Selenocysteine

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Selenocysteine[1]
Names
IUPAC name
Selenocysteine
Systematic IUPAC name
3-Selanyl-L-alanine (semisystematic name)
2-Amino-3-selanylpropanoic acid (fully systematic name)
Other names
L-Selenocysteine; Selenium-cysteine
Identifiers
3D model (
JSmol
)
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard
 100.236.386 Edit this at Wikidata
KEGG
UNII
  • InChI=1S/C3H7NO2Se/c4-2(1-7)3(5)6/h2,7H,1,4H2,(H,5,6)/t2-/m0/s1 checkY
    Key: ZKZBPNGNEQAJSX-REOHCLBHSA-N checkY
  • InChI=1/C3H7NO2Se/c4-2(1-7)3(5)6/h2,7H,1,4H2,(H,5,6)/t2-/m0/s1
    Key: ZKZBPNGNEQAJSX-REOHCLBHBZ
  • O=C(O)[C@@H](N)C[SeH]
  • Zwitterion: O=C([O-])[C@@H]([NH3+])C[SeH]
Properties
C3H7NO2Se
Molar mass 168.065 g·mol−1
Properties
Acidity (pKa) 5.24,[2] 5.43[3]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Selenocysteine (symbol Sec or U,

Selenoproteins contain selenocysteine residues. Selenocysteine is an analogue of the more common cysteine with selenium in place of the sulfur
.

Selenocysteine is present in several

tetraiodothyronine 5′ deiodinases, thioredoxin reductases, formate dehydrogenases, glycine reductases, selenophosphate synthetase 2, methionine-R-sulfoxide reductase B1 (SEPX1), and some hydrogenases). It occurs in all three domains of life, including important enzymes (listed above) present in humans.[6]

Selenocysteine was discovered in 1974[7] by biochemist Thressa Stadtman at the National Institutes of Health.[8]

Chemistry

Selenocysteine is the Se-analogue of cysteine. It is rarely encountered outside of living tissue (and is not available commercially) because it is very susceptible to air-oxidation. More common is the oxidized derivative

deprotonated at physiological pH.[10]

Structure

Selenocysteine has the same structure as cysteine, but with an atom of selenium taking the place of the usual sulfur. It has a selenol group. Like other natural proteinogenic amino acids, cysteine and selenocysteine have L chirality in the older D/L notation based on homology to D- and L-glyceraldehyde. In the newer R/S system of designating chirality, based on the atomic numbers of atoms near the asymmetric carbon, they have R chirality, because of the presence of sulfur or selenium as a second neighbor to the asymmetric carbon. The remaining chiral amino acids, having only lighter atoms in that position, have S chirality.)

Proteins which contain a selenocysteine residue are called selenoproteins. Most selenoproteins contain a single selenocysteine residue. Selenoproteins that exhibit catalytic activity are called selenoenzymes.[11]

Biology

Selenocysteine has a lower reduction potential than cysteine. These properties make it very suitable in proteins that are involved in antioxidant activity.[12]

Although it is found in the

3′ untranslated region (3′ UTR) of the mRNA and can direct multiple UGA codons to encode selenocysteine residues.[18]

Unlike the other amino acids, no free pool of selenocysteine exists in the cell. Its high reactivity would cause damage to cells.

tRNA
, which also functions to incorporate it into nascent polypeptides.

The primary and secondary structure of selenocysteine-specific tRNA, tRNASec, differ from those of standard tRNAs in several respects, most notably in having an 8-base-pair (bacteria) or 10-base-pair (eukaryotes)[Archaea?] 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-tRNASec is not used for translation because it is not recognised by the normal translation elongation factor (EF-Tu in bacteria, eEF1A in eukaryotes).[Archaea?]

Rather, the tRNA-bound seryl residue is converted to a selenocysteine residue by the pyridoxal phosphate-containing enzyme selenocysteine synthase. In eukaryotes and archaea, two enzymes are required to convert tRNA-bound seryl residue into tRNA selenocysteinyl residue: PSTK (O-phosphoseryl-tRNA[Ser]Sec kinase) and selenocysteine synthase.[20][21] Finally, the resulting Sec-tRNASec is specifically bound to an alternative translational elongation factor (SelB or mSelB (or eEFSec)), 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 bacteria, SelB) or an extra subunit (SBP2 for eukaryotic mSelB/eEFSec)[Archaea?] which bind to the corresponding RNA secondary structures formed by the SECIS elements in selenoprotein mRNAs.

Selenocysteine is decomposed by the enzyme selenocysteine lyase into L-alanine and selenide.[22]

As of 2021, 136 human proteins (in 37 families) are known to contain selenocysteine (selenoproteins).[23]

Selenocysteine derivatives γ-glutamyl-Se-methylselenocysteine and

Se-methylselenocysteine occur naturally in plants of the genera Allium and Brassica.[24]

Applications

Biotechnological applications of selenocysteine include use of 73Se-labeled Sec (half-life of 73Se = 7.2 hours) in

NMR, among others.[6]

See also

  • Pyrrolysine, another amino acid not in the basic set of 20.
  • Selenomethionine, another selenium-containing amino acid, which is randomly substituted for methionine.

References

  1. ^ Merck Index, 12th Edition, 8584
  2. PMID 6076213
    .
  3. ^
    PMID 27748600
    .
  4. ^ "Nomenclature and Symbolism for Amino Acids and Peptides". IUPAC-IUB Joint Commission on Biochemical Nomenclature. 1983. Archived from the original on 9 October 2008. Retrieved 5 March 2018.
  5. doi:10.1046/j.1432-1327.1999.news99.x
    .
  6. ^
    PMID 15967579
    .
  7. ^ "Stadtman Pioneer of Selenium Biochemistry - Office of NIH History and Stetten Museum". history.nih.gov. Retrieved 2023-04-06.
  8. S2CID 84982102
    .
  9. PMID 26090162
    .
  10. PMID 26949981
    .
  11. S2CID 32351033
    .
  12. S2CID 11002236
    .
  13. PMID 15069108
    .
  14. PMID 1838215
    .
  15. S2CID 976337
    .
  16. PMID 20488192
    .
  17. ISBN 978-0-387-89381-5
    .
  18. PMID 8344267
    .
  19. PMID 7959166
    .
  20. PMID 17194211
    .
  21. PMID 17142313
    .
  22. PMID 24987004
    .
  23. PMID 24194593. [1]
  24. ISBN 978-0-85404-190-9
    .

Further reading

External links

Scholia has a topic profile for Selenocysteine.
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