Thrombin complexed with fibrinogen, molecular model. The thrombin molecules (left and right, purple and green) are bound to the central part of the fibrinogen molecule (centre, multiple colours). The fibrinogen chains are the beta and gamma chains. Both structures are the ones found in humans. Thrombin is an enzyme 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, (with platelets) over a wound site.