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Mammalian pancreatic juices contain several inactive precursors of endopeptidases or peptide chain cutting enzymes. These include trypsinogen, chymotrypsinogens A and B, a component of pro-carboxypeptidase similar to chymotrypsinogen and pro-elastase. Once activated in the duodenum these enzymes have different specificities to the side chain immediately preceding the peptide bond to be cleaved. Trypsin is highly specific toward the binding of positively charged side chains of lysine and arginine, while chymotrypsin and elastase have specificity toward large hydrophobic and small aliphatic side chains, respectively.
Stroud et al. (Sat,) studied this question.