pepsin

pepsin, enzyme produced in the mucosal lining of the stomach that acts to degrade protein. 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 particularly effective in severing links between particular types of amino acids, collaborate to break down dietary proteins to their components, i.e., peptides and amino acids, which can be readily absorbed by the intestinal lining. In the laboratory studies pepsin is most efficient in cleaving bonds involving the aromatic amino acids, phenylalanine, tryptophan, and tyrosine. Pepsin is synthesized in an inactive form by the stomach lining; hydrochloric acid, also produced by the gastric mucosa, is necessary to convert the inactive enzyme and to maintain the optimum acidity (pH 1-3) for pepsin function. Pepsin and other proteolytic enzymes are used in the laboratory analysis of various proteins; pepsin is also used in the preparation of cheese and other protein-containing foods.

Powerful enzyme in gastric juice (see stomach) that partially digests proteins in food. Glands in the stomach lining make pepsinogen, a zymogen (enzyme precursor) converted to pepsin by the hydrochloric acid in gastric juice. Pepsin is active only in the acid environment of the stomach (pH 1.5–2.5 or less); it is ineffective in the intestine (pH 7, neutral). It is used commercially in some cheese making, in the leather industry to remove hair and residual tissue from hides, and in the recovery of silver from discarded photographic films by digesting the gelatin layer that holds the silver.

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Pepsin is a digestive protease released by the chief cells in the stomach that functions to degrade food proteins into peptides.

According to American Heritage Dictionary, pepsin derives from the Greek word pepsis, meaning digestion (peptein: to digest).

Pepsin was discovered by Theodor Schwann in 1836. It was the first animal enzyme to be discovered. In 1929, it became one of the first enzymes to be crystallized, by John H. Northrop.

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 carbon bonds by cleaving preferentially after the C-terminal of aromatic amino acids such as phenylalanine and tyrosine although it has broad specificity. It will not cleave at bonds containing valine, alanine, or glycine. 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, particularly those with a pH of 1.5 to 2.

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

See also

Other important digestive proteases are the pancreatic enzymes trypsin and chymotrypsin. Pepsin denatures if the pH is more than 5.0. Pepsin is potently inhibited by the peptide inhibitor pepstatin.

"Pepsin" was the common named for pepsin chewing gum, a popular digestive aid composed of gum and pepsin enzymes extracted from butchered hog stomachs. Dr. Eugene Beeman first introduced the product in 1891.

It is disputed whether or not the original recipe for Pepsi included enzyme pepsin.

Storage

Pepsins should be stored at very cold temperatures (between −20°C and −80°C) to prevent autolysis (self-cleavage). Autolysis may also be prevented by storage of pepsins at pH 11 or by using pepsins modified by e.g. reductive methylation. When the pH is adjusted back to pH 6 activity returns.

Outside links

• Pepsin A description from BRENDA database

References