The taxadiene-forming carbocation cascade.

Area of Science:

• Biochemistry
• Computational Chemistry
• Organic Chemistry

Background:

• Taxadiene is a precursor to the anticancer drug paclitaxel.
• The biosynthesis of taxadiene involves complex carbocation rearrangements.
• Previous mechanistic proposals for taxadiene synthase have been based on limited experimental data.

Purpose of the Study:

• To elucidate the complete reaction pathway for taxadiene biosynthesis using quantum chemical calculations.
• To reconcile theoretical findings with existing experimental data, including labeling studies.
• To propose a binding orientation for substrate analogues within taxadiene synthase.

Main Methods:

• Quantum chemical calculations were employed to determine the structures and energies of intermediates and transition states.
• The calculated pathway was compared with established experimental results from labeling experiments.
• Theoretical analysis was used to interpret X-ray crystal structure data of taxadiene synthase.

Main Results:

• A complete, energetically feasible pathway from geranylgeranyl diphosphate to taxadiene was established.
• The calculated pathway aligns with experimental labeling data but suggests novel conformations for carbocation intermediates.
• Subtle differences in bond formation synchronicity were identified compared to previous mechanistic models.
• A nonproductive binding orientation for 2-fluoro-geranylgeranyl diphosphate in taxadiene synthase was proposed based on theoretical results.

Conclusions:

• The study provides a detailed, theoretically validated mechanism for taxadiene biosynthesis.
• The findings offer new insights into the conformational flexibility of carbocation intermediates in terpene cyclization.
• The proposed nonproductive binding mode challenges existing interpretations of taxadiene synthase substrate interactions.