title: Interheme electron tunneling in cytochrome c oxidase.
abstract: Cytochrome c oxidase (CcO) is the terminal enzyme of the respiratory chain that catalyzes respiratory reduction of dioxygen (O(2)) to water in all eukaryotes and many aerobic bacteria. CcO, and its homologs among the heme-copper oxidases, has an active site composed of an oxygen-binding heme and a copper center in the vicinity, plus another heme group that donates electrons to this site. In most oxidoreduction enzymes, electron transfer (eT) takes place by quantum-mechanical electron tunneling. Here we show by independent molecular dynamics and quantum-chemical methods that the heme-heme eT in CcO differs from the majority of cases in having an exceptionally low reorganization energy. We show that the rate of interheme eT in CcO may nevertheless be predicted by the Moser-Dutton equation if reinterpreted as the average of the eT rates between all individual atoms of the donor and acceptor weighed by the respective packing densities between them. We argue that this modification may be necessary at short donor/acceptor distances comparable to the donor/acceptor radii.
authors.affiliation: Helsinki Bioenergetics Group, Structural Biology and Biophysics Programme, Institute of Biotechnology, University of Helsinki, PO Box 65, FI-00014 Helsinki, Finland. ville.kaila@nih.gov
authors.forename: Ville R I
authors.initials: VR
authors.lastname: Kaila
authors.forename: Mikael P
authors.initials: MP
authors.lastname: Johansson
authors.forename: Dage
authors.initials: D
authors.lastname: Sundholm
authors.forename: Mårten
authors.initials: M
authors.lastname: Wikström
mesh_terms: D003300:Copper / Q000737:chemistry; D004579:Electron Transport; D003576:Electron Transport Complex IV / Q000737:chemistry*; D004583:Electrons; D006418:Heme / Q000737:chemistry*; D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D011487:Protein Conformation; D011789:Quantum Theory
chemical_list: D006418:Heme; D003300:Copper; D003576:Electron Transport Complex IV; D010100:Oxygen