title: Metastable radical state, nonreactive with oxygen, is inherent to catalysis by respiratory and photosynthetic cytochromes bc1/b6f.
abstract: Oxygenic respiration and photosynthesis based on quinone redox reactions face a danger of wasteful energy dissipation by diversion of the productive electron transfer pathway through the generation of reactive oxygen species (ROS). Nevertheless, the widespread quinone oxido-reductases from the cytochrome bc family limit the amounts of released ROS to a low, perhaps just signaling, level through an as-yet-unknown mechanism. Here, we propose that a metastable radical state, nonreactive with oxygen, safely holds electrons at a local energetic minimum during the oxidation of plastohydroquinone catalyzed by the chloroplast cytochrome b6f This intermediate state is formed by interaction of a radical with a metal cofactor of a catalytic site. Modulation of its energy level on the energy landscape in photosynthetic vs. respiratory enzymes provides a possible mechanism to adjust electron transfer rates for efficient catalysis under different oxygen tensions.
authors.affiliation: Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Krakow, Poland.
authors.forename: Marcin
authors.identifier: 0000-0001-6813-5997
authors.initials: M
authors.lastname: Sarewicz
authors.affiliation: Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Krakow, Poland.
authors.forename: Łukasz
authors.initials: Ł
authors.lastname: Bujnowicz
authors.affiliation: Department of Biological Sciences, Purdue University, West Lafayette, IN 47907.
authors.forename: Satarupa
authors.initials: S
authors.lastname: Bhaduri
authors.affiliation: Department of Biological Sciences, Purdue University, West Lafayette, IN 47907.
authors.forename: Sandeep K
authors.initials: SK
authors.lastname: Singh
authors.affiliation: Department of Biological Sciences, Purdue University, West Lafayette, IN 47907.
authors.forename: William A
authors.initials: WA
authors.lastname: Cramer
authors.affiliation: Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Krakow, Poland; artur.osyczka@uj.edu.pl.
authors.forename: Artur
authors.initials: A
authors.lastname: Osyczka
mesh_terms: D002384:Catalysis; D045346:Cytochrome b6f Complex / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D014450:Electron Transport Complex III / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry; D010788:Photosynthesis; D016354:Rhodobacter capsulatus; D018724:Spinacia oleracea
chemical_list: D045346:Cytochrome b6f Complex; D014450:Electron Transport Complex III; D010100:Oxygen
keywords: cytochrome b6f; electron paramagnetic resonance; electron transport; reactive oxygen species; semiquinone