Journal
GLYCOBIOLOGY
Volume 25, Issue 3, Pages 319-330Publisher
OXFORD UNIV PRESS INC
DOI: 10.1093/glycob/cwu113
Keywords
beta-N-acetylglucosaminidase; catalytic mechanism; peptidoglycan; X-ray structure
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Funding
- CNRS
- French Infrastructure for Integrated Structural Biology (FRISBI) [ANR-10-INSB-05-01]
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Members of the GH73 glycosidase family cleave the beta-1,4-glycosidic bond between the N-acetylglucosaminyl (GlcNAc) and N-acetylmuramyl (MurNAc) moieties in bacterial peptidoglycan. A catalytic mechanism has been proposed for members FlgJ, Auto, AcmA and Atl(WM) and the structural analysis of FlgJ and Auto revealed a conserved alpha/beta fold reminiscent of the distantly related GH23 lysozyme. Comparison of the active site residues reveals variability in the nature of the catalytic general base suggesting two distinct catalytic mechanisms: an inverting mechanism involving two distant glutamate residues and a substrate-assisted mechanism involving anchimeric assistance by the C2-acetamido group of the GlcNAc moiety. Herein, we present the biochemical characterization and crystal structure of TM0633 from the hyperthermophilic bacterium Thermotoga maritima. TM0633 adopts the alpha/beta fold of the family and displays beta-N-acetylglucosaminidase activity on intact peptidoglycan sacculi. Site-directed mutagenesis identifies Glu34, Glu65 and Tyr118 as important residues for catalysis. A thorough bioinformatic analysis of the GH73 sequences identified five phylogenetic clusters. TM0633, FlgJ and Auto belong to a group of three clusters that conserve two carboxylate residues involved in a classical inverting acid-base mechanism. Members of the other two clusters lack a conserved catalytic general base supporting a substrate-assisted mechanism. Molecular modeling of representative members from each cluster suggests that variability in length of the beta-hairpin region above the active site confers ligand-binding specificity and modulates the catalytic mechanisms within the GH73 family.
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