the sixth site available to bind dioxygen (molecular oxygen). In addition, this enzyme binds 3 copper, magnesium, zinc, and several potassium and sodium ions. The two heme A groups in CCO are thought to readily exchange electrons between each other, the copper ions and the closely associated protein cytochrome c.

Both the formyl group and the isoprenoid side chain are thought to play important roles in conservation of the energy of oxygen