17-06: Strong Cellulase Inhibition by Non-Xylan Plant Hemicelluloses and Their Oligomers

Despite advances in pretreatments and reductions in enzymes cost, cellulosic biomass derived fuels via biological route are still expensive to be commercially viable. It appears that “no pretreatment will be effective enough” unless the highly active, inhibitors tolerant, and feedstocks agnostics enzymes cocktails are available at reasonable cost to overcome the cellulosic biomass macro/micro-level recalcitrance. Among inhibitors, non-lignin plant derived compounds released in pretreatment and/or enzymatic conversion can strongly retard cellulase action. In previous studies from our lab, it was shown that hemicellulose xylan and its oligomers (XOs) strongly inhibit cellulase activity, and cellulase inhibition by oligomers increased with chain length. Furthermore, oligomers were stronger inhibitors than xylan polymer or cellobiose at similar concentrations.  In this study, it was found that some non-xylan plant hemicelluloses even at a loading as low as 1.0 g/L inhibited cellulase activity for pure Avicel^® PH101 cellulose hydrolysis by more than 65%.  The inhibition increased significantly with hemicellulose loading, with up to a 90% drop in 120 h enzymatic conversion at 15 mg Accellerase^®1500 protein/g glucan.  Furthermore, the slurry containing these hemicellulose compounds required more than 6-8 times the amount of cellulase protein than for the control to realize the same conversions.  However, polymers of some hemicelluloses appeared to be stronger inhibitors than their oligomers. Further, interactions of these hemicellulose compounds with pure cellulase components were studied to understand the cause and mechanism for such strong inhibition, which would help in developing better enzyme cocktails and process strategies for effective conversion.