Describe and explain the oxygen dissociation curve of adult haemoglobin

Sourav PanOctober 30, 2024

Answered

The oxygen dissociation curve of adult hemoglobin (Hb) is a graphical representation that shows the relationship between the partial pressure of oxygen (pO₂) and the percentage saturation of hemoglobin with oxygen. This curve has a characteristic sigmoidal (S-shaped) shape, which reflects how hemoglobin’s affinity for oxygen changes with different pO₂ levels.

1. Shape and Characteristics of the Curve

• Sigmoidal Shape: The curve starts with a shallow slope at low pO₂ levels, then rises steeply in the middle range, and flattens out again at higher pO₂ levels. This S-shape indicates that hemoglobin’s affinity for oxygen is not constant but changes depending on the partial pressure of oxygen.
• Three Phases of Oxygen Binding:
  • At low pO₂ levels (e.g., in tissues with low oxygen), hemoglobin’s affinity for oxygen is low, so it does not readily bind to oxygen.
  • As pO₂ increases (e.g., as blood moves toward the lungs), hemoglobin’s affinity for oxygen increases, causing a rapid rise in oxygen saturation. This is due to the cooperative binding property of hemoglobin.
  • At high pO₂ levels (e.g., in the lungs), hemoglobin becomes nearly saturated with oxygen, and the curve plateaus.

2. Explanation of the Curve’s Sigmoidal Shape: Cooperative Binding

• Cooperative Binding: Hemoglobin is a tetrameric protein with four subunits, each containing an iron ion capable of binding an oxygen molecule. The binding of oxygen to one subunit increases the affinity of the remaining subunits for oxygen, making it easier for additional oxygen molecules to bind.
• As oxygen binds to hemoglobin, a conformational change occurs in the protein structure, increasing hemoglobin’s overall affinity for oxygen. This leads to the steep middle section of the curve, where small increases in pO₂ cause a large increase in oxygen saturation.

3. Physiological Implications of the Curve

• Oxygen Loading in the Lungs: At high pO₂ (about 100 mmHg in the lungs), hemoglobin is almost fully saturated with oxygen (~97-98%). This ensures that hemoglobin can pick up the maximum amount of oxygen in the lungs.
• Oxygen Unloading in Tissues: At lower pO₂ levels (about 40 mmHg in active tissues), hemoglobin’s affinity for oxygen decreases, causing it to release oxygen. This is crucial for supplying oxygen to tissues that need it for cellular respiration.
• Buffering Function: The sigmoidal shape allows hemoglobin to act as an oxygen buffer, picking up oxygen when it’s abundant (in the lungs) and releasing it when it’s scarce (in tissues), adapting to the oxygen demands of different environments.

4. Factors Shifting the Curve

• The oxygen dissociation curve can shift in response to factors that affect hemoglobin’s oxygen affinity:
  • Right Shift: Conditions such as increased temperature, elevated CO₂ (Bohr effect), high concentrations of 2,3-bisphosphoglycerate (2,3-BPG), or decreased pH cause a rightward shift. This indicates decreased affinity for oxygen, favoring oxygen release in active tissues.
  • Left Shift: Lower temperatures, decreased CO₂, lower 2,3-BPG levels, or increased pH result in a leftward shift, meaning increased affinity for oxygen, aiding in oxygen uptake in the lungs.