Oxidative Rearrangements of the Alkaloid Intermediate Geissoschizine

Plants can generate structural diversity by enzymatic rearrangement of a central intermediate. 19E-geissoschizine is one such chemically versatile intermediate that plays a central role in the biosynthesis of monoterpene indole alkaloids such as strychnine, ibogaine, and vinblastine. Here we report how 19E-geissoschizine undergoes oxidative transformations to generate four distinct alkaloid scaffolds through the action of three biosynthetic enzymes. Using in vitro enzymatic assays and gene silencing, we demonstrate how these three cytochrome P450 enzymes in the medicinal plant Catharanthus roseus transform 19E-geissoschizine into strychnos, sarpagan, akuammiline-type, and mavacurane-type alkaloids. We use mutational analysis to show how minimal changes to the active site of these similar enzymes modulate product specificity. This work highlights how substrate reactivity and enzyme mutations work synergistically to generate chemical diversity.