A rather odd amino acid, often called the 21st amino acid, which is a variant of cysteine with the trace element selenium instead of sulfur.

Not many living things use this amino acid. One such example is Brewer's Yeast.

Discovered in1986, selenocysteine is the 21st naturally occuring amino acid, and is coded for by the RNA codon UGA, which is normally a "stop" signal, but is modified in some organisms to create selenocysteine by a subsequent RNA loop, which is interpreted by a group of genes called the sel group, which are activated by the loop in the mRNA and produce molecule of tRNA for selenocysteine. The molecule itself is effectively a cysteine amino acid with the sulphur atom replaced by a selenium atom. It occurs in several enzymes and in a large number of organisms, including humans, but is much rarer than most of the other amino acids.

Interestingly a protein containing selenocysteine has been found in HIV-1, and may shed new light on the action of the virus, as it is known to deplete selenium levels within the body

Chemical structure:
        |  |  |
        O  H  H  

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