Analysis of binding of the family 2a carbohydrate-binding module from Cellulomonas fimi xylanase 10A to cellulose: specificity and identification of functionally important amino acid residues

The family 2a carbohydrate-binding module (CBM2a) of xylanase 10A from Cell ulomonas fimi binds to the crystalline regions of cellulose, It does not sh are binding sites with the N-terminal family 4 binding module (CBM4-1) from the cellulase 9B from C. fimi, a module that binds strictly to soluble sug ars and amorphous cellulose, The binding of CBM2a to crystalline matrices i s mediated by several residues on the binding face, including three promine nt, solvent-exposed tryptophan residues. Binding to crystalline cellulose w as analyzed by making a series of conservative (phenylalanine and tyrosine) and non-conservative substitutions (alanine) of each solvent-exposed trypt ophan (W17, W54 and W72). Other residues on the binding face with hydrogen bonding potential were substituted with alanine. Each tryptophan plays a di fferent role in binding; a tryptophan is essential at position 54, a tyrosi ne or tryptophan at position 17 and any aromatic residue at position 72, Ot her residues on the binding face, with the exception of N15, are not essent ial determinants of binding affinity. Given the specificity of CBM2a, the s tructure of crystalline cellulose and the dynamic nature of the binding of CBM2a, we propose a model for the interaction between the polypeptide and t he crystalline surface.