Which components in the water soluble faction inhibit enzymatic hydrolysis of steam pretreated biomass and how can we ameliorate this inhibition

Any biomass pretreatment, such as steam explosion, which is used to enhance accessibility to the cellulose will solubilize a certain amount of hemicellulose. To achieve high sugar concentrations it would be desirable to use the whole pretreated slurry, including both the cellulose and hemicellulose components. However, this whole slurry typically contains cellulase inhibitors (e.g. soluble mono/oligomeric sugars and phenolics) which limit the efficiency of enzyme hydrolysis of the cellulose. Our previous work has shown that hemicellulose derived monosaccharides are the major inhibitors, and interestingly, exhibit a different mode of inhibition than does glucose. To try to better understand the mechanism of sugar inhibition, the influence of various hemicellulose-derived monomeric sugars on the rate and extent of cellulose hydrolysis were assessed. The major enzyme activities (exo/endo-glucanase, β-glucosidase, xylanase activities, etc.) and their kinetics (adsorption/desorption, productive binding, processive moving) were extensively investigated using enzyme mixtures, purified enzyme monocomponents or individual enzyme binding/catalytic domains. We also evaluated how the presence of higher concentrations of sugars could influence major substrate characteristics such as accessibility, surface charge/morphology, crystallinity, degree of polymerization, etc., during hydrolysis. Those substrate characteristics that were most influenced by sugar inhibition were identified.  Potential solutions to deal with the high sugar inhibition of cellulose hydrolysis will be described.