Cytochrome c oxidase is the component of the respiratory chain that catalyzes the reduction of oxygen to water. Subunits 1-3 form the functional core of the enzyme complex. Co I is the catalytic subunit of the enzyme. Electrons originating in cytochrome c are transferred via the copper A center of subunit 2 and heme a of subunit 1 to the bimetallic center formed by heme a3 and copper B. This cytochrome c oxidase shows proton pump activity across the membrane in addition to the electron transfer. 4 Fe(II)-[cytochrome c] + 8 H(+)(in) + O2 = 4 Fe(III)-[cytochrome c] + 4 H(+)(out) + 2 H2O Binds 1 copper B ion per subunit. Binds 2 heme groups per subunit. Energy metabolism; oxidative phosphorylation. Under slightly air-limited conditions, the cytochrome a3 center of the enzyme is replaced by cytochrome o. This cytochrome c oxidase shows clear proton pump activity in addition to electron transfer across the membrane. Belongs to the heme-copper respiratory oxidase family.