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#selenocysteine synthesis

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Eukaryotes gene regulation

Conversion of Sep-tRNA Sec to Sec-tRNA Sec is by

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.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: SepSecS Converts Sep-tRNASec to Sec

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Eukaryotes gene regulation

Phosphorylation of Ser-tRNA Sec is catalyzed by

Conversion of Ser-tRNASec to phosphoseryl intermediate begins with O-phosphoseryl-tRNASec kinase PSTK, unique monomeric kinase using ATP to phosphorylate seryl hydroxyl still esterified to tRNA. PSTK harbors N-terminal kinase domain resembling phosphoserine kinases and C-terminal tRNASec-binding domain conferring strict specificity, discriminating against tRNASer via variable arm recognition. Phosphorylation generates high-energy phosphoester with enhanced leaving group capability for nucleophilic substitution by selenophosphate. In archaea and eukaryotes PSTK essential; bacteria bypass this step using SelA directly. Loss-of-function mutations abolish selenoprotein expression despite normal serine charging, underscoring priming role converting inert alcohol into activated precursor competent for selenium insertion.

Ref: NCBI Bookshelf, Amino Acid Metabolism: PSTK Phosphorylation of Ser-tRNASec

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Eukaryotes gene regulation

Enzyme charging tRNA Sec with serine is

Serine loading onto tRNASec uses ubiquitous seryl-tRNA synthetase SerRS, same dimeric class II enzyme charging canonical tRNASer isoacceptors. SerRS identity elements include long variable arm and discriminator base G73 recognized in tRNASec acceptor stem, compensating absence of typical G3:U70 marker. Reaction consumes ATP forming seryl-adenylate intermediate then transferring serine to A76 3' end. No separate SerRS isoform required for Sec pathway, reflecting economical reuse of translation machinery. Fidelity depends on downstream modifying enzymes PSTK and SepSecS rejecting unmodified tRNASec and tRNASer species. Hence canonical synthetase moonlights in selenoprotein pathway linking selenium utilization to central tRNA aminoacylation infrastructure across domains.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: SerRS Charging tRNASec

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