Understanding how noncatalytic carbohydrate binding modules can display specificity for xyloglucan

Area of Science:

• Biochemistry
• Structural Biology
• Plant Science

Background:

• Plant biomass is crucial for the carbon cycle and sustainable industries like biofuels.
• Carbohydrate-binding modules (CBMs) on plant cell wall-degrading enzymes target specific polysaccharides, enhancing catalytic efficiency.
• CBMs binding β-glucans often recognize cellulose, mixed-linked glucans, and xyloglucan by targeting common structural features.

Purpose of the Study:

• To investigate the unique binding properties of two closely related CBMs, CBM65A and CBM65B, from EcCel5A.
• To elucidate the structural basis for the specific recognition of xyloglucan by these CBMs.
• To understand the mechanism differentiating the binding of linear and mixed-linked β-glucans.

Main Methods:

• X-ray crystallography was used to determine the structures of CBM65A and CBM65B.
• Ligand binding assays were performed to assess the specificity of the CBMs for various β-glucans.
• Site-directed mutagenesis was employed to identify key residues involved in ligand binding.

Main Results:

• CBM65A and CBM65B exhibit a significant preference for xyloglucan compared to other β-glucans.
• The crystal structures reveal a β-sandwich fold where the ligand binding site is formed by the concave β-sheet.
• Five aromatic residues in the binding site mediate hydrophobic interactions with both the β-glucan backbone and the xylose side chains of xyloglucan, conferring specificity.
• CBM65A uses distinct polar residues (e.g., Gln(106)) for binding cellulose versus mixed-linked glucans.

Conclusions:

• CBM65A and CBM65B possess a unique mechanism for specifically targeting xyloglucan through hydrophobic interactions with its decorated side chains.
• These CBMs can differentiate between linear and mixed-linked β-glucans by utilizing specific polar and aromatic residues.
• The findings provide insights into the molecular basis of carbohydrate recognition and have implications for biomass deconstruction in biofuel production.