Chitins in living tissues occur in covalent combination

with either proteins or glucans

and are often cross-linked following the quinone tanning process.

Most part of the chitin, however, is highly ordered as crystallites

called nanofibrils, that are used for nanotechnological applications.

New aspects of chitin chemistry and enzymology

A chapter by Riccardo A.A. Muzzarelli

Published in the book

Binomium chitin-chitinase: recent issues

S. Musumeci and M.G. Paoletti, eds.

Nova Science Publishers, Inc.

Hauppauge NY, USA. 2009.

Introductory notes

In spite of the inherent insolubility of chitin in water, chitin aqueous systems include ethers obtained from alkali chitin such as O-carboxymethyl chitin and glycol chitin; 6-oxychitin and partially reacetylated chitin are also easy to prepare, while chitin oligomers are per se water-soluble. Useful organic solvents for chitin are dimethylacetamide-LiCl and hexafluoroisopropanol.

Chitinases are enzymes involved in growth, defense, aggression and feeding secreted by animals, fungi and bacteria; they are finding applications in agriculture, particularly after the genomes of plants such as rice Oryza sativa and pests such as Tribolium castaneum were fully elucidated.

Transgenic rice plants are endowed with novel and powerful chitinases that are promptly activated in case of infection by rice pathogens. Phytoparasitism has been put to profit by engaging Trichoderma harzianum in protecting plants against pathogens such as Rhizoctonia solani.

The importance of lysozyme is highlighted insofar as chitinases retain an ancient structural motif of lysozyme, and actually lysozyme is an enzyme ubiquitously present in the human body for defense against microbes and parasites. Unspecific enzymes such as cellulase, hemicellulase and lipase are currently used to prepare chitin oligomers to be used in the biomedical field, particularly in medication and drug delivery.

Chitin fibers in crustacean shells are associated with carbonate that diffuses and precipitates after the fibrous component has been excreted and stabilized; a parallel situation occurs with collagen fibers and calcium phosphate in bones. In both these tissues, the supporting organic component is made of preformed nanometer to micrometer-size elongated particles arranged into supramolecular structures with geometry analogous to those of certain liquid crystals. In compact bones, arthropod cuticles and plant cell walls, these structures exhibit the macroscopic features of a cholesteric phase, except fluidity. In most cases, collagen, chitin and cellulose can be extracted from the biological tissues and dispersed in aqueous media to form colloidal suspensions. At appropriate concentrations, liquid crystalline phases can be identified, indicating that rod-like or spindle-like particles tend to align cooperatively in these systems. The chitin nanofibrils are currently being used as reinforcing additives for plastics and biocompatible films.

Figure. Predicted structure of wheat chitinase. Three-dimensional structural model of the 33-kDa wheat chitinase with 10 alpha-helices (red), 3 beta-strands (orange) and 21 loop turns. Six conserved cysteine residues are shown to form 3 disulphide bonds (yellow). The catalytic glutamic acid residues Glu94 and Glu103 (green) identified in the loop sequence connected to beta-strand may be suggested for chitinase activity. Reprinted from Protein Expression and Purification 56. Singh A, Kirubakaran SI, Sakthivel N. Heterologous expression of new antifungal chitinase from wheat. Pages 100-109. Copyright (2007), with permission from Elsevier.