Haemoglobin is the oxygen transporter in erythrocytes. It consists of four polypeptide
chains chains held together by noncovalent interactions. Each chain contains
a haem group and one oxygen binding site therefore each haemoglobin molecule
can bind four oxygen molecule to itself. Haem is a porphyrin, it contains a
tetrapyrrole ring with an iron atom located at the center of this ring.