<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 5 <db_xref db="CAZY" dbkey="GH5"/> comprises enzymes with several known activities; endoglucanase (<db_xref db="EC" dbkey="3.2.1.4"/>); beta-mannanase (<db_xref db="EC" dbkey="3.2.1.78"/>); exo-1,3-glucanase (<db_xref db="EC" dbkey="3.2.1.58"/>); endo-1,6-glucanase (<db_xref db="EC" dbkey="3.2.1.75"/>); xylanase (<db_xref db="EC" dbkey="3.2.1.8"/>); endoglycoceramidase (<db_xref db="EC" dbkey="3.2.1.123"/>).</p><p>The microbial degradation of cellulose and xylans requires several types of enzymes. Fungi and bacteria produces a spectrum of cellulolytic enzymes (cellulases) and xylanases which, on the basis of sequence similarities, can be classified into families. One of these families is known as the cellulase family A [<cite idref="PUB00001778"/>] or as the glycosyl hydrolases family 5 [<cite idref="PUB00000503"/>]. One of the conserved regions in this family contains a conserved glutamic acid residue which is potentially involved [<cite idref="PUB00004956"/>] in the catalytic mechanism.</p>