Extra tyrosine in the carbohydrate-binding module of <i>Irpex lacteus</i> Xyn10B enhances its cellulose-binding ability

The xylanase (Xyn10B) that strongly adsorbs on microcrystalline cellulose was isolated from Driselase. The Xyn10B contains a Carbohydrate-binding module family 1 (CBM1) (<i>Irp</i>CBM_Xyn10B) at N-terminus. The canonical essential aromatic residues required for cellulose binding were conserved in <i>Irp</i>CBM_Xyn10B; however, its adsorption ability was markedly higher than that typically observed for the CBM1 of an endoglucanase from <i>Trametes hirsuta</i> (<i>Th</i>CBM_EG1). An analysis of the CBM-GFP fusion proteins revealed that the binding capacity to cellulose (7.8 μmol/g) and distribution coefficient (2.0 L/μmol) of <i>Irp</i>CBM_Xyn10B-GFP were twofold higher than those of <i>Th</i>CBM_EG1-GFP (3.4 μmol/g and 1.2 L/μmol, respectively), used as a reference structure. Besides the canonical aromatic residues (W24-Y50-Y51) of typical CBM1-containing proteins, <i>Irp</i>CBM_Xyn10B had an additional aromatic residue (Y52). The mutation of Y52 to Ser (<i>Irp</i>CBM_Y52S-GFP) reduced these adsorption parameters to 4.4 μmol/g and 1.5 L/μmol, which were similar to those of <i>Th</i>CBM_EG1-GFP. These results indicate that Y52 plays a crucial role in strong cellulose binding.