Fig. 3.

Schematic diagram of the reduction of ubiquinone. The complete oxidation of pyruvate generates 4 NADH molecules and 1 FADH₂, which are oxidised on complex I (I; NADH: ubiquinone oxidoreductase) and complex II (II; succinate: ubiquinone oxidoreductase) of the mitochondrial electron transport chain, respectively, while reducing ubiquinone (Q) to ubiquinol (QH₂). In contrast, one round of β-oxidation also forms 4 NADH, but 2 FADH₂. The second FADH₂ is oxidised by the electron transfer flavoprotein: ubiquinone oxidoreductase (ETF), which also contributes to the QH₂ pool. Additionally, the two mitochondrial dehydrogenases glycerol 3-phosphate dehydrogenase and dihydroorotate dehydrogenase (both not shown) can contribute to the QH₂ pool as part of glycerol metabolism and the de novo pyrimidine biosynthesis pathway, respectively. QH₂ is oxidised at complex III (coenzyme Q: cytochrome c oxidoreductase). Oxygen is reduced to water at complex IV (IV; cytochrome c oxidase). An ATPase synthase (V) synthesises ATP from ADP at Pi)