Structural basis for substrate recognition by Erwinia chrysanthemi GH30 glucuronoxylanase

Xylanase A from the phytopathogenic bacterium Erwinia chrysanthemi is classified as a glycoside hydrolase family 30 enzyme (previously in family 5) and is specialized for degradation of glucuronoxylan. The recombinant enzyme was crystallized with the aldotetraouronic acid beta-D-xylopyranosyl(1 -> 4)-[4-O-methyl-alpha-D-glucuronosyl-(1 -> 2)]-beta-D-xylopyranosyl-(1 -> 4)D-xylose as a ligand. The crystal structure of the enzyme-ligand complex was solved at 1.39 angstrom resolution. The ligand xylotriose moiety occupies subsites) 1,) 2 and) 3, whereas the methyl glucuronic acid residue attached to the middle xylopyranosyl residue of xylotriose is bound to the enzyme through hydrogen bonds to five amino acids and by the ionic interaction of the methyl glucuronic acid carboxylate with the positively charged guanidinium group of Arg293. The interaction of the enzyme with the methyl glucuronic acid residue appears to be indispensable for proper distortion of the xylan chain and its effective hydrolysis. Such a distortion does not occur with linear beta-1,4-xylooligosaccharides, which are hydrolyzed by the enzyme at a negligible rate.