Family 42 carbohydrate-binding modules display multiple arabinoxylan-binding interfaces presenting different ligand...

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Enzymes degrading plant cell walls have modular structures with catalytic and carbohydrate-binding modules (CBMs).
• CBMs are classified into 59 families in the Carbohydrate-Active enZYme (CAZy) database.
• Family 42 CBMs from Clostridium thermocellum are investigated for their role in plant cell wall degradation.

Purpose of the Study:

• To determine the crystal structure of CtCBM42A.
• To biochemically characterize CtCBM42B and CtCBM42C.
• To elucidate the binding mechanisms of CBM42 family members to plant polysaccharides.

Main Methods:

• X-ray crystallography was used to determine the structure of CtCBM42A.
• Biochemical assays were performed to characterize the binding properties of CtCBM42B and CtCBM42C.
• Comparative analysis of CBM42 binding mechanisms with homologous CBM13s was conducted.

Main Results:

• The crystal structure of CtCBM42A revealed a beta-trefoil fold with three sub-domains (alpha, beta, gamma), each possessing a potential carbohydrate-binding pocket.
• CtCBM42A, CtCBM42B, and CtCBM42C specifically bind to arabinose side-chains of arabinoxylans and arabinan.
• The gamma sub-domain of CtCBM42A is crucial for arabinoxylan binding, while CtCBM42B and CtCBM42C require the combined action of beta and gamma sub-domains for ligand binding.

Conclusions:

• CBM42 members target specific regions of the plant cell wall rich in arabinose side-chains.
• The binding mechanism of CBM42 family members differs from CBM13, highlighting diverse strategies for polysaccharide recognition.
• Structural and biochemical insights into CBM42 provide a foundation for understanding plant cell wall degradation by cellulosomal enzymes.