Editing Oxidative phosphorylation (section)

=== Alternative reductases and oxidases ===
Many eukaryotic organisms have electron transport chains that differ from the much-studied mammalian enzymes described above. For example, [[plant]]s have alternative NADH oxidases, which oxidize NADH in the cytosol rather than in the mitochondrial matrix, and pass these electrons to the ubiquinone pool.<ref>{{cite journal | vauthors = Rasmusson AG, Soole KL, Elthon TE | title = Alternative NAD(P)H dehydrogenases of plant mitochondria | journal = Annual Review of Plant Biology | volume = 55 | pages = 23–39 | year = 2004 | pmid = 15725055 | doi = 10.1146/annurev.arplant.55.031903.141720 }}</ref> These enzymes do not transport protons, and, therefore, reduce ubiquinone without altering the electrochemical gradient across the inner membrane.<ref>{{cite journal | vauthors = Menz RI, Day DA | title = Purification and characterization of a 43-kDa rotenone-insensitive NADH dehydrogenase from plant mitochondria | journal = The Journal of Biological Chemistry | volume = 271 | issue = 38 | pages = 23117–23120 | date = September 1996 | pmid = 8798503 | doi = 10.1074/jbc.271.38.23117 | s2cid = 893754 | doi-access = free }}</ref>

Another example of a divergent electron transport chain is the ''[[alternative oxidase]]'', which is found in [[plant]]s, as well as some [[fungus|fungi]], [[protist]]s, and possibly some animals.<ref>{{cite journal | vauthors = McDonald A, Vanlerberghe G | title = Branched mitochondrial electron transport in the Animalia: presence of alternative oxidase in several animal phyla | journal = IUBMB Life | volume = 56 | issue = 6 | pages = 333–341 | date = June 2004 | pmid = 15370881 | doi = 10.1080/1521-6540400000876 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Sluse FE, Jarmuszkiewicz W | title = Alternative oxidase in the branched mitochondrial respiratory network: an overview on structure, function, regulation, and role | journal = Brazilian Journal of Medical and Biological Research = Revista Brasileira de Pesquisas Medicas e Biologicas | volume = 31 | issue = 6 | pages = 733–747 | date = June 1998 | pmid = 9698817 | doi = 10.1590/S0100-879X1998000600003 | doi-access = free }}</ref> This enzyme transfers electrons directly from ubiquinol to oxygen.<ref>{{cite journal | vauthors = Moore AL, Siedow JN | title = The regulation and nature of the cyanide-resistant alternative oxidase of plant mitochondria | journal = Biochimica et Biophysica Acta (BBA) - Bioenergetics | volume = 1059 | issue = 2 | pages = 121–140 | date = August 1991 | pmid = 1883834 | doi = 10.1016/S0005-2728(05)80197-5 }}</ref>

The electron transport pathways produced by these alternative NADH and ubiquinone oxidases have lower [[adenosine triphosphate|ATP]] yields than the full pathway. The advantages produced by a shortened pathway are not entirely clear. However, the alternative oxidase is produced in response to stresses such as cold, [[reactive oxygen species]], and infection by pathogens, as well as other factors that inhibit the full electron transport chain.<ref>{{cite journal | vauthors = Vanlerberghe GC, McIntosh L | title = ALTERNATIVE OXIDASE: From Gene to Function | journal = Annual Review of Plant Physiology and Plant Molecular Biology | volume = 48 | pages = 703–734 | date = June 1997 | pmid = 15012279 | doi = 10.1146/annurev.arplant.48.1.703 }}</ref><ref>{{cite journal | vauthors = Ito Y, Saisho D, Nakazono M, Tsutsumi N, Hirai A | title = Transcript levels of tandem-arranged alternative oxidase genes in rice are increased by low temperature | journal = Gene | volume = 203 | issue = 2 | pages = 121–129 | date = December 1997 | pmid = 9426242 | doi = 10.1016/S0378-1119(97)00502-7 }}</ref> Alternative pathways might, therefore, enhance an organism's resistance to injury, by reducing [[oxidative stress]].<ref>{{cite journal | vauthors = Maxwell DP, Wang Y, McIntosh L | title = The alternative oxidase lowers mitochondrial reactive oxygen production in plant cells | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 96 | issue = 14 | pages = 8271–8276 | date = July 1999 | pmid = 10393984 | pmc = 22224 | doi = 10.1073/pnas.96.14.8271 | doi-access = free | bibcode = 1999PNAS...96.8271M }}</ref>