Iron is essential to human life and one must take it from several sources besides fortified cereal. The oxygen carriers in vertebrates are the proteins hemoglobin and myoglobin. Hemoglobin also plays a vital role in the transport of carbon dioxide and hydrogen atom. The capacity of these proteins to bind oxygen depends on the presence of a non plypeptide group, called heme.
Heme consists of an organic part and an iron atom.
The organic part, protoporphyrin has 4 nitrogens in the center of its ring , and the iron binds to the 4 nitrogens in a coordinate way. Only ferrohemoglobin can bind oxygen carrying it away from the lungs to the tissue cells, through the arterial blood. Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.