Binding specificity and thermodynamics of a family 9 carbohydrate-binding module from Thermotoga maritima xylanase 10A

The C-terminal family 9 carbohydrate-binding module of xylanase 10A from Th ermotoga maritima (CBM9-2) binds to amorphous cellulose, crystalline cellul ose, and the insoluble fraction of oat spelt xylan, The association constan ts (K-a) for adsorption to insoluble polysaccharides are 1 x 10(5) to 3 x 1 0(5) M-1. Of the soluble polysaccharides tested, CBM9-2 binds to barley bet a -glucan, xyloglucan, and xylan. CBM9-2 binds specifically to the reducing ends of cellulose and soluble polysaccharides, a property that is currentl y unique to this CBM. CBM9-2 also binds glucose, xylose, galactose, arabino se, cellooligosaccharides, xylooligosaccharides, maltose, and lactose, with affinities ranging from 10(3) M-1 for monosaccharides to 10(6) M-1 for dis accharides and oligosaccharides. Cellooligosaccharides longer than two gluc ose units do not bind with improved affinity, indicating that cellobiose is sufficient to occupy the entire binding site. In general, the binding reac tion is dominated by favorable changes in enthalpy, which are partially com pensated by unfavorable entropy changes.