Pepsin

Pepsin is an enzyme whose precursor form (pepsinogen) is released by the chief cells in the stomach and that degrades food proteins into peptides. It was discovered in 1836 by Theodor Schwann who also coined its name from the Greek word pepsis, meaning digestion (peptein: to digest). It was the first animal enzyme to be discovered, and, in 1929, it became one of the first enzymes to be crystallized, by John H. Northrop. Pepsin is a digestive protease, a member of the aspartate protease family.

Pepsin is one of three principal protein-degrading, or proteolytic, enzymes in the digestive system, the other two being chymotrypsin and trypsin. The three enzymes were among the first to be isolated in crystalline form. During the process of digestion, these enzymes, each of which is specialized in severing links between particular types of amino acids, collaborate to break down dietary proteins into their components, i.e., peptides and amino acids, which can be readily absorbed by the intestinal lining. Pepsin is most efficient in cleaving peptide bonds between hydrophobic and preferably aromatic amino acids such as phenylalanine, tryptophan, and tyrosine.

History
The term Pepsin was first coined by Theodor Schwann in the early 19th century. Scientists around this time began discovering many biochemical compounds that play a significant role in biological processes and pepsin was one of them. It was with the identification of a chemical agent found in the stomachs of animals, that scientist began looking into the digestive properties of organisms. This acidic substance that was able to convert nitrogen based foods into water soluble material was determined to be pepsin.

Precursor
Pepsin is expressed as a pro-form zymogen, pepsinogen, whose primary structure has an additional 44 amino acids.

In the stomach, chief cells release pepsinogen. This zymogen is activated by hydrochloric acid (HCl), which is released from parietal cells in the stomach lining. The hormone gastrin and the vagus nerve trigger the release of both pepsinogen and HCl from the stomach lining when food is ingested. Hydrochloric acid creates an acidic environment, which allows pepsinogen to unfold and cleave itself in an autocatalytic fashion, thereby generating pepsin (the active form). Pepsin cleaves the 44 amino acids from pepsinogen to create more pepsin. Pepsin will digest up to 20% of ingested amide bonds by cleaving preferentially after the N-terminal of aromatic amino acids such as phenylalanine, tryptophan, and tyrosine. Pepsin exhibits preferential cleavage for hydrophobic, preferably aromatic, residues in P1 and P1' positions. Increased susceptibility to hydrolysis occurs if there is a sulfur-containing amino acid close to the peptide bond, which has an aromatic amino acid. Pepsin cleaves Phe1Val, Gln4His, Glu13Ala, Ala14Leu, Leu15Tyr, Tyr16Leu, Gly23Phe, Phe24Phe and Phe25Tyr bonds in the B chain of insulin. Peptides may be further digested by other proteases (in the duodenum) and eventually absorbed by the body. Pepsin is stored as pepsinogen so it will only be released when needed, and does not digest the body's own proteins in the stomach's lining.

Pepsin functions best in acidic environments and is often found in an acidic environment, in particular those with a pH of 1.5 to 2. Pepsin denatures if the pH is more than 5.0.

Pepsin is said to have an optimum temperature between 37°C and 42°C in humans.

Pepsin is potently inhibited by the peptide inhibitor pepstatin. Pepsin is used for digestion of proteins.

Storage
Pepsins should be stored at very cold temperatures (between −80 °C and −20 °C) to prevent autolysis (self-cleavage).

Pepsin Inhibitors
Pepsin can be inhibited in two ways. The first method is introducing a pepsin inhibitor compound, and the second method is decreasing the acidity level to which the pepsin becomes inactive. This is usually around a pH of 4-5.

Pepstatin is a hydrophobic and insoluble compound that inhibits pepsin reaction by reducing the digestion of fibrin by pepsin. Antacids also inactivate pepsin by removing the acid that activates the pepsin. It also precipitates pepsin at pH 3-6. They are used in the treatment of gastrointestinal bleeding. Another type of pepsin inhibitor is 1,1-bis(diazoacetyl)-2-penylethane. This bisdiazoketone inactivates pepsin at pH around 5. The Reaction is usually accelerated by the presence of Cu(II), and the ph dependence of the process is consistent with the interaction of the enzyme with the metal complex of the carbine derived from the reagent.

Pepsin also goes through the process of feedback inhibition. When too much protein digestion occurs, the product slows down the reaction by inhibiting the pepsin that began the reaction.

Applications
Pepsin is commonly used in the preparation of F(ab')2 fragments from antibodies. In some assays, it is preferable to use only the antigen-binding (Fab) portion of the antibody. For these applications, antibodies may be enzymatically digested to produce either an Fab or an F(ab')2 fragment of the antibody. To produce an F(ab')2 fragment, IgG is digested with pepsin, which cleaves the heavy chains near the hinge region. One or more of the disulfide bonds that join the heavy chains in the hinge region are preserved, so the two Fab regions of the antibody remain joined together, yielding a divalent molecule (containing two antibody binding sites), hence the designation F(ab')2. The light chains remain intact and attached to the heavy chain. The Fc fragment is digested into small peptides. Fab fragments are generated by cleavage of IgG with papain instead of pepsin. Papain cleaves IgG above the hinge region containing the disulfide bonds that join the heavy chains, but below the site of the disulfide bond between the light chain and heavy chain. This generates two separate monovalent (containing a single antibody binding site) Fab fragments and an intact Fc fragment. The fragments can be purified by gel filtration, ion exchange, or affinity chromatography.

Fab and F(ab')2 antibody fragments are used in assay systems where the presence of the Fc region may cause problems. In tissues such as lymph nodes or spleen, or in peripheral blood preparations, cells with Fc receptors (macrophages, monocytes, B lymphocytes, and natural killer cells) are present which can bind the Fc region of intact antibodies, causing background staining in areas that do not contain the target antigen. Use of F(ab')2 or Fab fragments ensures that the antibodies are binding to the antigen and not Fc receptors. These fragments may also be desirable for staining cell preparations in the presence of plasma, because they are not able to bind complement, which could lyse the cells. F(ab')2, and to a greater extent Fab, fragments allow more exact localization of the target antigen, i.e., in staining tissue for electron microscopy. The divalency of the F(ab')2 fragment enables it to cross-link antigens, allowing use for precipitation assays, cellular aggregation via surface antigens, or rosetting assays.

Pepsin was also put into chewing gum, the first person to do so was Dr. Edward E. Beeman when he made beemans gum.

Genes
The following three genes encode identical human pepsinogen A enyzmes: 

A fourth human gene encodes gastricsin also known as pepsinogen C:

