Thrombin is a key clot promoter. It is an enzyme that presides over the conversion of a fibrinogen to fibrin. The coagulation cascade, is a series of proenzyme-enzyme conversions. Coagulation starts when a vessel is wounded. A vessel is wrapped in a layer that is full of cells containing Tissue Factor (TF). After injury, TF is exposed to blood, it binds with factor VIIa to form the first enzyme: TF:VIIa. This activates factor X to Xa, which in turn forms thrombin.