Chitin, the extracellular matrix polysaccharide of insects and arthropods is widely distributed in nature in all kingdoms of life and serves a variety of functions. After synthesis by membrane-bound chitin synthases, it is extensively remodeled before incorporation into divergent matrices with wide-ranging physical and biological properties. This chapter discusses the properties of a variety of insect enzymes and proteins involved in this process. Chitin remodeling involves chitin synthases, which make the nascent chitin chains, and chitin deacetylases that partially deacetylate some of the N-acetylglucosamine residues either randomly or sequentially to yield local chitosan-like regions. Other proteins secreted into the procuticle or the midgut help in the assembly of single chitin chains into larger crystalline aggregates that measure in a few 100 nanometers. They are further embedded in a complex matrix of cuticular proteins or become associated with proteins containing chitin-binding domains to constitute the laminar procuticle or the lattice-like peritrophic matrix. During molting, previously formed laminar cuticle or PM are decrystallized/depolymerized to unmask the chitin chains, which then are degraded by a mixture of chitinolytic enzymes consisting of chitinases and N-acetylglucosaminidases present in molting fluid or in gut secretions. Some of the degradation products may be recycled for the synthesis of new matrices. We present a model of chitin synthesis, assembly, and degradation and the roles of these chitin-remodeling enzymes in this overall process.
Keywords: Chitin; Chitin deacetylase; Chitin remodeling; Chitin synthase; Chitinase; Cuticular proteins; Knickkopf; N-acetylglucosaminidase; Peritrophic matrix.