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Title: Similar Transition States Mediate the Q-cycle and Superoxide Production by the Cytochrome bc1 Complex

Abstract

The cytochrome bc complexes found in mitochondria, chloroplasts and many bacteria catalyze a critical reaction in their respective electron transport chains. The quinol oxidase (Qo) site in this complex oxidizes a hydroquinone (quinol), reducing two one-electron carriers, a low-potential cytochrome b heme and a ''Rieske'' iron-sulfur cluster. The overall electron transfer reactions are coupled to transmembrane translocation of protons via a ''Q-cycle'' mechanism, which generates proton motive force for ATP synthesis. Since semiquinone intermediates of quinol oxidation are generally highly reactive, one of the key questions in this field is: how does the Qo site oxidize quinol without the production of deleterious side reactions including superoxide production? We attempt to test three possible general models to account for this behavior: (1) The Qo site semiquinone (or quinol:imidazolate complex) is unstable and thus occurs at a very low steady-state concentration, limiting O2 reduction; (2) the Qo site semiquinone is highly stabilized making it unreactive towards oxygen; and (3) the Qo site catalyzes a quantum mechanically-coupled two-electron/two proton transfer without a semiquinone intermediate. Enthalpies of activation were found to be almost identical between the uninhibited Q-cycle and superoxide production in the presence of Antimycin A in wild type. This behavior was alsomore » preserved in a series of mutants with altered driving forces for quinol oxidation. Overall, the data supports models where the rate-limiting step for both Q-cycle and superoxide production are essentially identical, consistent with model 1 but requiring modifications to models 2 and 3.« less

Authors:
; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
897679
Report Number(s):
PNWD-SA-7426
19832; TRN: US0701500
DOE Contract Number:  
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Biological Chemistry, 281(50):38459-38465
Country of Publication:
United States
Language:
English
Subject:
59 BASIC BIOLOGICAL SCIENCES; 60 APPLIED LIFE SCIENCES; 72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; ANTIBIOTICS; BACTERIA; CHLOROPLASTS; CYTOCHROMES; ELECTRON TRANSFER; ELECTRONS; HEME; MITOCHONDRIA; MODIFICATIONS; MUTANTS; OXIDASES; OXIDATION; OXYGEN; PROTONS; SYNTHESIS; TRANSLOCATION; Environmental Molecular Sciences Laboratory

Citation Formats

Forquer, Isaac P., Covian, Raul, Bowman, Michael K., Trumpower, Bernard, and Kramer, David M. Similar Transition States Mediate the Q-cycle and Superoxide Production by the Cytochrome bc1 Complex. United States: N. p., 2006. Web. doi:10.1074/jbc.M605119200.
Forquer, Isaac P., Covian, Raul, Bowman, Michael K., Trumpower, Bernard, & Kramer, David M. Similar Transition States Mediate the Q-cycle and Superoxide Production by the Cytochrome bc1 Complex. United States. doi:10.1074/jbc.M605119200.
Forquer, Isaac P., Covian, Raul, Bowman, Michael K., Trumpower, Bernard, and Kramer, David M. Fri . "Similar Transition States Mediate the Q-cycle and Superoxide Production by the Cytochrome bc1 Complex". United States. doi:10.1074/jbc.M605119200.
@article{osti_897679,
title = {Similar Transition States Mediate the Q-cycle and Superoxide Production by the Cytochrome bc1 Complex},
author = {Forquer, Isaac P. and Covian, Raul and Bowman, Michael K. and Trumpower, Bernard and Kramer, David M.},
abstractNote = {The cytochrome bc complexes found in mitochondria, chloroplasts and many bacteria catalyze a critical reaction in their respective electron transport chains. The quinol oxidase (Qo) site in this complex oxidizes a hydroquinone (quinol), reducing two one-electron carriers, a low-potential cytochrome b heme and a ''Rieske'' iron-sulfur cluster. The overall electron transfer reactions are coupled to transmembrane translocation of protons via a ''Q-cycle'' mechanism, which generates proton motive force for ATP synthesis. Since semiquinone intermediates of quinol oxidation are generally highly reactive, one of the key questions in this field is: how does the Qo site oxidize quinol without the production of deleterious side reactions including superoxide production? We attempt to test three possible general models to account for this behavior: (1) The Qo site semiquinone (or quinol:imidazolate complex) is unstable and thus occurs at a very low steady-state concentration, limiting O2 reduction; (2) the Qo site semiquinone is highly stabilized making it unreactive towards oxygen; and (3) the Qo site catalyzes a quantum mechanically-coupled two-electron/two proton transfer without a semiquinone intermediate. Enthalpies of activation were found to be almost identical between the uninhibited Q-cycle and superoxide production in the presence of Antimycin A in wild type. This behavior was also preserved in a series of mutants with altered driving forces for quinol oxidation. Overall, the data supports models where the rate-limiting step for both Q-cycle and superoxide production are essentially identical, consistent with model 1 but requiring modifications to models 2 and 3.},
doi = {10.1074/jbc.M605119200},
journal = {Journal of Biological Chemistry, 281(50):38459-38465},
number = ,
volume = ,
place = {United States},
year = {Fri Dec 15 00:00:00 EST 2006},
month = {Fri Dec 15 00:00:00 EST 2006}
}