Structural and Thermodynamic Dissection of Specific Mannan Recognition by a Carbohydrate Binding Module, TmCBM27

The C-terminal 176 amino acids of a Thermotoga maritima mannanase (Man5) constitute a carbohydrate binding module (CBM) that has been classified into CBM family 27. The isolated CBM27 domain, named Tm CBM27, binds tightly (K a s 10 5 –10 6 M −1 ) to β-1, 4-mannooligosaccharides, carob galactomannan, and konjac glucomannan, but not to cellulose (insoluble and soluble) or soluble birchwood xylan. The X-ray crystal structures of native Tm CBM27, a Tm CBM27-mannohexaose complex, and a Tm CBM27-6 3 ,6 4 -α-D-galactosyl-mannopentaose complex at 2.0 A, 1.6 A, and 1.35 A, respectively, reveal the basis of Tm CBM27's specificity for mannans. In particular, the latter complex, which is the first structure of a CBM in complex with a branched plant cell wall polysaccharide, illustrates how the architecture of the binding site can influence the recognition of naturally substituted polysaccharides.