Biosynthetic Mechanism of Lanosterol: A Completed Story

The remarkable biochemical conversion of acyclic 2,3-oxidosqualene to tetracyclic lanosterol (the common precursor to cholesterol and its relatives) with seven stereocenters constructed, catalyzed by oxidosqualene cyclase (OSC), has fascinated chemists for over a half century. Although many experimental and theoretical efforts have been reported, most of the studies focused on the initial cyclization while the subsequent cascade carbocation rearrangement and deprotonation were ignored, which determine the regio- and stereochemistry of the final product lanosterol associated with the function-related structure domains of cholesterol and its relatives. Herein we continue our previous works on the cyclization (Angew. Chem., Int. Ed. 2015, 54, 8693–8696); the mechanistic details of the remaining part of lanosterol biosynthesis, involving three hydride shifts, two methyl shift and one deprotonation, were investigated by quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations. We obtained the complete free energy profile for lanosterol biosynthesis, which is thermodynamically and kinetically reasonable for the sole formation of the desired product with byproducts effectively avoided. We identified some key factors such as electrostatic interactions and CH···π interactions that facilitate migration of the carbocation. We also identified the direct deprotonation precursor, intermediate I, which has been controversial in previous studies. More importantly, we found that the enzyme mediates the energy transfer from the initial cyclization to the subsequent rearrangements through the electric field of the active pocket, which guarantees the fidelity of the enzyme catalysis.