Hemoglobin

Hemoglobin is the oxygen-carrying protein that fills red blood cells and gives blood its red color. Adult hemoglobin is built from four protein chains, two alpha-globins and two beta-globins, and each chain holds a heme group with an iron ion at its center. Each iron binds one oxygen molecule, so one hemoglobin molecule can carry four, and a single red blood cell holds about 300 million hemoglobin molecules.

In the lungs, where oxygen is plentiful, hemoglobin loads up to form bright red oxyhemoglobin. In the tissues, where oxygen is scarce, it lets oxygen go and becomes darker deoxyhemoglobin. Hemoglobin also carries part of the carbon dioxide made by the tissues back to the lungs, bound to its protein chains as carbaminohemoglobin, although most carbon dioxide travels in the plasma as bicarbonate. When red blood cells wear out, macrophages break hemoglobin down, recycle the iron and amino acids, and turn the rest of the heme into bilirubin.

A low hemoglobin level is the defining sign of anemia. Changes in the globin genes cause inherited hemoglobin disorders: in sickle cell disease a single amino acid change in beta-globin produces hemoglobin S, which deforms red cells, and in beta thalassemia too little beta-globin is made.