Selenocysteine

Selenocysteine is a protein-building amino acid in which selenium replaces cysteine’s sulfur. In specific genetic contexts, the UGA codon directs its insertion instead of terminating translation.

UGA codon: UGA is an RNA stop codon that can also encode selenocysteine in specific genetic contexts. A downstream RNA signal and specialized factors let translation machinery read this codon as selenocysteine.
Cysteine: Cysteine is an amino acid with a sulfur-containing side chain that can form disulfide bonds. Replacing cysteine’s sulfur with selenium gives selenocysteine its defining chemical difference.
Selenoprotein: A selenoprotein is a protein containing at least one selenocysteine residue. These proteins are the products of selenocysteine’s specialized incorporation pathway.
Thressa Stadtman: Thressa Stadtman was an American biochemist who helped establish selenocysteine as a genetically encoded amino acid. Her research identified selenocysteine in proteins and clarified its biological role.
SECIS element: A SECIS element is an RNA structure that enables UGA recoding as selenocysteine. This signal helps recruit the machinery that inserts selenocysteine at UGA.
Selenium: Selenium is a chemical element that is essential in small amounts for many organisms. Its incorporation into an amino acid gives selenocysteine distinctive redox chemistry.
Glutathione peroxidase: Glutathione peroxidases are enzymes that reduce peroxides using glutathione or related reducing systems. Several family members use selenocysteine to catalyze peroxide reduction.
Selenocysteine discovery: Selenocysteine discovery is the identification of this amino acid as a naturally occurring component of proteins. Its recognition revealed that selenium can be incorporated directly during protein synthesis.
Selenocysteine tRNA: Selenocysteine tRNA is a specialized transfer RNA that carries selenocysteine to the ribosome. It delivers the amino acid while recognizing UGA in the required translation context.
Genetic code: The genetic code maps nucleotide codons to amino acids or translation termination signals. Selenocysteine is a rare exception to the usual assignment of UGA as a stop signal.

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