The Effects of Surfactant Pretreatment and Xylooligomers on Enzymatic Hydrolysis of Cellulose and Pretreated Biomass
There is a vital need to reduce enzyme costs to facilitate production of cost-competitive cellulosic ethanol and other products. End-product inhibition, declining substrate reactivity, enzyme denaturation, and non-productive binding of enzyme to lignin are among the factors that could account for the loss of enzyme effectiveness as enzymatic hydrolysis of lignocellulosic biomass proceeds. In addition, lignin and hemicelluloses, the other two most abundant components in lignocellulosic biomass besides cellulose, are believed to be responsible for high recalcitrance of lignocellulosic biomass to biological conversion. This study focused on investigating the effects of hemicelluloses on enzymatic hydrolysis and developing possible strategies to overcome their negative impacts. In the first part of the study, we identified that xylooligomers released from biomass during pretreatment and enzymatic hydrolysis are stronger inhibitors than long known for glucose and cellobiose. Furthermore, mixtures of xylooligomers of varying chain length were shown to dramatically decrease enzymatic conversion rates and yields of both pure cellulose and pretreated biomass. To clarify the relative importance of different xylooligomers, gel permeation chromatography (GPC) was applied to separate xylooligomers according to their chain length, and selected fractions were added to pure cellulose hydrolysis to determine their relative influence on rates and yields. The degree of inhibition was found to increase with xylooligomer degree of polymerization (DP), and cellulase exhibited a greater binding affinity for birchwood xylan than pure cellulose. All of these observations suggest that xylooligomers could competitively adsorb on cellulase, thereby reducing enzyme accessibility to cellulose and cellooligomers. Consequently, applying hemicellulase to hydrolyze xylooligomers to much less inhibitory xylose prior to adding cellulase reduced this undesirable binding and was more beneficial than adding hemicellulase and cellulase together.