In 2002 and 2003, the publication of complete genome sequences of the actinomycetes Streptomyces coelicolor A3(2) and Streptomyces avermitilis (108,
109) revealed that S. avermitilis has two genes (SAV_1288 and SAV_1850) while S. coelicolor A3(2) has one gene (SCO7070) encoding chitosanases of GH family 75.
More recently, the chitosanase encoded by the SAV_1850 chitosanase (also named SaCsn75A or Csn75A) have been expressed in recombinant E. coli and substrate-binding preference of this enzyme
has been investigated (122) (see also the "Cleavage specificity" section).
Currently, GH75 family includes some sixty enzymes of fungal or bacterial origin. The enzymes which were characterized by biochemical techniques showed exclusively a chitosanase activity.
The alignment of 10 GH75 protein sequences revealed two highly conserved clusters of amino acid residues: D[VI]DCDG and GEAS (see also (107)).
The same aligment showed that GH75 enzymes from fungal and actinomycetal origin are grouped in two well separated clusters, as shown on the unrooted phylogenetic tree below:
Directed mutagenesis experiments performed on the chitosanase from Fusarium solani f. sp. phaseoli SUF386 (110) as well as that of Aspergillus fumigatus
(107) demonstrated that these chitosanases operate via an inverting mechanism and use a pair of carboxylic acid as catalytic residues.
The chitosanases from F. solani SUF386 and A. oryzae end with a cysteine-rich module which is not present in the other enzymes belonging to GH 75 family.
We can only speculate about the structural properties of family 75 enzymes. However, predictions of the secondary structure from Fusarium solani SUF386 chitosanase and
those of Streptomyces avermitilis by an algorithm (111) reveal significant differences. These predictions suggest a high propensity for the formation
of beta-sheets in the case of F. solani while the same algorithm predicts a rich helical structure for both S. avermitilis chitosanases.