Selenocysteine: Facts, Discussion Forum, and Encyclopedia Article

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Selenocysteine

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	Selenocysteine
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	Topics Selenocysteine Topic Home Selenocysteine is an amino acidAmino acid In chemistry, an amino acid is any molecule that contains both amine and carboxyl functional groups.... that is present in several enzymeEnzyme Enzymes are proteins that accelerate, or catalyze, chemical reactions.... s (for example glutathione peroxidaseGlutathione peroxidase Glutathione peroxidase is a peroxidase found in the erythrocytes of mammals that helps prevent lipid peroxidation of the cel... s, tetraiodothyronine 5' deiodinaseTetraiodothyronine 5 deiodinase Tetraiodothyronine 5 deiodinase is an enzyme. The amino acid selenocysteine is one of its constituent parts.... s, thioredoxin reductaseThioredoxin reductase Thioredoxin Reductase are the only known enzymes to reduce thioredoxin.... s, formate dehydrogenaseFormate dehydrogenase Formate dehydrogenases are a set of enzymes that catalyse the oxidation of formate to bicarbonate, donating the electrons to... s, glycine reductaseGlycine reductase In enzymology, a glycine reductase is an enzyme that catalyzes the chemical reaction... s and some hydrogenaseHydrogenase Overview A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen.... s). Nomenclature The joint nomenclature committee of the IUPAC/IUBMB has officially recommended the three-letter symbol Sec and the one-letter symbol U for selenocysteine. Structure Selenocysteine has a structure similar to cysteineCysteine Cysteine is an a-amino acid with the chemical formula HO2CCHCH2SH.... , but with an atom of seleniumSelenium Selenium is a chemical element with atomic number 34, with the chemical symbol Se.... taking the place of the usual sulfurSulfur Sulfur or sulphur is the chemical element in the periodic table that has the symbol S and atomic number 16.... . Proteins that contain one or more selenocysteine residues are called selenoproteinSelenoprotein A selenoprotein is any protein that includes a selenocysteine residue.... s. Biology Unlike other amino acids present in biological proteinProtein Proteins are large organic compounds made of amino acids arranged in a linear chain and joined by peptide bonds.... s, however, it is not coded for directly in the genetic codeGenetic code The genetic code is the set of rules by which information encoded in genetic material is translated into proteins by livin... . Selenocysteine is encoded in a special way by a UGA codon, which is normally a stop codon. The UGA codon is made to encode selenocysteine by the presence of a SECIS elementSECIS element In biology, the SECIS element is a structural motif that directs the cell to translate UGA codons as selenocysteines.... (SElenoCysteine Insertion Sequence) in the mRNA. The SECIS element is defined by characteristic nucleotide sequences and secondary structure base-pairing patterns. In eubacteria, the SECIS element is located immediately following the UGA codon within the reading frame for the selenoprotein. In archaeaArchaea Archaea , also called Archaebacteria , is a major division of living organisms.... and in eukaryoteEukaryote \|-\| style = "background: pink; padding: 4px;" \| Animalia - Animals... s, the SECIS element is in the 3' untranslated region (3' UTR) of the mRNA, and can direct multiple UGA codons to encode selenocysteine residues. When cells are grown in the absence of selenium, translation of selenoproteins terminates at the UGA codon, resulting in a truncated, nonfunctional enzyme. Like the other amino acids used by cells, selenocysteine has a specialized tRNA. The primary and secondary structure of selenocysteine tRNA, tRNA(Sec), differ from those of standard tRNAs in several respects, most notably in having an 8-base (bacteria) or 9-base (eukaryotes) pair acceptor stem, a long variable region arm, and substitutions at several well-conserved base positions. The selenocysteine tRNAs are initially charged with serine by seryl-tRNA ligase, but the resulting Ser-tRNA(Sec) is not used for translation because it is not recognised by the normal translation factor (EF-Tu in bacteria, EF1-alpha in eukaryotes). Rather, the tRNA-bound seryl residue is converted to a selenocysteyl-residue by the pyridoxal phosphate-containing enzyme selenocysteine synthase. Finally, the resulting Sec-tRNA(Sec) is specifically bound to an alternative translational elongation factor (SelB or mSelB) which delivers it in a targeted manner to the ribosomes translating mRNAs for selenoproteins. The specificity of this delivery mechanism is brought about by the presence of an extra protein domain (in bacterial SelB) or an extra subunit (SBP-2 for eukaryotic mSelB) which bind to the corresponding RNA secondary structures formed by the SecIS elements in selenoprotein mRNAs. The SecIS elements of bacterial selenoproteins (as far as analysed) are located within the coding sequences immediately following the UGA codons for selenocysteine, those of Eukarya and Archaea are located in the 3' UTR of the respective mRNAs. In addition, at least one case has been described for an archaeal selenoprotein mRNA containing its SecIS in the 5' UTR. See also Sodium seleniteSodium selenite Sodium selenite, Na2SeO3, is a selenium compound.... PyrrolysinePyrrolysine Summary Pyrrolysine is a naturally-occurring genetically-coded amino acid.... , another amino acid not in the basic set of 20. Further reading