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.
Highlights
Cytochrome bc1 is one of the key enzymes of respiratory and photosynthetic electron transport chains
We proposed that the two populations of SQo reflect two configurations of the Qo site
We seem to face the opposite situation where several seemingly different SQo intermediates have been exposed. The differences concern both the properties of SQo species and the experimental conditions used to trap the SQo intermediates. This certainly does not make it easier for a general reader to follow the progress in understanding the mechanism of ubiquinol oxidation at the Qo site as it leaves space for various interpretations and mechanistic considerations that at this stage do not seem to converge into one generally accepted model of action
Summary
Cytochrome bc is one of the key enzymes of respiratory and photosynthetic electron transport chains. The architecture of the Qo site creates distinctly different conditions for ubiquinol oxidation: the substrate binds in between the two chains of cofactors and can experience simultaneous presence of two redox centres (FeS cluster and haem bL) ready to engage in electron transfers In this case, the two-electron reaction does not need to proceed through the relatively long-lived SQ intermediate. In 1981, de Vries et al [31] reported the detection of a new SQ in antimycin-inhibited submitochondrial particles under conditions of oxidant-induced reduction of haems b initiated by addition of fumarate/succinate to the membranes This SQ signal was antimycin-insensitive but disappeared after addition of British anti-Lewisite—a thiol-containing compound that disrupts the Rieske cluster in cytochrome bc and abolishes activity of the Qo site. The later work by Rich and co-workers [32] showed that under similar experimental conditions this SQ signal was not sensitive to inhibitors that block the activity of the Qo site (myxothiazol, MOA-stilbene or stigmatellin), but at the same time, it was at least partially sensitive to several inhibitors of complex I and II
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