Selenoprotein Synthesis

Abstract

Selenoproteins contain one or more residues of the nonstandard amino acid selenocysteine, which is an analog of cysteine in which a selenol group replaces a thiol. The majority of these proteins catalyzes some oxidation/reduction function in which the selenol of the selenocysteine that is present in the active site of the respective enzyme takes part in the reaction. The advantage of having a selenol instead of a thiol lies in the fact that it confers to these enzymes a higher kinetic efficiency. In some biological systems, selenoproteins may also fulfill a structural role because of their capacity to oligomerize proteins via the formation of diselenide or mixed disulfide–selenide bridges. The biosynthesis of selenoproteins is unique since the incorporation of selenocysteine occurs co-translationally by the ribosome and not posttranslationally. Selenocysteine insertion is DNA encoded, requires the function of a cognate transfer RNA (tRNA) and of a specific translation elongation factor different from elongation factor Tu. Selenocysteine, therefore, has been designated as the 21st amino acid.