Direct and indirect roles of His‐418 in metal binding and in the activity of β‐galactosidase (<i>E. coli</i>)
Bibliographic record
Abstract
The active site of ss-galactosidase (E. coli) contains a Mg(2+) ion ligated by Glu-416, His-418 and Glu-461 plus three water molecules. A Na(+) ion binds nearby. To better understand the role of the active site Mg(2+) and its ligands, His-418 was substituted with Asn, Glu and Phe. The Asn-418 and Glu-418 variants could be crystallized and the structures were shown to be very similar to native enzyme. The Glu-418 variant showed increased mobility of some residues in the active site, which explains why the substitutions at the Mg(2+) site also reduce Na(+) binding affinity. The Phe variant had reduced stability, bound Mg(2+) weakly and could not be crystallized. All three variants have low catalytic activity due to large decreases in the degalactosylation rate. Large decreases in substrate binding affinity were also observed but transition state analogs bound as well or better than to native. The results indicate that His-418, together with the Mg(2+), modulate the central role of Glu-461 in binding and as a general acid/base catalyst in the overall catalytic mechanism. Glucose binding as an acceptor was also dramatically decreased, indicating that His-418 is very important for the formation of allolactose (the natural inducer of the lac operon).
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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) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| 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".