Where do chitosans occur naturally?
Until recently, it was thought that chitosan occurs naturally only in a small sub-group of the fungi, the Zygomycetes. While other fungi contain chitin (and other glycans, glycoproteins, etc.) in their cell walls, the Zygomycetes appear to have replaced much of the chitin by chitosan. Recently though, it was found that other fungi, too, may contain chitosan in their cell walls, in particular pathogenic fungi, i.e. fungi causing disease in plants or humans. Such fungi appear to convert at least part of the chitin in their cell walls, in particular chitin exposed on the outer surface of the pathogenic hyphae, into chitosan upon penetrating into their host tissues, most likely in an attempt to evade the chitin-triggered immune system of their hosts. In all of these fungi, chitosan biosynthesis is thought to occur via a two-step enzymatic process, firstly involving chitin synthases, then chitin de-N-acetylases.