Peroxisome
24062
225025131
2008-07-11T14:54:50Z
PeroxisomeDB
7460076
[[Image:Peroxisome.jpg|300px|right|thumb|Basic structure of a peroxisome]]
'''Peroxisomes''' are [[ubiquitous]] [[organelle]]s in [[eukaryote]]s that participate in the metabolism of [[fatty acids]] and other metabolites. Peroxisomes have enzymes that rid the [[cell (biology)|cell]] of toxic [[peroxide]]s. They have a single [[lipid bilayer|lipid bilayer membrane]] that separates their contents from the [[cytosol]] (the internal fluid of the cell) and contain membrane proteins critical for various functions, such as importing proteins into the organelles and aiding in proliferation. Like [[lysosome]]s, peroxisomes are part of the [[secretory pathway]] of a cell, but they are much more dynamic and can replicate by enlarging and then dividing. Peroxisomes were identified as cellular organelles by the Belgian cytologist [[Christian de Duve]] in [[1967]]<ref name="pmid4389648">{{cite journal | author = de Duve C | title = The peroxisome: a new cytoplasmic organelle | journal = Proc. R. Soc. Lond., B, Biol. Sci. | volume = 173 | issue = 30 | pages = 71–83 | year = 1969 | pmid = 4389648 | doi = | issn = }}</ref> after they had been first described in a Swedish PhD thesis a decade earlier.<ref name="Rhodin_1954">{{cite journal | author = Rhodin, J | title = Correlation of ultrastructural organization and function in normal and experimentally changed proximal tubule cells of the mouse kidney | journal = Doctorate Thesis. Karolinska Institutet, Stockholm | volume = | issue = | pages = | year = 1954 | pmid = | doi = | issn = }}</ref>
== Occurrence and evolution==
Peroxisomes are found in virtually all eukaryotic cells. Peroxisomes contain enzymes for certain oxidative reactions, like the [[beta-oxidation]] of very-long-chain fatty acids. [[Prokaryotes]] lack peroxisomes. The enzymatic content of peroxisomes varies across species, but the presence of certain proteins common to many species has been used to suggest an [[Endosymbiotic theory|endosymbiotic]] origin; that is, peroxisomes evolved from bacteria that invaded larger cells as parasites, and very gradually evolved a symbiotic relationship.<ref name="pmid3916321">{{cite journal | author = Lazarow PB, Fujiki Y | title = Biogenesis of peroxisomes | journal = Annu. Rev. Cell Biol. | volume = 1 | issue = | pages = 489–530 | year = 1985 | pmid = 3916321 | doi = 10.1146/annurev.cb.01.110185.002421 }}</ref> However, this view has been challenged by recent discoveries. For example, peroxisome-less mutants can restore peroxisomes upon introduction of the wild-type gene, and peroxisomes have been observed to be formed from the [[endoplasmic reticulum]].<ref name="pmid16009135">{{cite journal | author = Hoepfner D, Schildknegt D, Braakman I, Philippsen P, Tabak HF | title = Contribution of the endoplasmic reticulum to peroxisome formation | journal = Cell | volume = 122 | issue = 1 | pages = 85–95 | year = 2005 | pmid = 16009135 | doi = 10.1016/j.cell.2005.04.025 }}</ref>
An evolutionary analysis of the peroxisomal [[proteome]] found homologies between the peroxisomal import machinery and the [[ERAD]] pathway in the [[endoplasmic reticulum]] <ref name="pmid16452116">{{cite journal | author = Schlüter A, Fourcade S, Ripp R, Mandel JL, Poch O, Pujol A | title = The evolutionary origin of peroxisomes: an ER-peroxisome connection | journal = Mol Biol Evol | volume = 23(4) | issue = | pages = 838-45 | year = 2006 | pmid = 16452116 | doi = 10.1093/molbev/msj103 }}</ref>, along with a number of metabolic enzymes that were likely recruited from the [[mitochondria]].<ref name="pmid16556314">{{cite journal | author = Gabaldón T, Snel B, van Zimmeren F, Hemrika W, Tabak H, Huynen MA | title = Origin and evolution of the peroxisomal proteome | journal = Biol. Direct | volume = 1 | issue = | pages = 8 | year = 2006 | pmid = 16556314 | doi = 10.1186/1745-6150-1-8 }}</ref> These results indicate that the peroxisome does not have an endosymbiotic origin; instead, it likely originates from the ER, and its proteins were recruited from pools existing within the primitive eukaryote, as quoted in the science textbook Biozone.
== Function ==
Peroxisomes contain oxidative [[enzyme]]s, such as [[catalase]], [[D-amino acid oxidase]], and [[uric acid oxidase]].<ref name="pmid1334030">{{cite journal | author = del Río LA, Sandalio LM, Palma JM, Bueno P, Corpas FJ | title = Metabolism of oxygen radicals in peroxisomes and cellular implications | journal = Free Radic. Biol. Med. | volume = 13 | issue = 5 | pages = 557–80 | year = 1992 | pmid = 1334030 | doi = | issn = }}</ref> 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 enzyme in the peroxisome, in turn uses this H<sub>2</sub>O<sub>2</sub> to oxidize other substrates, including [[phenols]], [[formic acid]], [[formaldehyde]], and [[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]] we drink is oxidized to [[acetaldehyde]] in this way. 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>
A major function of the peroxisome is the breakdown of [[fatty acid]] molecules, in a process called [[beta-oxidation]]. In this process, the fatty acids are broken down two carbons at a time, converted to [[Acetyl-CoA]], which is then transported back to the [[cytosol]] for further use. In animal cells, beta-oxidation can also occur in the mitochondria. In yeast and plant cells, this process is exclusive for the peroxisome.
The first reactions in the formation of [[plasmalogen]] in animal cells also occurs 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 of the reasons that many peroxisomal disorders lead to neurological disease.
Peroxisomes also play a role in the production of [[bile]] acids and proteins.
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 the generation of superoxide (O<sub>2</sub><small><sup>•-</sup></small>) and [[nitric oxide]] (<sup>•</sup>NO) radicals. <ref >{{cite journal | author = Corpas F.J., Barroso, J.B., del Río, L.A.| title = Peroxisomes as a source of reactive oxygen species and nitric oxide signal molecules in plant cells| journal = Trends Plant Sci | volume = 6 | issue = | pages = 145–150 | year = 2001| doi = 10.1016/S1360-1385(01)01898-2}}</ref>,<ref >{{cite journal | author = Corpas FJ et al.| title = Cellular and subcellular localization of endogenous nitric oxide in young and senescent pea plants | journal = Plant Physiol | volume = 136 | issue = | pages = 2722–2733| year = 2004| doi = 10.1104/pp.104.042812| pmid = 15347796}}</ref>.
== Protein import ==
Proteins are selectively imported into peroxisomes. Since the organelles contain no DNA or ribosomes and thus have no means of producing proteins, all of their proteins must be imported across the membrane. It is believed that necessary proteins enter through the [[endoplasmic reticulum]] during biogenesis as well as through membrane proteins.
A specific protein signal (PTS or [[peroxisomal targeting signal]]) of three amino acids at the ''[[C-terminus]]'' of many peroxisomal proteins signals the membrane of the peroxisome to import them into the organelle. Other peroxisomal proteins contain a signal at the ''[[N-terminus]]''. There are at least 32 known peroxisomal proteins, called [[peroxin]]s,<ref name="pmid17050007">{{cite journal | author = Saleem RA, Smith JJ, Aitchison JD | title = Proteomics of the peroxisome | journal = Biochim. Biophys. Acta | volume = 1763 | issue = 12 | pages = 1541–51 | year = 2006 | pmid = 17050007 | doi = 10.1016/j.bbamcr.2006.09.005| url = http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1858641&blobtype=pdf }}</ref> which participate in the process of importing proteins by means of ''[[ATP hydrolysis]]''. Proteins do not have to unfold to be imported into the peroxisome. The protein receptors, the peroxins ''[[PEX5]]'' and ''[[PEX7]]'', accompany their cargoes (containing a PTS1 or a PTS2, respectively) all the way into the peroxisome where they release the cargo and then return to the [[cytosol]] - a step named ''recycling''. Overall, the import cycle is referred to as the ''extended shuttle mechanism''.
Evidence now indicates that ATP hydrolysis is required for the recycling of receptors to the [[cytosol]]. Also, [[ubiquitination]] appears to be crucial for the export of PEX5 from the peroxisome, to the cytosol. Little is known about the import of PEX7, although it has helper proteins that have been shown to be ubiquitinated.
== Deficiencies ==
[[Peroxisomal disorders]] are a class of condtions that lead to disorders of [[lipid metabolism]]. One well-known example is [[Zellweger syndrome]].
Peroxisomes matrix proteins are synthesized on free ribosomes in the cytosol and that these proteins are imported posttranslationally in pre-existing peroxisomes.
==Genes==
Gene which encode peroxisomal proteins include:
* [[PEX1]]
* PEX2 - [[PXMP3]]
* [[PEX3]]
* [[PEX5]]
* [[PEX6]]
* [[PEX7]]
* [[PEX10]]
* [[PEX11A]], [[PEX11B]], [[PEX11G]]
* [[PEX12]]
* [[PEX13]]
* [[PEX14]]
* [[PEX16]]
* [[PEX19]]
* [[PEX26]]
==References ==
{{Reflist|2}}
==External links==
{{wikiversity3|Topic:Cell Biology|Peroxisomes|The Department of Cell Biology}}
*[http://www.peroxisomeDB.org PeroxisomeDB: Peroxisome-Database]
{{NCBI-scienceprimer}}
{{organelles}}
[[Category:Organelles]]
[[Category:Metabolism]]
[[ar:جسيم تأكسدي]]
[[bg:Пероксизома]]
[[id:Peroksisom]]
[[ca:Peroxisoma]]
[[cs:Peroxizom]]
[[de:Peroxisom]]
[[es:Peroxisoma]]
[[eo:Peroksisomo]]
[[fr:Peroxysome]]
[[hr:Peroksisom]]
[[it:Perossisoma]]
[[he:פראוקסיזום]]
[[lt:Peroksisoma]]
[[nl:Peroxisoom]]
[[ja:ペルオキシソーム]]
[[oc:Peroxisòma]]
[[pl:Peroksysom]]
[[pt:Peroxissoma]]
[[ru:Пероксисома]]
[[sr:Пероксизом]]
[[sh:Peroksizom]]
[[su:Peroksisom]]
[[fi:Peroksisomi]]
[[sv:Peroxisom]]
[[vi:Peroxisome]]
[[uk:Пероксисома]]
[[zh:过氧物酶体]]