Oxygen Dissociation Curves Show Haemoglobin's Oxygen Binding and Release Dynamics
- An oxygen dissociation curve shows the relationship between the partial pressure of oxygen (pO₂) and the percentage saturation of haemoglobin with oxygen.
- It provides insights into how haemoglobin picks up oxygen in the lungs and releases it in tissues.
- X-axis: Partial pressure of oxygen (pO₂) in kilopascals (kPa).
- Y-axis: Percentage saturation of haemoglobin with oxygen.
Key Features of the Oxygen Dissociation Curve
- At high pO₂ (lungs, ~13 kPa): Haemoglobin is nearly 100% saturated, meaning most haemoglobin molecules are carrying their full complement of four oxygen molecules.
- At low pO₂ (respiring tissues, ~5 kPa): Haemoglobin releases oxygen, reducing its saturation.
- The curve is S-shaped (sigmoid) rather than linear due to cooperative binding.
This pattern allows haemoglobin to efficiently pick up oxygen in the lungs and release it where it is needed in the body.
Cooperative Binding Is The Key to the Sigmoid Shape
- Haemoglobin is a tetrameric protein, consisting of four polypeptide chains, each containing a haem group capable of binding to one oxygen molecule.
- The binding of one oxygen molecule influences the binding of subsequent molecules, making haemoglobin’s oxygen affinity dynamic rather than fixed.
How Cooperative Binding Works
- Low Affinity at Low pO₂ (Tense or "T" State):
- At low pO₂, haemoglobin has a low affinity for oxygen because the haem groups are in an inactive conformation.
- The first oxygen molecule binds with difficulty, resulting in a shallow slope at the start of the curve.
- Increasing Affinity After the First Binding (Relaxed or "R" State):
- Once the first oxygen molecule binds, haemoglobin undergoes a conformational change, making it easier for the second and third oxygen molecules to bind.
- This causes a steep increase in the curve, meaning haemoglobin can rapidly load oxygen as pO₂ rises.
- Saturation Plateau at High pO₂:
- When most binding sites are occupied, the curve levels off because fewer binding sites are available.
- The last oxygen molecule binds more slowly as there are fewer empty haem groups.
This cooperative behaviour ensures that haemoglobin is highly sensitive to small changes in pO₂, which is crucial for efficient oxygen transport and delivery.
Analogy- Imagine fastening a stiff jacket.
- The first button takes effort because the fabric resists.
- But once the first button is secured, the fabric aligns, making it easier to fasten the next buttons.
- Similarly, the binding of the first oxygen molecule makes it easier for haemoglobin to bind subsequent oxygen molecules.
