A tale of two mechanisms: Clarification of the pathway for MBD4 catalyzed glycosidic bond cleavage using MD and QM/MM calculations
Bibliographic record
Abstract
DNA methylation to yield 5-methylcytosine (5mC) in CpG motifs plays a vital role in epigenetic regulation. However, deamination of 5mC results in canonical thymine (T) that requires methyl-CpG-binding domain protein 4 (MBD4) for repair. This important function has resulted in MBD4 being implicated in various human health disorders including MBD4-associated neoplasia syndrome and cancer resistance to 5-fluorouracil treatment. Nevertheless, the catalytic mechanism of MBD4 is poorly understood, with conflicting experimental observations resulting in multiple proposals. To provide atomic level structural details of the active site conformation and clarify the mechanistic pathway, this study uses a combination of adaptively biased molecular dynamics (abMD) simulations and quantum mechanics/molecular mechanics (QM/MM) calculations to map the MBD4 catalytic mechanism. Although our data indicate that the catalytic D560 residue is flexible in the active site, only one conformation facilitates 5mC excision. Despite some literature proposing the formation of a DNA−protein crosslinked intermediate, our modeling suggests catalysis is only viable through a deglycosylation mechanism that involves a water nucleophile attacking C1′ of T, with D560 activating the nucleophile and stabilizing the transition state and nucleobase departure facilitated by a network of hydrogen bonds. This proposal is fully consistent with experimental crystallographic, mutagenic, stereoscopic, and kinetic data, and aligns the MBD4 catalytic pathway with that characterized for several other monofunctional DNA glycosylases. By furthering our knowledge of MBD4 catalysis, this work will aid in the future development of treatments for MBD4-related genetic disorders and the rational design of transition state mimic inhibitors to enhance existing cancer therapies. • Key MBD4 residue D560 adopts many conformations, with one facilitating catalysis • MBD4 utilizes a direct hydrolysis mechanism for glycosidic bond cleavage • MBD4 uses multiple amino acid−substrate hydrogen bonds to stabilize the anionic leaving group • Computational mechanism aligns with experimental structural, kinetic, and mutational data • New insights can be used to enhance treatments for human diseases
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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".