Effects of inhibitory ligands on the aerobic carbon monoxide complex of cytochrome c oxidase

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Abstract
In the presence of both CO and O2, ox heart cytochrome c oxidase forms a 607 nm peak intermediate distinct from both the cytochrome a2+a32+CO and the cytochrome a3+a32+CO (mixed-valence) CO complexes. This aerobic CO compound is stable towards ferricyanide addition, but decomposed on treatment with ferric cytochrome a3 ligands such as formate, cyanide and azide. Addition of formate or cyanide induces transfer of the reducing equivalent to cytochrome a, but addition of azide gives rise to a complex with .alpha.-peak at 598 nm, not identical with any azide complex of the free enzyme, but possibly a cytochrome a32+NO complex produced by oxidative attack of partially reduced O2 on the azide. Although the initial reaction of O2 is with cytochrome a32+, the next step is not an oxidation of the ferrous cytochrome a3, but a transfer of O2 to a neighboring group, such as Cu+, to give Cu2+O2- or similar complexes. The aerobic CO complex is then identified as a3+a32+COCu2+O2-; a similar compound (Compound C) is formed by photolysis of a3+a32+CO (the mixed-valence CO complex) in the presence of O2 at low temperatures.