Characterization of the C-terminal extension of carboxysomal carbonic anhydrase from <i>Synechocystis</i> sp. PCC6803
Bibliographic record
Abstract
Sequence analysis of the carboxysomal carbonic anhydrase (CcaA) from Synechocystis PCC6803, Synechococcus PCC7942 and Nostoc ATCC29133, indicated high sequence identity to the β class of plant and bacterial carbonic anhydrases (CA), and conservation of the active site region. However, the cyanobacterial enzyme has a C -terminal extension of about 75 amino acids (aa) not found in the plant enzymes, and largely absent from other bacterial enzymes. Using recombinant DNA technology, genes encoding C -terminal truncation products of up to 127 aa were overexpressed in E. coli , and partially purified lysates were analysed for CA-mediated exchange of 18O between 13C18O2and H216O. Recombinant CcaA proteins with up to 60 aa removed (CcaAΔ60) were catalytically competent, but beyond this there was an abrupt loss of activity. CcaAΔ0, along with CcaAΔ40 and CcaAΔ60, also catalysed the hydrolysis of carbon oxysulfide (COS; an isoelectronic structural analogue of CO2), but CcaAΔ63 and CcaAΔ127 did not, indicating that truncations greater than 62 aa resulted in a general loss of catalytic competency. Analysis of protein-protein interaction using the yeast two-hybrid system revealed that CcaA did not interact with the large or small Rubisco subunits (RbcL and RbcS, respectively) of Synechocystis , but there was strong CcaA-CcaA interaction. This protein interaction also ceased with C -terminal truncations in CcaA greater than 60 aa. The correlation between loss of CcaA-CcaA interaction and CcaA catalytic activity suggests that the proximal portion of the C-terminal extension is required for oligomerization, and that this oligomerization is essential for catalysis by the cyanobacterial enzyme. Thus, the C-terminal extension may play an important role in the function of CA within cyanobacterial carboxysomes, which is not required by the higher plant enzymes.
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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".