Hemoglobin along with its close relative myoglobin are among the most important proteins in the body. Oxygen is a highly reactive molecule, and there are no amino acid side chains suited for the reversible binding of oxygen. There are, however, transition metals such as iron and copper that have a strong tendency to bind oxygen.
Heme is responsible for the characteristic red color of blood.
Hemoglobin is a tetrameric protein that contains four heme prosthetic. The R state has a much higher affinity for oxygen than the T state. In the absence of oxygen, however, the T state is more stable and conseqently the predominant conformation of hemoglobin. Hemoglobin is very important because it is the oxygen carrier in the blood. The hemoglobin tetramer can bind a total of four oxygen molecules.