title: Distinct properties of semiquinone species detected at the ubiquinol oxidation Qo site of cytochrome bc1 and their mechanistic implications.
abstract: The two-electron ubiquinol oxidation or ubiquinone reduction typically involves semiquinone (SQ) intermediates. Natural engineering of ubiquinone binding sites of bioenergetic enzymes secures that SQ is sufficiently stabilized, so that it does not leave the site to membranous environment before full oxidation/reduction is completed. The ubiquinol oxidation Qo site of cytochrome bc1 (mitochondrial complex III, cytochrome b6f in plants) has been considered an exception with catalytic reactions assumed to involve highly unstable SQ or not to involve any SQ intermediate. This view seemed consistent with long-standing difficulty in detecting any reaction intermediates at the Qo site. New perspective on this issue is now offered by recent, independent reports on detection of SQ in this site. Each of the described SQs seems to have different spectroscopic properties leaving space for various interpretations and mechanistic considerations. Here, we comparatively reflect on those properties and their consequences on the SQ stabilization, the involvement of SQ in catalytic reactions, including proton transfers, and the reactivity of SQ with oxygen associated with superoxide generation activity of the Qo site.
authors.affiliation: Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, Kraków, Poland.
authors.forename: Rafał
authors.identifier: 0000-0001-8424-6590
authors.initials: R
authors.lastname: Pietras
authors.affiliation: Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, Kraków, Poland.
authors.forename: Marcin
authors.initials: M
authors.lastname: Sarewicz
authors.affiliation: Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, Kraków, Poland artur.osyczka@uj.edu.pl.
authors.forename: Artur
authors.identifier: 0000-0002-1637-2365
authors.initials: A
authors.lastname: Osyczka
mesh_terms: D016227:Benzoquinones / Q000378:metabolism*; D014450:Electron Transport Complex III / Q000378:metabolism*; D010084:Oxidation-Reduction; D010940:Plant Proteins / Q000378:metabolism*; D010944:Plants / Q000378:metabolism*; D014451:Ubiquinone / Q000031:analogs & derivatives* / Q000378:metabolism
chemical_list: D016227:Benzoquinones; D010940:Plant Proteins; D014451:Ubiquinone; C025232:semiquinone radicals; D014450:Electron Transport Complex III; C003741:ubiquinol
keywords: Q cycle; complex III; cytochrome bc1; mitochondria; semiquinone; ubiquinol oxidation