ImageFile = L-selenocysteine-2D-skeletal.png
ImageFile1 = Selenocysteine-3D-vdW.png
IUPACName = L-Selenocysteine
Section1 = Chembox Identifiers
CASNo = 10236-58-5
SMILES = N [C@@H] (C [SeH] )C(O)=O
Section2 = Chembox Properties
Formula = C3H7NO2Se
MolarMass = 168.053 g/mol
Section3 = Chembox Hazards
Selenocysteine is an
amino acidthat is present in several enzymes (for example glutathione peroxidases, tetraiodothyronine 5' deiodinases, thioredoxin reductases, formate dehydrogenases, glycine reductases and some hydrogenases).
The joint nomenclature committee of the
IUPAC/ IUBMBhas officially recommended the three-letter symbol Sec and the one-letter symbol U for selenocysteine. [cite journal| author = IUPAC-IUBMB Joint Commission on Biochemical Nomenclature (JCBN) and Nomenclature Committee of IUBMB (NC-IUBMB) | title = Newsletter 1999 | year = 1999 | journal = European Journal of Biochemistry| volume = 264 | issue = 2 | pages = 607–609 | doi = 10.1046/j.1432-1327.1999.news99.x | url = http://www.chem.qmul.ac.uk/iubmb/newsletter/1999/item3.html | format = reprint, with permission]
Selenocysteine has a structure similar to
cysteine, but with an atom of seleniumtaking the place of the usual sulfur. Proteins that contain one or more selenocysteine residues are called selenoproteins.
Unlike other amino acids present in biological
proteins, Selenocysteine is not coded for directly in the genetic code. Instead, it 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 element(SElenoCysteine Insertion Sequence) in the mRNA. The SECIS element is defined by characteristic nucleotide sequences and secondary structure base-pairing patterns. In Bacteria, the SECIS element is located immediately following the UGA codon within the reading frame for the selenoprotein. In archaeaand in eukaryotes, 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.
Pyrrolysine, another amino acid not in the basic set of 20.
* cite journal
author = Boyce E. Cone, Rafael Martin Del Rio, Joe Nathan Davis, and Thressa C. Stadtman
title = Chemical Characterization of the Selenoprotein Component of Clostridial Glycine Reductase: Identification of Selenocysteine as the Organoselenium Moiety
year = 1976
journal = PNAS
volume = 73
issue = 8
pages = 2659–2663
doi = 10.1146/annurev.bi.65.070196.000503
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Selenium — (pronEng|səˈliniəm) is a chemical element with the atomic number 34, represented by the chemical symbol Se, an atomic mass of 78.96. It is a nonmetal, chemically related to sulfur and tellurium, and rarely occurs in its elemental state in nature … Wikipedia
SECIS element — In biology, the SECIS element (SECIS: selenocysteine insertion sequence ) is an RNA element around 60 nucleotides in length that adopts a stem loop structure. [cite journal | last = Walczak | first = R | coauthors = Westhof E, Carbon P, Krol A |… … Wikipedia