Selenocysteine's Biosynthesis

Most proteins are built from 20 types of amino acids. But some organisms, including humans, use a 21st amino acid, selenocysteine (shown), in their proteins. The selenium in selenocysteine is believed to play an essential protective role in the body, including preventing cancer and heart disease and delaying aging. Dolph L. Hatfield of the National Cancer Institute and his coworkers have now discovered the previously unknown selenocysteine biosynthetic pathway (PLoS Biol., DOI: 10.1371/journal.pbio.0050004). They used comparative genomics studies to identify proteins that occur only in organisms that use selenocysteine. When one of these proteins, soluble liver antigen, was given appropriate substrates, it catalyzed the synthesis of selenocysteine. With this selenocysteine synthase in hand, Hatfield and coworkers were able to elucidate the entire pathway of selenocysteine biosynthesis in mammals. The study suggests that "this pathway is also active in other eukaryotes and archaea that synthesize selenoproteins," the researchers note.