Selenocysteine

Selenocysteine has both a lower pKa (5.47) and a higher reduction potential than cysteine. These properties make it very suitable in proteins that are involved in anti-oxidant activity.
 * Selenocysteine ** (Sec or 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] ).
 * ~ Properties ||
 * [|Molecular formula] || C 3 H 7 NO 2 Se ||
 * [|Molar mass] || 168.05 g mol −1 ||

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. Such a mechanism is called translational recoding which efficiency depends on the selenoprotein to synthesize and on translation initiation factors. 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 typically 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.