Chymotrypsin serine protease, molecular model. Chymotrypsin is a digestive enzyme produced by the pancreas, shown here with a ribbon model for the protein and stick structures for the active site residues. Chymotrypsin is a serine protease, a common motif that uses a triad of serine (pink), histidine (yellow) and aspartic acid (orange) for its catalytic activity. The enzyme hydrolyses specifically the peptide bonds of tyrosine, tryptophan and phenylalanine residues.