Cytochrome c showing the Lys residues involved in intermolecular complex formation. The heme group (represented as a red bar in the thumbnail) along with the protein side chains that ligand it are rendered as balls and sticks, with the iron colored orange. Dark and light blue balls, respectively, mark the positions of Lys residues whose ε-amino groups are strongly and less strongly protected by cytochrome c₁ or cytochrome c oxidase against acetylation. Note that these Lys residues form a ring around the heme complex on one face of the protein. Change the Show Lysine Ring button to better see the Lysine ring. [Based on an X-ray structure by Harry Gray, Caltech.]