Globally Abundant Polymer: Chitin

Chitin is a globally abundant biopolymer, second only to cellulose
and possibly lignin in terms of biomass. Owing to extensive
hydrolytic activity mainly by soil and marine chitinolytic
microorganisms, chitin, similar in this respect to cellulose, is not accumulated
in the biosphere. Chitin, which is absent from plants and
vertebrates, is present to a small or large extent in most invertebrates,
notably in cuticles of arthropods, in primary septum and scar buds of
yeast, and in cell walls of most fi lamentous fungi. Chemically detectable
chitin has been verifi ed in 25-million-year-old insect fossils. Chitin
is almost invariably covalently or noncovalently associated with other
structural molecules in contact with the external environment; examples
include carbohydrate polymers in fungi and the cuticular proteins
that comprise up to 50% by weight of arthropod cuticles. The
chitoprotein supramolecular matrix occurs in peritrophic membranes
of insects and in the arthropod exoskeleton, where the rigid chitin
microfi brils contribute greatly to its mechanical strength.
STRUCTURE
Chitin is a large water-insoluble, linear aminocarbohydrate
homopolymer composed of β 1– 4 -linked N -acetyl- D-glucosamine units
with a three-dimensional α -helix confi guration . Intramolecular
hydrogen bondings stabilize the α -helical confi guration of the macromolecule.
In nature, chitin polymers coalesce extracellularly by intermolecular
hydrogen bonds to form crystalline microfi brils that may
appear in various polymorphs ( α , β , and γ ). The most abundant one in
insects is the antiparallel arrangement of the α -chitin polymorph. 

CHITIN SYNTHESIS
Chitin synthesis occurs throughout the insect’s life cycle and is
under hormonal control of ecdysteroids. Bursts of synthetic activity
that are associated with the buildup of the new cuticles occur in particular
at the last phase of embryonic development, and as larvae or
pupae molt. Chitin synthesis is the end result of a cascade of interconnected
biochemical and biophysical events that link the mobilization of
substrate molecules, polymerization by the enzyme chitin synthase, and
translocation of the nascent amino polymer across the plasma membrane
. Individual chitin chains coalesce outside the plasma
membrane, forming fi bril crystallites by intermolecular hydrogen
bonds. The UDP- N -acetyl- D -glucosamine substrate is the end point of
a series of biochemical transformations that include successive steps of
phosphorylation, amination, and acetylation of starting p recursors such
as trehalose or glucose. Chitin synthase is a relatively large membranebound
enzyme ( 170 kDa) with multiple transmembrane segments.
The active site of the enzyme faces the cytoplasm, and the catalysis
apparently involves linking together dimer amino sugar substrates. The
question of how chitin polymers are translocated across the cell membrane
remains unresolved. Hydrophobic transmembrane segments of
chitin synthase are i mplicated in this process.
chitin microfi brils are tightly associated with various cuticular proteins,
proteolytic activity accompanies and facilitates chitin hydrolysis.
Hydrolysis of chitin does not occur in the exocuticle, where sclerotization
of the cuticular protein takes place. Formation and secretion
of chitinases by epidermal cells, processes that are under hormonal
control, are vital for the molting process. The mono- and disaccharide
degradation products are absorbed by the epithelial cells and may be
recycled to serve for biosynthesis of the new chitin.
INHIBITION OF CHITIN SYNTHESIS
AND DEGRADATION
Because chitin is present in invertebrates (abundantly in arthropods)
and absent from vertebrates and plants, it is a logical target for selective
pest control. Peptidyl nucleosides (polyoxins, nikkomycins) isolated from
Streptomyces species, which structurally mimic the enzyme substrate,
act as highly potent competitive inhibitors of fungal and insect chitin
synthase. Acylurea compounds, discovered serendipitously by Dutch scientists
in 1972, inhibit chitin synthesis, resulting in deformed and weak
cuticles that cause molting failure and death by desiccation. Acylureas do
not inhibit the catalytic step of polymerization, and their exact biochemical
lesion is unresolved. It appears that the mode of action is associated
with the process of chitin translocation from site of catalysis across cell
membranes to the region of deposition and fi brillogenesis. The fi rst commercial
product reaching the market was difl ubenzuron (Dimilin)   which was followed by a large number of structurally similar bioactive
molecules. The acylurea compounds, which act as insect growth regulators,
are widely used in integrated pest management (IPM) programs.