Structure of cytochrome c oxidase. This enzyme consists of 13 polypeptide chains. Notice that most of the complex, as well as two major prosthetic groups (heme a and heme a3–CuB) are embedded in the membrane (α helices represented by vertical tubes). Heme a3–CuB is the site of the reduction of oxygen to water. The CuA/CuA prosthetic group is positioned near the intermembrane space to better accept electrons from cytochrome c. CO(bb) is a carbonyl group of the peptide backbone.
[Drawn from 2OCC.pdb.]