Strategies for Modulating the pH-Dependent Activity of a Family 11 Glycoside Hydrolase
Bibliographic record
Abstract
The pH-dependent activity of wild-type Bacillus circulans xylanase (BcX) is set by the pK(a) values of its nucleophile Glu78 and general acid/base Glu172. Herein, we examined several strategies to manipulate these pK(a) values and thereby shift the pH(opt) at which BcX is optimally active. Altering the global charge of BcX through random succinylation had no significant effect. Mutation of residues near or within the active site of BcX, but not directly contacting the catalytic carboxyls, either had little effect or reduced its pH(opt), primarily by lowering the apparent pK(a) value of Glu78. However, mutations causing the largest pK(a) changes also impaired activity. Although not found as a general acid/base in naturally occurring xylanases, substitution of Glu172 with a His lowered the pH(opt) of BcX from 5.6 to 4.7 while retaining 8% activity toward a xylobioside substrate. Mutation of Asn35, which contacts Glu172, to either His or Glu also led to a reduction in pH(opt) by ~1.2 units. Detailed pK(a) measurements by NMR spectroscopy revealed that, despite the opposite charges of the introduced residues, both the N35H and N35E forms of BcX utilize a reverse protonation mechanism. In this mechanism, the pK(a) value of the general acid is lower than that of the nucleophile, and only a small population of enzyme is in a catalytically competent ionization state. However, overall activity is maintained due to the increased strength of the general acid. This study illustrates several routes for altering the pH-dependent properties of xylanases, while also providing valuable insights into complex protein electrostatics.
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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 | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 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".