InterPro domain: IPR000726

O-Glycosyl hydrolases ( 3.2.1. ) are a widespread group of enzymes that hydrolyse the glycosidic bond between two or more carbohydrates, or between a carbohydrate and a non-carbohydrate moiety. A classification system for glycosyl hydrolases, based on sequence similarity, has led to the definition of 85 different families [ 1 , 2 ]. This classification is available on the CAZy (CArbohydrate-Active EnZymes) website.

Glycoside hydrolase family 19 GH19 comprises enzymes with only one known activity; chitinase ( 3.2.1.14 ).

Chitinases [ 3 ] are enzymes that catalyse the hydrolysis of the beta-1,4-N-acetyl-D-glucosamine linkages in chitin polymers. Chitinases belong to glycoside hydrolase families 18 or 19 [ 4 ]. Chitinases of family 19 (also known as classes I, II and IV) are enzymes from plants that function in the defence against fungal and insect pathogens by destroying their chitin-containing cell wall. Some family 19 chitinases are found in bacteria. Class I and II chitinases are similar in their catalytic domains. Class I chitinases have an N-terminal cysteine-rich, chitin-binding domain which is separated from the catalytic domain by a proline and glycine-rich hinge region. Class II chitinases lack both the chitin-binding domain and the hinge region. Class IV chitinases are similar to class I, but they are smaller in size due to certain deletions.

Despite any significant sequence homology with lysozymes, structural analysis reveals that family 19 chitinases, together with family 46 chitosanases, are similar to several lysozymes including those from T4-phage and from goose. The structures reveal that the different enzyme groups arose from a common ancestor glycohydrolase antecedent to the procaryotic/eucaryotic divergence [ 5 , 6 , 7 , 8 , 9 , 10 ].

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3. What's new in chitinase research? Experientia 48, 701-16
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