There are two main types of proteases secreted by the intestines:

• Endopeptidases break peptide bonds between specific amino acids in the middle of polypeptide chains.

• Exopeptidases break peptide bonds at the ends of polypeptide chains

Proteases are generally secreted in an inactive form to prevent them from digesting the protein within the cells which produce them

Pepsin is an endopeptidase which hydrolyses proteins to shorter - chain polypeptides.

• pH optimum of about 1.5 - 2.0

• Produced by specialised cells within the stomach walls.

• It is a substance in gastric juice which is secreted into the stomach. The hydrochloric acid in the juice begins its activation, then the small amount of pepsin formed quickly concerts the remaining pepsinogen into active pepsin by a process called autocatalysis

• It is activated by an enzyme enterokinase secreted by epithelial cells of the villi

• Once formed, trypsin can autocatalyse the activation of more trypsinogen. Like pepsin, trypsin is an endopeptidase which hydrolyses proteins to shorter - chain polypeptides. It has a pH optimum of about 7.5 - 8.8; this is provided by alkaline salts present in bile and in pancreatic and intestinal juices.

• Found within the cell membranes of the microvilli found on the surface of intestinal epithelium cells, or in the cytoplasm of the epithelial cells.

• They may be classified as:

  aminopeptidases if they work at the end of the chain that has an amino acid with a free amino group or carboxypeptidases if they work at the opposite end where the amino acid has a free carboxyl group.

1. proteins --> polypeptides + peptides

2. peptides --> amino acids + dipeptides