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Structure and Function of an Arabinoxylan-specific Xylanase

Lookup NU author(s): Professor Rick Lewis, Professor Harry Gilbert

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Abstract

The enzymatic degradation of plant cell walls plays a central role in the carbon cycle and is of increasing environmental and industrial significance. The enzymes that catalyze this process include xylanases that degrade xylan, a β-1,4-xylose polymer that is decorated with various sugars. Although xylanases efficiently hydrolyze unsubstituted xylans, these enzymes are unable to access highly decorated forms of the polysaccharide, such as arabinoxylans that contain arabinofuranose decorations. Here, we show that a Clostridium thermocellum enzyme, designated CtXyl5A, hydrolyzes arabinoxylans but does not attack unsubstituted xylans. Analysis of the reaction products generated by CtXyl5A showed that all the oligosaccharides contain an O3 arabinose linked to the reducing end xylose. The crystal structure of the catalytic module (CtGH5) of CtXyl5A, appended to a family 6 noncatalytic carbohydrate-binding module (CtCBM6), showed that CtGH5 displays a canonical (α/β)8-barrel fold with the substrate binding cleft running along the surface of the protein. The catalytic apparatus is housed in the center of the cleft. Adjacent to the −1 subsite is a pocket that could accommodate an L-arabinofuranose-linked α-1,3 to the active site xylose, which is likely to function as a key specificity determinant. CtCBM6, which adopts a β-sandwich fold, recognizes the termini of xylo- and gluco-configured oligosaccharides, consistent with the pocket topology displayed by the ligand-binding site. In contrast to typical modular glycoside hydrolases, there is an extensive hydrophobic interface between CtGH5 and CtCBM6, and thus the two modules cannot function as independent entities.


Publication metadata

Author(s): Correia MA, Mazumder K, Bras JL, Firbank SJ, Zhu Y, Lewis RJ, York WS, Fontes CM, Gilbert HJ

Publication type: Article

Journal: Journal of Biological Chemistry

Year: 2011

Volume: 286

Issue: 25

Pages: 22510-22520

Print publication date: 24/06/2011

ISSN (print): 0021-9258

ISSN (electronic): 1083-351X

Publisher: American Society for Biochemistry and Molecular Biology, Inc.

URL: http://dx.doi.org/10.1074/jbc.M110.217315

DOI: 10.1074/jbc.M110.217315


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