Sesquiterpene Synthase-Catalysed Formation of a New Medium-Sized Cyclic Terpenoid Ether from Farnesyl Diphosphate Analogues.

Florence Huynh,Robert L Jenkins,Daniel J Grundy,Rudolf K Allemann,David J Miller

doi:10.1002/cbic.201800218

Florence Huynh, Robert L Jenkins + Show 3 more

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https://doi.org/10.1002/cbic.201800218

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Abstract

Terpene synthases catalyse the first step in the conversion of prenyl diphosphates to terpenoids. They act as templates for their substrates to generate a reactive conformation, from which a Mg2+‐dependent reaction creates a carbocation–PPi ion pair that undergoes a series of rearrangements and (de)protonations to give the final terpene product. This tight conformational control was exploited for the (R)‐germacrene A synthase– and germacradien‐4‐ol synthase–catalysed formation of a medium‐sized cyclic terpenoid ether from substrates containing nucleophilic functional groups. Farnesyl diphosphate analogues with a 10,11‐epoxide or an allylic alcohol were efficiently converted to a 11‐membered cyclic terpenoid ether that was characterised by HRMS and NMR spectroscopic analyses. Further experiments showed that other sesquiterpene synthases, including aristolochene synthase, δ‐cadinene synthase and amorphadiene synthase, yielded this novel terpenoid from the same substrate analogues. This work illustrates the potential of terpene synthases for the efficient generation of structurally and functionally novel medium‐sized terpene ethers.

Highlights

Terpene synthases catalyse the first step in the conversion of prenyl diphosphates to terpenoids
Class II synthases form the initial carbocation by protonation of the distal double bond or an epoxide derivative thereof.[5a]. Class I terpene synthases comprise a mostly hydrophobic active site, surrounded by an ahelical barrel with two Mg2+-binding motifs at the entrance.[5a]. A large body of work on probing the chemical steps by using substrate analogues,[6] mutagenesis,[7] putative reaction intermediates[8] and X-ray crystallography[5a,9] combined with computational approaches[5c,10] has provided a detailed picture of the mechanisms, by which these enzymes catalyse their reactions.[5a,b] Co-crystal structures of aristolochene synthase with various substrate analogues, Mg2+ and PPi,[9c] in conjunction with molecular modelling,[10a] revealed the detailed physical steps that lead to the generation of the reactive Michaelis complex
The synthase first binds one Mg2+ ion and farnesyl diphosphate (FDP, 1); two more Mg2+ ions follow, and this closes the active site, forming the Michaelis complex.[10a]. Coordination of the diphosphate by the Mg2+ ions triggers the generation of a farnesyl cation (2) which is chaperoned by the active-site contour through a series of electrophilic ring closures and rearrangements