Does coenzyme Q10 carry electrons to complex III and support ATP production?

Coenzyme Q10 (CoQ10; ubiquinone/ubiquinol) has a central, established role in oxidative phosphorylation. The claim is strongly supported: CoQ10 is the shared mobile electron carrier linking major respiratory-chain inputs to complex III, and adequate tissue availability is required for efficient mitochondrial ATP generation.

Respiratory-chain and ATP-production mechanism

• Complex I transfers electrons from NADH to ubiquinone, and complex II transfers electrons from succinate to the same CoQ pool, reducing CoQ10 to ubiquinol.
• Ubiquinol then donates electrons at the complex III Qo site. Through the complex III Q-cycle, electrons are routed via the Rieske iron–sulfur protein/cytochrome c1 pathway and cytochrome b/Qi-site pathway.
• This electron flow supports proton translocation: complex I and complex III contribute to the inner-mitochondrial-membrane proton gradient, whereas complex II provides electrons but does not pump protons. ATP synthase uses this proton-motive force to generate ATP.

Biological and clinical significance

• Reduced tissue CoQ10 availability can restrict coupled respiration, electron flow into complex III, and oxidative ATP-production capacity. The functional consequence depends on the extent of depletion and the energy requirements of the tissue.
• Primary deficiency can arise from pathogenic variants affecting CoQ10 biosynthesis; secondary deficiency may occur in other genetic or acquired conditions. Marked impairment has been described around ≤30–40% of normal CoQ10 content, although this is not a universal diagnostic threshold.
• Plasma CoQ10—often reported with assay-dependent reference intervals around 0.5–1.7 μmol/L—can be affected by diet, lipoproteins, and supplements and may not reflect mitochondrial tissue stores. Skeletal-muscle assessment is more informative when a primary biosynthetic disorder is suspected.

Bottom line

• CoQ10 is essential for electron transfer from complexes I and II (and other dehydrogenases) to complex III and thereby for efficient oxidative ATP production. This biochemical necessity does not, by itself, establish that supplementation improves ATP production in people without confirmed deficiency.