18. Biochemistry | Globular Proteins

Structure Level of Myoglobin?

Tertiary (single polypeptide)

Structure Level of Hemoglobin?

Quaternary (tetramer: 4 polypeptides)

Subunits of Myoglobin?

1 α-helical chain (8 helices: A-H)

Subunits of Hemoglobin?

2 α-chains + 2 β-chains (HbA); variants include δ or γ chains

Heme Content of Myoglobin?

1 heme (porphyrin ring + Fe2+)

Heme Content of Hemoglobin?

4 hemes (one per subunit)

Histidine Coordination of Myoglobin?

Proximal His binds Fe2+; Distal His stabilizes O2

Histidine Coordination of Hemoglobin?

Same as myoglobin for each subunit

Location of Myoglobin?

Muscle cells

Location of Hemoglobin?

Red blood cells (systemic circulation)

Function of Myoglobin?

Stores O2; delivers to mitochondria (ETC Complex IV)

Function of Hemoglobin?

Transports O2 and CO2 between lungs and tissues

Variants of Hemoglobin?

HbA (Adult): α₂β₂ – ~95–98%; HbA₂ (Adult): α₂δ₂ – ~2–3%; HbF (Fetal): α₂γ₂ – higher O₂ affinity due to poor 2,3-BPG binding

Binding Curve of Myoglobin?

Hyperbolic (non-cooperative)

Binding Curve of Hemoglobin?

Sigmoidal (positive cooperativity)

Hill Coefficient of Myoglobin?

1 (non-cooperative)

Hill Coefficient of Hemoglobin?

~3 (cooperative)

O2 Affinity of Myoglobin?

High at all pO2

O2 Affinity of Hemoglobin?

Varies with pO2 due to T ⇄ R transition

Role of Myoglobin?

Extracts O2 from Hb and delivers to mitochondria

Role of Hemoglobin?

Delivers O2 from lungs to tissues

Conformations of Hemoglobin?

T (Taut) = low O2 affinity; R (Relaxed) = high O2 affinity

Trigger for T → R transition of Hemoglobin?

Movement of Fe2+ into the heme plane upon O2 binding

Describe the T (Taut) state of hemoglobin.

Low O2 affinity, deoxygenated, favors O2 release

Describe the R (Relaxed) state of hemoglobin.

High O2 affinity, oxygenated, favors O2 binding

Mnemonic for T vs. R states?

T = Tense, Taut, Tissue, Tired; R = Relaxed, Ready, Respiration, Rich in O2

What is the Bohr Effect?

↑ CO₂ or ↓ pH = reduced Hb affinity for O2

Which histidine mediates the Bohr effect?

C-terminal His; protonation interacts with Asp-94→ favors T form

Effect of ↑ [H⁺] (↓ pH, acidosis) on Hb O2 binding?

Decreases affinity→ Right shift (Bohr Effect's favors O2 release, T state)

Effect of ↑ CO₂ + transported as bicarbonate HCO3- in blood on Hb O2 binding?

Decreases affinity→ Right shift (Forms carbaminohemoglobin, stabilizes T form)

Effect of CO2 in lungs?

R form ↑ T form ↓

Effect of ↑ 2,3-BPG on Hb O2 binding?

Decreases affinity→ Right shift (Binds between β-chains, promotes O2 unloading s/p T form

Effect of ↑ Temperature on Hb O2 binding?

Decreases affinity→ Right shift (Enhances tissue O2 delivery)

Effect of ↓ [H⁺] (↑ pH, alkalosis) i.e. in lungs on Hb O2 binding?

Increases affinity→ Left shift (Favors R state in lungs)

Effect of CO on Hb O2 binding?

Artificially increases affinity→ Left shift (Prevents O2 release; loss of cooperativity)

Effect of HbF on Hb O2 binding?

Increases affinity→ Left shift (γ-chains do not bind 2,3-BPG→ better maternal O2 uptake)

CO Poisoning's Molecular Basis?

CO binds Hb 240x tighter than O2→ carboxyhemoglobin

CO Poisoning's Clinical Features?

Hypoxia, dizziness, seizures, lactic acidosis, death

CO Poisoning's Treatment?

100% O2 or hyperbaric O2

Sickle Cell Disease's Molecular Basis?

β-chain mutation Glu6Val (E6V)→ hydrophobic HbS aggregates → sickled RBCs

Sickle Cell Disease's Clinical Features?

Anemia, vaso-occlusive crises, pain in extremities

Sickle Cell Disease's Treatment?

Hydroxyurea (↑ HbF), supportive

Hemoglobin C Disease's Molecular Basis?

β-chain mutation Glu6Lys (E6K)

Hemoglobin C Disease's Clinical Features?

Mild hemolytic anemia

Hemoglobin C Disease's Treatment?

Supportive

Hb Hammersmith's Molecular Basis?

Phe42Ser (F42S)→ destabilizes heme pocket→ heme loss

Hb Hammersmith's Clinical Features?

Hemolytic anemia

Hb Savannah's Molecular Basis?

Gly24Val (G24V)→ helix disruption→ unstable structure

Hb Savannah's Clinical Features?

Hemolytic anemia

Hb Milwaukee's Molecular Basis?

Val67Glu (V67E)→ Fe2+ oxidized to Fe2+ (methemoglobin)

Hb Milwaukee's Clinical Features?

Cyanosis, chocolate-colored blood

Methemoglobinemia's Molecular Basis?

Fe3+ Hb accumulation; NADH-methemoglobin reductase deficiency

Methemoglobinemia's Clinical Features?

Cyanosis ('Blue People of Kentucky')

Methemoglobinemia's Treatment?

Methylene blue

α-Thalassemia's Molecular Basis?

Deletion of 1-4 α-globin genes (Chr 16)

α-Thalassemia's Severity?

(Deletion of) 1 (a-globin gene): Silent; 2: Trait; 3: HbH disease; 4: Hb Bart (fatal)

α-Thalassemia's Treatment?

Supportive

β-Thalassemia's Treatment?

Transfusions, chelation therapy

β-Thalassemia's Molecular Basis?

Mutation/deletion of 1-2 β-globin genes (Chr 11)

β-Thalassemia's Severity?

1: Minor (mild anemia); 2: Major (Cooley’s anemia)

HbA1c's Definition?

Non-enzymatic glycation of HbA in hyperglycemia→ diabetes marker

When does Hb bind O2?

At high O2 levels (lungs, O2 rich after breathing in)

When does myoglobin bind O2?

At low O2 levels (muscle, O2 poor after use) to deliver O to mitochondria ETC’s Complex 4 (cytochrome oxidase)

HbA1c's Treatment?

Glucose control

HbA1c's Clinical Features?

Chronic elevated HbA1c → diabetes mellitus indicator

A neonate is found to have higher oxygen affinity hemoglobin than his mother. Which of the following best explains this physiological adaptation?

A. Expression of β instead of γ chains in fetal hemoglobin
B. Lack of 2,3-BPG binding due to γ chains
C. Increased CO₂ binding to fetal hemoglobin
D. Presence of hydrophobic patches in fetal hemoglobin
E. Higher expression of α-thalassemia genes

B

HbF (α₂γ₂) has poor 2,3-BPG binding, which leads to higher O₂ affinity, allowing the fetus to extract O₂ from maternal blood.

• A. ❌ Fetal Hb has γ chains, not β chains.

• B. ✅ Correct—γ chains reduce 2,3-BPG binding → left shift.

• C. ❌ CO₂ transport does not increase O₂ affinity.

• D. ❌ Hydrophobic patches are seen in sickle cell HbS.

• E. ❌ Thalassemias reduce functional Hb, not increase affinity.

A 17-year-old male presents with severe anemia and a positive family history of blood disorders. Genetic testing reveals deletion of three α-globin genes. Which of the following is most likely present in his peripheral blood?

A. Carboxyhemoglobin
B. Hemoglobin S
C. Heinz bodies
D. Methemoglobin
E. HbA1c

C

3 α-gene deletions = HbH disease, which leads to β₄ tetramers forming Heinz bodies (precipitated Hb).

• A. ❌ Carboxyhemoglobin is from CO poisoning, unrelated to thalassemia.

• B. ❌ HbS is a β-chain point mutation (sickle cell).

• C. ✅ Heinz bodies are characteristic of HbH disease.

• D. ❌ Methemoglobin is Fe³⁺ Hb, unrelated to α-thalassemia.

• E. ❌ HbA1c is a diabetes marker, not relevant here.

A 32-year-old man is rescued from a house fire and arrives confused and short of breath. Pulse oximetry reads 98%, but arterial blood gas shows hypoxia. What is the most likely cause of this discrepancy?

A. High levels of 2,3-BPG
B. Increased CO₂ forming carbaminohemoglobin
C. Conversion of hemoglobin to methemoglobin
D. Formation of carboxyhemoglobin preventing O₂ release
E. Glycosylation of hemoglobin reducing O₂ affinity

D

Carboxyhemoglobin forms in CO poisoning, which prevents O₂ unloading, causes a left shift, and makes SpO₂ readings falsely high.

• A. ❌ 2,3-BPG affects tissue delivery but not pulse ox discrepancy.

• B. ❌ Carbaminohemoglobin forms with CO₂ but does not cause pulse ox artifact.

• C. ❌ Methemoglobinemia would decrease SpO₂ readings.

• E. ❌ HbA1c has no acute effect on O₂ delivery.