<p>O-Glycosyl hydrolases <db_xref db="EC" dbkey="3.2.1."/> are a widespread group of enzymes that hydrolyse the glycosidic bond between two or more carbohydrates, or between a carbohydrate and a non-carbohydrate moiety. A classification system for glycosyl hydrolases, based on sequence similarity, has led to the definition of 85 different families [<cite idref="PUB00004870"/>, <cite idref="PUB00005266"/>]. This classification is available on the CAZy (CArbohydrate-Active EnZymes) web site.</p><p>Glycoside hydrolase family 45 <db_xref db="CAZY" dbkey="GH45"/> comprises enzymes with only one known activity; endoglucanase (<db_xref db="EC" dbkey="3.2.1.4"/>).</p><p>The microbial degradation of cellulose and xylans requires several types ofenzymes such as endoglucanases, cellobiohydrolases (<db_xref db="EC" dbkey="3.2.1.91"/>)(exoglucanases), or xylanases (<db_xref db="EC" dbkey="3.2.1.8"/>) [<cite idref="PUB00000117"/>, <cite idref="PUB00003608"/>].Fungi and bacteria producea spectrum of cellulolytic enzymes (cellulases) and xylanases which, on thebasis of sequence similarities, can be classified into families. One of thesefamilies is known as the cellulase family K or as the glycosyl hydrolasesfamily 45 [<cite idref="PUB00000528"/>].The best conserved regions in these enzymes is located in the N-terminalsection. It contains an aspartic acid residue which has been shown [<cite idref="PUB00004155"/>] to actas a nucleophile in the catalytic mechanism.This also has several cysteines that are involved in forming disulphide bridges.</p>