A critical appraisal of the mitochondrial coenzyme Q pool

The function of the coenzyme Q (CoQ) pool in the inner mitochondrial membrane is reviewed in view of recent findings suggesting a supramolecular organization of the mitochondrial respiratory complexes. In spite of the structural evidence for preferential aggregations of the inner membrane components, most kinetic evidence is in favor of a dispersed organization based on random collisions of the small connecting redox components, in particular CoQ, with the individual complexes. The shape of the CoQ molecule in the pool, suggested to be a folded one, is in agreement with its very rapid lateral diffusion mobility in the membrane midplane. Since the structural evidence in favor of specific supercomplexes is rather strong, it cannot be excluded that electron transfer may follow either pool behavior or preferential channeling depending on the physiological conditions. Another function ascribed to the CoQ pool is the antioxidant action of the reduced CoQ molecules; although it cannot be excluded that protein-bound ubisemiquinones may be a source of oxygen radicals, particularly at the level of complex III, the available evidence suggests that the mitochondrial pool only behaves as an antioxidant under physiological conditions.