Oxidative phosphorylation is the concluding pathway in cellular respiration, the series of metabolic processes that convert chemical energy from glucose into adenosine triphosphate (ATP), a usable form of energy for the cell. A series of five protein complexes, each with increasing reduction potentials, located in the mitochondrial inner membrane forms the electron transport chain (ETC). Electrons are transferred from one complex to the next in a series of redox reactions which release energy used to pump protons from the mitochondrial matrix into the intermembrane space. As a result, an electrochemical gradient forms across the inner mitochondrial membrane. Complex V (ATP synthase) is the singular channel by which protons flow back into the matrix. ATP synthase uses this gradient to synthesize ATP from ADP and phosphate (Pi). Complex I is the NADH dehydrogenase complex responsible for the oxidation of NADH and the reduction of ubiquinone (coenzyme Q), transferring two electrons from NADH to the respiratory chain. Four protons are pumped into the intermembrane space as a result of this electron transfer, and a further two protons are pumped due to the reduction of ubiquinone to ubiquinol. Complex II is the succinate dehydrogenase complex responsible for the oxidation of succinate into fumarate and the reduction of ubiquinone, transferring two electrons from succinate to ubiquinone instead of directly to the ETC. No protons are pumped at this complex because succinate oxidation releases less energy than NADH oxidation. Complex III is the ubiquinol-cytochrome c oxidoreductase complex responsible for transferring electrons from ubiquinol to cytochrome c. Two protons are pumped into the intermembrane space as a result of the oxidization of one molecule of ubiquinol (a coenzyme that can carry two electrons) and the reduction of two molecules of cytochrome c (a heme protein that can carry only one electron). Complex IV is the cytochrome c oxidase complex responsible for transferring electrons to oxygen, the terminal electron acceptor, and reducing it to water. Four protons are pumped into the intermembrane space as a result of the electron transfer. In addition, the reduction of oxygen further contributes to the proton gradient due to its use of matrix protons. Complex V is the mitochondrial membrane F-Type ATP synthase which produces ATP from ADP in the presence of a proton gradient across the membrane (generated by electron transport complexes of the respiratory chain). An F-Type ATPase is composed of two domains: a catalytic core (where ATP is synthesized) and a proton channel.

Oxidative Phosphorylation References