Bulk of protein digestion is due to the pancreatic proteases. Several proteases are synthesized in the pancreas and secreted into the lumen of the small intestine.

The four major pancreatic proteases are trypsin, chymotrypsin, elastase and carboxypeptidase, which are synthesized and packaged into secretory vesicles as an the inactive proenzymes.

packaging of an inactive precursor is a way for the cells to safely handle these enzymes. In addition, the secretory vesicles also contain a trypsin inhibitor which serves as an additional safeguard.

Trypsinogen is activated by the enzyme enterokinase, which is embedded in the intestinal mucosa.

Once trypsin is formed it activates the other three enzymes, as well as additional molecules of
trypsinogen.

Trypsin, chymotrypsin and elastase, digest proteins into peptides and peptides into smaller peptides, but
they cannot digest proteins and peptides to single amino acids.