Adenine phosphoribosyltransferase isoforms of <i>Arabidopsis</i> and their potential contributions to adenine and cytokinin metabolism
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Bibliographic record
Abstract
Adenine phosphoribosyltransferase (APT; EC 2.4.2.7) is a constitutively expressed enzyme involved in the one-step salvage of adenine to AMP. The Arabidopsis thaliana genome contains five sequences annotated as encoding APT or APT-like enzymes. Three of these have now been cloned, over-expressed and compared using kinetic analyses. At a cytosolic pH, all bind adenine efficiently based on their Km values (0.8-2.6 &mgr;M), although APT1 metabolizes adenine at a rate 31-53 times faster than APT2 and APT3, respectively. Since APT also has a possible role in the interconversion of cytokinin bases to nucleotides, we characterized the activity of each isoform on zeatin, isopentenyladenine and benzyladenine. Based on their Km values, APT2 and APT3 had much higher affinities than APT1 for all three cytokinins (15-440 &mgr;M for APT2 and 3 vs. 1.8-2.5 mM for APT1); conversely the Vmax values for APT2 and APT3 on these CK substrates showed the opposite trend, being 4- to 19-fold lower than those of APT1. Anti-peptide antibodies for APT1, APT2, and APT3 were prepared and used to examine the subcellular localization of each isoform. Based on these results, APT1 and APT3 appear to be cytosolic, while the localization of APT2 was inconclusive although sequence analysis implies that APT2 is also cytosolic. Each isoform was modelled against the crystal structure of APT from Leishmania donovani, and structural differences in substrate specificity-determining domains have been found. The estimated kinetic activities of these APTs suggest that they contribute primarily to adenine recycling, although an involvement in cytokinin interconversion cannot be discounted.
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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 it