page_title: Oxidative phosphorylation
section_title: Succinate-Q oxidoreductase (complex II)
hierarchical_section_title: Oxidative phosphorylation / Eukaryotic electron transport chains / Succinate-Q oxidoreductase (complex II)
context_page_description: Oxidative phosphorylation is the metabolic pathway in which cells use enzymes to oxidize nutrients, thereby releasing the chemical energy of molecular oxygen, which is used to produce adenosine triphosphate. In most eukaryotes, this takes place inside mitochondria. Almost all aerobic organisms carry out oxidative phosphorylation. This pathway is so pervasive because the energy of the double bond of oxygen is so much higher than the energy of the double bond in carbon dioxide or in pairs of single bonds in organic molecules observed in alternative fermentation processes such as anaerobic glycolysis.
  During oxidative phosphorylation, electrons are transferred from electron donors to electron acceptors such as oxygen in redox reactions. These redox reactions release the energy stored in the relatively weak double bond of O₂, which is used to form ATP. In eukaryotes, these redox reactions are catalyzed by a series of protein complexes within the inner membrane of the cell's mitochondria, whereas, in prokaryotes, these proteins are located in the cell's intermembrane space. These linked sets of proteins are called electron transport chains.
context_section_description: Succinate-Q oxidoreductase, also known as complex II or succinate dehydrogenase, is a second entry point to the electron transport chain. It is unusual because it is the only enzyme that is part of both the citric acid cycle and the electron transport chain. Complex II consists of four protein subunits and contains a bound flavin adenine dinucleotide (FAD) cofactor, iron–sulfur clusters, and a heme group that does not participate in electron transfer to coenzyme Q, but is believed to be important in decreasing production of reactive oxygen species. It oxidizes succinate to fumarate and reduces ubiquinone. As this reaction releases less energy than the oxidation of NADH, complex II does not transport protons across the membrane and does not contribute to the proton gradient.
  In some eukaryotes, such as the parasitic worm Ascaris suum, an enzyme similar to complex II, fumarate reductase (menaquinol:fumarate
  oxidoreductase, or QFR), operates in reverse to oxidize ubiquinol and reduce fumarate. This allows the worm to survive in the anaerobic environment of the large intestine, carrying out anaerobic oxidative phosphorylation with fumarate as the electron acceptor. Another unconventional function of complex II is seen in the malaria parasite Plasmodium falciparum. Here, the reversed action of complex II as an oxidase is important in regenerating ubiquinol, which the parasite uses in an unusual form of pyrimidine biosynthesis.
caption: Complex II: Succinate-Q oxidoreductase.