Enzymatic epimerization of monoterpene indole alkaloids in kratom

Monoterpene indole alkaloids (MIAs) are a large, structurally diverse class of bioactive natural products. These compounds are biosynthetically derived from a stereoselective Pictet–Spengler condensation that generates a tetrahydro-β-carboline scaffold characterized by a 3S stereocenter. However, a subset of MIAs contains a noncanonical 3R stereocenter. Here we report the basis for 3R-MIA biosynthesis in Mitragyna speciosa (kratom). We discover the presence of the iminium species (20S)-3-dehydrocorynantheidine, which supports isomerization of 3S to 3R via oxidation and stereoselective reduction downstream of the initial Pictet–Spengler condensation. Isotopologue feeding experiments identify the sites for downstream MIA pathway biosynthesis as well as the oxidase/reductase pair that catalyzes this epimerization. This oxidase/reductase pair has broad substrate specificity, suggesting that this pathway may be responsible for the formation of many 3R-MIAs and downstream spirooxindole alkaloids in kratom. The elucidation of this epimerization mechanism allows biocatalytic access to a range of pharmacologically active spirooxindole alkaloid compounds.