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Discovery of a Plant Pictet-Spenglerase With R-Stereoselectivity.

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Researchers discovered a novel enzyme, Epi-STR, from Pogonopus speciosus that produces the R-epimer of strictosidine, a key plant alkaloid. This finding enables efficient synthesis of valuable R-epimers and offers new insights into enzyme stereoselectivity.

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Area of Science:

  • Biochemistry
  • Enzymology
  • Phytochemistry

Background:

  • Monoterpenoid indole alkaloids (MIAs) are crucial plant compounds.
  • Strictosidine is a central MIA precursor, synthesized by strictosidine synthase (STR).
  • All known STR enzymes exhibit 3S-stereoselectivity, limiting access to the 3R epimer.

Purpose of the Study:

  • To investigate the stereoselectivity of STR orthologs in medicinal plants.
  • To identify and characterize a novel STR enzyme producing the 3R epimer.
  • To elucidate the structural basis of STR stereoselectivity and engineer altered selectivity.

Main Methods:

  • Phylogenetic and structural analyses of STR orthologs.
  • Biochemical characterization of a novel STR enzyme (Epi-STR) from Pogonopus speciosus.
  • Site-directed mutagenesis and reciprocal amino acid substitutions to alter stereoselectivity.

Main Results:

  • A novel STR ortholog, Epi-STR, was identified, exclusively producing the 3R epimer of strictosidine (vincosidic acid).
  • Epi-STR accepts both secologanic acid and secologanin methyl ester as substrates.
  • Specific amino acid residues were identified as critical for conferring 3R-stereoselectivity.
  • Rational engineering successfully switched the stereoselectivity of both canonical STR and Epi-STR, demonstrating substrate-dependent stereoselectivity.

Conclusions:

  • Discovery of Epi-STR provides a method for cost-effective, epimer-pure production of the 3R-strictosidine precursor.
  • Mechanistic insights into enzyme stereoselectivity were gained, challenging previous assumptions.
  • Highlights the value of exploring understudied plant species for novel biochemical discoveries.