Enzyme TOT1 That Forms Paclitaxel's Oxetane Ring Identified; Baccatin III Produced in Tobacco

A joint research team from the Agricultural Genomics Institute at Shenzhen of the Chinese Academy of Agricultural Sciences (CAAS) and Peking University newly identified enzymes that make baccatin III, the core skeleton of the anticancer drug paclitaxel, and reconstituted the pathway in tobacco plants. Paclitaxel has a structure in which a side chain is attached to a highly oxidised diterpenoid skeleton, and although it has been studied for nearly half a century, gaps remained in the biosynthetic pathway of its skeleton.

A 'Bifunctional P450' That Builds the Oxetane Ring

The researchers found the cytochrome P450 enzyme TOT1 (taxane oxetanase 1) in Taxus mairei, a species of yew. This enzyme forms oxetane, the four-membered ring unique to paclitaxel, through an oxidative rearrangement reaction, a ring-forming mechanism that had not been known. Using a screening strategy based on the taxusin biosynthetic pathway, they also identified T9αH1, an enzyme that oxidises the C9 position of the taxane skeleton. By combining the newly found enzymes with existing ones, they artificially reconstituted a baccatin III production pathway in tobacco.

Paclitaxel has been produced by relying on raw material obtained from yew or on plant cell culture. With the skeleton pathway now connected, the result provides a starting point for research on plant- and microbe-based production. However, this achievement is at the stage of demonstrating pathway reconstitution, and it does not address production yield or full biosynthesis of paclitaxel with the side chain attached.