Hemoglobin

Haemoglobin (or hemoglobin) is the iron-containing oxygen-transport chemical in the red cells of the blood in mammals and other animals. (Some marine organisms use a different compound that contains copper instead of iron.)

The molecule is mostly protein: mutations in the gene for the haemoglobin protein result in the hereditary diseases sickle cell anaemia and thalassaemia, as well as a group of diverse but rare diseases called hemoglobinopathies.

At the core of the molecule is a heterocyclic ring, known as a porphyrin which holds an iron atom; this iron atom is responsible for the molecule's oxygen binding properties.

In adult humans, hemoglobin is a tetramer, consisting of two alpha and two beta subunits, and each subunit has a molecular weight of about 16,000, for a total molecular weight in the tetramer of about 64,000. In the tetrameric form of normal adult hemoglobin, the binding of oxygen is a cooperative process, with the binding affinity of hemoglobin for oxygen affected by the oxygen saturation of the molecule. As a consequence, the oxygen binding curve of hemoglobin is sigmoidal, or 'S' shaped, as opposed to the normal hyperbolic (noncooperative) curve.

The binding of oxygen as well is affected by molecules such as 2,3-diphosphoglycerate, which lowers the affinity of hemoglobin for oxygen. In people acclimated to high altitudes, the concentration of 2,3-diphosphoglycerate in the blood is increased, which allows these individuals to deliver a larger amount of oxygen to tissues under conditions of lower oxygen tension. This phenomenon, where molecule Y affects the binding of molecule X to a transport molecule H, is called a heterotropic allosteric effect.

When red cells reach the end of their life, they are broken down, and the haemoglobin molecule broken up and the iron recycled. When the porphyrin ring is broken up, the fragments are normally secreted in the bile by the liver. There is a genetic disorder, known as porphyria in which this mechanism fails to work properly. King George III of the United Kingdom was probably the most famous sufferer from this disease.