A hydroquinone is a compound with two hydroxyl (OH) groups bound to aromatic carbons, which can be reversibly oxidized by 2 electrons to a quinone (with two double bonded oxygens), which makes this family of compounds useful in electron transport chains.
The simplest phenol, hydroquinone, and quinone are shown below. These compounds are aromatic ring systems with one hydroxyl (OH) group, two hydroxyls, or two double-bonded oxygens, respectively. Under oxidizing conditions, a phenol can be oxidized to a quinone, and a quinone can be reduced by two electron and two protons (or two hydrogen atoms, H·) to a hydroquinone. This 2-electron process associated with quinone-to-hydroquinone transformation is easily reversible, which makes these molecules useful in biochemical redox reactions.
A common, or ubiquitous, quinone found in biological systems is ubiquinone, or coenzyme Q, which is an important two-electron acceptor in the electron transport chain. During cellular respiration, ubiquinone is reduced by NADH to its hydroquinone form, called ubiquinol (below to the right). In this example, NADH is the reducing agent because it acts as a hydride (H‒ or [H+ + 2e‒]) donor to reduce ubiquinone to ubiquinol. Therefore, the ubiquinone is the oxidizing agent, since it oxidizes NADH to NAD+.
Practice Questions
Khan Academy
MCAT Official Prep (AAMC)
Section Bank C/P Section Passage 1 Question 2
Practice Exam 2 C/P Section Passage 2 Question 6
Key Points
• “Quinone” is a term used to describe cyclohexadiendiones in general, and p‑benzoquinone in particular.
• “Hydroquinones” are produced by the reduction of quinones according to the following half‑reaction:
• “Ubiquinones” are naturally occurring quinones whose role is to transfer a pair of electrons from one substance to another in enzyme‑catalyzed reactions. Ubiquinones are also called coenzymes Q.
• Quinones can be reversibly reduced by 2 electrons (and two protons) to the respective hydroquinones, making them useful biomolecules in the electron transport chain.
Key Terms
Aromatic ring systems: Hydrocarbons which contain benzene, or some other related ring structure.
Phenol: A mildly acidic toxic white crystalline solid obtained from coal tar and used in chemical manufacture, and in dilute form (under the name carbolic ) as a disinfectant.
Hydroquinone: A crystalline compound made by the reduction of benzoquinone.
Ubiquinone: Aka “coenzyme Q”. One of a family of quinone molecules, which is an electron acceptor in the electron transport chain. The reduced, hydroquinone form is called ‘ubiquinol’.
Oxidation:The loss of electrons during a reaction by a molecule, atom or ion. Oxidation occurs when the oxidation state of a molecule, atom or ion is increased.
Reduction: Occurs when there is a gain of electrons or the oxidation state of an atom, molecule, or ion decreases.
Redox chemistry – OIL RIG: Acronym to remember Oxidation Is Loss, Reduction Is Gain (of electrons or hydrogen (written: H· or [H++e‒]).
Electron transport chain: A series of complexes that transfer electrons from electron donors to electron acceptors via redox reactions, and couples this electron transfer with the transfer of protons across a membrane. The electron transport chain is built up of peptides, enzymes, and other molecules.
Cellular respiration: A set of metabolic reactions and processes that take place in the cells of organisms to convert chemical energy from oxygen molecules or nutrients into adenosine triphosphate, and then release waste products.