Where are chitinases found in nature?

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Chitinases are widespread in nature, being produced by members of virtually all branches of the phylogenetic tree of life. Chitinases can be part of the immune system, protecting organisms from pathogenic fungi or e.g. nematodes, they can have nutritional roles for microbes able to live on chitin or chitosan, and they can have morphogenetic roles e.g. in the development of fungi containing chitin (or chitosans) in their cell walls. GH18 chitinases are best described from bacteria but only because their genes are most easily heterologously expressed so that sufficient quantities of pure enzyme can be produced for analysis, and site-directed mutagenesis can easily be performed to analyse the importance of specific amino acids for substrate binding and catalysis. GH19 chitinases were originally thought to be found exclusively in plants, but they have since also been found in some bacteria. Humans also contain two true chitinases, namely chitotriosidase and acidic mammalian chitinase, in addition to lysozyme which is also able to degrade chitin. Humans also contain a surprisingly large number of genes coding for inactive chitinases, inactive because one catalytically active amino acid is missing. These chitin binding proteins are thought to act as receptors for chitinous ligands, mediating their biological activities, but their natural ligands are unknown.