(200d) The Mechanism of Inhibition On Cellulose Hydrolysis by Different Chain Length Xylooligomers

There is a vital need to reduce enzyme costs to facilitate production of cost-competitive cellulosic ethanol. 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. However, our recent results suggest that xylooligomers that are released from biomass during pretreatment and enzymatic hydrolysis are stronger inhibitors than long established for glucose and cellobiose. Furthermore, mixtures of xylooligomers of varying chain length have been 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 hydrolysis rates and yields. The degree of inhibition was found to increase with xylooligomers degree of polymerization (DP). Moreover, cellulase exhibited a greater binding affinity on birchwood xylan than on 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 was more beneficial than supplementing cellulase with hemicellulase.