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.

Unlike other amino acids present in biological proteins, however, it is not coded for directly in the genetic code. Selenocysteine is encoded by a UGA codon, which is normally a stop codon but, in these exceptional proteins, is modified by a subsequent sequence in the mRNA molecule that encodes the enzyme. When cells are grown in the absence of selenium, protein translation terminates at the UGA codon, resulting in a truncated, nonfunctional enzyme.

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.