Biosynthetic Mechanism of Lanosterol: Cyclization

Area of Science:

• Biochemistry and Molecular Biology
• Computational Chemistry

Background:

• The biosynthesis of lanosterol, a crucial triterpenoid intermediate in cholesterol production, involves the cyclization of 2,3-oxidosqualene.
• Oxidosqualene cyclase (OSC) catalyzes this complex transformation, forming a tetracyclic structure from an acyclic precursor.
• The precise mechanism of this cyclization has been a long-standing question in chemistry and biology.

Purpose of the Study:

• To elucidate the detailed cyclization mechanism of 2,3-oxidosqualene to lanosterol.
• To investigate the role of oxidosqualene cyclase (OSC) in this biosynthetic pathway.
• To computationally validate the proposed reaction pathway using advanced simulation techniques.

Main Methods:

• State-of-the-art two-dimensional (2D) Quantum Mechanics/Molecular Mechanics (QM/MM) Molecular Dynamics (MD) simulations were employed.
• The simulations focused on the cyclization process from the acyclic 2,3-oxidosqualene to the tetracyclic lanosterol intermediate.
• Calculated reaction barriers were compared with experimental kinetic data.

Main Results:

• The study revealed a nearly concerted yet highly asynchronous cyclization of the A-C rings, forming a stable 6-6-5 ring intermediate.
• Subsequent ring expansion of the C-ring and formation of the D-ring yielded the 6-6-6-5 protosterol cation.
• The calculated reaction barrier for the rate-limiting step (≈22 kcal/mol) closely matched experimental kinetic findings.

Conclusions:

• The identified reaction mechanism provides a detailed step-by-step pathway for lanosterol biosynthesis.
• Computational results are highly consistent with existing experimental mutagenic data, supporting the proposed mechanism.
• This work offers significant insights into the enzymatic catalysis of complex natural product formation.