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

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Abstract
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 (CuB) 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 (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 a3 iron and moved to a temporary binding site midway between the CuB 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.
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Funding Information
  • Division of Chemistry (CHE-1404929)
  • National Center for Advancing Translational Sciences (P41RR001209)
  • Division of Biological Infrastructure (1231306, 1565180)
  • NIH Office of the Director (S10 OD023453)
  • Basic Energy Sciences (DE-AC02-76SF00515)
  • Arizona State University
  • National Institute of General Medical Sciences (GM095583, GM115773, GM126297, P41 GM103393, P41 GM139687)

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