Subtilisin is evolutionary unrelated to the chymotrypsin-clan, but shares the same catalytic mechanism utilising a catalytic triad, to create a nucleophilic serine. Subtilisin is a serine protease in prokaryotes. The combination of these three enzymes make an incredibly effective digestive team, and are primarily responsible for the digestion of proteins. The pocket that is in 'trypsin' and 'chymotrypsin' is now partially filled with valine and threonine, rendering it a mere depression, which can accommodate these smaller amino acid residues. (These amino acid residues form much of the connective tissues in meat).

Elastase is responsible for cleaving peptide bonds following a small neutral amino acid residue, such as Alanine, glycine and valine.

This can then interact with positively-charged residues such as arginine and lysine on the substrate peptide to be cleaved. Instead of having the hydrophobic pocket of the chymotrypsin, there exists an aspartic acid residue at the base of the pocket.

Trypsin is responsible for cleaving peptide bonds following a positively-charged amino acid residue.

Preferred residues include phenylalanine, tryptophan and tyrosine, which fit into a snug hydrophobic pocket.

Chymotrypsin is responsible for cleaving peptide bonds following a bulky hydrophobic amino acid residue.

Each of these digestive serine proteases targets different regions of a polypeptide chain, based upon the side chains of the amino acid residues surrounding the site of cleavage: The different enzymes, like most enzymes, are highly specific in the reactions they catalyze.

The differing aspect lies in the peptide bond which is being cleaved, which is called the scissile bond. All three of these enzymes are similar in structure, as shown through their X-ray structures. All three enzymes are synthesized by the pancreatic acinar cells, secreted in the small intestine and are responsible for catalyzing the hydrolysis of peptide bonds. The three serine proteases of the chymotrypsin-like clan that have been studied in greatest detail are chymotrypsin, trypsin, and elastase. Digestive serine proteases Members Chymotrypsin-clan

Bulky hydrophobic amino acids