FoTO1 is an epoxide isomerase in paclitaxel biosynthesis

Paclitaxel biosynthesis is limited by the instability of taxadiene-4(5)-epoxide, which readily diverts to the non-productive byproduct 5(12)-oxa-3(11)-cyclotaxane (OCT) instead of rearranging to taxadiene-5α-ol. Although FoTO1 suppresses OCT accumulation, its molecular function has been unclear. Here we identify FoTO1 as a dedicated epoxide isomerase that directs productive rearrangement. Biochemical characterization, site-directed mutagenesis, and QM/MM calculations reveal a pre-organized D68-D149 dyad that electrostatically activates epoxide ring opening and stereospecific rearrangement. Modular dissection of the C-terminal extension further reveals a functional partition between catalytic integrity and productive coupling with T5αOH, mediated by specific hydrophobic contacts that enforce precise geometric complementarity at the binary complex interface. These results demonstrate how electrostatic activation and enzyme association cooperate to control the fate of a highly reactive intermediate in paclitaxel biosynthesis.