<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>Family 56 (<db_xref db="CAZY" dbkey="GH56"/>) encompasses a group of hyaluronidases (<db_xref db="EC" dbkey="3.2.1.35"/>)that includes venom hyaluronidases and mammalian sperm surface proteins(PH-20).</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"/>]. During fertilisation, sperm must first penetrate a layer of cumulus cellsthat surrounds the egg, before reaching the zona pellucida. The cumuluscells are embedded in a matrix containing hyaluronic acid, which is formedprior to ovulation. PH-20 facilitates penetration of the cumulus cell layerby digesting hyaluronic acid.</p><p>The amino 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> Glycoside hydrolase, family 56, sperm surface protein PH20