Digestive enzyme

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Digestive enzymes are enzymes that break down food into its individual parts during digestion in the digestive system , in order to make it usable for the metabolism . The task of almost all of these enzymes is to break down long-chain molecules ( biopolymers ) such as proteins , carbohydrates , fats and nucleic acids into simpler, lower-molecular compounds ( amino acids , simple sugars, glycerol , fatty acids) using water .

properties

Salivary amylase , for example, works in the mouth ; later, when the food reaches the stomach , pepsin and hydrochloric acid work in the gastric juice , and the bile continues to break down in the duodenum . In the small intestine , lactases , maltase and saccharase act for carbohydrates and intestinal lipases for fats . Various peptidases are responsible for proteins , including the dipeptidyl peptidases . The pancreas produces many digestive enzymes in the acinar cells , including trypsin , chymotrypsin , carboxypeptidases , elastase , α-amylase , ribo- and deoxyribonucleases, and lipases , without which food cannot be used. So that their enzyme activity does not start at the place of manufacture, some digestive enzymes are produced as preprotein (more precisely: as zymogen ) and only converted into their active form at the site of action by proteolysis , for example pepsinogen, trypsinogen, chymotrypsinogen, procarboxypeptidase, proelastase. In addition, the protease inhibitor aprotinin is also formed in the pancreas of cattle , which inhibits the effects of trypsin and chymotrypsin in the pancreas. Digestive enzymes produced in the pancreas are secreted into the small intestine . After translation, secreted proteins have a signal sequence that enables secretion and is partially split off in the course of secretion.

The binding specificity of the enzyme results from the shape of the active center in the enzyme and the binding sites around it , in which the respective substrate of the enzyme is bound. The substrate specificity of digestive enzymes is so low compared to other enzymes that it can be more precisely described as substrate preference. Cleavage by digestive enzymes is hydrolysis , which means that the bond is cleaved by inserting a molecule of water into the bond, one side of the cleaved bond being given a hydroxyl group (OH - ) and the other side of the broken bond a proton (H + ).

history

In ancient times, Empedocles assumed that humans convert the food they eat through putrefaction . The first subdivision of the digestive functions was made by Diocles of Karystus . Later, different Galen then stomach digestion, small bowel digestion and processing by the liver from each other.

In the 17th century Johan Baptista van Helmont discovered that fermentation is necessary for digestion , which he viewed as a dynamic natural principle. His student Franciscus Sylvius discovered that the pancreas releases a juice that is necessary for digestion and that saliva secretion also has a similar effect. Building on this knowledge, Regnier de Graaf obtained the pancreatic juice from an artificial fistula and Johannes Bohn then demonstrated that this juice does not react acidic . François Magendie, however, then showed that the juice is alkaline .

René-Antoine Ferchault de Réaumur then finally dismissed the assumption that digestion is a mechanical process, but instead showed that it is a chemical process. Lazzaro Spallanzani then examined the gastric juice more closely and demonstrated its proteolytic properties . Jean Senebier used this knowledge in a different way: he treated poorly healing wounds and leg ulcers externally with gastric juice from animals.

In 1697 Georg Ernst Stahl suspected that saliva could split the starch , but could not prove it. Erhand Friedrich Leuch succeeded in doing this in 1831 . Louis Mialhe (1807-1886) began an attempt to isolate the amylase in 1845 and in 1863 Julius Cohnheim successfully continued this attempt.

Gabriel Gustav Velentin found out that pancreatic juice can break down sugar and in 1845 Apollinaire Bouchardat was able to prove this. In 1859 Heinrich Köbner showed that the intestinal juice can break down sucrose and in 1899 Rudolf Weinland discovered how lactose is broken down.

Mammalian digestive enzymes

Peptidases (enzymes thatcan cleave peptide bonds in proteins or peptides, synonymous with proteases )

  • Pepsin is used to break down proteins in the stomach and is made by the stomach lining. Pepsin mainly cuts proteins according to the amino acids leucine , phenylalanine , tyrosine or tryptophan , provided that this is not followed by proline . The optimal pH for pepsin is pH 2.
  • Trypsin causes the breakdown of proteins in the small intestine and is produced in the pancreas. It has the greatest specificity of the peptidases in digestion, ie it only cuts proteins according to basic amino acids such as lysine or arginine , provided that it is not followed by proline.
  • Chymotrypsin breaks down proteins in the small intestine and is produced in the pancreas. Chymotrypsin also has a milk-clotting effect. It cuts mainly for amino acids with a large and hydrophobic side chain such as phenylalanine, tyrosine and tryptophan, provided that this is not followed by proline. After the amino acids asparagine , histidine , methionine and leucine , chymotrypsin cleaves more slowly.
  • Chymosin (synonym Rennin ) is mainly formed in the stomach of babies and splits the peptide bond between the two amino acids phenylalanine and methionine, which is found in the milk protein κ- casein .
  • Pancreatic elastase breaks down proteins in the small intestine and is produced in the pancreas. It cuts mainly for small uncharged amino acids like glycine , alanine , leucine and valine , provided that they are not followed by proline. Among other things , it splits elastin , which is where the name comes from.
  • Aminopeptidase N is produced in the small intestine and breaks down proteins starting at their N terminus
  • Carboxypeptidase A breaks down proteins starting from their C -terminal , provided that the C -terminal amino acid is not arginine, lysine or proline and the preceding amino acid is not proline.
  • Carboxypeptidase B also breaks down proteins starting from their C terminus.

Glycosidases (enzymes thatbreak down polysaccharides like starch into monosaccharides like glucose )

Lipases (enzymes from the pancreas thatbreak down lipids in the small intestine), in particular

Nucleases (enzymes thatbreak down nucleic acids )

Digestive enzymes of other organisms

In some arthropods ( Heteroptera , Coleoptera , Crustacea and Acari ) peptidases from the group of cathepsins are formed as digestive enzymes , which in other animals only occur within cells.

Digestive enzymes outside the organism

Soaking up pizza by a fly

Some living things do some of their digestion outside their bodies, such as some bacteria , flies , spiders, and carnivorous plants . Flies salivate their food and then suck it up. In spiders, the digestive enzymes are injected into the prey after a bite and after a while they are sucked up again with the liquefied food. Bacillus subtilis and other species of the genus Bacillus probably secrete subtilisin for digestion. The α-lytic protease is presumably secreted by Lysobacter enzymogenes for digestion. Likewise were Protease A and Protease B from Streptomyces griseus described.

Carnivorous Plants

Most carnivorous plants produce their own digestive enzymes, but some rely on the help of other living things to do so. Heliamphora and the cobra lily rely completely on the formation of digestive enzymes by colonizing bacteria . Genlisea , Sarracenia , most pitcher plants , Cephalotus and some Heliamphora. Roridula use both their own and bacterial digestive enzymes for digestion. Pameridea , Setocoris and Metriocnemus are colonized by insects , whose excrement is ingested by the plant. Some pitcher plants ingest excrement from birds.

literature

Web links

Wiktionary: digestive enzyme  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ a b c d e f g h i j k l m n o p q r s t u v w x Donald Voet, Judith G. Voet: Biochemistry , 4th edition. John Wiley & Sons, 2010, ISBN 978-1-118-13993-6 . Pp. 169, 194, 367, 370, 373, 473, 508, 524, 533, 547, 942.
  2. ^ A b c Wolf-Dieter Müller-Jahncke , Christoph Friedrich , Ulrich Meyer: Medicinal history . 2., revised. and exp. Ed. Wiss. Verl.-Ges, Stuttgart 2005, ISBN 978-3-8047-2113-5 .
  3. ^ A b Jeremy M. Berg, John L. Tymoczko, Lubert Stryer: Stryer Biochemistry. 7th edition, Springer-Verlag, 2010, ISBN 978-1-429-22936-4 , p. 328.
  4. ↑ FJ Fuzita, MW Pinkse, PD Verhaert, AR Lopes: Cysteine ​​cathepsins as digestive enzymes in the spider Nephilengys cruentata. In: Insect biochemistry and molecular biology. Volume 60, May 2015, pp. 47-58, doi : 10.1016 / j.ibmb.2015.03.005 , PMID 25818482 .
  5. ^ W. Adlassnig, M. Peroutka, T. Lendl: Traps of carnivorous pitcher plants as a habitat: composition of the fluid, biodiversity and mutualistic activities. In: Annals of botany. Volume 107, number 2, February 2011, pp. 181-194, doi : 10.1093 / aob / mcq238 , PMID 21159782 , PMC 3025736 (free full text).
  6. ↑ T. Renner, CD Specht: Inside the trap: gland morphologies, digestive enzymes, and the evolution of plant carnivory in the Caryophyllales. In: Current opinion in plant biology. Volume 16, number 4, August 2013, pp. 436-442, doi : 10.1016 / j.pbi.2013.06.009 , PMID 23830995 , PMC 3820484 (free full text).
  7. ↑ R. Ravee, F. '. Mohd Salleh, HH Goh: Discovery of digestive enzymes in carnivorous plants with focus on proteases. In: PeerJ. Volume 6, 2018, p. E4914, doi : 10.7717 / peerj.4914 , PMID 29888132 , PMC 5993016 (free full text).
  8. ↑ a b c d The Carnivorous Plant FAQ: How do carnivorous plants digest their prey? In: sarracenia.com. Retrieved June 23, 2018 .
  9. ^ A b Carnivorous Plant Digestion and Nutrient Assimilation - ICPS. In: carnivorousplants.org. Retrieved June 23, 2018 .
  10. ↑ M. Saganová, B. Bokor, T. Stolárik, A. Pavlovič: Regulation of enzyme activities in carnivorous pitcher plants of the genus Nepenthes. In: Planta. [Electronic publication before going to press] May 2018, doi : 10.1007 / s00425-018-2917-7 , PMID 29767335 .
  11. ↑ K. Fukushima, X. Fang, D. Alvarez-Ponce, H. Cai, L. Carretero-Paulet, C. Chen, TH Chang, KM Farr, T. Fujita, Y. Hiwatashi, Y. Hoshi, T. Imai , M. Kasahara, P. Librado, L. Mao, H. Mori, T. Nishiyama, M. Nozawa, G. Pálfalvi, ST Pollard, J. Rozas, A. Sánchez-Gracia, D. Sankoff, TF Shibata, S. Shigenobu, N. Sumikawa, T. Uzawa, M. Xie, C. Zheng, DD Pollock, VA Albert, S. Li, M. Hasebe: Genome of the pitcher plant Cephalotus reveals genetic changes associated with carnivory. In: Nature ecology & evolution. Volume 1, number 3, February 2017, p. 59, doi : 10.1038 / s41559-016-0059 , PMID 28812732 .
  12. ↑ B. Anderson: Adaptations to foliar absorption of faeces: a pathway in plant carnivory. In: Annals of botany. Volume 95, number 5, April 2005, pp. 757-761, doi : 10.1093 / aob / mci082 , PMID 15728666 , PMC 4246731 (free full text).