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Protein structure: Haemoglobin

Haemoglobin is a globular protein formed of four polypeptide chains. The chains are held together by hydrogen bonds, ionic bonds and hydrophobic interactions.

Haemoglobin is the oxygen transport protein present in red blood cells.

The blue and red ribbons represent folded polypeptide chains in haemoglobin (left), each associated with a haem group (magnified on the right).
The blue and red ribbons represent folded polypeptide chains in haemoglobin (left), each associated with a haem group (magnified on the right).

Each of the polypeptide chains is tightly associated with a haem group which is a non-protein molecule. Haem is a hydrocarbon ring with an iron ion in the centre.

Haemoglobin contains four haem groups, and each of these groups can carry one oxygen molecule. Overall, haemoglobin can carry up to four oxygen molecules.

Haem binds to oxygen at high concentrations of the gas, such as in the blood vessels of the lungs, and then releases it elsewhere in the body.

Haemoglobin is water-soluble. The solubility is achieved through the arrangement of the amino acids in the tertiary structure. Amino acids with hydrophilic R-groups are arranged around the outside of the protein.