Selenoprotein
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A '''selenoprotein''' is any [[protein]] that includes a [[selenocysteine]] residue. Selenoproteins exist in all major forms of life, [[eukaryote]], [[eubacteria]] and [[archaea]]. Among [[eukaryote]]s, selenoproteins appear to be common in [[animal]]s, but rare or absent in other phyla (one has been identified in the green [[alga]] ''[[Chlamydomonas]]'', but none in other [[plant]]s or in [[fungi]]). Among [[eubacteria]] and [[archaea]], selenoproteins are only present in some lineages, while they are completely absent in many other phylogenetic groups. These observations have recently been confirmed by whole genome analysis, which shows the presence or absence of selenoprotein genes and accessory genes for the synthesis of selenoproteins in the respective organism.
Besides the [[selenocysteine]]-containing selenoproteins, there are also some selenoproteins known from bacterial species, which have [[selenium]] bound non-covalently. Most of these proteins are thought to contain a selenide-ligand to a [[molybdopterin]] cofactor at their active sites (e. g. [[nicotinate dehydrogenase]] of ''[[Eubacterium barkeri]]'', or [[xanthine dehydrogenase]]s). Selenium is also specifically incorporated into modified bases of some [[tRNA]]s (as 2-seleno-5-methylaminomethyl-uridine).
In addition, selenium occurs in proteins as unspecifically incorporated [[selenomethionine]], which replaces methionine residues. Proteins containing such unspecifically incorporated [[selenomethionine]] residues are not regarded as selenoproteins. However, replacement of all [[methionine]]s by [[selenomethionine]]s is a widely used recent technique in solving the phase problem during X-ray crystallographic structure determination of many proteins ([[MAD-phasing]]). While the exchange of [[methionine]]s by [[selenomethionine]]s appears to be tolerated (at least in bacterial cells), unspecific incorporation of [[selenocysteine]] in lieu of [[cysteine]] seems to be highly toxic. This may be one reason for the existence of a rather complicated pathway of [[selenocysteine]] biosynthesis and specific incorporation into selenoproteins, which avoids the occurrence of the free amino acid as intermediate. Thus, even if a [[selenocysteine]]-containing selenoprotein is taken up in the diet and used as selenium-source, the amino acid has to be degraded prior to synthesising a new [[selenocysteine]] for incorporation into a selenoprotein.
[[Selenium]] is a vital nutrient in [[animal]]s and [[humans]]. About 25 different [[selenocysteine]]-containing selenoproteins have so far been observed in human [[cell (biology)|cell]]s and tissues. Since lack of selenium deprives the cell's ability to synthesize selenoproteins, many health effects of low selenium intake are believed to be caused by the lack of on or more specific selenoproteins. In fact, 3 selenoproteins, TR1, TR3 and GPx4, have been shown to be essential in mice knockout experiments. On the other side, too much selenium in the diet causes toxic effects and leads to [[selenium poisoning]]. The threshold between essential and toxic concentrations of this element is rather narrow (the factor is in the range of 10-100).
==References==
* {{cite journal | author=G. V. Kryukov, S. Castellano, S. V. Novoselov, A. V. Lobanov, O. Zehtab, R. Guigó, and V. N. Gladyshev | title=Characterization of mammalian selenoproteomes | journal=Science | year=2003 | volume=300 | issue=5624 | pages= 1439–1443 | doi=10.1126/science.1083516 | pmid=12775843}}
* {{cite journal | author=Gregory V. Kryukov and Vadim N. Gladyshev | title=The prokaryotic selenoproteome | journal=EMBO Rep | year=2004 | volume=5 | issue=5 | pages= 538–543 | doi=10.1038/sj.embor.7400126}}
* {{cite journal | author=Matilde Maiorino, Valentina Boselloa, Fulvio Ursinia, Carlo Forestab, Andrea Garollab, Margherita Scapina, Helena Sztajerc, and Leopold Flohéc | title=Genetic variations of gpx-4 and male infertility in humans | journal=Biol Reprod | year=2003 | volume=68 | issue=4 | pages= 1134–1141 | doi=10.1095/biolreprod.102.007500 | pmid=12606444}}
[[Category:Selenium compounds]]
[[Category:Proteins]]
[[ca:Selenoproteïna]]