Quantum tunneling in DNA methylation: A computational study using QM/MM and molecular dynamics

Area of Science:

• Biochemistry
• Computational Chemistry
• Epigenetics

Background:

• DNA methylation is a key epigenetic mechanism regulating gene expression and chromatin structure.
• While often viewed as thermally driven, quantum tunneling's role in methyl transfer by DNA methyltransferases (DNMTs) is increasingly recognized.
• Understanding these mechanisms is crucial for comprehending gene regulation and disease development.

Purpose of the Study:

• To investigate the contribution of quantum mechanical tunneling to the methyl transfer reaction catalyzed by DNMTs.
• To employ multiscale computational methods to model the tunneling process within the enzymatic environment.
• To quantify the impact of tunneling on reaction rates and kinetic isotope effects (KIEs).

Main Methods:

• Utilized a multiscale computational approach combining density functional theory (DFT), QM/MM, and advanced tunneling formalisms (WKB and Ring Polymer Instanton theory).
• Applied Wentzel-Kramers-Brillouin (WKB) analysis as a baseline and Ring Polymer Instanton (RPI) theory with instanton-Hessian refinement for accurate tunneling calculations.
• Analyzed enzyme-aligned reaction coordinates, including Morse potential fits, to extract barrier properties and interpret tunneling dynamics.

Main Results:

• WKB analysis showed negligible tunneling without enzymatic influence, highlighting the necessity of the catalytic environment.
• Instanton + Hessian analysis for DNMT1 predicted a large KIE (≈ 8.17×10^3) for the pre-methylation state, indicating significant tunneling.
• RPI calculations for the methylated state yielded a modest KIE (≈ 6.28-6.32), suggesting suppressed isotope sensitivity post-methylation.

Conclusions:

• DNMT1 actively reshapes the reaction barrier to facilitate methyl group tunneling during the chemical step.
• The enzyme's catalytic environment is essential for enabling significant quantum tunneling in DNA methylation.
• Product-like geometries appear to reduce the sensitivity of the reaction rate to isotopic substitution after methylation.