Coenzyme Q Function in Mitochondria

In this chapter we provide a review with a focus on the function of Coenzyme Q (CoQ, ubiquinone) in mitochondria. The notion of a mobile pool of CoQ in the lipid bilayer as the vehicle of electrons from respiratory complexes has somewhat changed with the discovery of respiratory supramolecular units, in particular the supercomplex comprising Complexes I and III; in such assembly the electron transfer is thought to be mediated by direct channelling, and we provide evidence for a kinetic advantage on the transfer based on random collisions. The CoQ pool, however, has a fundamental function in establishing a dissociation equilibrium with bound CoQ, besides being required for electron transfer from other dehydrogenases to Complex III. CoQ bound to Complex I and to Complex III is also involved in proton translocation; although the mechanism of the Q-cycle is well established for Complex III, the involvement of CoQ in proton translocation by Complex I is still debated. This review also briefly examines some additional roles of CoQ, such as the antioxidant effect of its reduced form and its postulated action at the transcriptional level.