Detection of a Geminate Photoproduct of Bovine Cytochrome c Oxidase by Time-Resolved Serial Femtosecond Crystallography

Cytochrome c oxidase (CcO) is a large membrane-bound hemeprotein that catalyzes the reduction of dioxygen to water. Unlike classical dioxygen binding hemeproteins with a heme b group in their active sites, CcO has a unique binuclear center (BNC) composed of a copper atom (Cu_B) and a heme a₃ iron, where O₂ binds and is reduced to water. CO is a versatile O₂ surrogate in ligand binding and escape reactions. Previous time-resolved spectroscopic studies of the CO complexes of bovine CcO (bCcO) revealed that photolyzing CO from the heme a₃ iron leads to a metastable intermediate (Cu_B-CO), where CO is bound to Cu_B, before it escapes out of the BNC. Here, with a pump-probe based time-resolved serial femtosecond X-ray crystallography, we detected a geminate photoproduct of the bCcO-CO complex, where CO is dissociated from the heme a₃ iron and moved to a temporary binding site midway between the Cu_B and the heme a₃ iron, while the locations of the two metal centers and the conformation of Helix-X, housing the proximal histidine ligand of the heme a₃ iron, remain in the CO complex state. This new structure, combined with other reported structures of bCcO, allows for a clearer definition of the ligand dissociation trajectory as well as the associated protein dynamics.