Pepsin
Encyclopedia
Pepsin is an enzyme
whose precursor form (pepsinogen) is released by the chief cell
s in the stomach
and that degrades food protein
s 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 acid
s, 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 bond
s between hydrophobic
and preferably aromatic
amino acids such as phenylalanine
, tryptophan
, and tyrosine
.
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.
as a pro-form zymogen
, pepsinogen, whose primary structure
has an additional 44 amino acid
s.
In the stomach, chief cell
s release pepsinogen. This zymogen is activated by hydrochloric acid
(HCl), which is released from parietal cell
s 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 the digestion of proteins.
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.
. 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
.
A fourth human gene encodes gastricsin also known as pepsinogen C:
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
whose precursor form (pepsinogen) is released by the chief cell
Gastric chief cell
A gastric chief cell is a cell in the stomach that releases pepsinogen, gastric lipase and Chymosin...
s in the stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
and that degrades food protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s into peptides. It was discovered in 1836 by Theodor Schwann
Theodor Schwann
Theodor Schwann was a German physiologist. His many contributions to biology include the development of cell theory, the discovery of Schwann cells in the peripheral nervous system, the discovery and study of pepsin, the discovery of the organic nature of yeast, and the invention of the term...
who also coined its name from the Greek word
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
pepsis, meaning digestion
Digestion
Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones....
(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
Protease
A protease is any enzyme that conducts proteolysis, that is, begins protein catabolism by hydrolysis of the peptide bonds that link amino acids together in the polypeptide chain forming the protein....
, a member of the aspartate protease
Aspartate protease
Aspartic proteases are a family of protease enzymes that use an aspartate residue for catalysis of their peptide substrates. In general, they have two highly-conserved aspartates in the active site and are optimally active at acidic pH...
family.
Pepsin is one of three principal protein-degrading, or proteolytic, enzymes in the digestive system, the other two being chymotrypsin
Chymotrypsin
Chymotrypsin is a digestive enzyme that can perform proteolysis. Chymotrypsin preferentially cleaves peptide amide bonds where the carboxyl side of the amide bond is a tyrosine, tryptophan, or phenylalanine. These amino acids contain an aromatic ring in their sidechain that fits into a...
and trypsin
Trypsin
Trypsin is a serine protease found in the digestive system of many vertebrates, where it hydrolyses proteins. Trypsin is produced in the pancreas as the inactive proenzyme trypsinogen. Trypsin cleaves peptide chains mainly at the carboxyl side of the amino acids lysine or arginine, except when...
. 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 acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s, 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 bond
Peptide bond
This article is about the peptide link found within biological molecules, such as proteins. A similar article for synthetic molecules is being created...
s between hydrophobic
Hydrophobe
In chemistry, hydrophobicity is the physical property of a molecule that is repelled from a mass of water....
and preferably aromatic
Aromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...
amino acids such as phenylalanine
Phenylalanine
Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...
, tryptophan
Tryptophan
Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG...
, and tyrosine
Tyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...
.
History
The term Pepsin was first coined by Theodor SchwannTheodor Schwann
Theodor Schwann was a German physiologist. His many contributions to biology include the development of cell theory, the discovery of Schwann cells in the peripheral nervous system, the discovery and study of pepsin, the discovery of the organic nature of yeast, and the invention of the term...
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 expressedProtein expression
Protein expression is a subcomponent of gene expression. It consists of the stages after DNA has been translated into polypeptide chains, which are ultimately folded into proteins...
as a pro-form zymogen
Zymogen
A zymogen is an inactive enzyme precursor. A zymogen requires a biochemical change for it to become an active enzyme. The biochemical change usually occurs in a lysosome where a specific part of the precursor enzyme is cleaved in order to activate it...
, pepsinogen, whose primary structure
Primary structure
The primary structure of peptides and proteins refers to the linear sequence of its amino acid structural units. The term "primary structure" was first coined by Linderstrøm-Lang in 1951...
has an additional 44 amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s.
In the stomach, chief cell
Chief cell
In general, a chief cell is a cell which releases a precursor enzyme. There are two types of chief cells which are most commonly referenced:...
s release pepsinogen. This zymogen is activated by hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
(HCl), which is released from parietal cell
Parietal cell
Parietal cells, or oxyntic cells, are the stomach epithelium cells that secrete gastric acid and intrinsic factor.Acetylcholine and gastrin . The histamine receptors act by increasing intracellular cAMP, whereas the muscarinic and gastrin receptors increase intracellular Ca2+ levels...
s in the stomach lining. The hormone gastrin
Gastrin
In humans, gastrin is a peptide hormone that stimulates secretion of gastric acid by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the antrum of the stomach, duodenum, and the pancreas...
and the vagus nerve
Vagus nerve
The vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...
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
Phenylalanine
Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...
, tryptophan
Tryptophan
Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG...
, and tyrosine
Tyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...
. 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
Insulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
.
Peptides may be further digested by other proteases (in the duodenum
Duodenum
The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms anterior intestine or proximal intestine may be used instead of 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
Pepstatin
Pepstatin is a potent inhibitor of aspartyl proteases. It is a hexa-peptide containing the unusual amino acid statine , having the sequence Isovaleryl-Val-Val-Sta-Ala-Sta...
.
Pepsin is used for the 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 antibodyAntibody
An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...
. 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
Chewing gum
Chewing gum is a type of gum traditionally made of chicle, a natural latex product, or synthetic rubber known as polyisobutylene. For economical and quality reasons, many modern chewing gums use rubber instead of chicle...
, the first person to do so was Dr. Edward E. Beeman when he made beemans gum
Beemans gum
Beemans gum is a chewing gum invented by Ohio physician Dr. Edward E. Beeman in the late 19th century.-History:...
.
Genes
The following three genes encode identical human pepsinogen A enyzmes:A fourth human gene encodes gastricsin also known as pepsinogen C:
External links
- The MEROPSMeropsMerops may refer to:* Merops , a genus of bee-eaters.* MEROPS, an on-line database for peptidases.It may also refer to several figures from Greek mythology:* King of Ethiopia, husband of Clymene, who lay with Helios and bore Phaethon...
online database for peptidases and their inhibitors: Pepsin A A01.001, Pepsin B A01.002, Pepsin C (Gastricsin) A01.003 - Beemans Gum