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Arrangement of the hemes a and a3:CuB and CuA in CcO. The binuclear CuA center is located in Cox2 subunit (transparent green) and is the entrance site for electrons from reduced cytochrome c. Electrons are subsequently passed to the low-spin, bis-His heme a and then to the heterobimetallic heme a3:CuB center in Cox1 (transparent grey) where O2 reduction occurs. The axial ligands to the heme iron are highlighted along with respective residue numbers and subunits (PDB ID: 2OCC numbering).
