FIGURE 3.
Mechanism of Sec biosynthesis in eukaryotes and the Sec machinery-based pathway for synthesis of Cys. The pathway for Sec biosynthesis in eukaryotes is shown, in which phosphoseryl-tRNA kinase (PSTK) provides the phosphorylated intermediate PSer-tRNA[Ser]Sec serving as a substrate for SecS. Selenophosphate (H2SePO3−) generated by SPS2 from selenite and ATP is used as a donor of active Se for SecS (see top right portion of the figure for the final steps in Sec biosynthesis). The de novo synthesis of Cys using the Sec biosynthetic machinery is shown on the bottom right (see text for details).