InterPro domain: IPR001554

O-Glycosyl hydrolases ( 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 ]. This classification is available on the CAZy (CArbohydrate-Active EnZymes) website.

Glycoside hydrolase family 14 GH14 comprises enzymes with only one known activity; beta-amylase ( 3.2.1.2 ). A Glu residue has been proposed as a catalytic residue, but it is not known if it is the nucleophile or the proton donor.

Beta-amylase [ 3 , 4 ] is an enzyme that hydrolyses 1,4-alpha-glucosidic linkages in starch-type polysaccharide substrates so as to remove successive maltose units from the non-reducing ends of the chains. Beta-amylase is present in certain bacteria as well as in plants.

Three highly conserved sequence regions are found in all known beta-amylases. The first of these regions is located in the N-terminal section of the enzymes and contains an aspartate which is known to be involved in the catalytic mechanism [ 5 ]. The second, located in a more central location, is centred around a glutamate which is also involved in the catalytic mechanism [ 6 ].

The 3D structure of a complex of soybean beta-amylase with an inhibitor (alpha-cyclodextrin) has been determined to 3.0A resolution by X-ray diffraction [ 7 ]. The enzyme folds into large and small domains: the large domain has a (beta alpha)8 super-secondary structural core, while the smaller is formed from two long loops extending from the beta-3 and beta-4 strands of the (beta alpha)8 fold. The interface of the two domains, together with shorter loops from the (beta alpha)8 core, form a deep cleft, in which the inhibitor binds. Two maltose molecules also bind in the cleft, one sharing a binding site with alpha-cyclodextrin, and the other sitting more deeply in the cleft [ 8 ].

1. Conserved catalytic machinery and the prediction of a common fold for several families of glycosyl hydrolases. Proc. Natl. Acad. Sci. U.S.A. 92, 7090-4
2. Structures and mechanisms of glycosyl hydrolases. Structure 3, 853-9
3. [Primary structure and function of beta-amylase] Seikagaku 60, 211-6
4. Segments of amino acid sequence similarity in beta-amylases. Protein Seq. Data Anal. 1, 499-501
5. Identification of glutamic acid 186 affinity-labeled by 2,3-epoxypropyl alpha-D-glucopyranoside in soybean beta-amylase. J. Biochem. 105, 573-6
6. Residues essential for catalytic activity of soybean beta-amylase. Eur. J. Biochem. 221, 649-54
7. Three-dimensional structure of soybean beta-amylase determined at 3.0 A resolution: preliminary chain tracing of the complex with alpha-cyclodextrin. J. Biochem. 112, 541-6