What is chitinosanase?

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Chitinosanase is a recently described, unique chitosan hydrolase which is neither a chitinase nor a chitosanase. By definition, chitinases are able to cleave the GlcNAc-GlcNAc bond but not the GlcN-GlcN bond, while chitosanases can cleave the GlcN-GlcN bond but not the GlcNAc-GlcNAc bond. Consequently, chitinases can degrade GlcNAc oligomers but not GlcN oligomers, while chitosanases can degrade GlcN oligomers but not GlcNAc oligomers. Chitinosanase can cleave both bonds, but cannot degrade either type of oligomer. This is because chitinosanase has an absolute specificity for GlcN at the (-2) subsite (which is typical for almost all chitosanases) and for GlcNAc at the (-1) subsite (which is typical for GH18 chitinases), thus it cleaves "behind" (i.e. at the reducing site of) the diad GlcN-GlcNAc in partially acetylated chitosans. This unique specificity makes it ideally suited for fingerprinting analyses of chitosans. The enzyme has so far only been isolated from the fungus Alternaria alternata, and its gene is not yet known. It remains to be seen, thus, whether it is a new class of chitosan degrading enzymes, or rather a very peculiar chitinase or chitosanase.