Question

12 Which of the following accepts only 1 elec-
tron?
(A) Coenzyme Q
(B) FMN
(C) Cytochrome b
(D) FAD​

Answer 1

the mitochondrial electron-transport chain.

Coenzyme Q is a quinone derivative with a long isoprenoid tail. The number of five-carbon isoprene units in coenzyme Q depends on the species. The most common form in mammals contains 10 isoprene units (coenzyme Q10). For simplicity, the subscript will be omitted from this abbreviation because all varieties function in an identical manner. Quinones can exist in three oxidation states (Figure 18.10). In the fully oxidized state (Q), coenzyme Q has two keto groups. The addition of one electron and one proton results in the semiquinone form (QH·). The semiquinone form is relatively easily deprotonated to form a semiquinone radical anion (Q·-). The addition of a second electron and proton generates ubiquinol (QH2), the fully reduced form of coenzyme Q, which holds its protons more tightly. Thus, for quinones, electron-transfer reactions are coupled to proton binding and release, a property that is key to transmembrane proton transport.