Figure 1. Thrombin cleaves fibrinopeptides from the fibrinogen molecule to create fibrin monomers.

The cleavage of fibrinopeptides allows ‘knob-hole’ interactions between fibrin monomers, causing the fibrin monomers to polymerize and form protofibrils and fibers. The peptide GPRP competes for the ‘holes’ in the fibrin monomers and inhibits the ‘knob-hole’ interactions. If GPRP is present in large enough concentrations, the fibrin aggregates depolymerize and the clot dissolves. GPRP plays the role of a competitive inhibitor of fibrin polymerization, but can also push the reaction in the opposite direction.