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Probing the mechanism of ligand recognition in family 29 carbohydrate-binding modules

Lookup NU author(s): Dr James Flint, Dr David Bolam, Dr Edward Taylor, Professor Harry Gilbert

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Abstract

The recycling of photosynthetically fixed carbon, by the action of microbial plant cell wall hydrolases, is integral to one of the major geochemical cycles and is of considerable industrial importance. Non-catalytic carbohydrate-binding modules (CBMs) play a key role in this degradative process by targeting hydrolytic enzymes to their cognate substrate within the complex milieu of polysaccharides that comprise the plant cell wall. Family 29 CBMs have, thus far, only been found in an extracellular multienzyme plant cell wall-degrading complex from the anaerobic fungus Piromyces equi, where they exist as a CBM29-1:CBM29-2 tandem. Here we p̧resent both the structure of the CBM29-1 partner, at 1.5 Å resolution, and examine the importance of hydrophobic stacking interactions as well as direct and solvent-mediated hydrogen bonds in the binding of CBM29-2 to different polysaccharides. CBM29 domains display unusual binding properties, exhibiting specificity for both β-manno- and β-gluco-configured ligands such as mannan, cellulose, and glucomannan. Mutagenesis reveals that "stacking" of tryptophan residues in the n and n+2 subsites plays a critical role in ligand binding, whereas the loss of tyrosine-mediated stacking in the n+4 subsite reduces, but does not abrogate, polysaccharide recognition. Direct hydrogen bonds to ligand, such as those provided by Arg-112 and Glu-78, play a pivotal role in the interaction with both mannan and cellulose, whereas removal of water-mediated interactions has comparatively little effect on carbohydrate binding. The interactions of CBM29-2 with the O2 of glucose or mannose contribute little to binding affinity, explaining why this CBM displays dual gluco/manno specificity. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.


Publication metadata

Author(s): Flint JE, Bolam DN, Nurizzo D, Taylor EJ, Williamson M, Walters C, Davies G, Gilbert HJ

Publication type: Article

Publication status: Published

Journal: Journal of Biological Chemistry

Year: 2005

Volume: 280

Issue: 25

Pages: 23718-23726

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.M501551200

DOI: 10.1074/jbc.M501551200

PubMed id: 15784618


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