Clostridium thermocellum Xyn10B carbohydrate-binding module 22-2: The roleof conserved amino acids in ligand binding

The majority of plant cell wall hydrolases are modular enzymes which, in ad dition to a catalytic module, possess one or more carbohydrate-binding modu les (CBMs). These carbohydrate-active enzymes and their constituent modules have been classified into a number of families based upon amino acid seque nce similarity. The Clostridium thermocellum xylanase, Xyn10B, contains two CBMs that belong to family 22 (CBM22). The crystal structure of the C-term inal CBM22 (CBM22-2) was determined in a previous study [Charnock, S. J., e t al. (2000) Biochemistry 39, 5013-5021] and revealed a surface cleft which presents several conserved residues that are implicated in ligand binding. These amino acids have been substituted and the structure and biochemical properties of the mutants analyzed. The data show that R25A, W53A, Y103A, Y 136A, and E138A exhibit greatly reduced affinity for xylotetraose relative to that of the wild-type protein. Conversely, mutations Y103F and Y136F hav e little effect on ligand binding. Using thermodynamic, X-ray, and NMR meas urements on the mutants, we show that the cleft of CBM22-2 does indeed form the ligand-binding site. Trp 53 and Tyr 103 most likely participate in hyd rophobic stacking interactions with the ligand, while Glu 138 makes one or more important hydrogen bonds with the tetrasaccharide. Although Arg 25 and Tyr 136 are likely to form hydrogen bonds with the ligand, they are also s hown to play a critical role in maintaining the structural integrity of the binding cleft.