KLKB1 is a 638 amino acids protein that belongs to the peptidase S1 family and plasma kallikrein subfamily. It contains a peptidase S1 domain and four apple domains. The enzyme cleaves Lys-Arg and Arg-Ser bonds. It activates, in a reciprocal reaction, factor XII after its binding to a negatively charged surface. It also releases bradykinin from HMW kininogen and may also play a role in the renin-angiotensin system by converting prorenin into renin. It participates in the surface-dependent activation of blood coagulation, fibrinolysis, kinin generation and inflammation.Plasma prekallikrein deficiency causes a prolonged activated partial thromboplastin time in patients.