Selenocysteine
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| Selenocysteine | |
|---|---|
| |
| Image:Selenocysteine-3D-vdW.png | |
| IUPAC name | L-Selenocysteine |
| Identifiers | |
| CAS number | |
| SMILES | N[C@@H](C[SeH])C(O)=O |
| Properties | |
| Molecular formula | C3H7NO2Se |
| Molar mass | 168.053 g/mol |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references | |
Selenocysteine is an amino acid that is present in several enzymes (for example glutathione peroxidases, tetraiodothyronine 5' deiodinases, thioredoxin reductases, formate dehydrogenases, glycine reductases and some hydrogenases).
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.[1]
Structure
Selenocysteine has a structure similar to cysteine, but with an atom of selenium taking the place of the usual sulfur. Proteins that contain one or more selenocysteine residues are called selenoproteins.
Biology
Unlike other amino acids present in biological proteins, however, it is not coded for directly in the genetic code. 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 element (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 archaea and 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.
See also
- Sodium selenite
- Pyrrolysine, another amino acid not in the basic set of 20.
References
Further reading
- F. Zinoni, A. Birkmann, T. C. Stadtman and A. Bock (1986). "Nucleotide Sequence and Expression of the Selenocysteine-Containing Polypeptide of Formate Dehydrogenase (Formate-hydrogen-lyase-Linked) from Escherichia coli". PNAS 83 (13): 4650-4654. PMID 2941757.
- F. Zinoni, A. Birkmann, W. Leinfelder and A. Bock (1987). "Cotranslational Insertion of Selenocysteine into Formate Dehydrogenase from Escherichia coli Directed by a UGA Codon". PNAS 84 (10): 3156-3160.
- Boyce E. Cone, Rafael Martin Del Rio, Joe Nathan Davis, and Thressa C. Stadtman (1976). "Chemical Characterization of the Selenoprotein Component of Clostridial Glycine Reductase: Identification of Selenocysteine as the Organoselenium Moiety". PNAS 73 (8): 2659-2663. doi:10.1146/annurev.bi.65.070196.000503.ca:Selenocisteïna
de:Selenocysteineo:Selenocisteino fr:Sélénocystéine ko:셀레노시스테인 it:Selenocisteina lt:Selenocisteinas ja:セレノシステイン no:Selenocystein
Acknowledgement and Attribution Regarding Sources of Content
Some of the initial content on this page may be incorporated in part from copyleft sources in the public domain including wikis such as Wikipedia and AskDrWiki. Drug information for patients came from the The National Library of Medicine. Infectious disease information may have come from the Centers for Disease Control (CDC). Differential Diagnoses are drawn from clinicians as well as an amalgamation of 3 sources: 1.The Disease Database; 2. Kahan, Scott, Smith, Ellen G. In A Page: Signs and Symptoms. Malden, Massachusetts: Blackwell Publishing, 2004:3; 3. Sailer, Christian, Wasner, Susanne. Differential Diagnosis Pocket. Hermosa Beach, CA: Borm Bruckmeir Publishing LLC, 2002:7 .
