coenzyme Q10

**Structure and biochemistry** - Also called ubiquinone (oxidized form), ubisemiquinone (intermediate), and ubiquinol (reduced form, CoQH2). - It is a benzoquinone ring with a 10-unit isoprenoid tail. The tail makes it lipophilic, so it sits in the inner mitochondrial membrane and moves freely within it. - It is synthesized in all cells. Synthesis uses the mevalonate pathway, the same pathway that makes cholesterol: acetyl-CoA → HMG-CoA → mevalonate (HMG-CoA reductase, rate-limiting) → isoprenoids (farnesyl PP) → CoQ10. Tyrosine or phenylalanine supply the ring.

**Function** 1. Electron transport chain: CoQ accepts electrons from NADH via Complex I (NADH dehydrogenase) and from FADH2 via Complex II (succinate dehydrogenase). Glycerol-3-phosphate dehydrogenase and fatty acyl-CoA dehydrogenase (β-oxidation) also feed it. It passes electrons to Complex III (cytochrome bc1), which drives the Q cycle and proton pumping. Proton pumping builds the gradient that ATP synthase (Complex V) uses to make ATP. 2. Antioxidant: ubiquinol is a lipid-soluble antioxidant. It protects membranes and lipoproteins from lipid peroxidation and helps regenerate vitamin E. 3. Other roles include pyrimidine biosynthesis (dihydroorotate dehydrogenase) and membrane stabilization.

**Clinical significance** - **Statin myopathy:** Statins inhibit HMG-CoA reductase, which decreases CoQ10 synthesis. This is a proposed mechanism for myalgias, myopathy, and rhabdomyolysis. Meta-analyses and RCTs do not show consistent benefit from CoQ10 supplementation, so it is not guideline-recommended. Risk is higher with fibrate or niacin co-administration, CYP3A4 inhibitors, hypothyroidism, renal disease, and older age. Check CK if symptoms occur. - **Primary CoQ10 deficiency:** Autosomal recessive mutations in COQ genes (for example COQ2, PDSS1/2, ADCK3/COQ8A) cause a heterogeneous mitochondrial disease. Presentations include: - Steroid-resistant nephrotic syndrome (focal segmental glomerulosclerosis, FSGS) - Encephalomyopathy and seizures - Cerebellar ataxia - Lactic acidosis - Sensorineural hearing loss It is one of the few mitochondrial diseases that responds to treatment: high-dose oral CoQ10 supplementation. - **Mitochondrial disease context:** Impaired ETC function leads to lactic acidosis, "ragged red fibers" (with mitochondrial myopathies such as MELAS and MERRF), and involvement of high-energy organs (brain, muscle, heart, kidney). Mitochondrial inheritance is maternal, with heteroplasmy. - **Heart failure:** Supplementation (for example the Q-SYMBIO trial) has shown possible benefit, but it is not standard of care. - **Drug interactions:** CoQ10 may decrease the INR response to warfarin because its structure is similar to vitamin K. It may also lower blood pressure. - **Supplements:** It is generally well tolerated. Adverse effects are mild GI upset and insomnia.

**Board pearls** - ETC inhibitors (what blocks what): - Complex I: rotenone, amytal (amobarbital), metformin - Complex III: antimycin A - Complex IV: cyanide, carbon monoxide, azide - ATP synthase: oligomycin - Uncouplers (2,4-DNP, aspirin overdose, thermogenin/UCP-1) dissipate the proton gradient. - Statin + muscle pain → think CoQ10 depletion (the proposed mechanism); the stem may also ask for CK. - CoQ10 is the only mobile, non-protein (lipid-soluble) electron carrier in the inner membrane besides cytochrome c, which is a small protein in the intermembrane space.