Both myoglobin and hemoglobin contain a prosthetic group . Prosthetic groups are non-polypeptide groups. There are many different types. They are often found tightly bound to proteins and are usually required for activity. A protein that is missing its prosthetic group is called an apoprotein .
Many enzymes also contain prosthetic groups.
The prosthetic group in myoglobin and hemoglobin is called the heme . The iron is coordinated at four positions by heterocyclic nitrogens present on the protoporphyrin ring. Called ferrihemoglobin . Also called methemoglobin. Only the ferrous form is capable of transporting oxygen.