Structural analysis of glucuronoxylan specific Xyn30D and its attached CBM35 domain give insights into the role of modularity in specificity [Enzymology]

September 8th, 2014 by Sainz–Polo, M. A., Valenzuela, S. V., Gonzalez, B., Pastor, F. I. J., Sanz–Aparicio, J.

Glucuronoxylanase Xyn30D is a modular enzyme containing a family 30 glycoside hydrolase catalytic domain and an attached carbohydrate binding module of the CBM35 family. We present here the three-dimensional structure of the full-length Xyn30D at 2.4 A resolution. The catalytic domain folds into an (α/β)8 barrel with an associated β-structure, while the attached CBM35 displays a jellyroll β-sandwich including two calcium ions. Although both domains fold in an independent manner, the linker region makes polar interactions with the catalytic domain allowing a moderate flexibility. The ancillary Xyn30D-CBM35 domain has been expressed and crystallized and its binding abilities have been investigated by soaking experiments. Only glucuronic acid-containing ligands produced complexes, and their structures have been solved. A calcium dependent glucuronic acid binding site shows distinctive structural features as compared to other uronic acid specific CBM35s, as the presence of two aromatic residues delineating a wider pocket. The non-conserved Glu129 makes a bidentate link to calcium and defines region E, previously identified as specificity hot spot. The molecular surface of Xyn30D-CBM35 shows a unique stretch of negative charge distribution extending from its binding pocket that might indicate some oriented interaction with its target substrate. The binding ability of Xyn30D-CBM35 to different xylans was analyzed by affinity gel electrophoresis. Some binding was observed with rye glucuronoarabinoxylan in presence of calcium chelating EDTA, which would indicate that Xyn30D-CBM35 might establish interaction to other components of xylan, such as arabinose decorations of glucuronoarabinoxylan. A role in depolymerization of highly substituted chemically complex xylans is proposed.
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