An
amino acid used in several enzymes
? ([glutathione peroxidase]
?, [tetraikiodothyronine 5' deiodinase]
? and [formate dehydrogenase]
? for example). Selenocystein has a structure similar to cysteine
?, but with an atom of
selenium taking the place of the usual
sulfur. Unlike other amino acids used in biological
protein, however, it is not coded for directly in the
genetic code. It is coded for by a UGA
codon, which is normally a stop codon but in these exceptional cases is modified by a subsequent sequence in the
mRNA molecule encoding the enzyme. When cells are grown in the absence of selenium protein translation terminates normally at the UGA codon instead, resulting in a truncated and nonfunctional enzyme. The primary and secondary structure of selenocysteine
tRNAs differ from standard tRNAs in several respects, most notably an eight base pair acceptor stem, a long variable region arm, and substitutions at several well-conserved base positions.