Two Triterpene Synthases from Imperata cylindrica Catalyzing the Formation of a Pair of Diastereoisomers through Boat or Chair Cyclization

Imperata cylindrica is known to produce a pair of triterpenes, isoarborinol and fernenol, that exhibit identical planar structures but possess opposite stereochemistry at six of the nine chiral centers. These differences arise from a boat or a chair cyclization of the B‐ring of the substrate. Herein...

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Bibliographic Details
Published in:Chembiochem : a European journal of chemical biology 2021-06, Vol.22 (11), p.1992-2001
Main Authors: Naraki, Shingo, Kakihara, Mai, Kato, Sayuri, Saga, Yusuke, Mannen, Kazuto, Takase, Shohei, Takano, Akihito, Shinpo, Sayaka, Hosouchi, Tsutomu, Nakane, Takahisa, Suzuki, Hideyuki, Kushiro, Tetsuo
Format: Article
Language:English
Subjects:
Alkyl and Aryl Transferases - metabolism
Biocatalysis
Conformation
Cyclization
Diastereoisomers
fernenol
Genes
Histidine
Imperata cylindrica
isoarborinol
Molecular Structure
oxidosqualene cyclase
Planar structures
Poaceae - enzymology
Residues
Stereochemistry
Stereoisomerism
Substrates
triterpene
Triterpenes
Triterpenes - chemistry
Triterpenes - metabolism
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Summary:Imperata cylindrica is known to produce a pair of triterpenes, isoarborinol and fernenol, that exhibit identical planar structures but possess opposite stereochemistry at six of the nine chiral centers. These differences arise from a boat or a chair cyclization of the B‐ring of the substrate. Herein, we report the characterization of three OSC genes from I. cylindrica. IcOSC1 and IcOSC5 were identified as isoarborinol and fernenol synthases, respectively, while IcOSC3 was characterized as a multifunctional enzyme that produces glutinol and friedelin as its major products. Mutational studies of isoarborinol and fernenol synthases revealed that the residues surrounding the DCTAE motif partially affected the conformation of the B‐ring during cyclization. Additionally, the IcOSC1‐W255H mutant produced the rare triterpene boehmerol. The introduced histidine residue presumably ed a proton from the intermediary carbocation at C18 during the 1,2‐rearrangement. Expression analysis indicated that all OSC genes were highly expressed in stems. Controlling conformation during cyclization: I. cylindrica produces the diastereomeric triterpenes isoarborinol and fernenol depending on whether the B‐ring of oxidosqualene was in a boat or chair conformation during cyclization. Mutational studies on the two enzymes resulted in partial conversion of the products. Placing a histidine residue at a critical site resulted in the production of a rare triterpene.
ISSN:1439-4227
1439-7633
DOI:10.1002/cbic.202100077