Foetal haemoglobin and 2,3-bisphosphoglycerate
Foetal haemoglobin (HbF, the main haemoglobin before birth) binds 2,3-bisphosphoglycerate (a small molecule in red cells) poorly, so it holds oxygen more tightly than adult haemoglobin.
Remember
Foetal haemoglobin binds 2,3-bisphosphoglycerate poorly, so it has a higher oxygen affinity (it holds oxygen more tightly), which lets the foetus take oxygen from maternal blood.
Before birth, the foetus takes up its oxygen from maternal blood across the placenta. Blood leaving the placenta for the foetus is at a partial pressure of oxygen (the pressure from dissolved oxygen, which drives its movement) of only 30 to 35 mmHg.
The foetus copes by using foetal haemoglobin, made of two alpha and two gamma chains, in place of adult haemoglobin (HbA), which has two alpha and two beta chains. The gamma chains bind 2,3-bisphosphoglycerate (2,3-BPG) poorly. Normally 2,3-BPG stabilises the T state, the tense, low-affinity form of haemoglobin that lets go of oxygen. With little 2,3-BPG bound, HbF is held less firmly in that form, so it takes up oxygen more readily.
Its oxygen dissociation curve (the graph of how saturated haemoglobin is at each partial pressure of oxygen) is therefore shifted to the left of the adult curve. A left shift means haemoglobin holds on to oxygen, whereas a right shift means it gives oxygen up. Foetal blood holds more oxygen at the same pressure than maternal blood does, so oxygen passes from mother to foetus.
Two further features add to oxygen delivery: a higher haemoglobin concentration and a higher cardiac output per kilogram. HbF predominates until shortly after birth, and the switch from gamma-chain to beta-chain production is largely complete by 6 months after birth.
Key facts
- Structure
- HbF has two alpha and two gamma chains, whereas HbA has two alpha and two beta chains.
- Cause
- the gamma chains bind 2,3-BPG poorly, so the T state is less stabilised.
- Affinity
- HbF has a higher oxygen affinity than HbA.
- Curve
- the oxygen dissociation curve of HbF lies to the left of the adult curve.
- Direction
- a right shift means oxygen is released more readily, and a left shift means it is held more tightly.
- Switch
- the change from gamma-chain to beta-chain production is largely complete by 6 months after birth.
Exam traps
- Binding 2,3-bisphosphoglycerate more strongly than adult haemoglobin does not explain a higher oxygen affinity, because 2,3-BPG holds haemoglobin in its low-affinity T state, and foetal haemoglobin binds it poorly.
- A lower total haemoglobin concentration does not describe foetal blood: it has a higher concentration, which adds to oxygen delivery, and concentration is not a molecular property of the haemoglobin itself.
- Foetal haemoglobin does bind 2,3-bisphosphoglycerate poorly, but its oxygen dissociation curve is shifted to the left of the adult curve and not to the right, because a rightward shift means a lower oxygen affinity, so haemoglobin gives oxygen up.
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