How large should the QM region be in QM/MM calculations? The case of catechol O-methyltransferase

TL;DR

This study investigates how large the QM region should be in QM/MM enzyme simulations, revealing that including around 200-300 atoms is necessary for accurate catalytic property predictions, especially considering charge transfer effects.

Contribution

It introduces a charge shift analysis method to determine the minimal QM region size needed for accurate enzyme modeling, improving upon traditional distance-based selection.

Findings

Activation energy approaches chemical accuracy with ~500-600 atoms.

Charge transfer from protein residues influences the QM region size.

Charge shift analysis identifies 11-16 residues critical for accurate modeling.

Abstract

Hybrid quantum mechanical-molecular mechanical (QM/MM) simulations are widely used in studies of enzymatic catalysis. Until recently, it has been cost prohibitive to determine the asymptotic limit of key energetic and structural properties with respect to increasingly large QM regions. Leveraging recent advances in electronic structure efficiency and accuracy, we investigate catalytic properties in catechol O-methyltransferase, a representative example of a methyltransferase critical to human health. Using QM regions ranging in size from reactants-only (64 atoms) to nearly one-third of the entire protein (940 atoms), we show that properties such as the activation energy approach within chemical accuracy of the large-QM asymptotic limits rather slowly, requiring approximately 500-600 atoms if the QM residues are chosen simply by distance from the substrate. This slow approach to asymptotic limit is due to charge transfer from protein residues to the reacting substrates. Our large QM/MM calculations enable identification of charge separation for fragments in the transition state as a key component of enzymatic methyl transfer rate enhancement. We introduce charge shift analysis that reveals the minimum number of protein residues (ca. 11-16 residues or 200-300 atoms for COMT) needed for quantitative agreement with large-QM simulations. The identified residues are not those that would be typically selected using criteria such as chemical intuition or proximity. These results provide a recipe for a more careful determination of QM region sizes in future QM/MM studies of enzymes.