| Ubiquinone (coenzyme Q10) transfers electrons to complex III
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| Ubiquinone is so named, because it is ubiquitous in virtually all living systems. It is a small, lipid-soluble compound found in the inner membrane of animal and plant mitochondria and in the plasma membrane of bacteria. The primary form of mammalian ubiquinone contains a side chain of 10 isoprene units and is often called CoQ10. It diffuses in the inner membrane, accepts electrons from the four major mitochondrial flavoproteins, and transfers them to complex III (QH2-cytochrome c reductase). It can carry either one or two electrons (Fig. 8.8). It is also thought to be a major source of superoxide radicals in the cell (see Chapter 35).
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