Revealing a Stimulatory Effect of Lignin in Enzymatic Hydrolysis of Lignocellulose

The electrostatic and hydrophobic interactions between cellulases and lignin play essential roles in enzymatic hydrolysis of lignocellulsoic substrates for biochemical conversion of biomass to fuels and chemicals. The objective of this study was to identify an appropriate parameter to elucidate both inhibitory and stimulatory effects of organosolv lignins on enzymatic hydrolysis. Remarkably, we observed contrasting effects of hardwood organosolv lignin (EOL-SG) and softwood organosolv lignin (EOL-LP) on enzymatic hydrolysis of Avicel and pretreated substrates. The addition of EOL-SG significantly improved the 72 h hydrolysis yields of organosolv pretreated sweetgum (OPSG) and loblolly pine (OPLP) from 49.3% to 68.6% and from 41.2% to 60.8%, respectively. On the contrary, the addition of EOL-LP (8 g/L) decreased the 72 h hydrolysis yields of OPSG and OPLP to 42.0% and 38.1%, respectively. A strong correlation between the distribution coefficients of cellulase enzymes on lignins and the changes of hydrolysis yields indicated that the inhibitory or stimulatory effects of organosolv lignins on enzymatic hydrolysis were governed by the distribution coefficients (R). The different R values of EOL-SG and EOL-LP probably were related to the electrostatic interactions, hydrophobic interactions and hydrogen bondings between enzymes and lignin, which in turn were controlled by the physical and chemical natures of lignin. Our study suggested that organosolv lignins with low affinity to enzyme could be used as additives to enhance enzymatic hydrolysis.