Hemoglobin. Computer graphic of the hemoglobin molecule. Hemoglobin is the oxygen-carrying pigment in red blood cells. It consists of four globin polypeptide chains (alpha globin = blue, beta globin = yellow), each carrying a heme group (white) with a central iron atom, which binds to oxygen. Hemoglobin transports oxygen around the body by binding reversibly to oxygen when the local concentration is high (in the lungs) and releasing the oxygen when the local concentration falls (in other body tissues). The green structure is the amino acid glutamic acid at residue 6 on the beta chain. In sickle cell anemia this is replaced by valine due to a mutation.