Ubiquinone (Coenzyme Q) and the Respiratory Chain

Abstract

UBIQUINONE (coenzyme Q, Co Q), a lipid soluble benzoquinone which is localized principally in mitochondria^1,2, has been shown to undergo cyclic oxidation-reduction during the oxidation of substrates of the citric acid cycle³. Kinetic investigations^4,5 of the redox reactions, however, have shown that its rate of reduction is insufficient to account for the total electron flux through the system. Treatment of mitochondrial particles with acetone, which extracts ubiquinone, has been claimed to result in a loss of electron transport activity which could not be restored without the addition of ubiquinone or one of its homologues⁶. These experiments showed that ubiquinone functions as an electron carrier in the respiratory chain, but while the kinetic data suggest that it has a position in a branch pathway⁷, the extraction-reactivation experiments have been taken as proof of its obligatory involvement in a single sequence of electron transfer components⁸. In the course of a detailed investigation of the acetone extraction technique we have now obtained direct evidence that electron transport can occur in mitochondrial particles from which all the ubiquinone has been removed. This provides additional support for the concept that electron transfer occurs through a branched chain system with one pathway dependent on ubiquinone and at least one other pathway in which ubiquinone does not participate.