Fig. 1.

The CoQ pool in the mitochondrial inner membrane as essential link between metabolic pathways and cellular energy supply.

Various mitochondrial enzymes of diverse metabolic function feed electrons into the CoQ pool of the inner mitochondrial membrane. The electrons are then relayed to complex III of the ETC and thereby coupled to the maintenance of Δp and ATP production. a Complex I (NADH:Ubiquinone oxidoreductase) and complex II (succinate dehydrogenase) of the ETC chain are the canonical enzymes feeding electrons into the CoQ pool under normal conditions. The electrons are shuttled to complex III (ubiquinol:cytochrome c oxidoreductase) to be transferred onto cytochrome c and finally oxidised by complex IV (cytochrome c oxidase). By pumping protons from the matrix into the intermembrane space, complexes I, III & IV establish the mitochondrial protonmotive force (Δp), which is used for ATP synthesis by complex V (ATP-synthase) and various transport processes across the inner membrane. b The two electron carrier ubiquinone is a 1,4-benzoquinone, linked to an isoprene tail which in mammals consists of 9–10 isoprenyl subunits. Ubiquinone is reduced to ubiquinol by two electrons and acts as electron shuttle and antioxidant. c Numerous other mitochondrial oxidoreductases, such as SQR (hydrogen sulfide:ubiquinone oxidoreductase, catabolism of hydrogen sulphide), DHODH (dihydroorotate dehydrogenase, pyrimidine biosynthesis), CHDH (choline dehydrogenase, choline oxidation), G3PDH (glycerol 3-phosphate dehydrogenase, glycerol-3-phosphate shuttle), ETF-QO (electron-transferring-flavoprotein dehydrogenase, fatty acid oxidation) and PRODH (proline dehydrogenase, catabolism of proline) feed electrons into the mitochondrial CoQ pool. The electrons are funnelled by CoQ via complex III and cytochrome c to complex IV to reduce O₂ to H₂O.