Prothrombin is a protein made by the liver. It is one of several substances known as clotting (coagulation) factors. During the clotting process, prothrombin (coagulation factor II) is proteolytically cleaved by the prothrombinase enzyme complex to form thrombin. Thrombin in turn acts as a serine protease that converts soluble fibrinogen into insoluble strands of fibrin, as well as catalyzing many other coagulation-related reactions.