Coagulation factor XII circulates in blood as a zymogen. This single chain zymogen is converted to a two chain serine protease with an heavy chain (alpha factor XIIa) and a light chain. The heavy chain contains two fibronectin type domains, two epidermal growth factor (EGF) like domains, a kringle domain and a proline rich domain, whereas the light chain contains only a catalytic domain. On activation, further cleavages takes place in the heavy chain, resulting in the production of beta factor XIIa light chain and the alpha factor XIIa light chain becomes beta factor XIIa fraction is isolated by a two-step PEG heavy chain. Prekallikrein is cleaved by factor XII to precipitation procedure followed by form kallikrein, which then cleaves factor XII first to alpha factor XIIa and then to beta factor XIIa.