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Proteins are extremely large molecules. They are digested into polypeptides, then dipeptides and then amino acids.

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The general names for protein-digesting enzymes are protease and peptidase.

They break the peptide bonds joining amino acids together. The human intestine secretes a number of different proteases but they can be divided in two main types: endopeptidases and exopeptidases.

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Endopeptidases:

break peptide bonds between specific amino acids in the middle of polypeptide chains. They beak large polypeptides into smaller polypeptides.

Exopeptidases:

break peptide bonds at the ends of polypeptide chains, so producing amino acids.

Endopeptidases hydrolyse peptide bonds within the protein molecule, then exopeptidases hydrolyse the terminal or penultimate peptide bonds at the ends of these shorter peptides.

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

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Pepsinogen, the inactive precursor of the enzyme pepsin, is produced by specialised cells within the stomach walls.

It forms a component of gastric juice which is secreted into the stomach; the hydrochloric acid also present in the juice initiates its activation.

The first - formed pepsin then quickly completes the conversion of the remainder of the pepsinogen, in a process known as autocatalysis.

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

It has a pH optimum of about 1.5 - 2.0; this provided by the hydrochloric acid.

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Trypsinogen is an inactive form of the enzyme trypsin and is secreted by the exocrine glands in the pancreas.

It is activated to trypsin by the 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.

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Peptidases are exopeptidases which complete the digestion of proteins by releasing amino acids from the ends of the polypeptides produced by the action of pepsin and trypsin.

They are 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.

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See proteins at 3:06-4:28

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Digestion and Absorption for A-level Biology: Carbohydrates, proteins and lipids (micelles)

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Peptidases can be classified as aminopeptidases or carboxypeptidases.

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What type of reaction occurs in the breaking of peptide bonds? 

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Which is within the optimum pH for trypsin?

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Which is within the pH optimum of pepsin?

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The cell membranes of the microvilli on the surface of intestinal epithelium cells or in the cytoplasm of the epithelial cells

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Specialised cells within the stomach wall 

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Which of the following statements is true?

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Endopeptidases break bonds at the ends of the polypeptide

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Endopeptidases break bonds in the middle of the polypeptide

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What is the name of the inactive precursor of the enzyme pepsin? 