Submitted to: Enzyme and Microbial Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/6/2009
Publication Date: 1/1/2010
Citation: Ximenes, E., Kim, Y., Mosier, N., Dien, B.S., Ladisch, M. 2010. Inhibition of cellulases by phenols. Enzyme and Microbial Technology. 46(3-4):170-176. Interpretive Summary: Commercialization of cellulosic ethanol is a national priority and is one strategy for reducing imported petroleum and reducing net greenhouse gas emissions. A major technical barrier to biomass conversion is the cost of enzymes required for converting the pretreated biomass into sugars that can be fermented into ethanol. Even under ideal conditions, over 50 times more enzyme is needed to convert cellulosic biomass compared to corn starch into sugars even under ideal conditions. One reason for this disparity is the sensitivity of the cellulolytic enzymes to various compounds released during the necessary pretreatment and during the enzymatic hydrolysis. This study identifies these inhibitory chemicals and measures the associated decreases of enzyme activity. Furthermore, the study uses the important potential biomass feedstock of distillers’ grains, which is generated at existing corn ethanol production facilities. As such, this paper should be of interest to a broad group of people interested in expanding feedstocks used for ethanol production beyond starch.
Technical Abstract: The inhibition of enzymes by the end products that they make is a well-known phenomenon. Another form of inhibition is manifested by the decrease in hydrolysis of pretreated cellulosic material as the concentration of solid biomass material increases, even though the ratio of enzyme to cellulose is kept constant. Our work builds upon that of Mandels, Wyman, Cantarella, Berlin, and others to identify inhibitors which may help to explain this phenomenon for the hydrolysis of untreated and pretreated distillers' grains (wet cake), and cellobiose. Wet cake is a useful model substrate due to its low lignin content with a structural carbohydrate content that is otherwise similar to that found in lignocellulosic biomass. Possible inhibition effects are associated with hemicelluloses and starch or their hydrolysis products, including phenolic compounds. These soluble phenolics can therefore be measured while minimizing effects of other known interferences from lignin itself. Pretreatment was by the liquid hot water method where the cellulosic feedstock (wet cake) was cooked in liquid stillage at 160 deg C at its saturation vapor pressure. Enzyme hydrolysis was by commercial cellulases containing cellobiohydrolases, endoglucanases, and ß-glucosidase. Inhibitory effects were a function of the microbial source of the ß-glucosidase. ß-glucosidase from Trichoderma reesei is more susceptible to inhibition by oat spelt xylan hydrolysis products than ß-glucosidase from Aspergillus niger. Cellulase mixtures containing endo- and exo-components are inhibited by xylan and pectin hydrolysates as well as by vanillin, syringaldehyde, trans-cinnamic acid, and hydroxybenzoic acid.