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A family IIb xylan-binding domain has a similar secondary structure to a homologous family IIa cellulose-binding domain but different ligand specificity

Lookup NU author(s): Dr David Bolam, Emeritus Professor Harry Gilbert


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Background: Many enzymes that digest polysaccharides contain separate polysaccharide-binding domains. Structures have been previously determined for a number of cellulose-binding domains (CBDs) from cellulases. Results: The family lib xylan-binding domain 1 (XBD1) from Cellulomonas fimi xylanase D is shown to bind xylan but not cellulose. Its structure is similar to that of the homologous family IIa CBD from C. fimi Cex, consisting of two four- stranded β sheets that form a twisted 'β sandwich'. The xylan-binding site is a groove made from two tryptophan residues that stack against the faces of the sugar rings, plus several hydrogen-bonding polar residues. Conclusions: The biggest difference between the family IIa and IIb domains is that in the former the solvent-exposed tryptophan sidechains are coplanar, whereas in the latter they are perpendicular, forming a twisted binding site. The binding sites are therefore complementary to the secondary structures of the ligands cellulose and xylan. XBD1 and Cex(CBD) represent a striking example of two proteins that have high sequence similarity but a different function.

Publication metadata

Author(s): Simpson P, Bolam DN, Cooper A, Ciruela A, Hazlewood G, Gilbert HJ, Williamson M

Publication type: Article

Publication status: Published

Journal: Structure

Year: 1999

Volume: 7

Issue: 7

Pages: 853-864

Print publication date: 15/07/1999

ISSN (print): 0969-2126

ISSN (electronic): 1878-4186

Publisher: Cell Press


DOI: 10.1016/S0969-2126(99)80108-7

PubMed id: 10425686


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