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SUMMARY Triterpenoids (C30‐isoprenoids) represent a major group of natural products with various physiological functions in plants. Triterpenoids and their derivatives have medicinal uses owing to diverse bioactivities. Arjuna ( Terminalia arjuna ) tree bark accumulates highly oxygenated β‐amyrin‐derived oleanane triterpenoids (e.g., arjunic acid, arjungenin, and arjunolic acid) with cardioprotective roles. However, biosynthetic routes and enzymes remain poorly understood. We mined the arjuna transcriptome and conducted cytochrome P450 monooxygenase (P450) assays using Saccharomyces cerevisiae and Nicotiana benthamiana to identify six P450s and two P450 reductases for oxidative modifications of oleanane triterpenoids. P450 assays using oleananes revealed a greater substrate promiscuity of C‐2α and C‐23 hydroxylases/oxidases than C‐28 oxidases. CYP716A233 and CYP716A432 catalyzed β‐amyrin/erythrodiol C‐28 oxidation to produce oleanolic acid. C‐2α hydroxylases (CYP716C88 and CYP716C89) converted oleanolic acid and hederagenin to maslinic acid and arjunolic acid. CYP716C89 also hydroxylated erythrodiol and oleanolic aldehyde. However, CYP714E107a and CYP714E107b catalyzed oleanolic acid/maslinic acid/arjunic acid, C‐23 hydroxylation to form hederagenin, arjunolic acid and arjungenin, and hederagenin C‐23 oxidation to produce gypsogenic acid, but at a lower rate than oleanolic acid C‐23 hydroxylation. Overall, P450 substrate selectivity suggested that C‐28 oxidation is the first P450‐catalyzed oxidative modification in the arjuna triterpenoid pathway. However, the pathway might branch thereafter through C‐2α/C‐23 hydroxylation of oleanolic acid. Taken together, these results provided new insights into substrate range of P450s and unraveled biosynthetic routes of triterpenoids in arjuna. Moreover, complete elucidation and reconstruction of arjunolic acid pathway in S. cerevisiae and N. benthamiana suggested the utility of arjuna P450s in heterologous production of cardioprotective compounds.
Srivastava et al. (Mon,) studied this question.