Thrombin (Factor IIa) (Last Updated - 2/27/2005)

-formed via activation of prothrombin (Factor II) --> this reaction is catalyzed by calcium (Factor IV), phospholipids, Factor Va & Factor Xa
-thrombin in turn catalyzes multiple different factor activations:
     1) along with calcium (Factor IV) catalyzes the activation of fibrinogen (Factor I) to fibrin (Factor Ia)
     2) along with calcium (Factor IV) catalyzes the activation of Factor XIII to XIIIa
     3) catalyzes the activation of Factor V to Va --> so thrombin actually leads to its own activation via activating Factor Va (positive feedback loop)
     4) catalyzes the activation of Factor VIII to VIIIa
     5) catalyzes the activation of Factor XI to XIa

-click here to view an image of the clotting cascade

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References:

Goodman & Gilman's: The Pharmacologic Basis Of Therapeutics - 10th Edition - 2001. Chapter 55.