Q.34 Which of the following is/are TRUE about the electron carrier, ubiquinone (coenzyme Q)?
(A) Its ability to accept two electrons, one at a time, enables ubiquinone to function at the junction
between a 2-electron donor and a 1-electron acceptor
(B) Being small and hydrophobic, ubiquinone readily shuttles between protein-based electron
transfer complexes within the membrane
(C) Its hydrophilic nature and high affinity for protons enable ubiquinone to transport protons
readily within the intermembrane space of mitochondria.
(D) Its ability to interact with Heme C of cytochromes enables electron transport in the
mitochondrial membrane
Ubiquinone, also known as coenzyme Q, serves as a key mobile electron carrier in the mitochondrial electron transport chain (ETC). The correct answer to the MCQ is options (A) and (B) are true.
Electron Acceptance Mechanism
Ubiquinone accepts two electrons sequentially from donors like Complex I or II, first forming semiquinone radical (one electron) then ubiquinol (two electrons), bridging two-electron donors with one-electron acceptors like Complex III. This one-at-a-time transfer via the Q-cycle ensures efficient electron flow without free radical buildup.
Membrane Shuttling Role
Ubiquinone’s small size and hydrophobic isoprenoid tail allow diffusion within the inner mitochondrial membrane lipid bilayer, shuttling electrons between fixed complexes like I/II to III. Most CoQ remains unbound and mobile in the membrane pool.
Why Option C is False
Ubiquinone is hydrophobic, not hydrophilic, preventing proton transport in the aqueous intermembrane space; protons move via Complex pumps creating the gradient. Its lipid solubility confines it to the membrane.
Why Option D is False
Ubiquinone transfers electrons to Complex III’s Rieske Fe-S center and heme b cytochromes, not heme C (found in Complex IV or cytochrome c). Cytochrome c links III to IV separately.
