Mucins ( Latin mucus , “slime”) are the structural components of the mucus of organisms. These protective substances can be formed by many microorganisms (eponymous for slime mold ), plants and animals . They can be used externally (e.g. in prokaryotes , unicellular organisms , molluscs ) or to protect internal organs on the mucous membranes ( mucosa ). Mucins are glycoproteins , i.e. macromolecules made up of a central protein chain and (long) side chains made up of sugar compounds ( polysaccharides ). The polysaccharides give the mucins a high water-binding capacity and protect the central protein from enzymatic degradation ( proteolysis ) or the effects of acids (in the digestive system). Mucins play a role in the barrier function through the mucous membranes and in adhesion .
Naming and classification
The mucins are functionally divided into two groups: membrane-bound and secreted mucins.
Regardless of this, they are named or numbered consecutively in humans with the abbreviation “MUC” followed by a number (MUC1, MUC2, MUC3A etc.). In 2002, sixteen mucin genes were known in humans: MUC1 , MUC2, MUC3A, MUC3B, MUC4, MUC5AC, MUC5B, MUC6-9, MUC11-13, MUC16 and MUC17. However, this designation has been criticized because it suggests the existence of a family of genes, while the corresponding sequence homology between the genes of the various mucins is often missing.
MUC1, MUC3A ( Isoform 1), MUC3B and MUC4 are transmembrane proteins that contribute to the glycocalyx with their extracellular domains , MUC2, MUC5AC, MUC5B, MUC6 and MUC7 are secreted by the mucous membranes (“secretory mucins”) and MUC8 leaves do not clearly fit into one of these categories.
Mucins consist of a central protein which is glycosylated to a high degree, i.e. it is provided with covalently bound polysaccharides . The polysaccharides typically make up 60 to 80 percent by mass of the mucin. They have a very high molecular mass around 120 × 10 6 Daltons and often carry a high negative charge due to sialyl or sulfate groups . This anionic character and the hydroxyl groups within the polysaccharides allow the mucins to bind a lot of water and thus lead to the gel-like or slimy consistency of the mucins. The central protein is rich in the amino acids serine , threonine and proline . In all previously known mucins, it contains several tandem repeat domains, which are also the areas of the protein to which the polysaccharides are bound to the protein by O-glycosylation. These tandem repeat domains differentiate the "real" mucins from mucin-like glycoproteins such as GlyCAM1 or MadCAM1 , which play a role in adhesion.
The polysaccharide chain normally begins with an N-acetylgalactosamine , which is covalently linked to the protein backbone via the alcohol function of serine or threonine, i.e. via an oxygen atom, and consists of about 5–10 simple sugars per chain. This distinguishes the mucins from most other glycoproteins in which N-acetylglucosamine is linked to asparagine via an amide group . The first and probably also the second sugar of the oligosaccharide is determined by the structure of the underlying protein. However, the other sugars are independent of this and are presumably only determined by the concentrations of the corresponding glycosyltransferases .
It is likely that the individual mucins are linked to one another via disulfide bonds , which explains the occurrence of linear mucin chains with about twenty times the mass of a single mucin.
The ends of the polysaccharides are partially provided with sulfate groups or O-acetylated sialic acid groups in order to make it more difficult for bacteria to break down the mucins.
In the animal (and human) body, mucins are secreted (excreted) by the mucous membranes (e.g. eyes, bronchi, mouth, nose, stomach, intestines) to protect against chemical and mechanical effects from mucous glands . They are formed, for example, by secondary cells in the stomach . This secrete with bicarbonate loaded mucins (Bikarbonatbatterien belonging to the Bikarbonatsystem of the gastrointestinal tract include,) which hydrochloric acid trap or neutralize (gastric mucosal barrier). Other occurrences in: cartilage , tendons , skin , serum , vitreous bodies and as nubecula in urine .
MUC1, MUC2, MUC3, and MUC4 are the mucins mainly found in the colon . MUC2 has the largest proportion of secreted mucins. In the small intestine, MUC3 is the main mucin excreted.
Slimes or mucins were first described by Clamer Herman Hoffbauer (1734).
Until the 1970s, the term mucin was used for the major glycoprotein component in secreted mucus, which, however, was hardly characterized. At this time, the high content of mainly negatively charged carbohydrates (over 50 percent by mass) and the high content of the amino acids threonine and serine in the protein of the mucins were known. In the 1980s, however, the use of advanced spectroscopic methods (newer forms of mass spectrometry and nuclear magnetic resonance spectroscopy ) showed that mucins have a much more complex glycosylation than previously assumed. At around the same time, epitopes were found on mucins that were associated with tumors . In 1990, the DNA sequences of the first four genes for mucins were deciphered.
In medicine, mucins are used therapeutically as a component of artificial saliva .
An increased production of mucins is observed in many adenocarcinomas such as colon cancer , stomach cancer , lung cancer , prostate cancer or breast cancer . Mucins are also overexpressed in lung diseases such as asthma , bronchitis or chronic obstructive pulmonary disease ( COPD ) . In particular, the two mucins MUC1 and MUC4 were and are therefore intensively examined for their pathological function. These mucins are potential tumor markers for diagnostics as well as starting points for new forms of therapy.
- ↑ Trefoils and Mucins 2002. In: International Journal of Experimental Pathology. 83, 2002, pp. A1-A23, doi : 10.1046 / j.1365-2613.2002.t01-1-00226.x .
- ^ J. Dekker et al .: The MUC family: an obituary. In: Trends Biochem Sci . Volume 27, 2002, pp. 126-131. PMID 11893509 .
- ↑ Perez-Vilar J, Hill RL: The Structure and Assembly of Secreted Mucins. In: J Biol Chem . Volume 274, No. 45, 1999, pp. 31751-31754.
- ↑ a b F. G. Hanisch, S. Müller: MUC1: the polymorphic appearance of a human mucin. In: Glycobiology , Volume 10, 2000, pp. 439-449.
- ^ Drawing analogous to R. Xu, SR Hanson, Z. Zhang, YY Yang, PG Schultz, CH Wong: Site-specific incorporation of the mucin-type N-acetylgalactosamine-alpha-O-threonine into protein in Escherichia coli. In: Journal of the American Chemical Society. Volume 126, Number 48, December 2004, pp. 15654-15655, doi : 10.1021 / ja044711z , PMID 15571382 .
- ↑ JM Rhodes: Mucins and inflammatory bowel disease. In: QJ Med , Vol. 90, 1997, pp. 79-82. PMID 9068798 .
- ↑ AP Corfield et al .: Mucin degradation in the human colon: production of sialidase, sialate O-acetylesterase, N-acetylneuraminate lyase, arylesterase, and glycosulfatase activities by strains of fecal bacteria. In: Infection and Immunity . Volume 60, No. 10, 1992, pp. 3971-3978.
- ^ SE Miederer, O. Stadelmann: Physiology and physiopathology of gastric secretion. In: Med Klin. Volume 66, 1971, pp. 1587-1593. PMID 4943889 .
- ^ SE Miederer: The gastric mucosal barrier. In: Hepatogastroenterology , Volume 33, 1986, pp. 89-91.
- ↑ T. Shirazia et al .: Mucins and inflammatory bowel disease. In: Postgrad. Med. J. Vol. 76, 2000, pp. 473-478.
- ↑ KMAJ Tytgat et al .: Muc2 is the prominent secretory mucin in ulcerative colitis. ( Memento of the original from October 1, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. In: Netherl. J. Med. Volume 47, No. 2, 1995, p. 41.
- ↑ DR Mach et al .: Extracellular MUC3 mucin secretion follows adherence of Lactobacillus strains to intestinal epithelial cells in vitro. In: Good . Volume 52, 2003, pp. 827-833. PMC 1773687 (free full text).
- ^ CH Hoffbauer: De ignobili Muco Ingrato Multorum Nobilium Hospite. Italae, Magdeburg 1734.
- ↑ AP Singh et al .: MUC4 expression is regulated by cystic fibrosis transmembrane conductance regulator in pancreatic adenocarcinoma cells via transcriptional and post-translational mechanisms. In: Oncogene , Volume 26, 2007, pp. 30-41. PMID 16799633 .
- ↑ AP Singh et al .: Aberrant expression of transmembrane mucins, MUC1 and MUC4, in human prostate carcinomas. In: Prostate , Volume 66, 2006, pp. 421-429. PMID 16302265 .
- ^ AP Singh et al .: Emerging roles of MUC4 in cancer: a novel target for diagnosis and therapy. In: Cancer Research , Volume 67, 2007, pp. 433-436. PMID 17234748 .
- Ali MS et al .: Major secretory mucin expression in chronic sinusitis. In: Otolaryngology - Head and Neck Surgery . 133/2005, pp. 423-8.
- Singh AP et al .: Inhibition of MUC4 expression suppresses pancreatic tumor cell growth and metastasis. In: Cancer Research , 64/2004, pp. 622-30.