ubiquinol

**Structure and redox states:** Coenzyme Q (CoQ10, ubiquinone) is a lipid-soluble benzoquinone with a 10-unit isoprenoid tail that anchors it in the inner mitochondrial membrane. It exists in three states: ubiquinone (fully oxidized, Q), semiquinone (partially reduced, QH•), and ubiquinol (fully reduced, QH2). Ubiquinol forms when ubiquinone accepts 2 electrons and 2 protons.

**Role in the electron transport chain (ETC):** - Complex I (NADH dehydrogenase) transfers electrons from NADH to Q, forming QH2, and pumps protons. - Complex II (succinate dehydrogenase) transfers electrons from FADH2 to Q, forming QH2. It does not pump protons. - Other inputs include glycerol-3-phosphate dehydrogenase, fatty acyl-CoA dehydrogenase (electron-transfer flavoprotein), and dihydroorotate dehydrogenase. - Ubiquinol diffuses within the membrane to Complex III (cytochrome bc1). There it is oxidized back to Q through the Q cycle, passing electrons to cytochrome c and pumping protons into the intermembrane space. - The proton gradient drives ATP synthase (Complex V).

**Antioxidant function:** Ubiquinol is the only endogenously synthesized lipid-soluble antioxidant. It scavenges lipid peroxyl radicals and regenerates vitamin E (α-tocopherol), which protects membranes and LDL from peroxidation.

**Biosynthesis:** CoQ10 is made in all cells from tyrosine (benzoquinone ring) and the mevalonate pathway (isoprenoid tail). The mevalonate pathway is the same one that produces cholesterol, and HMG-CoA reductase is its rate-limiting step.

**Clinical correlations (high-yield):** - **Statins** inhibit HMG-CoA reductase, which may lower CoQ10 and may contribute to statin-associated myopathy. Evidence that CoQ10 supplementation helps is weak and inconsistent. - **Primary CoQ10 deficiency** is a rare autosomal recessive disorder. It can cause encephalomyopathy, seizures, cerebellar ataxia, steroid-resistant nephrotic syndrome, and lactic acidosis. It is treatable with CoQ10 or ubiquinol supplementation. - **ETC inhibitors:** - Rotenone inhibits Complex I. - Antimycin A inhibits Complex III. - Cyanide, CO, and azide inhibit Complex IV (cytochrome c oxidase). - Oligomycin inhibits ATP synthase. - Uncouplers (2,4-DNP, aspirin overdose, thermogenin/UCP1) dissipate the proton gradient. - **Mitochondrial disease:** Defects in the ETC can cause lactic acidosis and affect high-energy tissues (muscle, brain, heart). Examples are MELAS and Leigh syndrome. CoQ10 is sometimes used as adjunct therapy. - **Supplements:** Ubiquinol is marketed as a more bioavailable form than ubiquinone, but evidence of clinical benefit is limited. CoQ10 may reduce the anticoagulant effect of warfarin because of its structural similarity to vitamin K.

**Mnemonic:** Ubiquinone is the oxidized form and ubiquinol is the reduced form. Think "-ol" = alcohol/hydroxyl groups = reduced (hydroquinone).