Oligosaccharide binding to family 11 xylanases: both covalent intermediateand mutant product complexes display B-2,B-5 conformations at the active centre

The glycoside hydrolase sequence-based classification reveals two families of enzymes which hydrolyse the beta -1,4-linked backbone of xylan, xylanase s, termed families GH-10 and GH-11. Family GH-11 xylanases are intriguing i n that catalysis is performed via a covalent intermediate adopting an unusu al B-2,B-5 (boat) conformation, a conformation which also fulfils the stere ochemical constraints of the oxocarbenium ion-like transition state. Here, the 1.9 Angstrom structure of a nucleophile, E94A, mutant of the Xyn11 from Bacillus agaradhaerens in complex with xylotriose is presented. Intriguing ly, this complex also adopts the B-2,B-5 conformation in the -1 subsite, wi th the vacant space provided by the Glu --> Ala mutation allowing the sugar to adopt the alpha -configuration at C1. The structure of the covalent 2-d eoxy-2-fluoroxylobiosyl-enzyme intermediate has been extended to atomic (1. 1 Angstrom) resolution.