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영어

Plant expansins are capable of inducing plant cell wall extension and disrupting cellulose. BsEXLX1 found in Bacillus subtilis is a bacterial expansin, a structural homolog to a plant expansin ZmEXPB1. In our previous study, BsEXLX1was found to possess synergism with cellulase in the enzymatic hydrolysis of cellulose (Kim, E. S. et al., 2009, Biotechnol. Bioeng. 102(5):1342-1353). The thermodynamic analysis of binding of BsEXLX1 to micrcrystalline cellulose (e.g., Avicel) revealed that the binding mode of BsEXLX1 to Avicel was similar to those of other Type A surface-binding carbohydrate binding modules (CBMs). BsEXLX1 bound to Avicel in an entropy-driven mode possibly due to the increased mobility of water molecules released from the interface between the protein and the substrate. BsEXLX1 did not bind to soluble cellooligosaccharides, which was also similar to the behavior of other Type A CBMs. In addition, cellulose and xylan had much lower binding for BsEXLX1 than for CtCBD3, a Type A CBM. When the binding studies of BsEXLX1 were performed against pretreated or unpretreated Miscanthus x giganteus using CtCBD3, the amounts of BsEXLX1 bound to lignin-rich substrates were much higher compared to those of CtCBD3. Also, a binding competition assay demonstrated BsEXLX1 binding decreased in the presence of CtCBD3 against Avicel but not against alkali lignin. The mutagenesis of each of the three hydrophobic amino acid residues on the binding domain on the C-terminus of BsEXLX1 that are presumed to be responsible for cellulose binding did not affect the lignin binding activity but substantially decreased the cellulose binding activity. The findings in this work suggest that BsEXLX1 could be industrially applicable as a lignin blocker during the enzymatic hydrolysis of lignocellulose.