Thrombin (aka activated Factor II, Factor IIa, FIIa) is a coagulation protein that has many effects in the coagulation cascade. It is an serine protease (EC 3.4.21.5) that converts fibrinogen into fibrin.
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 activiation of the 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.
Factor XIII is also activated by thrombin. Factor XIIIa is a transglutaminase that catalyzes the formation of covalent bonds between lysine and glutamine residues in fibrin. The covalent bonds increase the stability of the fibrin clot.
In addition to its activity in the coagulation cascades, thrombin also promotes platelet activation, via activation of protease-activated receptors on the platlet.
Thrombin activates protein C, an inhibitor of the coagulation cascade. The activation of protein C is greatly enhanced following the binding of thrombin to thrombomodulin, an integral membrane protein expressed by endothelial cells. Activated protein C inactivates factors Va and VIIIa. Binding of activated protein C to protein S leads to a modest increase in its activity.
Activation of prothrombin is crucial in physiological and pathological coagulation. Various rare diseases involving prothrombin have been described.
An apparently quite common disorder (up to 5% in Western patients) is the substitution of adenine for guanine at position 20210 of the prothrombin gene. Although this falls outside the reading frame for the protein, it leads to high levels of prothrombin and a possibly increased risk of thrombosis (Poort et al 1996).
The prothrombin gene is located on the eleventh chromosome (11p11-q12).
Sources: DiaPharma, Wikipedia, PubMed