BIOCHEM 210 MODULE 3 EXAM 2026/2027 |
Protein Function: Hemoglobin, Myoglobin &
Oxygen Transport | Portage Learning
Comprehensive Assessment | Verified Q&A |
Pass Guaranteed - A+ Graded
SECTION I: MYOGLOBIN STRUCTURE & FUNCTION
Key Concepts: Monomeric structure, heme group, proximal/distal histidines, oxygen
storage, hyperbolic binding curve, high oxygen affinity
(Questions 1-10)
Q1: Myoglobin is a monomeric protein containing how many amino acid residues and
what type of prosthetic group?
A. 141 amino acids with a heme group containing Fe³⁺
B. 146 amino acids with a copper-containing heme
C. 153 amino acids with a protoporphyrin IX ring containing Fe²⁺
D. 153 amino acids with a magnesium-porphyrin complex
E. 141 amino acids with a zinc-containing prosthetic group
Correct Answer: C
,Rationale: Myoglobin consists of a single polypeptide chain of 153 amino acids (in
human myoglobin) with a molecular weight of approximately 17.8 kDa. It contains one
heme prosthetic group composed of protoporphyrin IX (a tetrapyrrole ring with four
methyl, two vinyl, and two propionate substituents) with a central ferrous iron (Fe²⁺).
The Fe²⁺ is essential for oxygen binding—ferric iron (Fe³⁺, metmyoglobin) cannot bind
O₂. Option A has the wrong count and oxidation state. Options B, D, and E describe
incorrect metals (copper, magnesium, zinc) not found in myoglobin heme. The heme
iron is the site of reversible oxygen binding.
Q2: In the heme group of myoglobin, the iron atom is coordinated to the protein via
which amino acid residue, and what is the geometry of this coordination?
A. Distal histidine (E7); ionic bond
B. Proximal histidine (F8); coordinate covalent bond
C. Lysine residue; hydrogen bond
D. Cysteine residue; disulfide bond
E. Glutamate residue; salt bridge
Correct Answer: B
Rationale: The heme iron is coordinated to the imidazole side chain of the proximal
histidine (His93, also called the F8 histidine based on its position in the 8th turn of the F
helix). This is a coordinate covalent bond where the nitrogen lone pair donates to the
iron. The iron has five coordination sites occupied: four by porphyrin nitrogens (in the
heme plane) and one by proximal histidine (below the plane). The sixth coordination site
(above the plane) is available for oxygen binding. The distal histidine (E7, His64) is
nearby but not coordinated to iron—it stabilizes the bound O₂ and prevents CO binding.
,Options A, C, D, and E describe incorrect residues or bond types. The proximal histidine
is essential for anchoring the heme and maintaining iron in the proper position.
Q3: What is the primary physiological function of myoglobin, and in which tissues is it
most abundant?
A. Oxygen transport in erythrocytes
B. Oxygen storage in cardiac and skeletal muscle
C. Carbon dioxide transport in plasma
D. pH buffering in the blood
E. Electron transport in mitochondria
Correct Answer: B
Rationale: Myoglobin functions as an oxygen storage and facilitated diffusion protein in
muscle tissues, particularly cardiac muscle and slow-twitch skeletal muscle (high
oxidative capacity). It binds oxygen released by hemoglobin in capillaries and stores it
for use during periods of high metabolic demand when oxygen delivery is insufficient.
Myoglobin releases oxygen only at very low partial pressures (P50 ~2-3 mmHg), making
it an effective oxygen reservoir. Option A describes hemoglobin. Option C describes
bicarbonate and carbamino mechanisms. Option D describes hemoglobin's buffering
capacity. Option E describes cytochromes. Myoglobin's high affinity ensures it loads
oxygen from hemoglobin but releases it only when intracellular PO₂ drops very low
(during intense exercise).
Q4: Which of the following best describes the oxygen binding curve of myoglobin and its
P50 value?
, A. Sigmoidal curve with P50 of approximately 26 mmHg
B. Hyperbolic curve with P50 of approximately 2-3 mmHg
C. Linear relationship with P50 of 100 mmHg
D. Biphasic curve with two distinct P50 values
E. Exponential curve with P50 of 40 mmHg
Correct Answer: B
Rationale: Myoglobin exhibits a hyperbolic (rectangular hyperbola) oxygen binding curve
because it is a monomer with a single oxygen binding site—there is no cooperativity.
The P50 (partial pressure at which myoglobin is 50% saturated) is approximately 2-3
mmHg (or torr), indicating very high oxygen affinity. This means myoglobin remains
saturated until tissue PO₂ drops to very low levels, ensuring oxygen is released only
when needed most. The hyperbolic curve follows the equation Y = [O₂]/(P50 + [O₂]),
where Y is fractional saturation. Option A describes hemoglobin's sigmoidal curve.
Options C, D, and E describe incorrect curve shapes and P50 values. The hyperbolic
curve reflects simple binding equilibrium without interaction between sites.
Q5: The distal histidine (E7) in myoglobin serves which important functional role?
A. It coordinates directly to the heme iron and is essential for oxygen binding
B. It stabilizes the bound oxygen molecule via hydrogen bonding and prevents carbon
monoxide binding
C. It catalyzes the conversion of O₂ to CO₂
D. It binds to the cell membrane to anchor myoglobin
Protein Function: Hemoglobin, Myoglobin &
Oxygen Transport | Portage Learning
Comprehensive Assessment | Verified Q&A |
Pass Guaranteed - A+ Graded
SECTION I: MYOGLOBIN STRUCTURE & FUNCTION
Key Concepts: Monomeric structure, heme group, proximal/distal histidines, oxygen
storage, hyperbolic binding curve, high oxygen affinity
(Questions 1-10)
Q1: Myoglobin is a monomeric protein containing how many amino acid residues and
what type of prosthetic group?
A. 141 amino acids with a heme group containing Fe³⁺
B. 146 amino acids with a copper-containing heme
C. 153 amino acids with a protoporphyrin IX ring containing Fe²⁺
D. 153 amino acids with a magnesium-porphyrin complex
E. 141 amino acids with a zinc-containing prosthetic group
Correct Answer: C
,Rationale: Myoglobin consists of a single polypeptide chain of 153 amino acids (in
human myoglobin) with a molecular weight of approximately 17.8 kDa. It contains one
heme prosthetic group composed of protoporphyrin IX (a tetrapyrrole ring with four
methyl, two vinyl, and two propionate substituents) with a central ferrous iron (Fe²⁺).
The Fe²⁺ is essential for oxygen binding—ferric iron (Fe³⁺, metmyoglobin) cannot bind
O₂. Option A has the wrong count and oxidation state. Options B, D, and E describe
incorrect metals (copper, magnesium, zinc) not found in myoglobin heme. The heme
iron is the site of reversible oxygen binding.
Q2: In the heme group of myoglobin, the iron atom is coordinated to the protein via
which amino acid residue, and what is the geometry of this coordination?
A. Distal histidine (E7); ionic bond
B. Proximal histidine (F8); coordinate covalent bond
C. Lysine residue; hydrogen bond
D. Cysteine residue; disulfide bond
E. Glutamate residue; salt bridge
Correct Answer: B
Rationale: The heme iron is coordinated to the imidazole side chain of the proximal
histidine (His93, also called the F8 histidine based on its position in the 8th turn of the F
helix). This is a coordinate covalent bond where the nitrogen lone pair donates to the
iron. The iron has five coordination sites occupied: four by porphyrin nitrogens (in the
heme plane) and one by proximal histidine (below the plane). The sixth coordination site
(above the plane) is available for oxygen binding. The distal histidine (E7, His64) is
nearby but not coordinated to iron—it stabilizes the bound O₂ and prevents CO binding.
,Options A, C, D, and E describe incorrect residues or bond types. The proximal histidine
is essential for anchoring the heme and maintaining iron in the proper position.
Q3: What is the primary physiological function of myoglobin, and in which tissues is it
most abundant?
A. Oxygen transport in erythrocytes
B. Oxygen storage in cardiac and skeletal muscle
C. Carbon dioxide transport in plasma
D. pH buffering in the blood
E. Electron transport in mitochondria
Correct Answer: B
Rationale: Myoglobin functions as an oxygen storage and facilitated diffusion protein in
muscle tissues, particularly cardiac muscle and slow-twitch skeletal muscle (high
oxidative capacity). It binds oxygen released by hemoglobin in capillaries and stores it
for use during periods of high metabolic demand when oxygen delivery is insufficient.
Myoglobin releases oxygen only at very low partial pressures (P50 ~2-3 mmHg), making
it an effective oxygen reservoir. Option A describes hemoglobin. Option C describes
bicarbonate and carbamino mechanisms. Option D describes hemoglobin's buffering
capacity. Option E describes cytochromes. Myoglobin's high affinity ensures it loads
oxygen from hemoglobin but releases it only when intracellular PO₂ drops very low
(during intense exercise).
Q4: Which of the following best describes the oxygen binding curve of myoglobin and its
P50 value?
, A. Sigmoidal curve with P50 of approximately 26 mmHg
B. Hyperbolic curve with P50 of approximately 2-3 mmHg
C. Linear relationship with P50 of 100 mmHg
D. Biphasic curve with two distinct P50 values
E. Exponential curve with P50 of 40 mmHg
Correct Answer: B
Rationale: Myoglobin exhibits a hyperbolic (rectangular hyperbola) oxygen binding curve
because it is a monomer with a single oxygen binding site—there is no cooperativity.
The P50 (partial pressure at which myoglobin is 50% saturated) is approximately 2-3
mmHg (or torr), indicating very high oxygen affinity. This means myoglobin remains
saturated until tissue PO₂ drops to very low levels, ensuring oxygen is released only
when needed most. The hyperbolic curve follows the equation Y = [O₂]/(P50 + [O₂]),
where Y is fractional saturation. Option A describes hemoglobin's sigmoidal curve.
Options C, D, and E describe incorrect curve shapes and P50 values. The hyperbolic
curve reflects simple binding equilibrium without interaction between sites.
Q5: The distal histidine (E7) in myoglobin serves which important functional role?
A. It coordinates directly to the heme iron and is essential for oxygen binding
B. It stabilizes the bound oxygen molecule via hydrogen bonding and prevents carbon
monoxide binding
C. It catalyzes the conversion of O₂ to CO₂
D. It binds to the cell membrane to anchor myoglobin
