The Breathing Molecule

The haemoglobin molecule is said to "breath' during oxygen uptake and release. Deoxyhaemoglobin is in the taut (T) configuration while the binding of oxygen produces a more relaxed (R) configuration by breaking salt links in the aip2 and a2pi interactions. The first oxygen molecule binds relatively weakly to haemoglobin, since more salt links must be broken

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Figure HA.1 Relationship between globin chains in haemoglobin

(and, therefore, more energy is required) compared with the subsequent two oxygen molecules and less energy still is required for the binding of the fourth oxygen molecule. This sequential increase in oxygen affinity explains the sigmoid shape of the oxygen dissociation curve of haemoglobin (Figure HA.2). Myoglobin being a single chain molecule has a hyperbolic oxygen dissociation curve.

CO2 binds to the terminal amino groups of haemoglobin as bicarbonate, which is formed in the red cell by the action of carbonic anhydrase:

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