Chitinases are digestive enzymes that break down glycosidic bonds in chitin. Because chitin composes the cell walls of fungi and exoskeletal elements of some animals (including worms and arthropods), chitinases are generally found in organisms that either need to reshape their own chitin or to dissolve and digest the chitin of fungi or animals.
Chitinivorous organisms include many bacteria (Aeromonads, Bacillus, Vibrio, among others), which may be pathogenic or detritivorous. They may attack arthropods, zooplankton or fungi; or they may degrade the remains of these organisms.
Fungi, such as Coccidioides immitis, are known to possess chitinases. This may be related to their typical role as detritivores and also to their potential as arthropod pathogens.
Plants may seem an unusual source for chitinase, but some of the archetypical chitinases have been characterized from plants (barley seed chitinase: PDB 1CNS, EC 18.104.22.168). Some plant chitinases are members of the pathogenesis related (PR) proteins which are induced after systemic acquired resistance induction (biotic and abiotic). Expression is mediated by the NPR1 gene and the salicylic acid pathway, both involved in resisting fungal and insect attack. Some may be required for creating fungal symbioses.
Chitin, like cellulose, has been thought of as abundant but difficult to digest. It is typically considered unavailable carbohydrate in animal diets, though certain fish can digest chitin to sugar; and dogma suggests that just as ruminants need bacteria to digest cellulose, chitin digestion would also require symbiosis and lengthy fermentations. As such, the discovery of animal, and particular mammalian and human chitinases is somewhat surprising. Actually, human chitinases appear in gastric juices. This is likely to be digestive chitinase, for catabolic activity.
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