Selenocysteine: the 21st amino acid

Abstract

SummaryGreat excitement was elicited in the field of selenium biochemistry in 1986 by the parallel discoveries that the genes encoding the selenoproteins glutathione peroxidase and bacterial formate dehydrogenase each contain an in-frame TGA codon within their coding sequence. We now know that this codon directs the incorporation of selenium, in the form of selenocysteine, into these proteins. Working with the bacterial system has led to a rapid increase in our knowledge of selenocysteine biosynthesis and to the exciting discovery that this system can now be regarded as an expansion of the genetic code. The prerequisites for such a definition are co-translational insertion into the polypeptide chain and the occurrence of a tRNA molecule which carries selenocysteine. Both of these criteria are fulfilled and, moreover, tRNA^"'' even has its own special translation factor which delivers it to the translating ribosome. It is the aim of this article to review the events leading to the elucidation of selenocysteine as being the 21st amino acid.

The Composition of Human Milk as a Model for the Design of Infant Formulas: Recent Findings and Possible Applications

“…Selenocysteine, on the other hand, is specifically inserted into glutathione peroxidase and other selenoproteins by a selenocysteine specific tRNA which differs from that for cysteine. Selenocysteine can therefore be seen as the 21st amino acid in terms of ribosome mediated protein synthesis (Bock et al 1991).…”

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“…It is beneficial because of its anti-oxidative properties, giving it a distinctive role in cancer prevention. In humans and animals, Se is an essential trace element and specifically incorporated to produce selenocysteine, the 21 st amino-acid [4], which in turn is used to form selenoproteins. Up to now, a few selenoproteins have been discovered: the group of enzymes of the glutathione peroxidase family, the selenoproteins P and W and the iodothyronine deiodinases [5], among others.…”

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“…This recoding event occurs in response to a specific set of trans factors and cis signals, which differ between eukaryotes and prokaryotes. Prokaryotes direct selenocysteine insertion at specific UGA codons using a 40 nt structured RNA sequence or SECIS element (Bock et al, 1991). The E. coli SelB protein, a homologue of the translation elongation factor EF-Tu, binds to the SECIS element using its C-terminal domain, bringing a UGA-decoding selenocysteine tRNA Sec to the UGA codon at the ribosomal A-site (Bock et al, 1991 ;Kromayer et al, 1996).…”

“…Prokaryotes direct selenocysteine insertion at specific UGA codons using a 40 nt structured RNA sequence or SECIS element (Bock et al, 1991). The E. coli SelB protein, a homologue of the translation elongation factor EF-Tu, binds to the SECIS element using its C-terminal domain, bringing a UGA-decoding selenocysteine tRNA Sec to the UGA codon at the ribosomal A-site (Bock et al, 1991 ;Kromayer et al, 1996). In eukaryote systems, the SECIS element is located in the 3h untranslated region (UTR) of the mRNA encoding the selenoprotein, distal to the UGA codon directing selenocysteine incorporation, and the trans factors are less well characterized (Berry et al, 1993(Berry et al, , 1991.…”