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 TmCBM27, binds tightly (K(a)s 10(5)-10(6) M-1) to beta(-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 TmCBM27, a TmCBM27-mannohexaose complex, and a TmCBM27-6(3), 6(4)-alpha-D-galactosyl-mannopentaose complex at 2.0 Angstrom, 1.6 Angstrom, and 1.35 Angstrom, respectively, reveal the basis of TmCBM27'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.