Hemoglobin carries oxygen from the lungs to the tissues. Four globin chains hold four hemes, and each heme binds one oxygen molecule. The four sites are not independent. The tetramer switches between a low-affinity T state and a high-affinity R state, so the binding curve is sigmoid. Acid, carbon dioxide and 2,3-BPG push the switch toward T. The molecule then releases oxygen where tissues need it.
- Where
- Cytoplasm of red blood cells, at about 5 mM tetramer. Related globins work in muscle, nerve and many invertebrates and plants.
- Power
- None. Hemoglobin spends no chemical fuel. The oxygen gradient and the allosteric switch do the work.
4
Chains in the working unit
126.6 mmHg
P50, standard conditions
18