Correct and incorrect nucleotide incorporation pathways in DNA polymerase beta

Understanding how DNA polymerases incorporate nucleotides with high fidelity is a key goal in molecular biology. Prior research has shown that DNA polymerase beta (pol beta) plays a central role in short-patch base excision repair. However, the exact energetic and structural changes during correct and incorrect nucleotide incorporation remain unclear. This gap motivated the need for a detailed mechanistic analysis of phosphoryl transfer reactions. While general models of DNA polymerase function exist, the specific proton transfer mechanisms and their energetic consequences have not been fully resolved. Computational methods have been used to model enzyme dynamics, but integrating these with quantum mechanical insights remains limited. The Grotthuss mechanism is known in proton transfer, but its role in DNA polymerase fidelity has not been clearly established. This study builds on prior computational work by introducing a novel protocol combining energy minimizations and mixed quantum mechanics/molecular mechanics simulations. The aim is to clarify how subtle structural differences in the active site affect catalytic efficiency and fidelity.

This study aimed to investigate the structural and energetic pathways of correct and incorrect nucleotide incorporation by DNA polymerase beta. The researchers focused on the phosphoryl transfer reactions during G:C and G:A base pair incorporations. They sought to determine how proton transfer mechanisms influence the fidelity of DNA synthesis. The motivation was to clarify the molecular basis of DNA polymerase fidelity using a computational approach. By comparing matched and mismatched base pairs, the study aimed to identify the energetic barriers that contribute to replication accuracy. The researchers also wanted to assess the role of conserved aspartate residues in the active site. Their goal was to provide a detailed model of the phosphoryl transfer process that could explain fidelity discrimination. This work addresses a gap in understanding how DNA polymerase beta achieves high fidelity through subtle structural changes.

The researchers employed a novel computational protocol involving energy minimizations and dynamics simulations. They used quasi-harmonic free energy calculations to model the phosphoryl transfer reactions. Mixed quantum mechanics/molecular mechanics (QM/MM) simulations were applied to capture the electronic and structural details of the active site. The study focused on the proton transfer mechanisms during G:C and G:A nucleotide incorporation. The Grotthuss hopping mechanism was analyzed to understand how protons move between water molecules and conserved aspartate residues. The team compared the structural and energetic profiles of matched and mismatched base pairs. They evaluated the free energy of activation for each step of the phosphoryl transfer process. The simulations allowed them to track transient intermediates and identify the rate-limiting steps in the reaction pathways.

The study found that the rate-limiting step in the G:C system is the initial proton hop, with a free energy of activation of at least 17 kcal/mol. This value closely matches the experimentally measured k(pol) values, suggesting a strong correlation between computational and empirical data. In the G:A system, the closed ternary complex of the enzyme is significantly less stable, leading to reduced catalytic efficiency. The activation energy for the initial nucleophilic attack is much higher in the G:A system compared to the G:C system. This difference is attributed to the deprotonation of the terminal DNA primer O3'H group. The conserved aspartate residues in the active site play a key role in proton transfer via the Grotthuss mechanism. The simulations revealed transient intermediates in the phosphoryl transfer pathways for both systems. These findings suggest that subtle structural differences between matched and mismatched base pairs lead to significant changes in catalytic performance.

The authors propose that the fidelity of DNA polymerase beta arises from structural and energetic differences in the active site during correct and incorrect nucleotide incorporation. Their findings suggest that the initial proton hop is the rate-limiting step in the G:C system. The G:A system shows a much higher activation energy and reduced stability of the closed ternary complex. These differences may explain the observed fidelity discrimination in pol beta. The study supports the involvement of a Grotthuss hopping mechanism in proton transfer between water molecules and conserved aspartate residues. The computational model aligns well with experimentally measured k(pol) values. The researchers propose that subtle changes in the active site geometry significantly affect catalytic performance. These conclusions are based on the observed structural and energetic profiles of the phosphoryl transfer reactions.