Stoichiometry and redox behaviour of metals in cytochrome‐c oxidase

The early observation of extra copper in preparations of cytochrome‐c oxidase has recently lead to a renewed interest in its stoichiometry and possible redox function. In various, pure preparations (heme A contents close to the theoretical value of 9.79 nmol/mg protein for the 13‐subunit bovine enzyme) protein‐related metal stoichiometries of 3 Cu, 2 Fe, 1 Zn, 1 Mg/monomer with M_r 204266 were determined. Despite the presence of five potential redox metal ions, reductive and reoxidative titrations indicate the presence of only four one‐electron‐accepting/donating species in the ligand‐free enzyme. Participation of two copper ions in a binuclear copper site acting as a one‐electron acceptor may explain both the observed copper stoichiometry and the redox behaviour. The homology of the C‐terminal sequence of subunit II with one of the copper‐binding sites in nitrous‐oxide reductases provides possible ligands for complexing two copper ions in a binuclear center.