X-ray structure of the NO-bound Cu_B in bovine cytochrome c oxidase

The X-ray crystallographic structure of nitric oxide-treated bovine heart cytochrome c oxidase (CcO) in the fully reduced state has been determined at 50 K under light illumination. In this structure, nitric oxide (NO) is bound to the CcO oxygen-reduction site, which consists of haem and a Cu atom (the haem a₃–Cu_B site). Electron density for the NO molecule was observed close to Cu_B. The refined structure indicates that NO is bound to Cu_B in a side-on manner.