<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>This entry represents the catalytic TIM beta/alpha barrel common to many different families of glycosyl hydrolases found in all groups of organisms including viruses and Gene Transfer Agents (GTA) [<cite idref="PUB00055430"/>]. In the GTA of <taxon tax_id="1061">Rhodobacter capsulatus</taxon> (Rhodopseudomonas capsulata) a glycosyl hydrolase domain is associated with ORFg15 (RCAP_rcc01698) [<db_xref db="SWISSPROT" dbkey="D5AU04"/>, see Fig.1, in <cite idref="PUB00055431"/>]. </p><p>Structures have been determined for several proteins containing this glycosyl hydrolase domain, including family 13 glycosyl hydrolases (such as alpha-amylase) [<cite idref="PUB00021988"/>], beta-glycanases [<cite idref="PUB00027698"/>], family 1 glycosyl hydrolases (such as beta-glucosidase) [<cite idref="PUB00027699"/>], type II chitinases [<cite idref="PUB00026805"/>], 1,4-beta-N-acetylmuraminidases [<cite idref="PUB00026374"/>], and beta-N-acetylhexosaminidases [<cite idref="PUB00027171"/>].</p>