Editing Oxidative phosphorylation (section)

=== Succinate-Q oxidoreductase (complex II) ===
[[File:Complex II.svg|250px|thumb|right|Complex II: [[Succinate - coenzyme Q reductase|Succinate-Q oxidoreductase]].]]

[[Succinate - coenzyme Q reductase|Succinate-Q oxidoreductase]], also known as ''complex II'' or ''succinate dehydrogenase'', is a second entry point to the electron transport chain.<ref>{{cite journal | vauthors = Cecchini G | title = Function and structure of complex II of the respiratory chain | journal = Annual Review of Biochemistry | volume = 72 | pages = 77–109 | year = 2003 | pmid = 14527321 | doi = 10.1146/annurev.biochem.72.121801.161700 }}</ref> 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.<ref>{{cite journal | vauthors = Yankovskaya V, Horsefield R, Törnroth S, Luna-Chavez C, Miyoshi H, Léger C, Byrne B, Cecchini G, Iwata S | title = Architecture of succinate dehydrogenase and reactive oxygen species generation | journal = Science | volume = 299 | issue = 5607 | pages = 700–704 | date = January 2003 | pmid = 12560550 | doi = 10.1126/science.1079605 | s2cid = 29222766 | bibcode = 2003Sci...299..700Y }}</ref><ref>{{cite journal | vauthors = Horsefield R, Iwata S, Byrne B | title = Complex II from a structural perspective | journal = Current Protein & Peptide Science | volume = 5 | issue = 2 | pages = 107–118 | date = April 2004 | pmid = 15078221 | doi = 10.2174/1389203043486847 }}</ref> It oxidizes [[succinic acid|succinate]] to [[Fumaric acid|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.

{{NumBlk|:|<chem>{Succinate} + Q -> {Fumarate} + QH2</chem>|{{EquationRef|2}}}}

In some eukaryotes, such as the [[parasitic worm]] ''[[Large roundworm of pigs|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.<ref>{{cite journal | vauthors = Kita K, Hirawake H, Miyadera H, Amino H, Takeo S | title = Role of complex II in anaerobic respiration of the parasite mitochondria from Ascaris suum and Plasmodium falciparum | journal = Biochimica et Biophysica Acta (BBA) - Bioenergetics | volume = 1553 | issue = 1–2 | pages = 123–139 | date = January 2002 | pmid = 11803022 | doi = 10.1016/S0005-2728(01)00237-7 | doi-access = free }}</ref> 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.<ref>{{cite journal | vauthors = Painter HJ, Morrisey JM, Mather MW, Vaidya AB | title = Specific role of mitochondrial electron transport in blood-stage Plasmodium falciparum | journal = Nature | volume = 446 | issue = 7131 | pages = 88–91 | date = March 2007 | pmid = 17330044 | doi = 10.1038/nature05572 | s2cid = 4421676 | bibcode = 2007Natur.446...88P }}</ref>