Structure of a mannan-specific family 35 carbohydrate-binding module: evidence for significant conformational changes upon ligand binding

Enzymes that digest plant cell wall polysaccharides generally contain non-catalytic, carbohydrate-binding modules (CBMs) that function by attaching the enzyme to the substrate, potentiating catalytic activity. Here, we present the first structure of a family 35 CBM, derived from the Cellvibrio japonicus beta-1,4-mannanase Man5C. The NMR structure has been determined for both the free protein and the protein bound to mannopentaose. The data show that the protein displays a typical beta-jelly-roll fold. Ligand binding is not located on the concave surface of the protein, as occurs in many CBMs that display the jelly-roll fold, but is formed by the loops that link the two beta-sheets of the protein, similar to family 6 CBMs. In contrast to the majority of CBMs, which are generally rigid proteins, CBM35 undergoes significant conformational change upon ligand binding. The curvature of the binding site and the narrow binding cleft are likely to be the main determinants of binding specificity. The predicted solvent exposure of O6 at several subsites provides an explanation for the observed accommodation of decorated mannans. Two of the key aromatic residues in Man5C-CBM35 that interact with mannopentaose are conserved in mannanase-derived CBM35s, which will guide specificity predictions based on the primary sequence of proteins in this CBM family.