Englerin A, a guaiane sesquiterpene diester isolated from the East African plant Phyllanthus engleri, is a potent and selective agonist of TRPC4/TRPC5 calcium channels and a much-used chemical probe and anticancer lead, but its supply depends on multi-step synthesis or trace plant isolation. A 2020 chemo-biological semisynthesis (Siemon et al., JACS 142:2760) engineered the fungal cyclase FgJ02895 in Escherichia coli and Saccharomyces cerevisiae to produce guaia-6,10(14)-diene as a microbial building block, from which englerin A was reached in a short chemical sequence. That work explicitly left the central scientific problem open: the native enzymatic pathway that makes englerin A in P. engleri has never been identified, so no fully biosynthetic (fermentation-only) route to englerin A exists. No transcriptomic, genomic, or terpene-synthase study of P. engleri has appeared to date, no study reconstitutes the pathway, and microbial production of the alternative cyclization product guaia-6,9-diene, like engineering of FgJ02895 itself, also remains unrealized.

Background

Englerin A is a sesquiterpene — a 15-carbon terpenoid built from three isoprene units — belonging to the guaiane family, meaning its carbon skeleton is a fused 5/7 bicyclic (hydroazulene) system. It carries a stereochemically dense oxygenation pattern: an epoxide bridging the guaiane skeleton and two ester groups attached directly to it — a glycolate ester whose hydroxyl remains free, and a cinnamate ester — with the core defining seven stereocenters. It was first isolated and reported at the end of 2008 from the root bark of Phyllanthus engleri, a tree native to Tanzania, and found to inhibit the growth of renal cancer cell lines with striking selectivity. In 2015 it was shown to be a potent and selective activator of the TRPC4 and TRPC5 ion channels — TRPC (transient receptor potential canonical) channels are non-selective cation channels that gate calcium and sodium entry — and most of englerin A's cytotoxic and pharmacological effects are now attributed to this agonism, which has made the compound a widely used tool in ion-channel biology and a starting point for anticancer and analgesic drug research. Because P. engleri is a slow-growing wild tree and englerin A accumulates only in trace amounts, plant extraction cannot supply research demand; numerous total syntheses exist (typically 8–15 steps) but are too long for economical scale-up. This supply problem motivated the 2020 work of Siemon et al.: they screened fungal sesquiterpene synthases (enzymes that cyclize farnesyl diphosphate, FPP, the universal 15-carbon terpene precursor, into a hydrocarbon skeleton) and identified FgJ02895 from Fusarium graminearum as an efficient producer of the hydrocarbon guaia-6,10(14)-diene. Using CRISPR/Cas9 genome editing, they engineered the mevalonate pathway (the metabolic route that supplies terpene precursors) of Escherichia coli and Saccharomyces cerevisiae to boost FPP supply, produced guaia-6,10(14)-diene at high titer, and converted it chemically in a few steps (selective hydration, dihydroxylation, epoxidation, esterification) into englerin A, oxyphyllol, and orientalol E — an efficient chemo-biological route, but one that still requires synthetic chemistry for all oxygenation and esterification steps. The fundamental biosynthetic question remains unanswered: in the plant, terpene synthases, cytochrome P450 monooxygenases (heme enzymes that install hydroxyl and epoxide groups on terpene skeletons), and acyltransferases of the BAHD family (enzymes that form ester bonds using CoA-activated acids) would have to act in sequence from FPP to englerin A, yet none of the responsible P. engleri genes has been identified — no genomic or transcriptomic resource exists for this species, and no enzyme from any organism has been shown to make englerin A or its oxidized guaiane intermediates.

Problem Statement

Identify the complete native biosynthetic pathway to englerin A in Phyllanthus engleri — the full set of enzymes that, starting from farnesyl diphosphate, constructs the guaiane sesquiterpene skeleton, installs the hydroxyl and epoxide groups, and attaches the glycolate and cinnamate esters, together with the plant genes that encode them — and functionally establish which reaction each identified enzyme catalyzes, what products it forms, and the order in which the enzymes act. The enzyme classes suggested by comparable plant sesquiterpene pathways — a sesquiterpene synthase, cytochrome P450 monooxygenases, and acyltransferases of the BAHD family — are a working hypothesis only, and the native pathway may use enzyme classes other than these. The pharmacological profiling of the related guaianes (-)-oxyphyllol and (+)-orientalol E, also flagged by the source, is an adjacent separable question and is not part of this completion boundary; likewise, a fermentation-only production process for englerin A, microbial production of the alternative cyclization product guaia-6,9-diene, and protein engineering of the fungal guaia-6,10(14)-diene synthase FgJ02895 are motivating context or adjacent engineering strands, and none of them — alone or in combination — constitutes a solution to this problem.

References

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