Practice question

O-phosphoseryl-tRNASec transformed to selenocysteinyl-tRNASec by SepSecS, also called Sec synthase SEPSECS, pyridoxal phosphate-dependent enzyme catalyzing beta-replacement reaction. Active site PLP forms internal aldimine with conserved lysine, transaldimination yields phosphoserine Schiff base, phosphate elimination facilitated by base creates aminoacrylate intermediate attacked by selenophosphate. Selenophosphate H2SePO3- synthesized by SPS2 serves as selenium donor, generating selenocysteine attached to tRNA ready for translation. SepSecS specifically recognizes tRNASec long variable arm, rejecting phosphoseryl-tRNASer, preventing off-target production. This final step completes on-tRNA synthesis of twenty-first amino acid, coupling selenium metabolism to protein synthesis machinery elegantly.