The the family 5 of glycoside hydrolases is very large: it includes more than 4000 proteins displaying a variety of enzymatic activities. Enzymes from this family has been isolated from a large spectrum of organisms and act on β-linked oligosaccharides, polysaccharides and glycoconjugates. In 2012, Aspeborg and coworkers (118), using a bioinformatic approach, classified the catalytic modules sequences from this family into 51 distinct subfamilies. All GH5 members with chitosanolytic activities described so far are of bacterial origin and belong to the GH5_2 subfamily.
To date, only a few enzymes with chitosanase activity has been identified and confirmed by biochemical studies. In 2003, Tanabe and coworkers (65) characterized two chitosanases (ChoI et ChoII) secreted by the actinomycete Streptomyces griseus 6037 HUT. Work on ChoII has shown that this enzyme operates via a retention mechanism and has transglycosylase activity (65). The same observations were also made for ChoI. Studies on cleavage specificity of ChoI and ChoII revealed (65) that these enzymes hydrolyze not only partially acetylated chitosan, but also carboxymethyl cellulose (CMC) with retention of the anomeric form. It should be mentioned that the relative activity of ChoII on CMC is only 29 % compared to its activity on chitosan. The sequence of CHOII is highly homologous to the endoglucanase E-5 of Thermobifida fusca YX which also belongs to the family 5 of glycoside hydrolase.