UBIQUINONE
Ubiquinone is one example of a biologically active quinone. Ubiquinone is also called coenzyme Q
and is a vital electron carrier associated with Complexes I, II, and III of the electron transport chain.
Ubiquinone is the most oxidized form that this molecule takes physiologically: it can also be
reduced to ubiquinol upon the acceptance of electrons, as shown in Figure 5.16. This oxidation–
reduction capacity allows the molecule to perform its physiological function of electron transport.
Figure 5.16. Ubiquinone (Coenzyme Q) and Ubiquinol
Ubiquinone is the oxidized form; when it picks up electrons, it is converted to its reduced form
(ubiquinol).BRIDGE
Coenzyme Q plays a role in Complexes I, II, and III of the electron transport chain. In Complex
III, it is the main player in the Q cycle, which contributes to the formation of the proton-
motive force across the inner mitochondrial membrane. The respiratory complexes are
discussed in Chapter 10 of MCAT Biochemistry Review.