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The Structure of a Streptomyces avermitilis alpha-L-Rhamnosidase Reveals a Novel Carbohydrate-binding Module CBM67 within the Six-domain Arrangement

Lookup NU author(s): Dr Adam Jackson, Professor Harry Gilbert

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Abstract

alpha-L-Rhamnosidases hydrolyze alpha-linked L-rhamnosides from oligosaccharides or polysaccharides. We determined the crystal structure of the glycoside hydrolase family 78 Streptomyces avermitilis alpha-L-rhamnosidase (SaRha78A) in its free and L-rhamnose complexed forms, which revealed the presence of six domains N, D, E, F, A, and C. In the ligand complex, L-rhamnose was bound in the proposed active site of the catalytic module, revealing the likely catalytic mechanism of SaRha78A. Glu(636) is predicted to donate protons to the glycosidic oxygen, and Glu(895) is the likely catalytic general base, activating the nucleophilic water, indicating that the enzyme operates through an inverting mechanism. Replacement of Glu(636) and Glu(895) resulted in significant loss of alpha-rhamnosidase activity. Domain D also bound L-rhamnose in a calcium-dependent manner, with a K-D of 135 mu M. Domain D is thus a non-catalytic carbohydrate binding module (designated SaCBM67). Mutagenesis and structural data identified the amino acids in SaCBM67 that target the features of L-rhamnose that distinguishes it from the other major sugars present in plant cell walls. Inactivation of SaCBM67 caused a substantial reduction in the activity of SaRha78A against the polysaccharide composite gum arabic, but not against aryl rhamnosides, indicating that SaCBM67 contributes to enzyme function against insoluble substrates.


Publication metadata

Author(s): Fujimoto Z, Jackson A, Michikawa M, Maehara T, Momma M, Henrissat B, Gilbert HJ, Kaneko S

Publication type: Article

Publication status: Published

Journal: Journal of Biological Chemistry

Year: 2013

Volume: 288

Issue: 17

Pages: 12376-12385

Print publication date: 26/04/2013

Online publication date: 13/03/2013

ISSN (print): 0021-9258

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

URL: http://dx.doi.org/10.1074/jbc.M113.460097

DOI: 10.1074/jbc.M113.460097


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