Binding Specificity and Thermodynamics of a Family 9 Carbohydrate-Binding Module from Thermotoga maritima Xylanase 10A

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Abstract
The C-terminal family 9 carbohydrate-binding module of xylanase 10A from Thermotoga maritima (CBM9-2) binds to amorphous cellulose, crystalline cellulose, and the insoluble fraction of oat spelt xylan. The association constants (Ka) for adsorption to insoluble polysaccharides are 1 × 105 to 3 × 105 M-1. Of the soluble polysaccharides tested, CBM9-2 binds to barley β-glucan, xyloglucan, and xylan. CBM9-2 binds specifically to the reducing ends of cellulose and soluble polysaccharides, a property that is currently unique to this CBM. CBM9-2 also binds glucose, xylose, galactose, arabinose, cellooligosaccharides, xylooligosaccharides, maltose, and lactose, with affinities ranging from 103 M-1 for monosaccharides to 106 M-1 for disaccharides and oligosaccharides. Cellooligosaccharides longer than two glucose units do not bind with improved affinity, indicating that cellobiose is sufficient to occupy the entire binding site. In general, the binding reaction is dominated by favorable changes in enthalpy, which are partially compensated by unfavorable entropy changes.