An enzyme is a protein that catalyzes a biochemical reaction. It converts a substrate into a product. Each enzyme has a structure adapted to its function and its activity is dependent on an optimum temperature and pH. Factor XII (FXII) is a glycoprotein synthesized by the liver. FXII participates in the contact phase which initiates the intrinsic pathway of coagulation. Activated on contact with a negatively charged surface, it becomes capable of activating prekallikrein and kallikrein (amplified by KHPM) then FXI to FXIa in the presence of KHPM. The FXIa thus formed activates the FXII in FXIIa, amplifying the reaction.