FIGURE 12-21 Oxidized and reduced forms of coenzyme Q (CoQ), which can carry two protons and two electrons. Because of its long hydrocarbon “tail” of isoprene units, CoQ, also called ubiquinone, is soluble in the hydrophobic core of phospholipid bilayers and is very mobile. Reduction of CoQ to the fully reduced form, QH₂ (dihydroquinone), occurs in two steps with a half-reduced free-radical intermediate, called semiquinone.