InterPro: IPR001439 Glycoside hydrolase, family 56, sperm surface protein PH20

O-Glycosyl hydrolases EC: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 [1, 2, 3]. This classification is available on the CAZy (CArbohydrate-Active EnZymes) web site [4]. Because the fold of proteins is better conserved than their sequences, some of the families can be grouped in clans.

Family 56 (GH56) encompasses a group of hyaluronidases (EC:3.2.1.35) that includes venom hyaluronidases and mammalian sperm surface proteins (PH-20).

PH-20 is required for sperm adhesion to the egg zona pellucida; it is located on both the sperm plasma membrane and acrosomal membrane [5]. During fertilisation, sperm must first penetrate a layer of cumulus cells that surrounds the egg, before reaching the zona pellucida. The cumulus cells are embedded in a matrix containing hyaluronic acid, which is formed prior to ovulation. PH-20 facilitates penetration of the cumulus cell layer by digesting hyaluronic acid.

The amino acid sequence of the mature protein contains 468 amino acids, and includes six potential N-linked glycosylation sites and twelve cysteines, eight of which are tightly clustered near the C terminus [5].