- Haemoglobin is the oxygen transport protein found in red blood cells.
- Each molecule has four subunits, and each subunit holds a haem group that binds one oxygen molecule.
- So one haemoglobin molecule can carry up to four oxygen molecules when fully saturated.
- Foetal haemoglobin (HbF) and adult haemoglobin (HbA) differ in their polypeptide chains.
- HbA has two alpha and two beta chains, while HbF has two alpha and two gamma chains.
- The gamma chains bind 2,3-bisphosphoglycerate (2,3-BPG) much less strongly than the beta chains of adult haemoglobin.
- Since 2,3-BPG lowers oxygen affinity, weaker binding leaves HbF with a higher oxygen affinity than HbA at the same partial pressure of oxygen.
On an oxygen dissociation curve, this higher affinity places the HbF curve to the left of the adult curve (see B3.1.13 for the base curve).
- At the placenta, maternal and foetal blood run close together, allowing gas exchange.
- Because HbF has the higher affinity, oxygen leaves maternal haemoglobin and binds to foetal haemoglobin.
- This one-way transfer supplies the foetus with enough oxygen for growth and aerobic respiration, even though oxygen partial pressure in the womb is low.
- After birth, HbF is gradually replaced by adult haemoglobin over the first months as the baby breathes for itself.
- The sigmoid curve and cooperative binding of oxygen are covered in B3.1.13.
- The Bohr shift, which moves the adult curve to the right in active tissues, is covered in B3.1.12.
- Which polypeptide chains does foetal haemoglobin have that adult haemoglobin does not?
- Why does weaker 2,3-BPG binding give foetal haemoglobin a higher oxygen affinity?
- How does the affinity difference move oxygen from mother to foetus at the placenta?
- In which direction does the foetal haemoglobin curve sit relative to the adult curve, and why?
