A QM/QM Investigation of the hUNG2 Reaction Surface: The Untold Tale of a Catalytic Residue
Bibliographic record
Abstract
Human uracil-DNA glycosylase (hUNG2) is a base excision repair enzyme that removes the damaged base uracil from DNA through hydrolytic deglycosylation of the nucleotide. In the present study, the mechanism of hUNG2 is thoroughly investigated using ONIOM(MPWB1K/6-31G(d):PM3) active-site models to generate reaction potential energy surfaces. Active-site models that differ in the hydrogen-bonding arrangement of the nucleophilic water molecule and/or protonation state of His148 are considered. The large barrier calculated using the model with a cationic His148 verifies that this residue is neutral in the early stages of the reaction. The reaction pathways predicted by two models with a neutral His148 are consistent with a wealth of experimental data on the enzyme, including mutational studies, which supports our approach. On the basis of our calculations, we propose a complete mechanism for the chemical step of hUNG2. In the first part of the reaction, His268, Asn204, and a water molecule work together to stabilize the negative charge forming on the uracil moiety. Subsequently, either Asp145 or His148 can act as the general base that activates the water nucleophile depending on the binding orientation of the water molecule in the active site. However, we propose that His148 preferentially acts as the general base. Therefore, in agreement with previous proposals, we assign the primary function of Asp145 to electrostatic stabilization of the positive charge developing on the sugar moiety during the reaction, which is also consistent with a growing theory that the primary function of active-site carboxylate groups present in many glycosylases is transition state stabilization. Most importantly, our work explains, for the first time, the role of His148 in the chemical step and provides additional support for the inclusion of this amino acid in the list of residues (Asp145 and His268) essential to the chemical step of the hUNG2 mechanism.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.003 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".