Hemoglobin and Myoglobin

Compare and Contrast Myoglobin vs. Hemoglobin

Myoglobin:

• Structure: Monomeric protein (153 amino acids).

• Location: Found in muscle tissue.

• Function: Binds O₂; high affinity for O₂ at all pO₂ levels.

• O₂ Binding Curve: Hyperbolic (indicating no cooperativity).

• Heme Group: 1 heme group per protein.

Hemoglobin:

• Structure: Tetrameric protein (141 and 143 amino acids for α and β subunits).

• Location: Found in red blood cells.

• Function: Binds O₂; cooperatively binds O₂.

• O₂ Binding Curve: Sigmoidal (indicating cooperativity).

• Heme Group: 4 heme groups, one in each subunit

Describe the Heme Binding and Function

Heme Binding Pocket:

• Structure: Heme group is bound in a hydrophobic pocket between the C, F, and E helices of each subunit in Hb.

• Iron Coordination: Fe²⁺ at the center of the heme binds to O₂.

• Prevention of Oxidation: Proximal histidine (His93) coordinates with Fe²⁺ to prevent oxidation to Fe³⁺.

Key Residues for O₂ Binding:

• Proximal Histidine (His93): Binds to Fe²⁺.

• Distal Histidine (His64): Stabilizes O₂ binding via hydrogen bonding.

What is cooperativity in Hemoglobin?

The binding of O₂ to one subunit increases the affinity of the other subunits for O₂

What is the T-state (Tense State)?

• Low O₂ affinity.

• O₂ binding is weak at all 4 sites.

What is the R-state (Relaxed State)?

• High O₂ affinity.

• O₂ binding is strong at all 4 sites.

How does O₂ Binding relate the the R-State?

• First binding stabilizes the R-state, facilitating additional O₂ bindings.

• Results in a sigmoidal O₂ binding curve.

Describe the O₂ Binding Curves for Myoglobin

• Curve Type: Hyperbolic.

• Affinity: High affinity for O₂ at all pO₂ values.

• Function: Stores O₂ in muscles.

Describe the O₂ Binding Curves for Hemoglobin

• Curve Type: Sigmoidal.

• Affinity: Shows cooperative binding; affinity increases as more O₂ binds

Describe the O₂ Binding Curves for Fetal Hemoglobin (HbF)

• Structure: α₂𝛾₂ tetramer (γ subunits instead of β).

• Affinity: Higher affinity for O₂ than adult Hb.

• O₂ Transfer: Traps O₂ released by maternal Hb due to absence of His143β, which prevents BPG binding

What is The Bohr Effect?

The impact of pH and CO₂ on Hb’s O₂ binding.

Describe mechanisms that affect of the Bohr Effect?

• Acidity (lower pH) decreases O₂ affinity, shifting the curve to the right.

• Protons (Bohr Protons): Bind at the C-terminus and N-terminus of Hb subunits, stabilizing the T-state.

• Carbamate Formation: CO₂ binds to the N-terminus of the β-chain, further stabilizing the T-state and promoting O₂ release

What is the BPG (2,3-Bisphosphoglycerate) Effect

BPG binds to the central cavity of Hb in the T-state and stabilizes the T-state, reducing Hb’s affinity for O₂

How does High Altitude relate to the BPG (2,3-Bisphosphoglycerate) Effect

• Increased BPG levels enhance O₂ unloading at tissues despite low O₂ availability.

• Increases P50 (lower O₂ affinity).

What are the Similarities Between Myoglobin and Hemoglobin?

Structural Similarity:

• Both proteins have similar heme-binding sites.

• Hemoglobin: Tetrameric (four subunits).

• Myoglobin: Monomeric (single subunit).

Sequence Similarity:

• Hemoglobin α and β subunits share structural homology with myoglobin.

What is the Sickle Cell Mutation (HbS)?

Glu6 → Val in the β-chain of Hb.

What is the consequence of the Sickle Cell Mutation (HbS)?

• In T-state (deoxygenated), Val6 binds to a hydrophobic pocket in another Hb molecule, causing fiber formation.

• Fiber Formation: Results in sickle-shaped cells that impair blood flow and flexibility.

• Deoxygenation: As pO₂ decreases, fibers form, causing capillary blockages and reduced cell flexibility.

How do Hemoglobin and Myoglobin Function in Different Conditions?

High Altitude:

• BPG Levels: Increase to facilitate O₂ release to tissues despite lower pO₂.

• Fetal Hemoglobin: Higher affinity for O₂ allows better O₂ trapping from maternal blood.

Exercise:

• Bohr Effect: Increased CO₂ and lactic acid production lower blood pH, promoting O₂ release from Hb.

• BPG Increase: Exercise also increases BPG, enhancing O₂ delivery to active muscles.