Chitin

Source: Wikipedia, the free encyclopedia.
For other uses, see Chitin (disambiguation).
Not to be confused with chiton or keratin.
Structure of the chitin molecule, showing two of the N-acetylglucosamine units that repeat to form long chains in β-(1→4)-linkage.
Haworth projection of the chitin molecule.
A close-up of the wing of a leafhopper; the wing is composed of chitin.

Chitin (

molluscs and in some nematodes and diatoms.[2][3]
It is also synthesised by at least some fish and lissamphibians.[4] Commercially, chitin is extracted from the shells of crabs, shrimps, shellfish and lobsters, which are major by-products of the seafood industry.[2][3] The structure of chitin is comparable to cellulose, forming crystalline nanofibrils or whiskers. It is functionally comparable to the protein keratin. Chitin has proved useful for several medicinal, industrial and biotechnological purposes.[3][5]

Etymology

The English word "chitin" comes from the French word chitine, which was derived in 1821 from the Greek word χιτών (khitōn) meaning covering.[6]

A similar word, "chiton", refers to a marine animal with a protective shell.

Chemistry, physical properties and biological function

Chemical configurations of the different monosaccharides (glucose and N-acetylglucosamine) and polysaccharides (chitin and cellulose) presented in Haworth projection

The structure of chitin was determined by Albert Hofmann in 1929. Hofmann hydrolyzed chitin using a crude preparation of the enzyme chitinase, which he obtained from the snail Helix pomatia.[7][8][9]

Chitin is a modified

polymers
, giving the chitin-polymer matrix increased strength.

A cicada emerges from its chitinous nymphal exoskeleton.

In its pure, unmodified form, chitin is translucent, pliable, resilient, and quite tough. In most

molluscs, chitin produces a much stronger composite. This composite material is much harder and stiffer than pure chitin, and is tougher and less brittle than pure calcium carbonate.[10] Another difference between pure and composite forms can be seen by comparing the flexible body wall of a caterpillar (mainly chitin) to the stiff, light elytron of a beetle (containing a large proportion of sclerotin).[11]

In butterfly wing scales, chitin is organized into stacks of

scales (five to fifteen micrometres thick) that diffusely reflect white light. These scales are networks of randomly ordered filaments of chitin with diameters on the scale of hundreds of nanometres, which serve to scatter light. The multiple scattering of light is thought to play a role in the unusual whiteness of the scales.[13][14] In addition, some social wasps, such as Protopolybia chartergoides, orally secrete material containing predominantly chitin to reinforce the outer nest envelopes, composed of paper.[15]

deacetylation of chitin by treatment with sodium hydroxide. Chitosan has a wide range of biomedical applications including wound healing, drug delivery and tissue engineering.[2][3] Due to its specific intermolecular hydrogen bonding network, dissolving chitin in water is very difficult.[16]Chitosan (with a degree of deacetylation of more than ~28%), on the other hand, can be dissolved in dilute acidic aqueous solutions below a pH of 6.0 such as acetic, formic and lactic acids. Chitosan with a degree of deacetylation greater than ~49% is soluble in water[17][18]

Humans and other mammals

Humans and other mammals have chitinase and chitinase-like proteins that can degrade chitin; they also possess several immune receptors that can recognize chitin and its degradation products, initiating an immune response.[19]

Chitin is sensed mostly in the lungs or

T helper cells.[19] Keratinocytes in skin can also react to chitin or chitin fragments.[19]

Plants

Plants also have receptors that can cause a response to chitin, namely chitin elicitor receptor kinase 1 and chitin elicitor-binding protein.[19] The first chitin receptor was cloned in 2006.[20] When the receptors are activated by chitin, genes related to plant defense are expressed, and jasmonate hormones are activated, which in turn activate systemic defenses.[21] Commensal fungi have ways to interact with the host immune response that, as of 2016, were not well understood.[20]

Some pathogens produce chitin-binding proteins that mask the chitin they shed from these receptors.[21][22] Zymoseptoria tritici is an example of a fungal pathogen that has such blocking proteins; it is a major pest in wheat crops.[23]

Fossil record

For more on the preservation potential of chitin and other biopolymers, see taphonomy.

Chitin was probably present in the exoskeletons of Cambrian arthropods such as trilobites. The oldest preserved chitin dates to the Oligocene, about 25 million years ago, consisting of a beetle encased in amber.[24]

Uses

Agriculture

Chitin is a good inducer of plant defense mechanisms for controlling diseases.[25] It has potential for use as a soil fertilizer or conditioner to improve fertility and plant resilience that may enhance crop yields.[26][27]

Industrial

Chitin is used in many industrial processes. Examples of the potential uses of chemically modified chitin in food processing include the formation of edible films and as an additive to thicken and stabilize foods and food emulsions.[28][29] Processes to size and strengthen paper employ chitin and chitosan.[30][31]

Research

How chitin interacts with the

vaccine adjuvant due to its ability to stimulate an immune response.[2][19]

Chitin and chitosan are under development as

bandages, surgical thread, and materials for allotransplantation.[2][16][35] Sutures made of chitin have been experimentally developed, but their lack of elasticity and problems making thread have prevented commercial success so far.[36]

biodegradable plastic.[37] Chitin nanofibers are extracted from crustacean waste and mushrooms for possible development of products in tissue engineering, drug delivery and medicine.[2][38]

Chitin has been proposed for use in building structures, tools, and other solid objects from a

biopolymers in the chitin are suggested as the binder for the regolith aggregate to form a concrete-like composite material
. The authors believe that waste materials from food production (e.g. scales from fish, exoskeletons from crustaceans and insects, etc.) could be put to use as feedstock for manufacturing processes.

See also

References

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  7. ^ Hofmann, A. (1929). Über den enzymatischen Abbau des Chitins und Chitosans [On the enzymatic degradation of chitin and chitosan] (Thesis). Zurich, Switzerland: University of Zurich.
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External links

Scholia has a profile for chitin (Q161219).
  • Media related to Chitin at Wikimedia Commons
  • The dictionary definition of chitin at Wiktionary
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