Bacterial-type Phosphoenolpyruvate Carboxylase (PEPC) Functions as a Catalytic and Regulatory Subunit of the Novel Class-2 PEPC Complex of Vascular Plants
Bibliographic record
Abstract
Phosphoenolpyruvate carboxylase (PEPC) is a tightly regulated anaplerotic enzyme situated at a major branch point of the plant C metabolism. Two distinct oligomeric classes of PEPC occur in the triglyceride-rich endosperm of developing castor oil seeds (COS). Class-1 PEPC is a typical homotetramer composed of identical 107-kDa plant-type PEPC (PTPC) subunits (encoded by RcPpc3), whereas the novel Class-2 PEPC 910-kDa hetero-octameric complex arises from a tight interaction between Class-1 PEPC and distantly related 118-kDa bacterial-type PEPC (BTPC) polypeptides (encoded by RcPpc4). Here, COS BTPC was expressed from full-length RcPpc4 cDNA in Escherichia coli as an active PEPC that exhibited unusual properties relative to PTPCs, including a tendency to form large aggregates, enhanced thermal stability, a high K(m)((PEP)), and insensitivity to metabolite effectors. A chimeric 900-kDa Class-2 PEPC hetero-octamer having a 1:1 stoichiometry of BTPC:PTPC subunits was isolated from a mixture of clarified extracts containing recombinant RcPPC4 and an Arabidopsis thaliana Class-1 PEPC (the PTPC, AtPPC3). The purified Class-2 PEPC exhibited biphasic PEP saturation kinetics with high and low affinity sites attributed to its AtPPC3 and RcPPC4 subunits, respectively. The RcPPC4 subunits: (i) catalyzed the majority of the Class-2 PEPC V(max), particularly in the presence of the inhibitor l-malate, and (ii) also functioned as Class-2 PEPC regulatory subunits by modulating PEP binding and catalytic potential of its AtPPC3 subunits. BTPCs appear to associate with PTPCs to form stable Class-2 PEPC complexes in vivo that are hypothesized to maintain high flux from PEP under physiological conditions that would otherwise inhibit Class-1 PEPCs.
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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 | 0.000 |
| 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 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".