haemoglobin

One of a group of globular proteins occurring widely in animals as oxygen carriers in blood. Vertebrate haemoglobin comprises two pairs of polypeptide chains, known as α-chains and β-chains (forming the globin protein), with each chain folded to provide a binding site for a haem group. Each of the four haem groups binds one oxygen molecule to form oxyhaemoglobin. Dissociation occurs in oxygen-depleted tissues: oxygen is released and haemoglobin is reformed. The haem groups also bind other inorganic molecules, including carbon monoxide (to form carboxyhaemoglobin). In vertebrates, haemoglobin is contained in the red blood cells (erythrocytes).