Editing Peroxisome (section)

== Metabolic functions ==
A major function of the peroxisome is the breakdown of [[very long chain fatty acid]]s through [[beta oxidation]]. In animal cells, the long fatty acids are converted to [[Medium-chain triglyceride|medium chain fatty acids]], which are subsequently shuttled to [[mitochondria]] where they eventually are broken down to carbon dioxide and water. In yeast and plant cells, this process is carried out exclusively in peroxisomes.<ref name="alberts">{{cite book | vauthors = Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P | title = Molecular Biology of the Cell | edition = Fourth | publisher = Garland Science | location = New York | year = 2002 | isbn = 978-0-8153-3218-3 | chapter  = Chapter 12: Peroxisomes | chapter-url = https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.2194 }}</ref><ref name=":0">{{Cite journal|last1=Schrader|first1=Michael|last2=Kamoshita|first2=Maki|last3=Islinger|first3=Markus|date=Mar 2019|title=Organelle interplay—peroxisome interactions in health and disease|journal=Journal of Inherited Metabolic Disease|volume=43|language=en|issue=1|pages=71–89|doi=10.1002/jimd.12083|pmid=30864148|pmc=7041636|issn=1573-2665|doi-access=free}}</ref>

The first reactions in the formation of [[plasmalogen]] in animal cells also occur in peroxisomes. Plasmalogen is the most abundant phospholipid in [[myelin]]. Deficiency of plasmalogens causes profound abnormalities in the myelination of [[neuron|nerve cells]], which is one reason why many [[peroxisomal disorders]] affect the nervous system.<ref name="alberts"/> Peroxisomes also play a role in the production of [[bile]] acids important for the absorption of fats and fat-soluble vitamins, such as vitamins A and K. Skin disorders are features of genetic disorders affecting peroxisome function as a result.<ref name=":0" />

The specific metabolic pathways that occur exclusively in mammalian peroxisomes are:<ref name="pmid16756494" />

* α-oxidation of [[phytanic acid]]
* β-oxidation of very-long-chain and polyunsaturated fatty acids
* biosynthesis of plasmalogens
* conjugation of cholic acid as part of bile acid synthesis

Peroxisomes contain oxidative [[enzyme]]s, such as [[D-amino acid oxidase]] and [[uric acid oxidase]].<ref name="pmid1334030">{{cite journal | vauthors = del Río LA, Sandalio LM, Palma JM, Bueno P, Corpas FJ | title = Metabolism of oxygen radicals in peroxisomes and cellular implications | journal = Free Radical Biology & Medicine | volume = 13 | issue = 5 | pages = 557–80 | date = November 1992 | pmid = 1334030 | doi = 10.1016/0891-5849(92)90150-F }}</ref> However the last enzyme is absent in humans, explaining the disease known as [[gout]], caused by the accumulation of uric acid. Certain enzymes within the peroxisome, by using molecular oxygen, remove hydrogen atoms from specific organic substrates (labeled as R), in an oxidative reaction, producing [[hydrogen peroxide]] (H<sub>2</sub>O<sub>2</sub>, itself toxic):

:<math>\mathrm{RH}_\mathrm{2} + \mathrm{O}_\mathrm{2} \rightarrow \mathrm{R }+ \mathrm{H}_2\mathrm{O}_2</math>

Catalase, another peroxisomal enzyme, uses this H<sub>2</sub>O<sub>2</sub> to oxidize other substrates, including [[phenols]], [[formic acid]], [[formaldehyde]], and [[Alcohol (chemistry)|alcohol]], by means of the peroxidation reaction:

:<math>\mathrm{H}_2\mathrm{O}_2 + \mathrm{R'H}_2 \rightarrow \mathrm{R'} + 2\mathrm{H}_2\mathrm{O}</math>, thus eliminating the poisonous hydrogen peroxide in the process.

This reaction is important in liver and kidney cells, where the peroxisomes detoxify various toxic substances that enter the blood. About 25% of the [[ethanol]] that humans consume by drinking alcoholic beverages is oxidized to [[acetaldehyde]] in this way.<ref name="alberts"/> In addition, when excess H<sub>2</sub>O<sub>2</sub> accumulates in the cell, catalase converts it to H<sub>2</sub>O through this reaction:

:<math>2\mathrm{H}_2\mathrm{O}_2 \rightarrow 2\mathrm{H}_2\mathrm{O} + \mathrm{O}_2</math>

In higher plants, peroxisomes contain also a complex battery of antioxidative enzymes such as [[superoxide dismutase]], the components of the [[ascorbate-glutathione cycle]], and the NADP-dehydrogenases of the pentose-phosphate pathway. It has been demonstrated that peroxisomes generate [[superoxide]] (O<sub>2</sub><sup>•−</sup>) and [[nitric oxide]] (<sup>•</sup>NO) radicals.<ref name="pmid11286918">{{cite journal | vauthors = Corpas FJ, Barroso JB, del Río LA | title = Peroxisomes as a source of reactive oxygen species and nitric oxide signal molecules in plant cells | journal = Trends in Plant Science | volume = 6 | issue = 4 | pages = 145–50 | date = April 2001 | pmid = 11286918 | doi = 10.1016/S1360-1385(01)01898-2 | bibcode = 2001TPS.....6..145C }}</ref><ref name="pmid15347796">{{cite journal | vauthors = Corpas FJ, Barroso JB, Carreras A, Quirós M, León AM, Romero-Puertas MC, Esteban FJ, Valderrama R, Palma JM, Sandalio LM, Gómez M, del Río LA | display-authors = 6 | title = Cellular and subcellular localization of endogenous nitric oxide in young and senescent pea plants | journal = Plant Physiology | volume = 136 | issue = 1 | pages = 2722–33 | date = September 2004 | pmid = 15347796 | pmc = 523336 | doi = 10.1104/pp.104.042812 }}</ref>

There is evidence now that those reactive oxygen species including peroxisomal H<sub>2</sub>O<sub>2</sub> are also important signaling molecules in plants and animals and contribute to healthy aging and age-related disorders in humans.<ref>{{cite journal | vauthors = Lismont C, Revenco I, Fransen M | title = Peroxisomal Hydrogen Peroxide Metabolism and Signaling in Health and Disease | journal = International Journal of Molecular Sciences | volume = 20 | issue = 15 | pages = 3673 | date = July 2019 | pmid = 31357514 | pmc = 6695606 | doi = 10.3390/ijms20153673 | doi-access = free }}</ref>

The peroxisome of plant cells is polarised when fighting fungal penetration. Infection causes a [[glucosinolate]] molecule to play an antifungal role to be made and delivered to the outside of the cell through the action of the peroxisomal proteins (PEN2 and PEN3).<ref name="pmid19095900">{{cite journal | vauthors = Bednarek P, Pislewska-Bednarek M, Svatos A, Schneider B, Doubsky J, Mansurova M, Humphry M, Consonni C, Panstruga R, Sanchez-Vallet A, Molina A, Schulze-Lefert P | display-authors = 6 | title = A glucosinolate metabolism pathway in living plant cells mediates broad-spectrum antifungal defense | journal = Science | volume = 323 | issue = 5910 | pages = 101–6 | date = January 2009 | pmid = 19095900 | doi = 10.1126/science.1163732 | bibcode = 2009Sci...323..101B | s2cid = 38423996 | doi-access = free }}</ref>

Peroxisomes in mammals and humans also contribute to anti-viral defense.<ref>{{cite journal | vauthors = Dixit E, Boulant S, Zhang Y, Lee AS, Odendall C, Shum B, Hacohen N, Chen ZJ, Whelan SP, Fransen M, Nibert ML, Superti-Furga G, Kagan JC | display-authors = 6 | title = Peroxisomes are signaling platforms for antiviral innate immunity | journal = Cell | volume = 141 | issue = 4 | pages = 668–81 | date = May 2010 | pmid = 20451243 | pmc = 3670185 | doi = 10.1016/j.cell.2010.04.018  }}</ref> and the combat of pathogens <ref>{{cite journal | vauthors = Di Cara F, Bülow MH, Simmonds AJ, Rachubinski RA | title = Dysfunctional peroxisomes compromise gut structure and host defense by increased cell death and Tor-dependent autophagy | journal = Molecular Biology of the Cell | volume = 29 | issue = 22 | pages = 2766–2783 | date = November 2018 | pmid = 30188767 | pmc = 6249834 | doi = 10.1091/mbc.E18-07-0434 }}</ref>