enzyme that causes blood clot formation
[thromâ²bin]. an enzyme formed from prothrombin, calcium, and thromboplastin in plasma during the clotting process. Thrombin causes fibrinogen to change to fibrin, which is essential in the formation of a clot.Also called factor IIa. See also blood clot, prothrombin. Hematology A key clot promoting enzyme that converts fibrinogen to fibrin and protects against fibrinolysis by activating thrombin-activatable fibrinolysis inhibitor.thromâ²bin]. an enzyme formed from prothrombin, calcium, and thromboplastin in plasma during the clotting process. Thrombin causes fibrinogen to change to fibrin, which is essential in the formation of a clot.