<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 56 <db_xref db="CAZY" dbkey="GH56"/> comprises enzymes with only one known activity; hyaluronidase <db_xref db="EC" dbkey="3.2.1.35"/>.</p><p>The venom of <taxon tax_id="7460">Apis mellifera</taxon> (Honeybee) contains several biologically-active peptides andtwo enzymes, one of which is a hyaluronidase [<cite idref="PUB00004814"/>]. The amino acid sequenceof bee venom hyaluronidase contains 349 amino acids, and includes fourcysteines and a number of potential glycosylation sites [<cite idref="PUB00004814"/>]. The sequenceshows a high degree of similarity to PH-20, a membrane protein of mammaliansperm involved in sperm-egg adhesion, supporting the view that hyaluronidasesplay a role in fertilisation [<cite idref="PUB00004814"/>].</p><p>PH-20 is required for sperm adhesion to the egg zona pellucida; it islocated on both the sperm plasma membrane and acrosomal membrane [<cite idref="PUB00003064"/>]. Theamino acid sequence of the mature protein contains 468 amino acids, andincludes six potential N-linked glycosylation sites and twelve cysteines,eight of which are tightly clustered near the C terminus [<cite idref="PUB00003064"/>].</p>