InterPro: IPR001968 Glycoside hydrolase, family 56

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.

Glycoside hydrolase family 56 GH56 comprises enzymes with only one known activity; hyaluronidase EC:3.2.1.35.

The venom of Apis mellifera (Honeybee) contains several biologically-active peptides and two enzymes, one of which is a hyaluronidase [5]. The amino acid sequence of bee venom hyaluronidase contains 349 amino acids, and includes four cysteines and a number of potential glycosylation sites [5]. The sequence shows a high degree of similarity to PH-20, a membrane protein of mammalian sperm involved in sperm-egg adhesion, supporting the view that hyaluronidases play a role in fertilisation [5].

PH-20 is required for sperm adhesion to the egg zona pellucida; it is located on both the sperm plasma membrane and acrosomal membrane [6]. 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 [6].