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Mannose foraging by Bacteroides thetaiotaomicron: Structure and specificity of the β-mannosidase, BtMan2A

Lookup NU author(s): Dr Louise Tailford, Dr Claire Dumon, Emeritus Professor Harry Gilbert

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Abstract

The human colonic bacterium Bacteroides thetaiotaomicron, which plays an important role in maintaining human health, produces an extensive array of exo-acting glycoside hydrolases (GH), including 32 family GH2 glycoside hydrolases. Although it is likely that these enzymes enable the organism to utilize dietary and host glycans as major nutrient sources, the biochemical properties of these GH2 glycoside hydrolases are currently unclear. Here we report the biochemical properties and crystal structure of the GH2 B. thetaiotaomicron enzyme BtMan2A. Kinetic analysis demonstrates that BtMan2A is a β-mannosidase in which substrate binding energy is provided principally by the glycone binding site, whereas aglycone recognition is highly plastic. The three-dimensional structure, determined to a resolution of 1.7 Å, reveals a five-domain structure that is globally similar to the Escherichia coli LacZ β-galactosidase. The catalytic center is housed mainly within a (β/α)8 barrel although the N-terminal domain also contributes to the active site topology. The nature of the substrate-binding residues is quite distinct from other GH2 enzymes of known structure, instead they are similar to other clan GH-A enzymes specific for mannoconfigured substrates. Mutagenesis studies, informed by the crystal structure, identified a WDW motif in the N-terminal domain that makes a significant contribution to catalytic activity. The observation that this motif is invariant in GH2 mannosidases points to a generic role for these residues in this enzyme class. The identification of GH-Aclan and GH2 specific residues in the active site of BtMan2A explains why this enzyme is able to harness substrate binding at the proximal glycone binding site more efficiently than mannan-hydrolyzing glycoside hydrolases in related enzyme families. The catalytic properties of BtMan2A are consistent with the flexible nutrient acquisition displayed by the colonic bacterium. © 2007 by The American Society for Biochemistry and Molecular Biology, Inc.


Publication metadata

Author(s): Tailford LE, Money VA, Smith NL, Dumon C, Davies GJ, Gilbert HJ

Publication type: Article

Publication status: Published

Journal: Journal of Biological Chemistry

Year: 2007

Volume: 282

Issue: 15

Pages: 11291-11299

ISSN (print): 0021-9258

ISSN (electronic): 1083-351X

Publisher: American Society for Biochemistry and Molecular Biology

URL: http://dx.doi.org/10.1074/jbc.M610964200

DOI: 10.1074/jbc.M610964200

PubMed id: 17287210


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