Abstract
Selenium, an essential trace element for humans, presents as a selenocysteine residue in selenoproteins, such as glutathione peroxidase, thioredoxin reductase, and selenoprotein P. Selenocysteine is co-translationally incorporated into selenoproteins at an in-frame UGA codon, which usually serves as a stop codon, within the coding region of the mRNA and therefore is designated as the 21st amino acid in the genetic code. The translation of selenocysteine codon in mRNA during selenoprotein biosynthesis requires selenocysteyl (Sec)-tRNASec. In general, aminoacyl-tRNA is synthesized from an amino acid and the corresponding tRNA isoacceptor in an ATP-dependent reaction catalyzed by an aminoacyl-tRNA synthetase. However, Sec-tRNASec, unlike cognate aminoacyl-tRNA, is synthesized by an indirect tRNA-dependent amino acid formation mechanism. In this article, functions of several selenoproteins and enzymes involved in the biosynthetic pathway of selenoproteins are reviewed. In addition, the reaction mechanism of a selenium-specific enzyme, selenocysteine lyase, is discussed.
