Ubiquinone (shown below) is an important molecule in the electron transport chain, where it receives electrons in complex I originating from NADH. What is the structure of the product formed when electrons are added to ubiquinone?

Question

Ubiquinone (shown below) is an important molecule in the electron transport chain, where it receives electrons in complex I originating from NADH.  What is the structure of the product formed when electrons are added to ubiquinone?

Quizplus · Accepted Answer

11ecba2d_127a_4479_a3bf_9366e17f18e4_MD0008_00 Oxidation-reduction (redox) reactions occur when electrons are transferred from one atom to another. A molecule is oxidized when it loses electrons and the number of carbon-heteroatom bonds increases. A molecule is reduced when it gains electrons and the number of carbon-heteroatom bonds decreases. Reduction can also be viewed as an increase in the number of bonds to hydrogen. Ubiquinone is the fully oxidized form of coenzyme Q, which participates in the electron transport chain in the mitochondrial membrane. The two electrons that ubiquinone receives from NADH reduce the two carbonyls (C=O) in ubiquinone to hydroxyl (-OH) groups. The carbonyl carbons each have two carbon-heteroatom bonds, and when ubiquinone gains electrons, the number of carbon-heteroatom bonds decreases to one. Electrons are also distributed to the six-membered ring, making it aromatic. Therefore, the structure of ubiquinol contains two hydroxyl groups on an aromatic ring. (Choice A) The ring should be an aromatic ring rather than the diene present in the six-membered ring of ubiquinone. (Choice B) Both carbonyl carbons should be reduced to hydroxyls (-OH), and the ring should be an aromatic ring rather than the diene present in ubiquinone. (Choice C) This structure shows the over-reduction of one carbonyl carbon with complete removal of the oxygen atom. Both carbonyl carbons should have been reduced to hydroxyls (-OH), and the ring should be an aromatic ring. Educational objective: Oxidation-reduction reactions result in the transfer of electrons from one atom to another. Oxidation is a loss of electrons and an increase in the number of carbon-heteroatom bonds; reduction is a gain of electrons and a decrease in the number of carbon-heteroatom bonds. In the electron transport chain, ubiquinone receives two electrons from NADH, which reduces the carbonyl carbons to hydroxyl groups and forms an aromatic ring, yielding ubiquinol.

Ubiquinone (Shown Below) Is an Important Molecule in the Electron