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 3 S stereocenter. However, a subset of MIAs contains a noncanonical 3 R stereocenter. Here we report the basis for 3 R -MIA biosynthesis in Mitragyna speciosa (kratom). We discover the presence of the iminium species (20 S )-3-dehydrocorynantheidine, which supports isomerization of 3 S to 3 R 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 3 R -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.