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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 systemdigestive system,
in the animal kingdom, a group of organs functioning in digestion and assimilation of food and elimination of wastes. Virtually all animals have a digestive system. In the vertebrates (phylum Chordata, subphylum Vertebrata) the digestive system is very complex.
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, the other two being chymotrypsinchymotrypsin
, proteolytic, or protein-digesting, enzyme active in the mammalian intestinal tract. It catalyzes the hydrolysis of proteins, degrading them into smaller molecules called peptides. Peptides are further split into free amino acids.
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 and trypsintrypsin,
enzyme that acts to degrade protein; it is often referred to as a proteolytic enzyme, or proteinase. Trypsin is one of the three principal digestive proteinases, the other two being pepsin and chymotrypsin.
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. 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., peptidespeptide,
organic compound composed of amino acids linked together chemically by peptide bonds. The peptide bond always involves a single covalent link between the α-carboxyl (oxygen-bearing carbon) of one amino acid and the amino nitrogen of a second amino acid.
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 and amino acidsamino acid
, any one of a class of simple organic compounds containing carbon, hydrogen, oxygen, nitrogen, and in certain cases sulfur. These compounds are the building blocks of proteins.
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, 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.



a proteolytic enzyme of the hydrolase class, found in the gastric juice of mammals, birds, reptiles, and most species of fish. Pepsin breaks down proteins and peptides. It was first recognized in 1836 by T. Schwann and isolated in crystal form in 1930 by J. Northrop.

Pepsin is a globular protein with a molecular weight of approximately 34,500. A pepsin molecule is a polypeptide chain consisting of 340 amino-acid residues, three disulfide bonds (—S—S—), and phosphoric acid. The isoelectric point of pepsin is approximately equal to pH 1.0. Therefore, pepsin is stable in a strongly acid medium and reaches its maximum activity at pH 1–2, the pH of gastric juice. It undergoes denaturation at pH 6.0.

Pepsin is an endopeptidase, that is, an enzyme that splits the central peptide bonds in protein and peptide molecules, except keratins and other scleroproteins, to form simpler peptides and free amino acids. It very rapidly hydrolyzes peptide bonds formed by the aromatic amino acids tyrosine and phenylalanine; however, unlike the proteolytic enzymes trypsin and chymotrypsin, it does not exhibit a strong specificity.

Pepsin is produced by the gastric chief cells in the form of inactive pepsinogen. Pepsinogen is then converted to pepsin by the splitting off of several peptides, including a pepsin inhibitor, from the N-terminal section of pepsinogen. The activation process involves several stages and is catalyzed by the hydrochloric acid in gastric juice and by pepsin itself (autocatalysis).

Pepsin is used in the laboratory investigation of the primary structure of proteins, in cheese-making, and in the treatment of certain gastrointestinal diseases.



A proteolytic enzyme found in the gastric juice of mammals, birds, reptiles, and fishes.


, pepsine
a proteolytic enzyme produced in the stomach in the inactive form pepsinogen, which, when activated by acid, splits proteins into peptones