An endopeptidase is a type of protease, an enzyme from a large group that degrades proteins. Enzymes are proteins that make reactions happen much more quickly. Proteins are comprised of chains of amino acids, linked by peptide bonds. This linkage connects the carboxyl terminus of one amino acid with the amino terminus of the next. Endopeptidases cleave the peptide bonds of the amino acids within the protein, in contrast to exopeptidases, which cleave at the ends of the protein.
Endopeptidases are found in all classes of organisms and have a wide range of biological activities. They are involved in the digestion of protein in food. This includes the enzymes pepsin, trypsin, and chymotrypsin. Proteases are also involved in cellular signaling by breaking down other proteins, like antibodies or hormones. They can switch pathways on or off.
Proteases are usually first made as a larger molecule that is inactive. This protects the cell that synthesizes it from being damaged. Once the protease has been delivered to its target, for instance the stomach, a piece of the molecule is removed. This activates the protease.
Because of their myriad of roles in cellular function, there is much medical interest in endopeptidase activity. One example of this is prolyl endopeptidase, which cleaves specifically after the amino acid proline. It has been associated with psychological disorders, such as depression, mania, and schizophrenia. There is clinical interest in prolyl endopeptidase inhibitors as possible anti-depressants.
Another example is neutral endopeptidase, which has several other names. It is also known as neprilysin and common acute lymphoblastic leukemia antigen (CALLA). This protease degrades small secreted peptides, including the peptide that has been implicated as a cause of Alzheimer’s Disease, and several important signaling peptides. Neutral endopeptidase is sometimes used as a cancer marker, but its role in cancer is unclear. Inhibitors have been developed to aid in pain relief and the control of high blood pressure.
The endopeptidases are placed in different families, depending on the structure of their active site and the conditions they prefer. There are serine proteases, which have the amino acid serine at their active site. Members of this family include the digestive proteases trypsin and chymotrypsin, along with prolyl endopeptidase. One inhibitor used frequently in biochemical research labs is the highly toxic compound phenylmethanesulfonylflouride (PMSF). It is used during protein isolation and purification to inhibit serine protease activity, which can degrade the protein being purified.
Cysteine proteases have a sulfur group at their active site and are common in fruit. These enzymes are found in meat tenderizers. Papain is an example of such an endopeptidase and is used to treat bee and wasp stings.
The active site of aspartic proteases generally contains two aspartate groups. Metalloendopeptidases require a metal cofactor for activity. Neutral endopeptidases are part of this family, requiring zinc for activity.