<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 35 <db_xref db="CAZY" dbkey="GH35"/> comprises enzymes with only one known activity; beta-galactosidase (<db_xref db="EC" dbkey="3.2.1.23"/>).</p><p>Mammalian beta-galactosidase is a lysosomal enzyme (gene GLB1) which cleaves the terminal galactose from gangliosides, glycoproteins, and glycosaminoglycans and whose deficiency is the cause of the genetic disease Gm(1) gangliosidosis (Morquio disease type B).</p><p>One of the best conserved regions in these enzymes contains a glutamic acid residue which, on the basis of similarities with other families of glycosyl hydrolases, probably acts as the proton donor in the catalytic mechanism. This signature spans the region contain the putative active site glutamic acid residue, it is the second glutamic acid residue of the two in the pattern [<cite idref="PUB00004870"/>]. </p>