Haemoglobin and Oxygen Transport: Cooperative Binding and Adaptations
- Haemoglobin is the oxygen transport protein found in red blood cells.
- Each molecule of haemoglobin has four subunits, each containing a haem group that can bind to one molecule of oxygen.
- This means a single haemoglobin molecule can transport up to four oxygen molecules.
Cooperative Binding in Haemoglobin
- Cooperative binding means that when one oxygen molecule binds to a haem group, the haemoglobin undergoes conformational changes that increase its affinity for oxygen at the other haem groups.
- Conversely, when one oxygen molecule dissociates, the affinity of haemoglobin for oxygen decreases, making it easier for the remaining oxygen molecules to dissociate.
- The haemoglobin molecule exists in two primary states:
- R State: Fully oxygenated with all four oxygen molecules bound.
- T State: Unsaturated with no oxygen bound.
- Think of cooperative binding as a chain reaction.
- Once haemoglobin starts binding oxygen, it gets progressively better at picking up more oxygen
Haemoglobin Saturation and Partial Pressure
- The percentage saturation of haemoglobin depends on the partial pressure of oxygen (pO₂) in the surrounding environment.
- At high pO₂ levels, such as in the lungs (10–13 kPa), haemoglobin becomes fully saturated (100%).
- At low pO₂ levels, such as in respiring tissues (below 10 kPa), haemoglobin readily releases oxygen to meet the tissue's needs.
- In the alveoli of the lungs, haemoglobin picks up oxygen where pO₂ is high.
- In active muscles, where pO₂ is low, haemoglobin releases oxygen to support aerobic respiration
Why Cooperative Binding is Crucial
- Efficiency in Oxygen Delivery: Haemoglobin can shift from fully saturated to partially unsaturated over a narrow range of pO₂ levels, ensuring rapid oxygen unloading in tissues where it’s needed most.
- Without this adaptation, haemoglobin would not deliver oxygen efficiently, particularly in muscle and other active tissues where oxygen demands are high.
Foetal Haemoglobin: Adapted for Oxygen Transfer
- Foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin.
- This allows it to extract oxygen from the maternal blood via the placenta, ensuring the foetus receives sufficient oxygen for growth and development.
- After birth, foetal haemoglobin is replaced by adult haemoglobin within a few months as the baby begins breathing independently.
At the placenta, oxygen dissociates from maternal haemoglobin and binds to foetal haemoglobin due to its higher oxygen affinity.
Self review- Why does haemoglobin exhibit cooperative binding, and how does this improve oxygen delivery?
- How does partial pressure influence haemoglobin saturation in the lungs versus respiring tissues?
- Why does foetal haemoglobin have a higher affinity for oxygen than adult haemoglobin?
