10-01: The role of softwood and cane lignins in enzymatic hydrolysis

The efficient use of cellulases in hydrolysis of pretreated lignocellulosic biomass is limited due to the presence of lignin. Lignin is reported to be covalently bound to hemicelluloses and to form lignin-carbohydrate complexes. Lignin is also known to bind hydrolytic enzymes non-specifically, thus reducing their action on carbohydrate substrates. The composition and location of residual lignin seem therefore to be important for optimizing the enzymatic hydrolysis of lignocellulosic substrates. The use of lignin modifying enzymes may have potential in the modification or partial removal of lignin from the biomass.

Laccases are well known for the modification of lignin by oxidative reactions. Previously, it has been shown that the response of lignin to the hydrolysis of lignocellulose varies depending on the structure and origin of the lignin. In this study, the effect of lignin modification by laccase on the hydrolysis of pre-treated spruce and giant cane (Arundo donax) was evaluated. The substrates were first treated with laccase and then hydrolyzed with commercial cellulases. With spruce, laccase modification improved the hydrolysis yield but surprisingly with giant cane, laccase had an adverse effect. The binding properties of cellulases on the untreated and laccase treated lignins were further studied on isolated lignins. The laccase treatment reduced the binding of enzymes on modified spruce lignin, whereas on giant cane lignins the differences were not as clear. Further research on the structural differences of the lignins to understand the underlying mechanisms is ongoing.