Two cysteines, two histidines, and one methionine are ligands of a binuclear purple copper center.

Cytochrome c oxidase contains a copper center, CuA, which is involved in electron transfer from cytochrome c to the oxygen-reducing active site. This center is distinct from types 1, 2, and 3 copper sites and related only to a purple copper center in nitrous oxide reductase. At present it is not clear whether this site is mononuclear or is comprised of two copper atoms in a mixed valence (Cu(I)-Cu(II)) configuration. Here we use a model of CuA, engineered into a structurally related but initially copperless protein, to study the structure of this copper center. The results from biochemical analysis, site-directed mutagenesis, and electrospray mass spectrometry support the binuclear model. Two cysteines, two histidines, and one methionine are the major ligands of two coppers. Substitution of these residues results in either a complete loss of color or dramatic changes in the absorbance spectrum. In contrast, substitution of the invariant glutamate residue, which is located between the copper-binding cysteines, leads to a minor perturbation of the optical spectrum.