Selenocysteine

 

Source: Wikipedia

Selenocysteine is a structure formed when the sulfur in cysteine is replaced by selenium. As a result of this substitution, selenocysteine has a lower reduction potential than cysteine and is more suitable as an antioxidant. Selenocysteine is a crucial amino acid in the composition of selenoproteins, which are found in both prokaryotes and eukaryotes. Consequently, selenocysteine is now considered one of the basic amino acids that make up proteins. In other words, proteins are no longer composed of just 20 amino acids, but 21!

A well-known selenoprotein is glutathione peroxidase, which is responsible for reducing hydrogen peroxide (H2O2) to water. Although selenocysteine is a basic amino acid in protein composition, unlike the other twenty amino acids, it doesn't have its own genetic codon. Instead, it uses UGA (a stop codon) along with a roughly sixty-base "selenocysteine insertion sequence" (SECIS element) as a signal for inserting selenocysteine. In prokaryotes, the SECIS element is located close to the UGA codon. In archaea and eukaryotes, the SECIS element is often found in the 3' untranslated region (3'-UTR) of the messenger RNA (mRNA).

Source: Wikipedia

When selenium is absent, the translation of selenoproteins stops at the UGA codon, resulting in incomplete, non-functional proteins. Only when selenium is present can complete proteins be produced. Selenocysteine was discovered in 1976 by Thressa Stadtman of the National Institutes of Health (NIH). She and her husband were the first married couple at NIH, each with their own laboratory, and the two labs also collaborated.

For the origin of cysteine, please refer to "Cysteine and Bladder." The "seleno-" in selenocysteine obviously comes from selenium, though "seleno-" is not a prefix. However, due to its association with selenoproteins, it seems to have been widely adopted.

References:

Wikipedia. Selenocysteine, Thressa Stadtman, SECIS element.