Page - (000212) - in Biomedical Chemistry: Current Trends and Developments

discovered in 1986. Sec is identical to cysteine (Cys), except that it contains selenium instead of sulphur. Selenocysteine is recognized as the 21st amino acid in ribosome-mediated protein synthesis and its specific incorporation is directed by the UGA codon (Stadtman, 1996). Figure 3.1.1: Structures of natural amino acids. Figure 3.1.2: Structures of selenocysteine and pyrrolysine. Selenium is in the same family below sulfur in the periodic table and the two elements share many similar chemical properties. Sec has a distinct functional advantage because the selenol group is more fully ionized than the thiol group of Cys at physiological pH (biosynthesis of Sec and de novo synthesis of Cys can be observed in Fig. 3.1.3). When Sec is replaced with Cys, the catalytic activity of a selenoenzyme is drastically reduced (Stadtman, 2000). A big surprise was the discovery of a 22nd amino acid in methane- producing archaea of the family Methanosarcinaceae in 2002, pyrrolysine (Pyl, Fig. 3.1.2). It is genetically encoded in a similar manner