Myoglobin is a heme containing protein responsible for storing oxygen. The heme in myoglobin is in the form of an iron complexed with protoporphyrin IX. The protection of an oxygen-binding metal from irreversible oxidation is the functional reason for the existence of myoglobin and hemoglobin. These two molecules provide environments in which the first step of an oxidation reaction (the binding of oxygen) is permitted, but the final step (oxidation) is blocked.