Catalytic Control in the Facile Proton Transfer in Taxadiene Synthase

Enzymes are highly efficient and usually very specific biocatalysts. However, some enzyme families, such as terpene synthases, are inherently promiscuous due to the extremely challenging chemistry they have evolved to tackle. Here we focus on one such enzyme, taxadiene synthase (TXS), which produces taxa-4(5),11(12)-diene, a key precursor to the chemotherapy agent taxol. A central chemical step in the biosynthesis of taxa-4(5),11(12)-diene by TXS is an intramolecular proton transfer. The inherent out of enzyme energetics for this facile proton transfer dictates a two-step proton transfer as the most favorable pathway, raising the question as to why an enzyme would prefer an indirect pathway that leaves it prone to side-product formation. In the current work, we employ hybrid quantum and molecular mechanical classical and path-integral simulations to address the nature of the intramolecular proton transfer in TXS, and we find that in the enzyme the direct proton transfer is slightly preferred over the indirect two-step pathway. This suggests that the enzyme might have evolved to favor a simpler, direct mechanistic pathway, thereby asserting chemical control by reducing its promiscuity. Understanding the underpinnings of such chemical control is likely to be important when attempting to design natural products in nonenzymatic environments.