Haemoglobin molecule. Computer artwork showing the tertiary structure of a haemoglobin monomer subunit, with its active site at centre. Haemoglobin is a metalloprotein that transports oxygen around the body in red blood cells. Each molecule consists of iron-containing haem groups and globin proteins. Each globin protein (red) is wrapped around a haem group (blue tubes), protecting it from being oxidised and destroyed by the oxygen it is intended to transport. The iron (gold sphere) within each haem group reversibly bonds to oxygen (white) and carbon dioxide in the blood. Four of these monomers usually bind together to form one large haemoglobin molecule.