Hemoglobin

Hemoglobin is the iron-containing protein in red blood cells that carries oxygen from the lungs to the tissues and returns some carbon dioxide on the way back. It gives blood its red color, and each red cell holds some 270 million molecules of it.

Adult hemoglobin is a tetramer of four chains, two alpha and two beta, and each chain holds one heme group with an iron atom that binds one oxygen molecule. Max Perutz solved the structure by X-ray crystallography over a period of decades, and he shared the 1962 Nobel Prize in Chemistry with John Kendrew. Fetal hemoglobin has gamma chains in place of beta chains and binds oxygen more tightly, which helps oxygen to pass across the placenta.

Binding of oxygen to one subunit changes the shape of the others and makes them bind more easily, so hemoglobin loads fully in the lungs and unloads readily in tissues. This cooperative behavior is the classic example of allosteric regulation. Carbon dioxide, acid and the molecule 2,3-bisphosphoglycerate lower its affinity for oxygen, which is known as the Bohr effect.

A single amino acid change in the beta chain causes sickle cell disease, and reduced production of chains causes the thalassemias. Hemoglobin variants are common where malaria has been widespread, because they protect against the parasite. The globin genes arose by gene duplication, and similar oxygen carriers are found in many animals, with hemolymph using other pigments in some invertebrates.

• Heme — the iron-containing group
• Sickle Cell Disease — a disease of the beta chain
• Allosteric Regulation — the principle behind its binding curve