Perturbation of the Cu_A Site in Cytochrome‐c Oxidase of Paracoccus denitrificans by Replacement of Met227 with Isoleucine

Subunit II of cytochrome‐c oxidase contains a redox centre, Cu_A, with unusual spectroscopic properties; this site consists of two copper atoms and acts as the entry point for electrons from cytochrome c. We have constructed a site–directed mutant of cytochrome‐c oxidase from Paracoccus denitrificans in which the Cu_A site has been disturbed by replacement of Met227 with isoleucine. The purified, fully assembled enzyme complex has been investigated with various techniques including metal analysis, EPR and visible spectroscopies, steady‐state and fast kinetics. The stoichiometry of the metals in the enzyme remains unchanged but a clear perturbation of the Cu_A site can be observed in the EPR and near‐infrared optical spectra. It is concluded that in the mutant Cu_A is still binuclear but that the two nuclei are no longer equivalent, converting the delocalized [Cu(1.5).…Cu(1.5)] centre of the wild type into a localized [Cu(I).…Cu(II)] system. Changes in the overall kinetics of the mutant are correlated with a diminished electron transfer rate between Cu_A and heme a.