Selenocysteine

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Selenocysteine

Selenocysteine (pronounced: /ˌsɛlɪnoʊˈsɪstiːn/) is a naturally occurring amino acid that is present in several enzymes (selenoproteins).

Etymology

The term "selenocysteine" is derived from the Greek word "selēnē" meaning moon and the English word "cysteine". The element selenium was named after the moon, and selenocysteine is so named because it is a cysteine analogue with a selenium-containing selenol group in place of the sulfur-containing thiol group.

Structure

Selenocysteine has the formula H2NCH2CH2SeH. Its structure is similar to that of cysteine, but with an atom of selenium taking the place of the usual sulfur, forming a selenol group which is deprotonated at physiological pH.

Biological Role

Selenocysteine is present in several enzymes, including glutathione peroxidases, thioredoxin reductases, and iodothyronine deiodinases. It is encoded directly into protein by a UGA codon, which is normally a stop codon in other organisms, but is modified in organisms that use selenocysteine.

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