STOICHIOMETRY AND REDOX BEHAVIOR 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 a nd possible redox function. In various, pure preparations (heme A cont ents 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 th e presence of five potential redox metal ions, reductive and reoxidati ve titrations indicate the presence of only four one-electron-acceptin g/donating species in the ligand-free enzyme. Participation of two cop per ions in a binuclear copper site acting as a one-electron acceptor may explain both the observed copper stoichiometry and the redox behav iour. The homology of the C-terminal sequence of subunit II with one o f the copper-binding sites in nitrous-oxide reductases provides possib le ligands for complexing two copper ions in a binuclear center.