7-73: Kinetic and comparative study of binding of a bacterial expansin (BsEXLX1) to cellulose and pretreated lignocellulose

Plant expansin proteins that induce plant cell wall extension showed synergistic activity with cellulase during cellulose hydrolysis. BsEXLX1 originating from Bacillus subtilis is a structural homolog of a β-expansin produced by Zea mays. The Langmuir isotherms of the binding of BsEXLX1 to Avicel demonstrated that the binding mode of BsEXLX1 to Avicel was similar to those of other Type A surface-binding carbohydrate binding modules (CBMs) to microcrystalline cellulose, and it was an entropy-driven binding similar to a Type A CBM, CBD_Cex, binding to bacterial microcrystalline cellulose. BsEXLX1 did not bind cellooligosaccharides, like the typical Type A CBMs. In addition, cellulose and xylan had much lower binding capacities for BsEXLX1 than for the Type A CBM, CtCBD3. The binding characterization of BsEXLX1 was also carried using various pretreated or unpretreated Miscanthus x giganteus and CtCBD3. The amounts of BsEXLX1 bound to lignin-rich substrates such as alkali lignin and acid-pretreated, cellulase-digested M. x. giganteus were much higher than those of CtCBD3. A binding competition assay between BsEXLX1 and CtCBD3 revealed that the binding activity of BsEXLX1 to Avicel was reduced by the presence of CtCBD3. However, the presence of CtCBD3 did not affect the binding of BsEXLX1 to alkali lignin. These results imply the preferential binding of BsEXLX1 to lignin. Therefore, the bacterial expansin could be used as a lignin blocker in the enzymatic hydrolysis of lignocellulose.