Dismutase Activity of ADP-<scp>l</scp>-<i>glycero</i>-<scp>d</scp>-<i>manno-</i>heptose 6-Epimerase: Evidence for a Direct Oxidation/Reduction Mechanism
Bibliographic record
Abstract
The first positive evidence for the utilization of a direct C-6' ' oxidation/reduction mechanism by ADP-l-glycero-d-manno-heptose 6-epimerase is reported here. The epimerase (HldD or AGME, formerly RfaD) operates in the biosynthetic pathway of l-glycero-d-manno-heptose, which is a conserved sugar in the core region of lipopolysaccharide (LPS) of Gram-negative bacteria. The stereochemical inversion catalyzed by the epimerase is interesting as it occurs at an "unactivated" stereocenter that lacks an acidic C-H bond, and therefore, a direct deprotonation/reprotonation mechanism cannot be employed. Instead, the epimerase employs a transient oxidation strategy involving a tightly bound NADP(+) cofactor. A recent study ruled out mechanisms involving transient oxidation at C-4' ' and C-7' ' and supported a mechanism that involves an initial oxidation directly at the C-6' ' position to generate a 6' '-keto intermediate (Read, J. A., Ahmed, R. A., Morrison, J. P., Coleman, W. G., Jr., Tanner, M. E. (2004) J. Am. Chem. Soc. 126, 8878-8879). A subsequent nonstereospecific reduction of the ketone intermediate can generate either epimer of the ADP-heptose. In this work, an intermediate analogue containing an aldehyde functionality at C-6' ', ADP-beta-d-manno-hexodialdose, is prepared in order to probe the ability of the enzyme to catalyze redox chemistry at this position. It is found that incubation of the aldehyde with a catalytic amount of the epimerase leads to a dismutation process in which one-half of the material is oxidized to ADP-beta-d-mannuronic acid and the other half is reduced to ADP-beta-d-mannose. Transient reduction of the enzyme-bound NADP(+) was monitored by UV spectroscopy and implicates the cofactor's involvement during catalysis.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.005 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.001 | 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 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 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 teacher head, 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".