Selenocysteine is a rare biological amino acid found in several enzymes. Its structure is similar to cysteine, but it has a selenium atom replacing the usual sulfur atom, thus forming a selenol group. Selenocysteine is a stronger acid and has a higher reduction potential than cysteine. These properties make it optimal for incorporation in proteins involved in antioxidant activity.
Selenocysteine is not directly coded for in the genetic code; rather, it is encoded by a UGA codon, which is normally a stop codon. The UGA codon is made to encode selenocysteine by the presence of a SECIS (SElenoCysteine Insertion Sequence) element in the coding 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 in the reading frame for the selenoprotein. In archaea and eukaryotes, the SECIS element is in the...