The name of the enzyme you are looking for is thrombin. Thrombin is a serine protease enzyme that is used in the regulation of blood flow by inducing clotting. The actual clotting of blood is done by the glycoprotein fibrinogen. Fibrinogen is readily soluble protein that is then converted by the enzyme thrombin into fibrin. Fibrin is a less soluble form of fibrinogen and thus induces the clotting effect in blood. Blood that has had the plasma removed from it is called serum and cannot clot because all of these biomolecules are found in the plasma and have been removed from the sample.