Haemoglobin Structure and Function

• Composition:
  • Haemoglobin is a protein composed of four polypeptide chains, each containing one haem group that can bind one O₂ molecule.
• Oxygen Binding:
  • Each haemoglobin molecule can bind up to four O₂ molecules (Hb + 4O₂ → HbO₄).
• Role:
  • Transports oxygen from the lungs to body tissues for aerobic respiration.

Key Terms

• Partial Pressure:
  • A measure of the concentration of a gas in a mixture of gases.

• Percentage Saturation:
  • The percentage of haemoglobin molecules that are bound with oxygen.

• Dissociation Curve:
  • A graph showing haemoglobin’s oxygen saturation versus oxygen partial pressure.

Haemoglobin Dissociation Curve

• Characteristics:
  • S-shaped Curve:
    • Low O₂ pressures: Haemoglobin has low saturation.
    • High O₂ pressures: Haemoglobin approaches full saturation (95-97% in lungs).
• Behavior:
  • In Lungs (High O₂ Pressure):
    • Haemoglobin binds O₂ effectively.
  • In Tissues (Low O₂ Pressure):
    • Haemoglobin releases O₂ (e.g., muscles at ~20-25% saturation).
• Cooperative Binding:
  • Binding of one O₂ molecule changes haemoglobin’s shape, making it easier for additional O₂ molecules to bind, resulting in the steep rise in the curve.

The Bohr Shift

Introduction

• The Bohr Shift is a critical physiological mechanism that facilitates efficient oxygen delivery to tissues based on their metabolic activity.
• Named after the Danish physiologist Christian Bohr, who first described the phenomenon in 1904, the Bohr Shift explains how hemoglobin’s affinity for oxygen changes in response to varying concentrations of carbon dioxide (CO₂) and pH levels within the blood.

Definition

• Bohr Shift:
  A shift in the oxygen-hemoglobin dissociation curve caused by changes in blood pH and CO₂ concentration, resulting in decreased hemoglobin affinity for oxygen and enhanced oxygen release to tissues.

Hemoglobin and Oxygen Binding

• Hemoglobin (Hb):
  A protein in red blood cells responsible for transporting oxygen from the lungs to body tissues and facilitating the return transport of carbon dioxide from tissues to the lungs.
• Oxygen-Hemoglobin Dissociation Curve:
  A graph that depicts the relationship between the partial pressure of oxygen (pO₂) and the percentage saturation of hemoglobin with oxygen. The curve is sigmoidal (S-shaped), indicating cooperative binding.

Mechanism of the Bohr Shift

Factors Influencing the Bohr Shift

1. Carbon Dioxide (CO₂) Concentration:
   • Increased CO₂:
     • Produced by active tissues during metabolism.
     • Diffuses into red blood cells and reacts with water to form carbonic acid (H₂CO₃) via the enzyme carbonic anhydrase.
     • H₂CO₃ dissociates into bicarbonate ions (HCO₃⁻) and hydrogen ions (H⁺).
2. pH Levels:
   • Decreased pH (Acidosis):
     • Higher concentration of H⁺ ions lowers blood pH.
     • H⁺ ions bind to hemoglobin, causing conformational changes that reduce hemoglobin’s affinity for oxygen.
3. Temperature:
   • Increased Temperature:
     • Often accompanies active metabolism.
     • Enhances the Bohr Shift by further decreasing hemoglobin’s oxygen affinity.

Detailed Process

1. At Active Tissues:
   • High Metabolic Activity:
     • Produces more CO₂ and heat.
     • Leads to increased CO₂ concentration and decreased pH in the surrounding blood.
2. Within Red Blood Cells:
   • CO₂ Conversion:
     • CO₂ + H₂O ↔ H₂CO₃ (catalyzed by carbonic anhydrase) ↔ H⁺ + HCO₃⁻.
3. Effect on Hemoglobin:
   • H⁺ Binding:
     • H⁺ ions attach to hemoglobin, stabilizing the T-state (low oxygen affinity).
   • Conformational Change:
     • Haemoglobin’s structure goes through a change, which shifts it from being a molecule with a high oxygen affinity to one with a lower affinity, facilitating oxygen release.
4. Oxygen Release:
   • Enhanced Dissociation:
     • Reduced affinity allows hemoglobin to release more oxygen where it’s needed most.

Physiological Importance

• Efficient Oxygen Delivery:
  Ensures that oxygen is preferentially released in tissues with high metabolic demands, such as muscles during exercise or active brain regions.
• Adaptation to Metabolic Changes:
  Allows the circulatory system to dynamically adjust oxygen delivery based on real-time metabolic activity and environmental conditions.
• Coordination with Respiratory System:
  Promotes the uptake of CO₂ in the tissues and its removal via the lungs, maintaining acid-base balance in the blood.

The Oxygen-Hemoglobin Dissociation Curve and the Bohr Shift

• Shifts in the Curve:
  • Right Shift (Bohr Shift):
    • Caused by increased CO₂, decreased pH, increased temperature.
    • Indicates decreased affinity of hemoglobin for oxygen, promoting oxygen release.
  • Left Shift:
    • Caused by decreased CO₂, increased pH, decreased temperature.
    • Indicates increased affinity of hemoglobin for oxygen, promoting oxygen uptake.

Mathematical Representation

• Hill Equation:
  Describes the cooperativity of oxygen binding to hemoglobin.

• Y: Fraction of oxygen-bound hemoglobin.
  • pO₂: Partial pressure of oxygen.
  • P₅₀: Partial pressure at which hemoglobin is 50% saturated.
  • n: ​ (the Hill coefficient) quantifies the degree of cooperativity in binding (for human hemoglobin). It​ is typically around 3).
  • S is the fractional saturation of hemoglobin (i.e. the fraction of binding sites occupied by oxygen),

• Impact of Bohr Shift on P₅₀:
  • Right Shift:
    • Increases P₅₀, meaning higher pO₂ is required for 50% saturation, indicating lower affinity.
  • Left Shift:
    • Decreases P₅₀, meaning lower pO₂ is required for 50% saturation, indicating higher affinity.

Clinical Relevance

1. Chronic Obstructive Pulmonary Disease (COPD):
   • Impact:
     • Impaired gas exchange leads to increased CO₂ levels.
     • Enhances the Bohr Shift, promoting oxygen release to compensate for reduced oxygen uptake.
2. Metabolic Acidosis:
   • Definition:
     • Condition characterized by decreased blood pH due to excessive acid production or loss.
   • Effect:
     • Amplifies the Bohr Shift, increasing oxygen delivery to tissues.
3. Carbon Monoxide Poisoning:
   • Mechanism:
     • CO binds to hemoglobin with higher affinity than oxygen, reducing oxygen delivery.
   • Relation to Bohr Shift:
     • The presence of CO exacerbates the Bohr Shift effects by further decreasing hemoglobin’s oxygen affinity.
4. Sickle Cell Anemia:
   • Effect on Oxygen Release:
     • Abnormal hemoglobin structure affects the Bohr Shift, impairing oxygen delivery to tissues.

Fetal Hemoglobin (HbF) and the Bohr Shift

• Fetal Hemoglobin (HbF):
  • Higher Oxygen Affinity:
    • Facilitates efficient oxygen uptake from maternal blood across the placenta.
• Bohr Shift in Fetuses:
  • Enhanced Oxygen Delivery:
    • The Bohr Shift ensures that oxygen is preferentially released to fetal tissues, complementing the high affinity of HbF for oxygen.

Mechanism of the Bohr Effect in Muscle Tissue

A. Conditions in Active Muscle Tissue

• High CO₂ Production: Muscle cells produce CO₂ as a by-product of aerobic metabolism.
• Increased H⁺ Concentration: CO₂ reacts with water to form carbonic acid (H₂CO₃), which dissociates into H⁺ and bicarbonate ions (HCO₃⁻), lowering pH.
• Lower pH (Acidosis): Accumulation of H⁺ ions decreases the pH in the local environment.

B. Impact on Hemoglobin (Hb)

• Reduced O₂ Affinity: Elevated CO₂ and H⁺ bind to hemoglobin, inducing conformational changes that decrease Hb’s affinity for O₂.
• Enhanced O₂ Release: With lower affinity, O₂ is more readily released from hemoglobin to meet the metabolic demands of muscle cells.

Physiological Significance

• Efficient Oxygen Delivery: Ensures that active muscles receive more O₂ when they need it most during increased activity.
• Facilitation of CO₂ Removal: Promotes the uptake of CO₂ from muscle cells for transport back to the lungs.

Key Factors Influencing the Bohr Effect in Muscles

• CO₂ Levels: Higher CO₂ concentration enhances the Bohr Effect.
• pH Levels: Lower pH (higher H⁺ concentration) strengthens the Bohr Effect.
• Temperature: Increased temperature in active muscles can further reduce Hb’s O₂ affinity.

Oxygen Dissociation Curves: Myoglobin vs. Hemoglobin

1. Myoglobin Curve:
   • Hyperbolic shape.
   • Myoglobin has a very high affinity for oxygen and binds it strongly, even at low partial pressures of oxygen (pO₂).
   • This high affinity ensures myoglobin effectively picks up oxygen released by hemoglobin in tissues.
2. Hemoglobin Curve:
   • Sigmoidal shape due to cooperative binding (oxygen binding increases affinity for subsequent oxygen molecules).
   • At the tissue level, hemoglobin’s oxygen affinity is reduced by factors such as:
     • Higher carbon dioxide levels.
     • Lower pH (Bohr effect).
     • Higher temperature.
     • Presence of 2,3-bisphosphoglycerate (2,3-BPG).

Why is Myoglobin’s Curve Left-Shifted?

• Higher Oxygen Affinity:
  • Myoglobin’s role is to store oxygen in muscles and release it only when pO₂ is critically low (e.g., during intense exercise).
  • This requires it to bind oxygen strongly, even when hemoglobin is releasing it.
• Non-Cooperative Binding:
  • Myoglobin has a single heme group and does not exhibit cooperative binding, unlike hemoglobin. Its oxygen-binding affinity remains consistently high across a range of pO₂ values.
• Comparison to Tissue-Level Hemoglobin:
  • In tissues, hemoglobin’s dissociation curve shifts rightward due to the Bohr effect (lower pH, higher CO₂), reducing its oxygen affinity.
  • Myoglobin’s higher affinity ensures it can still extract oxygen released by hemoglobin under these conditions.

Physiological Significance

• The left-shifted curve of myoglobin relative to hemoglobin ensures:
  1. Efficient oxygen transfer from hemoglobin (lower affinity) to myoglobin (higher affinity) in the tissues.
  2. Oxygen is retained in muscle cells until critically needed during hypoxia or high-energy demand.

Bohr compared with the Haldane Effect

Haldane Effect:

• The Haldane Effect describes the influence of oxygenation on hemoglobin’s ability to bind and transport carbon dioxide and protons (H⁺).
• Specifically, it refers to the increased capacity of deoxygenated hemoglobin to carry CO₂ and H⁺, and conversely, the reduced ability of oxygenated hemoglobin to carry these molecules.

Key Points:

• Deoxygenated Hemoglobin (Hb): Binds more CO₂ and H⁺.
• Oxygenated Hemoglobin (HbO₂): Binds less CO₂ and H⁺.

• This effect facilitates the uptake of CO₂ in tissues and its release in the lungs, enhancing overall gas exchange efficiency.

2. Mechanism of the Haldane Effect

a. CO₂ Transport in the Blood

CO₂ is transported in the blood in three primary forms:

1. Dissolved CO₂: Approximately 5-10% of total CO₂ is dissolved directly in plasma.
2. Carbaminohemoglobin (HbCO₂): About 20-30% binds directly to hemoglobin.
3. Bicarbonate Ions (HCO₃⁻): The majority (60-70%) is converted to bicarbonate ions in plasma.

b. Role of Hemoglobin in CO₂ Transport

• Deoxygenated Hemoglobin:
  • Enhanced CO₂ Binding: When hemoglobin releases oxygen to the tissues, it becomes deoxygenated (Hb). This state increases its affinity for CO₂ and H⁺, promoting the formation of carbaminohemoglobin and facilitating the conversion of CO₂ to bicarbonate.
  • Chloride Shift: As bicarbonate ions accumulate in red blood cells, they are exchanged for chloride ions (Cl⁻) in plasma to maintain electrical neutrality, a process known as the chloride shift.
• Oxygenated Hemoglobin:
  • Reduced CO₂ Binding: In the lungs, oxygen binds to hemoglobin, converting it to oxyhemoglobin (HbO₂). This oxygenation decreases hemoglobin’s affinity for CO₂ and H⁺, promoting the release of CO₂ from hemoglobin.
  • Facilitated CO₂ Release: Lower CO₂ affinity in the lungs aids in the expulsion of CO₂ during exhalation.

c. Interaction with Protons (H⁺)

• Deoxygenated Hemoglobin:
  • Buffering Capacity: Deoxygenated hemoglobin can bind free H⁺ ions, helping to buffer blood pH by mitigating acidity.
• Oxygenated Hemoglobin:
  • Reduced Buffering: Oxygenated hemoglobin has a decreased ability to bind H⁺, which can influence blood pH regulation.

3. Physiological Significance of the Haldane Effect

a. Enhanced Gas Exchange Efficiency

• In Tissues (Peripheral):
  • Oxygen Release: Oxygen is released from hemoglobin to meet metabolic demands.
  • CO₂ Uptake: Simultaneously, the deoxygenated state of hemoglobin enhances CO₂ binding and transport from tissues to the blood.
• In Lungs (Pulmonary):
  • Oxygen Uptake: Oxygen binds to hemoglobin, increasing its oxygen saturation.
  • CO₂ Release: The oxygenated state reduces hemoglobin’s affinity for CO₂, facilitating its release into the alveoli for exhalation.

b. Maintenance of Blood pH

• Buffering Action: By binding H⁺ ions, deoxygenated hemoglobin helps maintain blood pH, especially in actively metabolizing tissues where lactic acid production can increase acidity.

c. Complementary Role with the Bohr Effect

• Bohr Effect: Describes how increased CO₂ and H⁺ concentrations (lower pH) in tissues decrease hemoglobin’s affinity for oxygen, promoting oxygen release.
• Haldane Effect: Complements the Bohr Effect by enhancing CO₂ uptake in deoxygenated blood and facilitating its release in oxygenated blood.

Together, these effects ensure that oxygen delivery and CO₂ removal are tightly coupled to the metabolic activity of tissues.

Fetal Hemoglobin (HbF)

Definition

• Fetal Hemoglobin (HbF):
  A type of hemoglobin predominant in the fetus, consisting of two alpha (α) and two gamma (γ) globin chains (α₂γ₂).

Structure and Function

• Higher Oxygen Affinity:
  HbF has a greater affinity for oxygen compared to adult hemoglobin (HbA, α₂β₂), enabling efficient oxygen uptake from the maternal blood across the placenta.

Physiological Significance

• Placental Gas Exchange:
  Facilitates the transfer of oxygen from maternal blood (low HbF affinity) to fetal blood (high HbF affinity), optimizing oxygen uptake by fetal tissues.
• Efficient Oxygen Transfer:
  The high affinity of HbF for oxygen ensures that the developing fetus receives adequate oxygen from the mother’s blood, even when maternal oxygen levels are lower.

Carbon Dioxide Transport in Blood

• As Hydrogencarbonate Ions (HCO₃⁻):
  • 85% of CO₂ is converted to HCO₃⁻ ions in red blood cells.
  • Chloride Shift: HCO₃⁻ diffuses out of cells; Cl⁻ ions enter to maintain ionic balance.
• Dissolved CO₂ in Plasma:
  • 5% of CO₂ remains as dissolved molecules in blood plasma.
• As Carbaminohaemoglobin:
  • 10% of CO₂ binds directly to haemoglobin in red blood cells, forming carbaminohaemoglobin.
• Reverse Reaction in Lungs:
  • Lower CO₂ concentration in lungs drives the reverse reaction, releasing CO₂ from blood to alveoli for exhalation.

Key Terms in CO₂ Transport

• Carbonic Anhydrase:
  • An enzyme in red blood cells that catalyzes the reaction converting CO₂ and H₂O to carbonic acid (H₂CO₃).
• Chloride Shift:
  • The exchange of Cl⁻ ions for HCO₃⁻ ions between plasma and red blood cells to maintain ionic balance.
• Carbaminohaemoglobin:
  • A compound formed when CO₂ binds directly to haemoglobin.

Common Errors in Understanding Gas Transport

1. Oxyhaemoglobin Release:
   • Incorrect: Oxyhaemoglobin gradually releases O₂ as it passes from the lungs to a muscle.
   • Correct: O₂ is primarily released in tissues where the partial pressure of O₂ is low.
2. Function of Arteries:
   • Incorrect: Arteries pump blood around the body.
   • Correct: Arteries transport blood; the heart pumps it.
3. Red Blood Cells Surface Area:
   • Incorrect: Red blood cells have a large surface area for oxygen attachment.
   • Correct: RBCs’ surface area aids in diffusion of O₂, not direct attachment.
4. CO₂ Transport:
   • Incorrect: Most CO₂ is transported in plasma solution.
   • Correct: Most CO₂ is transported as HCO₃⁻ ions, not dissolved directly.