Selenocysteine

Selenocysteine (Se-Cys) 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.

Structure
Selenocysteine has a structure similar to that of cysteine, but with an atom of selenium taking the place of the usual sulfur, forming a selenol group. Proteins that contain one or more selenocysteine residues are called selenoproteins.

Biology
Selenocysteine has both a lower pKa and a higher reduction potential than cysteine. These properties make it very suitable in proteins that are involved in anti-oxidant activity.

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. When cells are grown in the absence of selenium, translation of selenoproteins terminates at the UGA codon, resulting in a truncated, nonfunctional enzyme. 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 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.

Unlike the other amino acids, no free pool of selenocysteine exists in the cell. Its high reactivity would incur damage to cells. Instead, cells store selenium in the less reactive selenide form (H2Se). Selenocysteine synthesis occurs on a specialized tRNA, which also functions to incorporate it into nascent polypeptides. 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 10-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, eEF1A in eukaryotes). Rather, the tRNA-bound seryl residue is converted to a selenocysteine-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 (a.k.a. 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 bacterial SelB) or an extra subunit (SBP2 for eukaryotic mSelB/eEFSec) which bind to the corresponding RNA secondary structures formed by the SECIS elements in selenoprotein mRNAs.