ubiquitous organelles in eukaryotes that participate in the metabolism of fatty acidsand other metabolites. Peroxisomes have enzymes that rid the cell of toxic peroxides. They have a single 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 lysosomes, peroxisomes are part of the secretory pathwayof 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 Duvein 1967cite 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 = ] after they had been first described in a Swedish PhD thesis a decade earlier.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 = ]
Occurrence and evolution
Peroxisomes are found in virtually all eukaryotic cells. Peroxisomes contain enzymes for certain oxidative reactions, like the
beta-oxidationof very-long-chain fatty acids. Prokaryoteslack 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 origin; that is, peroxisomes evolved from bacteria that invaded larger cells as parasites, and very gradually evolved a symbiotic relationship.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 ] However, this view has been challenged by recent discoveries.cite journal | author = Fagarasanu A, Fagarasanu M, Rachubinski, RA | title = Maintaining peroxisome populations: a story of division and inheritance| journal = Annu. Rev. Cell Dev. Biol. | volume = 23 | issue = | pages = 321-344| year = 2007 | pmid = 17506702 | doi = 10.1146/annurev.cellbio.23.090506.123456] 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.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 ]
An evolutionary analysis of the peroxisomal
proteomefound homologies between the peroxisomal import machinery and the ERADpathway in the endoplasmic reticulumcite 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 ] , along with a number of metabolic enzymes that were likely recruited from the mitochondria.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 ] 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.
Peroxisomes contain oxidative
enzymes, such as catalase, D-amino acid oxidase, and uric acid oxidase.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 = ] 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(H2O2, itself toxic):: Catalase, another enzyme in the peroxisome, in turn uses this H2O2 to oxidize other substrates, including phenols, formic acid, formaldehyde, and alcohol, by means of the peroxidation reaction: :, 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
ethanolwe drink is oxidized to acetaldehydein this way. In addition, when excess H2O2 accumulates in the cell, catalase converts it to H2O through this reaction::
A major function of the peroxisome is the breakdown of
fatty acidmolecules, 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 cytosolfor 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
plasmalogenin animal cells also occurs in peroxisomes. Plasmalogen is the most abundant phospholipid in myelin. Deficiency of plasmalogens causes profound abnormalities in the myelination of 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
bileacids and proteins.
In higher plants, peroxisomes contain also a complex battery of antioxidativeenzymes such as superoxide dismutase, the components of the ascorbate-glutathione cycle, and the NADP-dehydrogenases of the pentose-phosphate pathway. It hasbeen demonstrated the generation of superoxide (O2•-) and
nitric oxide(•NO) radicals. 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] ,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] .
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 reticulumduring 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 peroxins,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 ] 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, ubiquitinationappears 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.
Peroxisomal disordersare a class of conditions 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.
Gene which encode peroxisomal proteins include:
* PEX2 -
PEX11A, PEX11B, PEX11G
* [http://www.peroxisomeDB.org PeroxisomeDB: Peroxisome-Database]
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