Chitin | Wikipedia audio article


Chitin (C8H13O5N)n ( KY-tin), a long-chain
polymer of N-acetylglucosamine, is a derivative of glucose. It is a primary component of cell walls in
fungi, the exoskeletons of arthropods, such as crustaceans and insects, the radulae of
molluscs, cephalopod beaks, and the scales of fish and lissamphibians. The structure of chitin is comparable to another
polysaccharide—cellulose, forming crystalline nanofibrils or whiskers. In terms of function, it may be compared to
the protein keratin. Chitin has proved useful for several medicinal,
industrial and biotechnological purposes.==Etymology==
The English word “chitin” comes from the French word chitine, which was derived in 1821 from
the Greek word χιτών (chiton), meaning covering.A similar word, “chiton”, refers
to a marine animal with a protective shell.==Chemistry, physical properties and biological
function==The structure of chitin was determined by
Albert Hofmann in 1929.Chitin is a modified polysaccharide that contains nitrogen; it
is synthesized from units of N-acetyl-D-glucosamine (to be precise, 2-(acetylamino)-2-deoxy-D-glucose). These units form covalent β-(1→4)-linkages
(like the linkages between glucose units forming cellulose). Therefore, chitin may be described as cellulose
with one hydroxyl group on each monomer replaced with an acetyl amine group. This allows for increased hydrogen bonding
between adjacent polymers, giving the chitin-polymer matrix increased strength. In its pure, unmodified form, chitin is translucent,
pliable, resilient, and quite tough. In most arthropods, however, it is often modified,
occurring largely as a component of composite materials, such as in sclerotin, a tanned
proteinaceous matrix, which forms much of the exoskeleton of insects. Combined with calcium carbonate, as in the
shells of crustaceans and 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. 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).In butterfly
wing scales, chitin is organized into stacks of gyroids constructed of chitin photonic
crystals that produce various iridescent colors serving phenotypic signaling and communication
for mating and foraging. The elaborate chitin gyroid construction in
butterfly wings creates a model of optical devices having potential for innovations in
biomimicry. Scarab beetles in the genus Cyphochilus also
utilize chitin to form extremely thin 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. In addition, some social wasps, such as Protopolybia
chartergoides, orally secrete material containing predominantly chitin to reinforce the outer
nest envelopes, composed of paper.Chitosan is produced commercially by deacetylation
of chitin; chitosan is soluble in water, while chitin is not.Nanofibrils have been made using
chitin and chitosan.==Health effects==
Chitin-producing organisms like protozoa, fungi, arthropods, and nematodes are often
pathogens in other species.===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 in a pathogen-associated molecular pattern, initiating an immune response.Chitin
is sensed mostly in the lungs or gastrointestinal tract where it can activate the innate immune
system through eosinophils or macrophages, as well as an adaptive immune response through
T helper cells. Keratinocytes in skin can also react to chitin
or chitin fragments. According to in vitro studies, chitin is sensed
by receptors, such as FIBCD1, KLRB1, REG3G, Toll-like receptor 2, CLEC7A, and mannose
receptors.The immune response can sometimes clear the chitin and its associated organism,
but sometimes the immune response is pathological and becomes an allergy; allergy to house dust
mites is thought to be driven by a response to chitin.===Plants===
Plants also have receptors that can cause a response to chitin, namely chitin elicitor
receptor kinase 1 and chitin elicitor-binding protein. The first chitin receptor was cloned in 2006. When the receptors are activated by chitin,
genes related to plant defense are expressed, and jasmonate hormones are activated, which
in turn activate systematic defenses. Commensal fungi have ways to interact with
the host immune response that as of 2016 were not well understood.Some pathogens produce
chitin-binding proteins that mask the chitin they shed from these receptors. Zymoseptoria tritici is an example of a fungal
pathogen that has such blocking proteins; it is a major pest in wheat crops.==Fossil record==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 scorpion encased in amber.==Uses=====
Agriculture===Chitin is a good inducer of plant defense
mechanisms for controlling diseases. It has also been assessed as a fertilizer
that can improve overall crop yields.===Industrial===
Chitin is used in industry in many 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. Processes to size and strengthen paper employ
chitin and chitosan.==Research==
How chitin interacts with the immune system of plants and animals has been an active area
of research, including the identity of key receptors with which chitin interacts, whether
the size of chitin particles is relevant to the kind of immune response triggered, and
mechanisms by which immune systems respond. Chitin and chitosan have been explored as
a vaccine adjuvant due to its ability to stimulate an immune response.Chitin and chitosan are
under development as scaffolds in studies of how tissue grows and how wounds heal, and
in efforts to invent better bandages, surgical thread, and materials for allotransplantation. Sutures made of chitin have been explored
for many years, but as of 2015, none were on the market; their lack of elasticity and
problems making thread have prevented commercial development.In 2014, a method for using chitosan
as a reproducible form of biodegradable plastic was introduced. Chitin nanofibers are extracted from crustacean
waste and mushrooms for possible development of products in tissue engineering, medicine,
and industry.==See also==
Biopesticide Chitobiose
Lorica Sporopollenin
Tectin

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