Illustration of a haemoglobin molecule (HbAB). Haemoglobin is composed of four subunits, two alpha (pink surface) and two beta (blue surface). HbAB is the main functional constituent of the red blood cell, serving as the oxygen-carrying protein. In the centre, the bisphosphoglyceric acid (BGP) binding site is shown (yellow surface). When bound, BPG decreases the affinity for oxygen of HbAB. The figure also shows the haem groups of beta chains, in rod model, occupied by oxygen and carbon monoxide molecules.