The Asp272–Glu282 Region of Platelet Glycoprotein Ibα Interacts with the Heparin-binding Site of α-Thrombin and Protects the Enzyme from the Heparin-catalyzed Inhibition by Antithrombin III
Bibliographic record
Abstract
Platelet glycoprotein Ib (GpIb) mediates interaction with both von Willebrand factor and thrombin. Thrombin binds to GpIb via its heparin-binding site (HBS) (De Candia, E., De Cristofaro, R., De Marco, L., Mazzucato, M., Picozzi, M., and Landolfi, R. (1997) Thromb. Haemostasis 77, 735-740; De Cristofaro, R., De Candia, E., Croce, G., Morosetti, R., and Landolfi, R. (1998) Biochem. J. 332, 643-650). To identify the thrombin-binding domain on GpIbalpha, we examined the effect of GpIbalpha(1-282), a GpIbalpha fragment released by the cobra venom mocarhagin on the heparin-catalyzed rate of thrombin inhibition by antithrombin III (AT). GpIbalpha(1-282) inhibited the reaction in a dose-dependent and competitive fashion. In contrast, the GpIbalpha(1-271) fragment, produced by exposing GpIbalpha(1-282) to carboxypeptidase Y, had no effect on thrombin inhibition by the heparin-AT complex. Measurements of the apparent equilibrium constant of the GpIbalpha(1-282) binding to thrombin as a function of different salts (NaCl and tetramethyl-ammonium chloride) concentration (0.1-0.2 M) indicated a large salt dependence (Gamma(+/-) = -4.5), similar to that pertaining to the heparin binding to thrombin. The importance of thrombin HBS in its interaction with GpIbalpha was confirmed using DNA aptamers, which specifically bind to either HBS (HD22) or the fibrinogen recognition site of thrombin (HD1). HD22, but not HD1, inhibited thrombin binding to GpIbalpha(1-282). Furthermore, the proteolytic derivative gamma(T)-thrombin, which lacks the fibrinogen recognition site, binds to GpIbalpha via its intact HBS in a reaction that is inhibited by HD22. Neither alpha- nor gamma(T)-thrombin bound to GpIbalpha(1-271), suggesting that the Asp(272)-Glu(282) region of GpIbalpha may act as a "heparin-like" ligand for the thrombin HBS, thereby inhibiting heparin binding to thrombin. It was also demonstrated that intact platelets may dose-dependently inhibit the heparin-catalyzed thrombin inhibition by AT at enzyme concentrations <5 nM. Altogether, these findings show that thrombin HBS binds to the region of GpIbalpha involving the Asp(272)-Glu(282) segment, protecting the enzyme from the inactivation by the heparin-AT system.
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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) | 0.001 | 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 | 0.001 |
| Insufficient payload (model declined to judge) | 0.002 | 0.001 |
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".