Coordinated iron and copper ions serve vital rolls in the enzyme complexes of the respiratory chain. One important type of prosthetic group are iron-sulfur clusters in which iron is coordinated to sulfhydral groups of cycsteine as well as inorganic sulfides. In these groups the iron atoms interchange from the reduced form, Fe2+ , to the oxidized form, Fe3+. Three of the enzyme complexes of the respiratory chain are cytochromes, which means they are electron transferring proteins containing heme groups. A heme group consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Reduction of the iron corresponds to electron delocalization over the entire porphyrin network.