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Different enzymes in the digestive system break down food molecules, with most digestion occurring in the small intestine. Protein digestion involves endopeptidases in the stomach and small intestine, followed by exopeptidases to produce single amino acids. Exopeptidases, such as carboxypeptidase and aminopeptidase, have specific modes of action and break bonds between certain amino acids. Dipeptidases cleave specific pairs of amino acids.
Within the digestive system, there are many different enzymes that break down food molecules. Each of these enzymes has a different role and some only act in a specific organ under specific conditions. Most of the digestion takes place within the small intestine by a large number of different enzymes. Protein digestion is an example of a complex process that is carried out in different organs by different groups of enzymes. Exopeptidases are a group of enzymes involved in the complete digestion of proteins.
The digestion of a protein molecule is very specific, as the enzymes involved only break the bonds next to certain amino acids present within the peptide chain. When a peptide bond forms, it is always between the amino end of one amino acid and the carboxyl end of another. When the amino acid sequence for a particular peptide chain, or protein, is given, it is usually read from the amino end, N-terminus, to the last amino acid, which has a free carboxyl, or C-terminus. Initially, although the protein is being broken down, very few amino acids are produced.
Proteins are large molecules and their digestion has a series of steps, starting in the stomach with pepsin, one of three endopeptidases. Once the partially digested proteins move from the stomach to the small intestine, the other two endopeptidases, trypsin and chymotrypsin, continue to break down the proteins. These three enzymes divide the long peptide strands that make up the protein into variable lengths. Endopeptidases are so named because they break the peptide bonds found within the protein.
To complete the digestive process to produce single amino acids from a protein chain, an exopeptidase is required. Each exopeptidase breaks the bond between the final amino acid and the rest of the chain. There are several exopeptidases, each of which has a highly specific mode of action. Where the amino acid is joined to the rest of the peptide chain and which amino acids are joined play a role in determining which exopeptidase will break the bond.
Carboxypeptidase is an exopeptidase that breaks the bond between the penultimate and last amino acid at the C-terminal end. Another exopeptidase, aminopeptidase, performs the same action but at the N terminus. Other exopeptidases, called dipeptidases, cleave particular pairs of amino acids. For example, a dipeptidase will only break the bond between a glycine bound to a leucine. Another dipeptidase will only act on a peptide bond between two glycine amino acids linked together.
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