Design of a pH-dependent cellulose-binding domain

Area of Science:

• Biochemistry
• Protein Engineering
• Carbohydrate Binding

Background:

• Protein-carbohydrate interactions often involve aromatic amino acids like tyrosine, phenylalanine, and tryptophan interacting with sugar rings.
• The cellulose-binding domain (CBD) of Cel7A cellobiohydrolase from Trichoderma reesei typically binds crystalline cellulose via tyrosine residues and is pH-insensitive.
• Histidine residues are rarely implicated in such binding interactions.

Purpose of the Study:

• To investigate the role of histidine residues in mediating protein-carbohydrate binding interactions.
• To determine if histidine protonation influences the pH-dependency of binding.
• To engineer histidine residues into the Cel7A CBD and assess their impact on cellulose binding.

Main Methods:

• Site-directed mutagenesis was employed to replace tyrosine residues with histidine in the Cel7A CBD.
• The binding affinity of wild-type and mutant CBDs to crystalline cellulose was evaluated across different pH conditions.

Main Results:

• All engineered histidine mutants displayed a significant pH-dependency in their cellulose binding, unlike the wild-type.
• While mutant binding at optimal pH was generally lower than the wild-type, the Y31H mutant showed binding levels comparable to the wild-type.
• The introduction of histidine residues fundamentally altered the binding characteristics, making them sensitive to pH.

Conclusions:

• Histidine residues can mediate pH-dependent protein-carbohydrate interactions, a departure from the typical role of tyrosine.
• Engineering histidine into the Cel7A CBD demonstrates a novel mechanism for modulating cellulose-binding affinity through pH sensitivity.
• This research opens avenues for designing carbohydrate-binding proteins with tunable binding properties.