Our December 2025 Quick Question was a quiz that asked, “What plasma control protein acts against FIIa & FXa?” Here’s how our 34 respondents fared…
- Antithrombin: 27 (79%)
- Protein C: 5 (15%)
- Protein S: 1 (3%)
- Protein Z: 0
- TFPI: 1 (3%)
We scored well on this question. Antithrombin (AT), when activated by therapeutic heparin or native heparan sulfate, forms a covalent bond with FIIa (thrombin) or FXa, click for AT function figure. Unlike heparin, the direct anticoagulants apixaban and rivaroxaban bypass antithrombin and directly inactivate FXa, and the direct anticoagulant dabigatran directly inactivates FIIa.
PC is a serine protease that, when bound to cofactor PS, binds and digests FVa and FVIIIa. PC and PS are vitamin K-dependent proteins.
PZ is a little-studied vitamin K-dependent factor that binds ZPI. ZPI inhibits FXa and FXIa. (Han K, Fiehler R, Broze GI. Characterization of the protein Z-dependent protease inhibitor. Blood. 2000:96:3049–55.)
TFPI directly inhibits FVIIa and FXa.
Acquired control protein deficiencies may be associated with thrombophilia.
No comments here.