Cytochrome C with heme.

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. Reduction of the iron corresponds to electron delocalization over the entire porphyrin network.

The design of cytochrome c is more than a billion years old, common to all eukaryotic cells.