part of mitochondrion mitochondrial respiratory chain complex IV A protein complex located in the mitochondrial inner membrane that forms part of the mitochondrial respiratory chain. Contains the 13 polypeptide subunits of cytochrome c oxidase, including cytochrome a and cytochrome a3. Catalyzes the oxidation of reduced cytochrome c by dioxygen (O2). GO:0005751 GO:0005752 cellular_component go.owl pro.owl respiratory chain complex IV inner mitochondrial membrane protein complex