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.