Component of the cytochrome c oxidase, the last enzyme in the mitochondrial electron transport chain which drives oxidative phosphorylation. The respiratory chain contains 3 multisubunit complexes succinate dehydrogenase (complex II, CII), ubiquinol-cytochrome c oxidoreductase (cytochrome b-c1 complex, complex III, CIII) and cytochrome c oxidase (complex IV, CIV), that cooperate to transfer electrons derived from NADH and succinate to molecular oxygen, creating an electrochemical gradient over the inner membrane that drives transmembrane transport and the ATP synthase. Cytochrome c oxidase is the component of the respiratory chain that catalyzes the reduction of oxygen to water. Electrons originating from reduced cytochrome c in the intermembrane space (IMS) are transferred via the dinuclear copper A center (CU(A)) of subunit 2 and heme A of subunit 1 to the active site in subunit 1, a binuclear center (BNC) formed by heme A3 and copper B (CU(B)). The BNC reduces molecular oxygen to 2 water molecules using 4 electrons from cytochrome c in the IMS and 4 protons from the mitochondrial matrix. 4 Fe(II)-[cytochrome c] + 8 H(+)(in) + O2 = 4 Fe(III)-[cytochrome c] + 4 H(+)(out) + 2 H2O Component of the cytochrome c oxidase (complex IV, CIV), a multisubunit enzyme composed of a catalytic core of 3 subunits and several supernumerary subunits. The complex exists as a monomer or a dimer and forms supercomplexes (SCs) in the inner mitochondrial membrane with ubiquinol-cytochrome c oxidoreductase (cytochrome b-c1 complex, complex III, CIII). A stretch of 270 kb of the mitochondrial genome is duplicated within the centromere of chromosome 2 resulting in the duplication of the gene. The expression of this duplicated gene (At2g07687) is not demonstrated. It is also probably not RNA edited and therefore differs in all the positions known to be edited. Belongs to the cytochrome c oxidase subunit 3 family.