Focus protein: Haemoglobin
Carrying oxygen around the body
Oxygen moves out of your lungs dissolved in your blood. Haemoglobin, a globular protein, contains special oxygen carriers. Each protein contains four haem groups, an iron atom cradled in a ring of four nitrogen atoms. This allows blood to carry 70 times as much oxygen as blood plasma alone.
Like many globular proteins, the globin chain has stretches of alpha helix, which the protein chain coils around itself under the influence of electrostatic bonds, hydrogen bonds and van der Waals forces between amino acid atoms. The alpha helix, a very common secondary structure, is more tightly wound than the triple helix of collagen.
The way that the four subunits of haemoglobin interact, its quaternary structure, holds the key to its function. The four subunits each contain one oxygen-binding site. When the first iron atom takes up an oxygen molecule in the blood capillaries surrounding the lungs, that subunit changes shape. The other three then shift and ‘grab hold’ of any passing oxygen. In the tissues that need the oxygen, the process is reversed. Iron gives the protein and blood its dark red colour.
Horse haemoglobin was the second protein for which scientists solved the 3D structure. The first, myoglobin, binds oxygen in muscle tissue and has a very similar structure but just one amino acid chain. Many forms of haemoglobin and related proteins have been sequenced – in animals, plants and even bacteria. Comparing the sequences indicates that they all descended from a single ancestral globin gene, which first evolved billions of years ago.
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