Abstract
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 a(3)-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.