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 digestion takes place within the small intestine by a large number of different enzymes. The digestion of proteins is an example of a complex process that is carried out in different organs by different groups of enzymes. Exopeptidases are one 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 beside certain amino acids found within the peptide chain. When a peptide bond is formed, it is always between the amino end of one amino acid and the carboxyl end of another. When the amino acid sequence is given for a particular peptide chain, or protein, it is usually read from the amino end, N terminal, to the last amino acid, which has a free carboxyl, or the C terminal. Initially, although the protein is being broken down, very few amino acids are produced.
Proteins are large molecules and their digestion has a number of steps, beginning in the stomach by 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 split the long peptide strands that make up the protein into varying lengths. Endopeptidases are so named as they break down the peptide bonds found within the protein.
To complete the digestive process to produce single amino acids from a protein chain, an exopeptidase is needed. Each exopeptidase breaks the bond between the end amino acid and the rest of the chain. There are several different 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 together plays a role in determining which exopeptidase will break the bond.
Carboxypeptidase is an exopeptidase that breaks the bond between the second last and the last amino acid at the C terminal end. Another exopeptidase, aminopeptidase, carries out the same action but at the N terminal end. Other exopeptidases, called dipeptidases, break apart particular pairs of amino acids. For example, one dipeptidase will only break the bond between a glycine linked with a leucine. Another dipeptidase will only act on a peptide bond between two glycine amino acids linked together.