Iron. It is at the center of the structure. And because they'll delete my answer if I don't add on: Hemoglobin is the oxygen binding protein of red blood cells and is a globular protein with quaternary structure. Hemoglobin consists of four polypeptide subunits; 2 alpha chains and two beta chains. Hemoglobin transports oxygen in the blood from the lungs to the rest of the body.
