Prooxidant functions of coenzyme Q.

Abstract

Ubiquinone (oxidized coenzyme Q, Q, Q10) shares its biological role in membrane-associated redox reactions with a variety of other redox carriers such as dehydrogenases, non-heme-iron proteins, and cytochromes. The cooperation of this biological quinone with respiratory enzymes is often emphasized through the synonymous name “coenzyme Q.” Peculiarities arise from the lack of transition metals, which in contrast to the electron carriers mentioned above, do not participate in redox-shuttle activities of coenzyme Q. Another peculiarity is the lipophilicity of coenzyme Q, which allows free movement between reductants and oxidants of a membrane. The chemistry of ubiquinone reduction and ubiquinol (reduced coenzyme Q, QH2, Q10H2) oxidation requires the stepwise acceptance and transfer of two single electrons associated with the addition or release of two single protons. These special qualities are widely used in biological membranes for linear electron transfer and transmembrane proton translocation. In mitochondria, under certain conditions linear electron transfer from the semireduced form of coenzyme Q (ubisemiquinone radical(s), SQ) to native oxidants of the respiratory chain (Rieske iron sulfur protein and cytochromes) may run out of control, thereby establishing a permanent source of oxygen radical release (Fig. 1A).