Selenoproteins and Thyroid Gland
Selenium is present in proteins in form of two amino acids, i.e., selenocysteine (Sec) or selenomethionine (SeMet). Sec is incorporated during translation of selenoproteins by a well-conserved highly specific process requiring Sec-loaded tRNA and the decoding of the stop codon UGA as Sec-insertion signal. SeMet is incorporated randomly into newly synthesized proteins at AUG codons specifying methionine insertion as the translational machinery is unable to discriminate Met-tRNA from SeMet-tRNA. Consequently, any given protein can become a SeMet-containing protein by incorporation of SeMet instead of Met during translation, while selenoproteins always contain Sec at genetically predetermined specified positions. Currently, 25...