Cytochrome C oxidase is a molecule that none of us air-breathing creatures can live without. It's also really interesting. This protein complex is a dimer of two smaller complexes. Each of the smaller complexes contains 13 different proteins and two heme groups.
Oxidized Cytochrome C Oxidase. Each protein chain is colored differently. Arrows point to the active site in subunit I.
The two heme groups are both part of the cytochrome c oxidase subunit I protein. The DNA sequence of this protein is used for many types of DNA barcoding.