Occurrence of Chitin

As the major, organic, skeletal substance of invertebrates, chitin is found principally in the phyla Annelida (segmented worms), Arthropoda, and MoUusca, and, to a lesser extent, in Coelenterata and Nematoda (unsegmented worms). A critical compilation of the reported occurrences of chitin amongst these lower animals, especially the Arthropoda, has been made by Richards. This author reports that chitin has been found in the body wall 

The protein of soft cuticles (arthropodin ) can largely be removed by extraction with hot water. A further, smaller portion (sclerotin) can be 

Sot ee Proportion (% of organic fraction dry weight) Chitin Protein Rrferences 

Cancer pagurus cuticle can be dispersed in hot, aqueous solutions of lithium thiocyanate and be reprecipitated without separating the chitin and protein components. The stability of the complex under these conditions would suggest the presence of primary bonding. Thus, some ehitin-protein bonding does exist in arthropod cuticle, but its exact nature and its physiological significance or its involvement in chitin biosynthesis (or both) remain uncertain. 

In summary, the arthropod cuticle is not chitinous and can probably best be described as a plasticized, protein sheet, variously subdivided to give a laminar structure by (a) the addition of waxes at the outer surfaces and chitin in the inner parts, and (b) hardening due to impregnation with calcium salts or further polymerization with polyphenols, or both. 

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A number of important papers could not be cited in this chapter, due to the length limitations and the specific target of the chapter. For example, the antimicrobial activity of chitosans [349], the chitinolytic enzymes, the preparation of cosmetics, and the occurrence of chitin in fungi [350] are some of the subjects not dealt with specifically here, notwithstanding their importance. 

Orpin CG (1977) Occurrence of chitin in cell-walls of rumen organisms neocallimastix-frontalis, piromonas-communis and sphaeromonas-communis. J Gen Microbiol 99 215-218... 

Isolated chitins are highly ordered copolymers of 2-acetamido-2-deoxy-/3-D-glucose and 2-amino-2-deoxy-j6-D-glucose. The occurrence of the latter is explained by the fact that in vivo chitin is covalently finked to proteins via the nitrogen atom of approximately one repeating unit out of ten, therefore upon isolation a degree of deacetylation close to 0.10 is found. Chito-biose, 0-(2-amino-2-deoxy-j6-D-glucopyranosyl)-(l 4)-2-amino-2-deoxy-... 

Brief, general reviews of chitin chemistry exist,as do accounts of fungal chitin. The occurrence and function of chitin in arthropod cuticles are dealt with at length by Richards. The purpose of the present Chapter is to present a more comprehensive account of the chemistry and biochemistry of chitin. 

The occurrence of amino sugars in polymeric forms in marine biota, e.g. as mucopolysaccharides and glycoproteins, is undisputed, chitin (N-acetyl... 

H. Rudiger and H.-J. Gabius, Plant lectins Occurrence, biochemistry, functions and applications, Gly CO conjugate J., 18 (2001) 589-613 for recent applications of chitin, see, for instance, Y. Kato, H. Onishi, and Y. Machida, Application of chitin and chitosan derivatives in the pharmaceutical field, Curr. Pharm. Biotechnol, 4 (2003) 303-309. 

A similar position exists with most other polysaccharides. In general, the same chemical structures, produced by prokaryotes and eukaryotes are unlikely to be related in terms of their biosynthesis. Within the eukaryotes, evolutionary relationships may well exist where similar polymers are found in species that are known, from other evidence, to be closely related. Likewise, the present-day pattern of occurrence of a polymer such as chitin may well suggest that a single t3q>e of mechanism exists for its synthesis and that this has persisted in some phyla, but has been lost during the evolution of others. Where a polysaccharide seems to reappear after an evolutionary gap, there is always a question as to whether this is a re-emergence of the old synthetic pathway, or the acquisition of a new one. It should not be forgotten that the relative simplicity of polysaccharide structure does make the latter quite possible, in principle at least. 

As indicated in the first part of the chapter, this type of classification based on the occurrence of polysaccharides offers some disadvantages, particularly in that some materials such as chitin, hyaluronic acid, and cellulose occur in plants or microorganisms as well as in animals. However, until more chemical work is done, a strictly chemical classification does not seem feasible. The conflicting classifications and nomenclature in this field need standardization and agreement. 

The occurrence of extracts of in-sect cuticles with chitinolytie activity suggests that the enzymes might be eoneerned in the metabolism of chitin. However, the demonstration (Powning and Irzykiewicz, 1967) that such extracts also attack ilf. lysodeikticus cell walls makes a distinction between cliitinase and muramidase aetivitie.s impossible without further study. 

Last years the concern of the scientists and contributors to chitin, chitosan and chitincontaining connections has increased. It is connected to their widespread occurrence in the nature, paiticulai properties, and also feasibility in many areas of a national economy. The raw sources for obtaining chitincontaining of products are the testas of crabs, lobsters, shrimps, and also cabbage-weeds, funguses. 

Thus, based on material applications, the following polymers are important natural rubber, coal, asphaltenes (bitumens), cellulose, chitin, starch, lignin, humus, shellac, amber, and certain proteins. Figure 4 shows the primary structures of some of the above polymers. For detailed information on their occurrence, conventional utilization, etc., refer to the references cited previously. 

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