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Thrombin (activated Factor II) is a coagulation protein that has many effects in the coagulation cascade. It is in fact a serine protease, so it cleaves proteins at certain locations. Its main effect is to turn fibrinogen into fibrin.

Thrombin is produced from prothrombin, which is essentially the inactive state of this protein and produced in the liver. Activated coagulation factors X and V are responsible for the conversion of prothrombin to thrombin.

Prothrombin is also called factor II and requires vitamin K to be produced.

The active form of factor II, thrombin, has a number of effects on other coagulation proteins. Its most direct effect on clot production is its conversion of fibrinogen into fibrin, but it also feeds back on a number of locations in the coagulation cascade.

Thrombin feeds back, activating factor XI and factor VIII, increasing the flow through the intrinsic pathway of coagulation. Thrombin also activates factor V of the common pathway, which quickly increases its own production from prothrombin.

Factor XIII, the coagulation protein that stabilises the fibrin clot, is also activated. This increases the speed the fibrin in the clot is covalently linked to itself.

As well as the coagulation pathways, thrombin also promotes platelet aggregration.

Interestingly, thrombin activates protein C which inhibits coagulation. This happens on the surface of the endothelium, on a protein called thrombomodulin.