The haemoglobin molecule is made of 4 polypeptide chains
Each has a haem group attached, called the prosthetic group.
In the centre of the haem group is the iron ion associating with one oxygen molecule.
BUT for haemoglobin, it can carry 4 oxygen molecules because there are 4 polypeptide chains so 4 haem groups.
As the first haem group combines to an oxygen molecule, the shape of the protein changes exposing the next haem group making it easier to pick up another oxygen molecule.
This is known as conformational change.
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