Thrombin (activated Factor II (IIa)) is a coagulation protein that has many effects in the coagulation cascade. It is a serine protease () that converts soluble fibrinogen into insoluble strands of fibrin.

Generation

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Thrombin is produced by the enzymatic cleavage of two sites on prothrombin by activated Factor X (Xa). The activity of factor Xa is greatly enhanced by binding to activated Factor V (Va), termed the prothrombinase complex. Prothrombin is produced in the liver and is post-translationally modified in a vitamin K-dependent reaction that converts ten glutamic acids on prothrombin to gamma-carboxyglutamic acid (Gla). In the presence of calcium, the gamma-carboxyglutamic acid residues promote the binding of thrombin to phospholipid bilayers. Deficiency of vitamin K or administration of the anticoagulant warfarin inhibits the production of gamma-carboxyglutamic acid residues, slowing the activation of the coagulation cascade.

Action

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Coagulation cascade

Thrombin converts fibrinogen to an active form that assembles into fibrin. Thrombin also activates factor XI, factor V and factor VIII. This positive feedback accelerates the production of thrombin.

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