Selenocysteine

Selenocysteine is an amino acid that is present in several enzymes (glutathione peroxidase, tetraiodothyronine 5' deiodinase, and formate dehydrogenase for example). Selenocysteine has a structure similar to cysteine, but with an atom of selenium taking the place of the usual sulfur. Proteins that include a selenocysteine residue are called selenoproteins.

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 surrounds the UGA codon. In archaea and in eukaryotes, the SECIS element is in the 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 differ from those of standard tRNAs in several respects, most notably in having an 8-base pair acceptor stem, a long variable region arm, and substitutions at several well-conserved base positions.