Browsing by Author "Dang, Thu Thuy T."

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    Elucidation of the noscapine biosynthetic pathway in opium poppy
    (2014-09-30) Dang, Thu Thuy T.; Facchini, Peter J.
    Noscapine is a phthalideisoquinoline alkaloid found in the latex of opium poppy (Papaver somniferum). Noscapine has long been used as a cough suppressant and extensively investigated for its potential anticancer effect. Thanks to its long of history of safe use as an antitussive, rapid absorption after oral administration, and apoptosis-inducing effect on a number of cancer cell lines, noscapine has an advantage over other tubulin-binding anticancer natural products such as the well-established taxanes. Despite its isolation from opium more than two centuries ago, the complete biosynthesis of noscapine has not been elucidated until very recently. My project aims at understanding the biochemical genetics that underlies noscapine biosynthesis in opium poppy by identifying and functional characterizing noscapine biosynthetic candidate genes using comparative transcriptomics and metabolomics. Transcriptomes of eight different opium poppy chemotypes that display different noscapine profiles were used to mine for gene candidates that are potentially involved in noscapine pathway. The search resulted in the discovery of three O-methyltransferases and four cytochrome P450s, all of which were fully characterized biochemically and confirmed to be involved in the noscapine pathway by virus-induced gene silencing approach. It has been shown that the noscapine pathway starts by the 9-O-methylation of scoulerine catalyzed by scoulerine 9-O-methyltransferase to yield tetrahydrocolumbamine, which is then oxidized by canadine synthase to form canadine. N-methylation of canadine catalysed by tetrahydroprotoberberine N-methyltransferase gives rise to N-methylcanadine, which is acted upon by CYP82Y1 to form hydroxy-N-methylcanadine. CYP82X2 further oxidizes hydroxy-N-methylcanadine to (S)-1,13-dihydroxy-N-methylcanadine, which then serves as a substrate for AT1, yielding (S)-1-hydroxy-13-O-acetyl-N-methylcanadine. CYP82X1 then oxidizes (S)-1-hydroxy-13-O-acetyl-N-methylcanadine to form 1,8-dihydroxy-3-O-acetyl-N-methylcanadine, which spontaneously rearranges to yield 4’-desmethyl-3-acetylpapaveroxine. Ester hydrolysis catalysed by CXE1 prompts the formation of a hemiacetal moiety that triggers the conversion to the phthalideisoquinoline scaffold. Noscapine synthase (NOS or SDR1) catalyzes the final step in the formation of noscapine from narcotine hemiacetal.