Two sequentially different, seemingly unrelated α-amylase families exist, known as family-13 and family-57 glycosyl hydrolases.

They are defined as carbohydrolases that hydrolyze partially N-acetylated chitosan, splitting the β-1,4-glycosidic linkages except the N-acetyl-glucosamine homo-linkages (Glc NAc-Glc NAc) [1, E1].

From the view point of sequence and folding similarities chitosanases belong to families 46, 75 or 80 in the classification of glycosyl hydrolases [2, E2]. It is produced by the SIB Swiss Institute Bioinformatics.

Chitosanases of families 46 and 80 are enzymes from bacteria and Chlorella viruses that share a conserved active site-related module [3]. There are no restrictions on its use by non-profit institutions as long as its content is in no way modified.

This module includes several amino acids essential for enzyme activity and (or) stability as shown by site-directed mutagenesis studies. Usage by and for commercial entities requires a license agreement.

The recent analysis of these families, coupled with the burgeoning number of 3D structures, provides a detailed insight into the structure, function and catalytic mechanism of these enzymes.

They are extremely common enzymes with roles in nature including degradation of biomass such as cellulose (cellulase), hemicellulose, and starch (amylase), in anti-bacterial defense strategies (e.g., lysozyme), in pathogenesis mechanisms (e.g., viral neuraminidases) and in normal cellular function (e.g., trimming mannosidases involved in N-linked glycoprotein biosynthesis).Deficiency in specific lysosomal glycoside hydrolases can lead to a range of lysosomal storage disorders that result in developmental problems or death.Glycoside hydrolases are found in the intestinal tract and in saliva where they degrade complex carbohydrates such as lactose, starch, sucrose and trehalose.Glycoside hydrolases are classified into EC 3.2.1 as enzymes catalyzing the hydrolysis of O- or S-glycosides.Glycoside hydrolases can also be classified according to the stereochemical outcome of the hydrolysis reaction: thus they can be classified as either retaining or inverting enzymes.The detailed analysis of the recently determined sequence of the α-amylase from methanogenic archaeon α-amylase from family 57 while it obviously contains most of the sequence fingerprints characteristic for α-amylase family 13.