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

Cytochrome coxidase (CcO) is a large membrane-bound hemeprotein that catalyzes the reduction of dioxygen to water. Unlike classical dioxygen binding hemeproteins with a heme bgroup in their active sites, CcO has a unique binuclear center (BNC) composed of a copper atom (CuB) and a heme a3iron, where O2binds and is reduced to water. CO is a versatile O2surrogate 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 a3iron leads to a metastable intermediate (CuB-CO), where CO is bound to CuB, 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 a3iron and moved to a temporary binding site midway between the CuBand the heme a3iron, while the locations of the two metal centers and the conformation of Helix-X, housing the proximal histidine ligand of the heme a3iron, 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.