Haemoglobin is called conjugated protein because it consists of a basic protein globulin and a non-protein called haem. The haemoglobin, when exposed to high partial pressure of oxygen, combines with it to form oxyhaemoglobin, which carries four molecules of oxygen loosely bound to the four Fe2+ ions. When this oxyhaemoglobin reaches the tissues where there is low oxygen pressure, oxyhaemoglobin dissociates into oxygen and deoxyhemoglobin.
