Selenocysteine (Sec) is the 21st amino acid in the genetic code and this selenium containing amino acid is cotranslationally incorporated into selenium-containing proteins, designated selenoproteins, in response to the codon, UGA (1–3). Although UGA is normally a termination codon that dictates the cessation of protein synthesis, it is also used as a Sec codon by numerous organisms in each of the 3 domains of life: eubacteria, archaea, and eukaryotes. Of the >500 genomes sequenced in eubacteria, only ∼20% encode the machinery for inserting Sec into protein, and in archaea, ∼10% have this machinery. In eukaryotes, the Sec insertion machinery has been found in a number of lower organisms such as green algae, kinetoplastida, and slime molds and it is widespread in animals but absent in fungi and higher plants.
The biosynthesis of Sec in Escherichia coli proceeds as follows. The enzyme that synthesizes Sec, Sec synthase (designated SelA in eubacteria and SecS in eukaryotes and archaea), is a pyridoxal phosphate dependent protein. This enzyme interacts with seryl-tRNA[Ser]Sec and removes the hydroxyl group from the seryl moiety to yield aminoacrylyl- (dehydroalanyl-) tRNA[Ser]Sec as an intermediate. Dehydroalanyl-tRNA[Sec]Sec then accepts the active selenium donor, which was identified in eubacteria as monoselenophosphate, to form selenocysteyl-tRNA[Ser]Sec. Selenophosphate is synthesized from selenide and ATP by E. coli selenophosphate synthetase. Here you can see a recent cryoEM structure of the Selenocysteine synthase- SelA from Escherichia coli (PDB code: 8UZW)

#molecularart ... #aminoacid ... #selenocysteine ... #synthase ... #multimer ... #cryoem

Structure rendered with @proteinimaging and depicted with @corelphotopaint