Thrombin protein, computer model. Thrombin is a protein involved in the blood coagulation (clotting) process. It converts fibrinogen (a soluble plasma glycoprotein synthesised in the liver) to fibrin, a protein that is polymerised to form a mesh, or clot, (in conjunction with platelets) over a wound site. In this model the molecule's shape (secondary structure) is seen, comprising alpha helices (coils), beta sheets (arrows) and linking regions (lines).