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Differential oligosaccharide recognition by evolutionarily-related beta-1,4 and beta-1,3 glucan-binding modules
Alisdair B Boraston1, Didier Nurizzo, Valerie Notenboom
1Protein Engineering Network of Centres of Excellence, Edmonton, Alberta, Canada T6G 2S2.
Journal of Molecular Biology
|June 25, 2002
Summary
Carbohydrate-binding modules (CBMs) are key to enzyme function. This study reveals how structural differences in related CBMs enable specific polysaccharide recognition, highlighting the role of binding site topography and water interactions.
Area of Science:
- Biochemistry
- Structural Biology
- Enzymology
Background:
- Enzymes acting on complex carbohydrates often feature modular structures.
- Carbohydrate-binding modules (CBMs) are crucial components, but closely related CBMs can exhibit distinct polysaccharide specificities.
- Understanding the structural basis for this specificity is essential for enzyme engineering and biotechnology.
Purpose of the Study:
- To elucidate the structural and thermodynamic basis for differential polysaccharide recognition by related family 4 CBMs.
- To investigate how conserved folds accommodate distinct ligand specificities.
- To provide a structural rationale for the specific binding of beta-1,4 and beta-1,3 linked gluco-oligosaccharides by CBMs from Cellulomonas fimi and Thermotoga maritima.
Main Methods:
- X-ray crystallography to determine the three-dimensional structures of CBMs in complex with oligosaccharides.
- Isothermal titration calorimetry (ITC) to study the thermodynamics of ligand binding.
- Comparative structural analysis of CBMs and their binding sites.
Main Results:
- Two related family 4 CBMs, Cel9B and Lam16A, exhibit distinct binding preferences for beta-1,4 and beta-1,3 linked gluco-oligosaccharides, respectively.
- A conserved set of aromatic and polar residues forms the core interaction sites within the CBMs.
- Variations in loop structures create unique binding-site topographies that match the conformations of their respective ligands.
- Thermodynamic data reveal differential involvement of water molecules in the binding interactions.
Conclusions:
- Structural plasticity in loop regions of CBMs allows for adaptation to different polysaccharide structures, despite a conserved core fold.
- The interplay between conserved binding residues, variable loop conformations, and water dynamics dictates ligand specificity.
- This detailed structural understanding can inform the design of novel enzymes with tailored carbohydrate-binding properties.