An EPR study of the photodissociation reactions of oxidised cytochrome c oxidase-nitric oxide complexes.

Complexes of oxidised cytochrome c oxidase with NO in the absence and presence of ligands such as formate, fluoride and cyanide are photodissociable. After photodissociation at 10 K the EPR spectrum of the high-spin cytochrome a3+3 in the absence of ligands or in the presence of fluoride or formate disappears - as does the EPR spectrum of the low-spin cytochrome a3+3 in the presence of cyanide. The action spectra of the photodissociation reaction of these complexes show slight differences but all have maxima at 640-660 nm and below 400 nm, and are assigned to a diamagnetic Cu+B-NO+ complex. The differences in the action spectra in the presence of various ligands are due to binding of these anions to the cytochrome (a3-CuB) couple. The disappearance of the cytochrome a3 signal upon photodissociation of the Cu+B-NO+ complex is explained by a magnetic interaction between cytochrome a3+3 and Cu2+B in the photodissociated complex. The temperature at which NO recombines with Cu2+B is about 30 K and slightly affected by the presence of added ligands. It is suggested that in the oxidised ligand-cytochrome c oxidase complexes the coupling ligand between cytochrome a3+3 and Cu2+B is cyanide, fluoride and formate. The observation that two ligands may bind simultaneously to the cytochrome a3-CuB couple leads to further support for the notion that during turnover of cytochrome c oxidase both metal ions are involved in binding and reduction of oxygen.