Figure 1. The biosynthetic pathway for oxindole types of monoterpenoid indole alkaloids (MIAs) in Cephalanthus occidentalis and Mitragyna speciosa. (IMAGE)
Caption
In C. occidentalis, strictosidine aglycone is reduced by ajmalicine synthase (CoAJS) to form ajmalicine with a 3S stereochemistry. Subsequent tandem oxidation and reduction by the heteroyohimbine/yohimbine/corynanthe C3-oxidase and C3-reductase pair (CoHYC3O and CoHYC3R) invert the stereochemistry at C3 from S to R. The resulting 3-epi-ajmalicine is then converted by C. occidentalis oxindole synthase (CoOIS) via oxidative cleavage and rearrangement into mitraphylline and isomitraphylline, both bearing spirooxindole structures. CoOIS also catalyzes a dehydrogenation reaction on 3-epi-ajmalicine, producing 3-dehydro-ajmalicine. In M. speciosa, homologous enzymes catalyze analogous transformations, producing oxindole derivatives such as isocorynoxeine and 3-epi-corynoxeine from structurally related corynanthe type precursors.
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