Oxidative phosphorylation is the production of ATP using energy derived from the transfer of electrons in an electron transport system and occurs by chemiosmosis. The process is accomplished though oxidation-reduction reactions in the mitochondria. During oxidative phosphorylation, electrons are transferred from electron donors to electron acceptors, referred to as the electron transport chain. The flow of electrons from NADH to O2 through protein complexes located in the mitochondrial inner membrane leads to the pumping of protons out of the mitochondrial matrix. The resulting uneven distribution of protons generates a pH gradient and a transmembrane electrical potential that creates a proton-motive force. ATP is synthesized when protons flow back to the mitochondrial matrix through an enzyme complex (Complex V). The oxidation of fuels and the phosphorylation of ADP are coupled by the proton gradient across the inner mitochondrial membrane.
Oxidative phosphorylation consists of five protein-lipid enzyme complexes (Complex I - V) located in the mitochondrial inner membrane that contain flavins (FMN, FAD), quinoid compounds (coenzyme Q10, CoQ10) and transition metal compounds (iron-sulfur clusters, hemes, protein-bound copper). These enzymes are designated complex I (NADH:ubiquinone oxidoreductase, EC 1.6. 5.3), complex II (succinate:ubiquinone oxidoreductase, EC 18.104.22.168), complex III (ubiquinol:ferrocytochrome c oxidoreductase, EC 22.214.171.124), complex IV (ferrocytochrome c:oxygen oxidoreductase or cytochrome c oxidase, EC 126.96.36.199), and complex V (ATP synthase, EC 188.8.131.52). Complex I transports electrons from NADH to ubiquinone. Complex II catalyzes the oxidation of succinate to fumarate and transfers electrons to ubiquinone pool of respiratory chain. Complex III transfers electrons from ubiquinol to cytochrome c coupled with the transfer of electrons across inner mitochondrial membrane. Complex IV, the final step in the electron transport chain, is the reduction of molecular oxygen by electrons derived from cytochrome c. Complex V, the final enzyme in the oxidative phosphorylation pathway, couples a proton gradient generated by respiratory chain to ATP synthesis where protons flow from intermembrane mitochondrial space to the matrix.