2,3-BPG and altitude acclimatisation
Rising 2,3-BPG, a molecule made in red cells, lowers haemoglobin's affinity (how tightly it holds oxygen), so more oxygen is released to the tissues, as after adapting to altitude.
Remember
When tissue oxygen is persistently low, red cells make more 2,3-BPG, so haemoglobin holds oxygen less tightly and gives up more of it to the tissues, as in acclimatisation (adapting to living at high altitude).
Memory aid
more 2,3-BPG, looser grip, more oxygen let go
More 2,3-BPG gives a looser grip on oxygen, so more oxygen is let go to the tissues.
Haemoglobin can hold on to oxygen tightly or loosely. How tightly it holds oxygen is its affinity. A small molecule called 2,3-BPG (2,3-bisphosphoglycerate) is made inside red blood cells as a by-product of glycolysis, the breakdown of glucose for energy.
2,3-BPG binds to the centre of haemoglobin and holds it in the T state, the tense form that holds oxygen weakly. That lowers haemoglobin's affinity for oxygen, so more oxygen is released in the peripheral tissues. On the oxygen dissociation curve, which plots haemoglobin saturation against oxygen pressure, the curve moves to the right.
Red cells make more 2,3-BPG when tissue oxygen is persistently low. A person who has acclimatised (adapted) to living at high altitude, where the air holds less oxygen, shows this. It also happens in congestive heart failure, where blood flow to the tissues is reduced.
The extra oxygen released helps to make up for the shortfall in tissue oxygen. Other adaptations to altitude, such as more red cells, also help, but the rise in 2,3-BPG is one that changes how readily haemoglobin gives up oxygen.
Key facts
- Where it comes from
- made inside red cells as a by-product of glycolysis, the breakdown of glucose for energy
- What it does to haemoglobin
- binds haemoglobin that is not holding oxygen and holds it in the T state, the low-affinity form
- Effect on affinity
- lowers haemoglobin's affinity for oxygen, so more oxygen is released to the tissues
- Effect on the curve
- shifts the oxygen dissociation curve to the right and raises the P50 (the oxygen pressure at which haemoglobin is half saturated)
- When it rises
- when tissue oxygen is persistently low, as in acclimatisation to high altitude or congestive heart failure
- Oxygen capacity
- each haemoglobin molecule can still bind up to four oxygen molecules, because 2,3-BPG changes how tightly oxygen is held and not how many sites there are
Exam traps
- A fall in 2,3-BPG does not move the curve to the right, because less 2,3-BPG shifts it to the left, and acclimatisation raises 2,3-BPG.
- 2,3-BPG does not raise affinity by stabilising haemoglobin that is holding oxygen, because it binds deoxygenated haemoglobin, the form not holding oxygen, and stabilises the low-affinity form.
- 2,3-BPG does not reduce how many oxygen molecules haemoglobin can carry, because it changes how tightly oxygen is held (affinity), not how many sites there are (capacity).
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