [CORRECT]
