Glycosyl hydrolase family 43, such as Selenomonas ruminantium beta-D-xylosidase SXA
This glycosyl hydrolase family 43 (GH43) includes enzymes that have been characterized to mainly have beta-1,4-xylosidase (beta-D-xylosidase;xylan 1,4-beta-xylosidase; EC 3.2.1.37) activity, including Selenomonas ruminantium (Xsa;Sxa;SXA), Bifidobacterium adolescentis ATCC 15703 (XylC;XynB;BAD_0428) and Bacillus sp. KK-1 XylB. They are part of an array of hemicellulases that are involved in the final breakdown of plant cell-wall whereby they degrade xylan. They hydrolyze beta-1,4 glycosidic bonds between two xylose units in short xylooligosaccharides. These are inverting enzymes (i.e. they invert the stereochemistry of the anomeric carbon atom of the substrate) that have an aspartate as the catalytic general base, a glutamate as the catalytic general acid and another aspartate that is responsible for pKa modulation and orienting the catalytic acid. These enzymes possess an additional C-terminal beta-sandwich domain that restricts access for substrates to a portion of the active site to form a pocket. The active-site pockets comprise of two subsites, with binding capacity for two monosaccharide moieties and a single route of access for small molecules such as substrate. A common structural feature of GH43 enzymes is a 5-bladed beta-propeller domain that contains the catalytic acid and catalytic base. A long V-shaped groove, partially enclosed at one end, forms a single extended substrate-binding surface across the face of the propeller.
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