A Cytochrome P450 Enzyme Catalyses Oxetane Ring Formation in Paclitaxel Biosynthesis

Oxetane synthase (TmCYP1), a novel cytochrome P450 enzyme from Taxus×media cell cultures, has been functionally characterized to efficiently catalyse the formation of the oxetane ring in tetracyclic taxoids. Transient expression of TmCYP1 in Nicotiana benthamiana using 2α,5α,7β,9α,10β,13α-hexaacetoxytaxa-4(20),11(12)-diene (1) as a substrate led to the production of a major oxetane derivative, 1β-dehydroxybaccatin IV (1 a), and a minor 4β,20-epoxide derivative, baccatin I (1 b). However, feeding the substrate decinnamoyltaxinine J (2), a 5-deacetylated derivative of 1, yielded only 5α-deacetylbaccatin I (2 b), a 4β,20-epoxide. A possible reaction mechanism was proposed on the basis of substrate-feeding, ²H and ¹⁸O isotope labelling experiments, and density functional theory calculations. This reaction could be an intramolecular oxidation-acetoxyl rearrangement and the construction of the oxetane ring may occur through a concerted process; however, the 4β,20-epoxide might be a shunt product. In this process, the C5-O-acetyl group in substrate is crucial for the oxetane ring formation but not for the 4(20)-epoxy ring formation by TmCYP1. These findings provide a better understanding of the enzymatic formation of the oxetane ring in paclitaxel biosynthesis.