X-ray structure of azide-bound fully oxidized cytochrome c oxidase from bovine heart at 2.9 Å resolution

Two azide ions were identified, one between the Fe and Cu atoms in the O₂-reduction site and the other at the transmembrane surface of the enzyme, in the crystal structure of the azide-bound form of bovine heart cytochrome c oxidase at 2.9 Å resolution. Two geometries, a μ-1,3 type geometry between the Fe and Cu atoms and a terminal geometry on the Fe atom, are equally possible for an azide ion in the O₂-­reduction site. The other azide molecule was hydrogen bonded to an amide group of an asparagine and a hydroxyl group of tyrosine in a μ-1,1 type geometry. The antisymmetric infrared bands arising from these azide ions, which show essentially identical intensity [Yoshikawa & Caughey (1992), J. Biol. Chem. 267, 9757–9766], strongly suggest terminal binding of the azide to Fe. The electron density of all three imidazole ligands to Cu_B was clearly seen in the electron-density map of the azide-bound form of bovine heart enzyme, in contrast to the crystal structure of the azide-bound form of the bacterial enzyme [Iwata et al. (1995), Nature (London), 376, 660–669], which lacks one of the three imidazole ligands to Cu_B.