Selenocysteine
Selenocysteine (Sec) is called the 21st amino acid[1]. It is incorporated into rare proteins in all domains of life, and is essential for life. When the UGA stop codon is accompanied by a suitable signal, it is translated as Sec instead of stopping translation.
ImportanceImportance
Translation from UGA Stop CodonTranslation from UGA Stop Codon
Structure and SynthesisStructure and Synthesis
Cysteine (Cys) has a sulfur-containing side chain -CH2-SH. In selenocysteine, the sulfur is replaced with selenium, making the side chain -CH2-SeH. However, as explained below, Sec is not synthesized from Cys, but rather from Ser (sidechain -CH2-OH), by the replacement of oxygen with selenium.
Sec differs from the 20 standard amino acids because, in all domains of life, it lacks its own tRNA synthetase, and is synthesized from Ser covalently linked to tRNASec.
The crystal structure of the complex that converts Ser-tRNASec to Sec-tRNASec (3hl2) was solved in 2009[2]. This consists of O-Phosphoseryl-tRNA:selenocysteinyl-tRNA synthase (SepSecS) complexed to tRNASec, phosphoserine, and thiophosphate. The authors conclude that this structure, together with enzyme assays, supports a pyridoxal phosphate-dependent mechanism.
Notes and ReferencesNotes and References
- ↑ Atkins JF, Gesteland RF. The twenty-first amino acid. Nature. 2000 Sep 28;407(6803):463, 465. PMID:11028985 doi:10.1038/35035189
- ↑ Palioura S, Sherrer RL, Steitz TA, Soll D, Simonovic M. The human SepSecS-tRNASec complex reveals the mechanism of selenocysteine formation. Science. 2009 Jul 17;325(5938):321-5. PMID:19608919 doi:325/5938/321