The X-Ray Crystallographic Structure of Endoxylanase Xyn10C from Xanthomonas axonopodis at 1.6 Angstrom Resolution

The recent biochemical characterization of the xylanases of glycoside hydrolase family 10 (GH10) has identified a distinctive endolytic mode of action, hydrolyzing the β-1,4 xylan chain at a specific site directed by the position of an α-1,2-linked glucuronate moiety. Xylanase C (XynC), the GH10 xylanase from Xanthomonas axonopodis, has been cloned, overexpressed and crystallized. Crystals grew in 100 mM sodium phosphate citrate, 200 mM NaCl, 25 % (w/v) PEG 8000, 5 % (v/v) glycerol, pH 5.5. The crystals belonged to the orthorhombic space group C2221 and had unit cell parameters of 71 Å, 80 Å and 161 Å and α = β = γ = 90°. Diffraction data were collected to 1.6-Å resolution and an initial phasing model was created using molecular replacement techniques. Multiple rounds of model building and refinement were performed using Coot and Phenix, respectively. The protein was found to possess the typical (β/α)8 fold observed in all members of GH10. A large loop on the aglycone side of the active site was found to adopt a unique conformation. This loop was in a position that would prevent it from binding substrates larger than three xylose units in size and would be expected to produce a xylobiose and a free xylose upon cleavage of such a trimer. This work will help to identify the structural determinants that grant the exceptional specificity of this enzyme and the role it plays in the biological depolymerization and processing of glucuronoxylan