Commercial fungal cellulases used in biomass-to-biofuels processes can be grouped into three general classes: native, augmented, and engineered. Colorimetric assays for general glycoside hydrolase activities showed distinct differences in enzyme binding to lignin for each enzyme activity. Native cellulase preparations demonstrated low binding of endo- and exocellulases, high binding of xylanase, and moderate binding for β-D-glucosidases. Engineered cellulase formulations exhibited low binding of exocellulases, very strong binding of endocellulases and β-D-glucosidase, and mixed binding of xylanase activity. The augmented cellulase had low binding of exocellulase, high binding of endocellulase and xylanase, and moderate binding of β-D-glucosidase activities. Bound and unbound activities were correlated to general molecular weight ranges of proteins as measured by loss of proteins bands in bound fractions on SDS-PAGE gels. Lignin-bound high molecular weight bands correlated to binding of β-D-glucosidase activity. Whereas β-D-glucosidases demonstrated high binding in many cases, they have been shown to remain active. Bound low molecular weight bands correlated to xylanase activity binding. Contrary to other literature, exocellulase activity did not show strong lignin binding. The variation in enzyme activity binding between these three classes of cellulases preparations indicates that it is possible to alter the binding of specific glycoside hydrolase activities during the enzyme formulation process. It remains unclear whether or not loss of endocellulase activity to lignin binding is problematic for biomass conversion.
Keywords: Lignin; cellulase; protein binding; xylanase; β-D-glucosidase.