fig2

Figure 2. Schematic diagram of flavocoenzymes in mitochondrial energy metabolism. The OXPHOS system is a 5-enzyme complex which encompasses the mitochondrial respiratory chain (Complexes I-IV), Complex V, and two mobile electron shuttles (coenzyme Q10 and cytochrome c). Electrons derived from oxidation of pyruvate mediated by pyruvate dehydrogenase (PDH) and fatty acid oxidation are transferred via NADH to Complex I (FMN-dependent NADH-ubiquinone oxidoreductase), while electrons from succinate in the Krebs cycle, amino acid metabolism, and fatty acid oxidation are transferred to Complex II (FAD-dependent succinate-ubiquinone oxidoreductase) via FADH2. Electrons are subsequently transferred to ubiquinone (Coenzyme Q10) and then to Complex III (reduced CoQ-cytochrome c reductase), and via cytochrome c to cytochrome c oxidase (COX) (Complex IV), the terminal oxidase of the RC before finally reducing molecular oxygen to water. The free energy liberated during this sequential electron transfer is used to generate an electrochemical gradient of protons, which is finally used by Complex V (ATP synthase or F1F0 ATPase) to drive ATP synthesis from ADP and inorganic phosphate. RF: Riboflavin; RF kinase: riboflavin kinase; Q: Coenzyme Q10; CytC: cytochrome c oxidase; FMN: flavin mononucleotide; FAD: flavin adenine dinucleotide