Structural basis for cellulose binding by the type A carbohydrate-binding module 64 of Spirochaeta thermophila

Spirochaeta thermophila secretes seven glycoside hydrolases for plant biomass degradation that carry a carbohydrate-binding module 64 (CBM64) appended at the C-terminus. CBM64 adsorbs to various β1-4-linked pyranose substrates and shows high affinity for cellulose. We present the first crystal structure of a CBM64 at 1.2 Å resolution, which reveals a jelly-roll-like fold corresponding to a surface-binding type A CBM. Modeling of its interaction with cellulose indicates that CBM64 achieves association with the hydrophobic face of β-linked pyranose chains via a unique coplanar arrangement of four exposed tryptophan side chains.