<P> Proteases are used throughout an organism for various metabolic processes . Acid proteases secreted into the stomach ( such as pepsin ) and serine proteases present in duodenum ( trypsin and chymotrypsin ) enable us to digest the protein in food . Proteases present in blood serum ( thrombin , plasmin , Hageman factor , etc . ) play important role in blood - clotting , as well as lysis of the clots , and the correct action of the immune system . Other proteases are present in leukocytes ( elastase , cathepsin G ) and play several different roles in metabolic control . Some snake venoms are also proteases , such as pit viper haemotoxin and interfere with the victim 's blood clotting cascade . Proteases determine the lifetime of other proteins playing important physiological role like hormones , antibodies , or other enzymes . This is one of the fastest `` switching on '' and `` switching off '' regulatory mechanisms in the physiology of an organism . </P>
based on the residues in the active site what family of proteases does pepsin belong to
Acid proteases