Synthetic production of the naturally occurring anti-cancer medicine paclitaxel may be one step closer with the discovery of the enzymes responsible for producing baccatin III, a complex precursor in the biosynthesis of the drug. Previous methods for synthesizing paclitaxel in the lab take up to 30 steps, the use of hazardous reagents, and sourcing of baccatin III from yew trees or plant cell culture. Now a team of researchers in China report just nine genes are needed to produce baccatin III in tobacco plants.
Paclitaxel, sold under the brand name Taxol, is used to treat several types of cancers and was discovered in the Pacific yew in the 1960s. Synthesis of baccatin III with a view to producing the drug in the lab has frustrated researchers for decades, however. According to co-author Xiaoguang Lei, a chemist at the Beijing National Laboratory for Molecular Sciences, baccatin III biosynthesis is complicated because each enzyme can produce multiple substrates. By expressing a sub-family of cytochrome P450 genes, called CYP725A, in cell cultures and feeding those cells known substrates the team could untangle which genes were involved in each step.
Baccatin III is synthesized by the Pacific yew and can be used to produce Taxol. Understanding which enzymes are involved in its synthesis has helped chemists produce baccatin III in tobacco plants, which could in theory be engineered to produce the compound at much higher concentrations.
According to Lei, the crucial missing steps for baccatin III synthesis were the enzymes responsible for oxetane ring formation and C9 oxidation. A new bifunctional enzyme the team named taxane oxetanase (TOT) was responsible for the former. ‘TOT is a bifunctional oxygenase that directly converts the alkene moiety into the epoxide and the oxetane ring,’ said Lei. The formation of the epoxide ring was previously thought to be an essential intermediate for the oxetane ring, but this discovery revises that hypothesis. The team then identified an enzyme called T9αH which performed the C9 oxidation. In total, they identified nine genes and their enzymes required to produce baccatin III – far fewer than previous estimates of up to 14. To confirm the pathway, they expressed the genes in modified tobacco plants, producing baccatin III.
‘For now, this is a significant step forward in the potential for synthetic production of paclitaxel, which could have a major impact on cancer treatment,’ said Lei. The findings open up new possibilities for large-scale production of the drug, potentially reducing the reliance on yew trees and addressing supply chain issues. The research could pave the way for more accessible and sustainable production of this important cancer medicine.
