Cytochrome oxidase contains the following metal ion for transfer of four electrons from cytochrome c to O2:

Reduced cytochrome c is oxidized by Complex IV (cytochrome c oxidase), with the concomitant reduction of O2 to two molecules of water. This transfer of four electrons from cytochrome c to O2 involves two heme groups, a and a3, and Cu. Electrons are passed initially to a Cu center (CuA), which contains 2Cu atoms linked to two protein cysteine-SH groups (resembling an Fe-S), then in sequence to heme a, heme a3, a second Cu center, CuB, which is linked to heme a3, and finally to O2. Ref: Botham K.M., Mayes P.A. (2011). Chapter 13. The Respiratory Chain & Oxidative Phosphorylation. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.