There are four hemes in the large structure of hemoglobin, one per chain.
Shown is the heme cofactor present in every hemoglobin protein. The heme is a vital part of the molecule that binds oxygen molecules for transport
around throughout the bloodstream. Although the iron ion at the core binds to oxygen, the rest of the surrounding structure is there to protect and
stabilize the binding site. Lack of iron is a cause of anemia, which develops due to a low number of red blood cells (and hemoglobin) in the blood.
In the window on the right, red represents oxygen atoms, gray represents carbon, white represents hydrogen, blue represents nitrogen, and orange
represents the critical iron ion.