Unraveling the multivalent binding of a marine family 6 carbohydrate-binding module with its native laminarin ligand.

UNLABELLED: Laminarin is an abundant brown algal storage polysaccharide. Marine microorganisms, such as Zobellia galactanivorans, produce laminarinases for its degradation, which are important for the processing of this organic matter in the ocean carbon cycle. These laminarinases are often modular, as is the case with ZgLamC which has an N-terminal GH16 module, a central family 6 carbohydrate-binding module (CBM) and a C-terminal PorSS module. To date, no studies have characterized a true marine laminarin-binding CBM6 with its natural carbohydrate ligand. The crystal structure of ZgLamCCBM6 indicates that this CBM has two clefts for binding sugar (variable loop site, VLS; and concave face site, CFS). The ZgLamCCBM6 VLS binds in an exo-manner and the CFS interacts in an endo-manner with laminarin. Isothermal titration calorimetry (ITC) experiments on native and mutant ZgLamCCBM6 confirm that these binding sites have different modes of recognition for laminarin, in agreement with the 'regional model' postulated for CBM6-binding modules. Based on ITC data and structural data, we propose a model of ZgLamCCBM6 interacting with different chains of laminarin in a multivalent manner, forming a complex cross-linked protein-polysaccharide network. DATABASE: PDB code 5FUI.