Thus, some lysozymes could be considered as good chitinases and reciprocally some chitinases can cleave peptidoglycan, the natural substrate of lysozymes. However there is no obvious amino acid sequence similarity found between these two types of enzymes. On the other hand, a different enzyme, chitosanase, also hydrolyses polymer of GlcNAc, but with specificity for a partial or full deacetylation of chitin, named chitosan. The differences in substrate specificity of these enzymes, and occasionally in their catalytic mechanism, make them belong to different protein LY2835219 families with different E.C. number. All these proteins could be considered, to a large extent, as chitinolytic enzymes, i.e. enzymes that are able to hydrolyze derivatives of chitin. Chitinolytic enzymes are widely distributed in the tissues and body fluids of animals, plants and microorganisms and also in the soil- and bio-spheres of the earth. Chitinases are key enzymes in plant defence systems against fungal infection. They are classified on the basis of amino acid sequence in two different GH families, namely GH18 and GH19. Chitinases of GH18 are encountered in all living organisms whereas those of GH19 are mainly found in plants. Proteins of these two GH families significantly differ both in their three-dimensional structures and in their enzymatic mechanisms. Lysozymes are widely spread throughout nature. They are used by plants and higher organisms as a first defence mechanism against bacterial invasion. Since its discovery by Fleming in 1922, lysozyme has been extensively studied. It was one of the first proteins to be completely sequenced and one of the first enzymes for which the X-ray structure was determined. Several classes of lysozymes have been identified on the basis of their sequence similarities. The best known ones are of the Ctype, the G-type and the V-type. Chitosanases are classified in GH46. Most of these enzymes are found in microorganisms and few are found in virus. Although chitinases and chitosanases hydrolyze chemically similar substrates that differ only by an acetyl group, no sequence similarities were found between members of these two families. Polysaccharide-hydrolyzing enzymes commonly use two catalytic residues, a general-acid and a nucleophile/ base residue, and they basically perform their function through two different reaction mechanisms, a single-displacement mechanism with a net inversion of an anomeric carbon configuration and a double-displacement mechanism with a net retention of a substrate configuration. Whereas the catalytic general-acid residue is localized in equivalent positions in the lysozyme superfamily, the general-base residues are not well structurally conserved in the five families, and even in an extreme case, such as in GH23 and GH46 families, no residue with general base function has been identified. Finally, with the exceptions of GH22 lysozymes that are retaining enzymes.