Foetal haemoglobin and oxygen transfer across the placenta
Foetal haemoglobin (HbF) has two alpha and two gamma globin chains (protein subunits) and a higher oxygen affinity than maternal haemoglobin, meaning it holds oxygen more tightly.
Remember
Gamma chains bind 2,3-BPG (a small molecule made in red cells) poorly, so foetal haemoglobin has a higher oxygen affinity than maternal haemoglobin and can take up oxygen across the placenta.
Haemoglobin is built from four globin chains (protein subunits), and each chain carries a haem group, which binds oxygen. Adult haemoglobin (HbA) has two alpha chains and two beta chains. Foetal haemoglobin (HbF) keeps the two alpha chains but has two gamma chains in place of the beta chains.
At the placenta, the oxygen pressure in foetal blood is about 20 mm Hg, while maternal blood there is about 35 to 50 mm Hg. The pressure difference is small, so the foetus needs haemoglobin that holds oxygen more tightly than the mother's does.
HbF achieves this through reduced binding of 2,3-BPG, a small molecule made in red cells. In HbA, 2,3-BPG binds in the centre of the molecule and holds it in the low-affinity T state, so oxygen is released more easily. The gamma chains bind 2,3-BPG poorly, so HbF is not pushed into that state and has a higher oxygen affinity (it holds oxygen more tightly). Its oxygen dissociation curve (a curve whose position, left or right, is set by affinity) lies to the left of the maternal curve, meaning HbF is more saturated with oxygen than maternal haemoglobin at any given oxygen pressure, so oxygen moves from maternal to foetal haemoglobin.
After birth, production switches from gamma chains to beta chains, and affinity falls to the adult level.
Key facts
- Foetal haemoglobin
- has two alpha chains and two gamma chains.
- Adult haemoglobin
- has two alpha chains and two beta chains.
- Gamma chains
- bind 2,3-BPG poorly, so HbF stays in the high-affinity state.
- Oxygen pressure at the placenta
- is about 20 mm Hg in foetal blood and about 35 to 50 mm Hg in maternal blood.
- Oxygen dissociation curve
- of foetal haemoglobin lies to the left of the maternal curve.
- After birth
- production switches from gamma chains to beta chains, and affinity falls to the adult level.
Exam traps
- A right-shifted oxygen dissociation curve means lower affinity, which would reduce oxygen uptake from maternal blood, the opposite of what the foetus needs.
- Stronger 2,3-BPG binding does not raise oxygen affinity, because it stabilises the low-affinity state, which makes oxygen easier to release. Foetal haemoglobin binds less 2,3-BPG than adult haemoglobin, not more.
- An increased number of alpha chains compared with adult haemoglobin would not explain a higher affinity, because both types contain two alpha chains and the difference lies in the other two chains.
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