The pathway of CO binding to cytochrome c oxidase. Can the gateway be closed?

Addition of cyanide to the CO complex of cytochrome oxidase reduces the apparent photosensitivity of the Fe-CO bond. This effect is not seen with azide, or when cyanide is added to ferromyoglobin-CO. It is proposed that cyanide binds to CuB, and restricts the passage of CO out of the protein. This restriction favors geminate recombination of CO and ferrocytochrome a3, thereby lowering the apparent quantum yield for CO photolysis. The apparent Kd of cyanide for CuB is 15.4 mM. These data support a direct role for CuB in ligand binding by cytochrome c oxidase.