Chapter 43 – Chitin

Publisher Summary This chapter discusses synthesis and degradation of chitin biopolymer. Chitin 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 filamentous fungi. 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 configuration. Intramolecular hydrogen bondings stabilize the α-helical configuration of the macromolecule. In nature, chitin polymers coalesce extracellularly by intermolecular hydrogen bonds to form crystalline microfibrils 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 occurs throughout the insect's life cycle and is under hormonal control of ecdysteroids. 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. Further degradation of chitin is physiologically crucial for normal growth and development of insects. Chitin is degraded by the joint action of chitinase, which yields oligomeric fragments, and exochitinase, or β-N-acetylglucosaminidase, which hydrolyzes terminal polymers or dimers. 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 filamentous fungi. Chemically detectable chitin has been verified 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 microfibrils contribute greatly to its mechanical strength.