Enzymes chitin depolymerization

Strong acids cleave C. into D- glucosamine (chitosa-mine) and acetic acid. On decomposition with alkalis, acetates and the weakly basic deacetylated, partially depolymerized, and crystallizable chitosan are formed the latter can form gels and films. The chiti-nases (EC 3.2.1.14) found in snail stomachs, some mold fungi, and bacteria can - like some lysozymes -dissolve C. The thus formed chitobiose, the disaccharide of P-1,4-linked NAG, is then cleaved to monomers by chitobiase (EC 3.2.1.30). Tlie formation of C. is catalyzed by the enzyme chitin synthase (EC 2.4.1.16). The activity of, e.g., the insecticide diflubenzurone is based on inhibition of this enzyme. 

The enzymatic depolymerization of chitin by chitinases has been investigated for a few decades. Chitinases, a class of glycosyl hydrolyases, have been found in a variety of organisms ranging from bacteria to animals. Chitinases belong to two major families of carbohydrate enzymes, family 18 and family 19, based on the amino acid sequences (CAZY http //www.cazy.org). Both families of enzymes differ in their amino acid sequences, three-dimensional structures, and catalytic mechanisms (Fukamizo 2000). Prior to the family classification, plant chitinases are divided into five classes on the basis of their primary structures. Classes 1,11, and IV chitinases are included in family 19, whereas classes III and V belong to family 18. 

Overcoming the problems of acid hydrolysis, scientists have adopted different enzymatic hydrolysis methods using different types of enzymes for the preparation of COS. The polysaccharide structure of the chitin or chitosan is converted to oligosaccharide form by specific enzymatic depolymerization or nonspecific enzymatic depolymerization. 

Currently, commercial chitin and chitosan are extracted from industrial shellfish processing wastes (shrimp, crab, lobster). The seasonal character of those raw materials and the variability of the composition of the organisms make the process of chitin extraction rather expensive with low reprodudbillty. Moreover, they are subjected to environmental variations that impact on the products supply and quality [14,40,116]. Chitin is extracted from crustacean shells by the use of strong adds and/or bases that can cause deacetylation and depolymerization of chitin [119]. Alternative methods include the use of enzymes or proteolytic microorganisms (e.g.. Pseudomonas malto-philia, Bacillus subtilis. Streptococcus faecium, Aspergillus oryzae) that hydrolyze shellfish proteins and leave the associated chitin intact [119]. 

A number of endohexosamiriidases other than those considered in earlier sections have been described. Typically, such activities have been detected in extracts by the depolymerization of chitin or some derivative thereof, but only rarely have the constituent enzymes been separated and characterized. 

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