Haemoglobin is a protein that binds oxygen when there is lots of oxygen available and releases oxygen when there is a low partial pressure of oxygen.Many different organisms have different haemoglobins which differ in structure and their affinity for oxygen.Haemoglobin has a quaternary structure. It is made of four polypeptide chains. Two alpha chains and two beta chains are held together by hydrogen bonds and other weak interactions to form a globular molecule. Haemoglobin also has a non-protein haem prosthetic group attached to each polypeptide chain. The haem prosthetic group contains iron, Fe2+, which binds oxygen.