Selenocysteine

Selenocysteine (Sec or U) is a rare amino acid which is present in some enzymes.

Structure of selenocysteine

As with all amino acids, selenocysteine is composed of a COOH group and an NH₂group which are involved in peptide bonds. Its side chain contains an atom of selenium: CH₂-SeH.

Function of selenocysteine

There is no free selenocysteine in the cell. Selenocysteine is produced from serine, which has previously been bound to a specific transfer RNA whose anti codon pairs with the UGA codon of the messenger RNA.

This codon is usually a stop codon which causes arrest in protein synthesis when it is read by the ribosome. A specific mechanism involving proteins and a loop and stem on the messenger RNA leaves a place where selenocysteine can be incorporated.