Question

Explain why trypsin and chymotrypsin break peptide bonds between different amino acids.

Answer 1

The three serine proteases of the chymotrypsin-like clan that have been studied in greatest detail are chymotrypsin, trypsin, and elastase. 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. All three of these enzymes are similar in structure, as shown through their X-ray structures. The differing aspect lies in the peptide bond that is being cleaved; this is called the scissile bond. The different enzymes, like most enzymes, are highly specific in the reactions they catalyze. 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:

* Chymotrypsin is responsible for cleaving peptide bonds following a bulky hydrophobic amino acid residue. Preferred residues include phenylalanine, tryptophan, and tyrosine, which fit into a snug hydrophobic pocket.

* Trypsin is responsible for cleaving peptide bonds following a positively-charged amino acid residue. Instead of having the hydrophobic pocket of the chymotrypsin, there exists an aspartic acid residue at the base of the pocket. This can then interact with positively-charged residues such as arginine and lysine on the substrate peptide to be cleaved.

* Elastase is responsible for cleaving peptide bonds following a small neutral amino acid residue, such as Alanine, glycine, and valine. (These amino acid residues form much of the connective tissues in meat). 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.

The combination of these three enzymes make an incredibly effective digestive team and are primarily responsible for the digestion of proteins.