Bifunctional Catalysis by CDP-ribitol Synthase: Convergent Recruitment of Reductase and Cytidylyltransferase Activities in <i>Haemophilus influenzae</i> and <i>Staphylococcus aureus</i>
Bibliographic record
Abstract
CDP-ribitol synthase catalyzes the formation of CDP-ribitol from ribulose 5-phosphate, NADPH, and CTP. CDP-ribitol is an activated precursor for the synthesis of virulence-associated polysaccharides in the capsule of the Gram-negative pathogen Haemophilus influenzae and in the cell walls of Gram-positive pathogens including Staphylococcus aureus. We showed previously that CDP-ribitol synthase activity in H. influenzae is catalyzed by the bifunctional enzyme Bcs1 in a two-step reaction with reduction preceding cytidylyl transfer [Zolli, M., et al. (2001) Biochemistry 40, 5041-5048]. In the work reported here, we predicted a CDP-ribitol synthesis locus in S. aureus tandemly arranged as tarI, encoding an orthologue of the cytidylyltransferase domain of Bcs1, and tarJ, coding for an analogue of the reductase domain of Bcs1. We have shown the formation of a functional CDP-ribitol synthase complex between TarI and TarJ. Steady-state mechanistic studies of the CDP-ribitol synthases TarIJ and Bcs1 revealed that the analogous reductases and orthologous cytidylyltransferases undergo ordered mechanisms. The sequence of substrate binding and product release of the orthologous cytidylyltransferases differed. Steady-state analysis of the reductase and cytidylyltransferase activities of TarIJ indicated a 100-fold difference in the turnover where the primary reductase was rate limiting. Rapid mixing experiments revealed the presence of approximately 12 microM ribitol 5-phosphate at steady state, 100-fold lower than the observed K(m) for this intermediate. Analysis of the approach to steady state suggested that channeling was not occurring in the coupled enzyme complex and was an unlikely driving force in the convergent recruitment of reductase and cytidylyltransferase activities in the two CDP-ribitol synthases.
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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 | 0.000 | 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 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".