Lignin in enzymatic hydrolysis of lignocellulosic biomass: a double-edged sword

The interactions between lignin and cellulase enzymes play essential roles in the effective hydrolysis of lignocellulosic biomass.  Delignification has long been associated with pretreatment efficiency and substrate digestibility. However, recently we found that lignin is a double-edged sword. On the contrary to its negative role in limiting lignocellulosic hydrolysis, lignin can enhance the enzymatic hydrolysis significantly depending on its physicochemical structure. In this study, organosolv pretreated sweetgum (OPSG) were used to quantitatively elucidate the positive role of lignin on enzymatic hydrolysis. We observed that the addition of extractable lignin (from organosolv process) increased the 72-h hydrolysis yield of OPSG remarkably by 19% under 10 FPU, while the initial hydrolysis rate was kept the same at 1.33 g/L/h. Furthermore, we examined the enzymes distribution during the hydrolysis. The free enzymes in solution increased from 0.05 to 0.07 mg/mL at the end of hydrolysis. This suggested that extractable lignin could potentially reduce the non-productive binding and facilitate enzymatic hydrolysis. In addition, we removed bulk lignin in OPSG from 11% to 4% by NaClO₂, and then compared the initial rate and the 72-h hydrolysis yield.  The 72-h hydrolysis yield increased from 70% to 91% and the initial rate increased by 65%. It indicated that bulk lignin limited both the initial rate and the final hydrolysis yield significantly. The results of our work suggested extractable lignin and bulk lignin play distinct roles in enzymatic hydrolysis and turning bulk lignin to extractable lignin in pretreatment has great potential to improve the effectiveness of enzymatic hydrolysis.