Abstract:
:The cytochrome bc1 and related complexes are essential energy-conserving components of mitochondrial and bacterial electron transport chains. They orchestrate a complex sequence of electron and proton transfer reactions resulting in the oxidation of quinol, the reduction of a mobile electron carrier, and the translocation of protons across the membrane to store energy in an electrochemical proton gradient. The enzyme can also catalyze substantial rates of superoxide production, with deleterious physiological consequences. Progress on understanding these processes has been hindered by the lack of observable enzymatic intermediates. We report the first direct detection of a semiquinone radical generated by the Q(o) site using continuous wave and pulsed EPR spectroscopy. The radical is a ubisemiquinone anion and is sensitive to both specific inhibitors and mutations within the Q(o) site as well as O2, suggesting that it is the elusive intermediate responsible for superoxide production. Paramagnetic interactions show that the new semiquinone species is buried in the protein, probably in or near the Q(o) site but not strongly interacting with the 2Fe2S cluster. The semiquinone is substoichiometric, even with conditions optimized for its accumulation, consistent with recently proposed models where the semiquinone is destabilized to limit superoxide production. The discovery of this intermediate provides a critical tool to directly probe the elusive chemistry that takes place within the Q(o) site.
journal_name
Proc Natl Acad Sci U S Aauthors
Cape JL,Bowman MK,Kramer DMdoi
10.1073/pnas.0702621104subject
Has Abstractpub_date
2007-05-08 00:00:00pages
7887-92issue
19eissn
0027-8424issn
1091-6490pii
0702621104journal_volume
104pub_type
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