Access of cellulase to cellulose and lignin for poplar solids produced by leading pretreatment technologies

Abstract

Adsorption of cellulase on solids resulting from pretreatment of poplar wood by ammonia fiber expansion (AFEX), ammonia recycled percolation (ARP), controlled pH, dilute acid (DA), flowthrough (FT), lime, and sulfur dioxide (SO(2)) and pure Avicel glucan was measured at 4 degrees C, as were adsorption and desorption of cellulase and adsorption of beta-glucosidase for lignin left after enzymatic digestion of the solids from these pretreatments. From this, Langmuir adsorption parameters, cellulose accessibility to cellulase, and the effectiveness of cellulase adsorbed on poplar solids were estimated, and the effect of delignification on cellulase effectiveness was determined. Furthermore, Avicel hydrolysis inhibition by enzymatic and acid lignin of poplar solids was studied. Flowthrough pretreated solids showed the highest maximum cellulase adsorption capacity (sigma(solids) = 195 mg/g solid) followed by dilute acid (sigma(solids) = 170.0 mg/g solid) and lime pretreated solids (sigma(solids) = 150.8 mg/g solid), whereas controlled pH pretreated solids had the lowest (sigma(solids) = 56 mg/g solid). Lime pretreated solids also had the highest cellulose accessibility (sigma(cellulose) = 241 mg/g cellulose) followed by FT and DA. AFEX lignin had the lowest cellulase adsorption capacity (sigma(lignin) = 57 mg/g lignin) followed by dilute acid lignin (sigma(lignin) = 74 mg/g lignin). AFEX lignin also had the lowest beta-glucosidase capacity (sigma(lignin) = 66.6 mg/g lignin), while lignin from SO(2) (sigma(lignin) = 320 mg/g lignin) followed by dilute acid had the highest (301 mg/g lignin). Furthermore, SO(2) followed by dilute acid pretreated solids gave the highest cellulase effectiveness, but delignification enhanced cellulase effectiveness more for high pH than low pH pretreatments, suggesting that lignin impedes access of enzymes to xylan more than to glucan, which in turn affects glucan accessibility. In addition, lignin from enzymatic digestion of AFEX and dilute acid pretreated solids inhibited Avicel hydrolysis less than ARP and flowthrough lignin, whereas acid lignin from unpretreated poplar inhibited enzymes the most. Irreversible binding of cellulase to lignin varied with pretreatment type and desorption method.

Cellulase adsorption and relationship to features of corn stover solids produced by leading pretreatments

“…The greater cellulose accessibility for AFEX pretreated solids suggests that disruption of lignincarbohydrates linkages plays a greater role in enhancing accessibility than xylan/lignin removal. However, consistent with another finding (Kumar and Wyman, 2009b), the data here for selective lignin removal supports the hypothesis that lignin controls xylan accessibility, which consequently does not have a major impact on cellulase adsorption. The high cellulose accessibility for AFEX and lime pretreated solids could account for their good glucose release reported in the literature at cellulase loadings of 15 and 60 FPU (Wyman et al, 2005a).…”

“…Consistent with the findings here, another study (Kumar and Wyman, 2009b) showed that delignification of solids with a xylan content of <3% resulting from pretreatments at lower pH with dilute acid and SO 2 had a negligible impact on glucose release. On the other hand, delignification of solids produced by high pH pretreatments, such as AFEX and ARP, that contained a significant amount of xylan enhanced glucose release.…”

“…Consequently, several studies have evaluated corn stover pretreatment and enzymatic digestion (Kaar and Holtzapple, 2000;Wyman, 2003, 2004a,b;Lloyd and Wyman, 2005;Yang and Wyman, 2004). However, although cellulase adsorption onto solids is the primary step for enzymatic saccharification of pretreated corn stover Lynd et al, 2002;Wyman et al, 2005b;Zhang and Lynd, 2004) and the hydrolysis rate and/or yield is often claimed to be directly related to the amount of adsorbed enzymes (Jeoh et al, 2007;Karlsson et al, 1999;Kotiranta et al, 1999;Kumar and Wyman, 2009b), little information is available on substrate-enzyme interactions, including cellulase adsorption kinetics and the accessibility of enzyme to lignin and cellulose in pretreated biomass. Similarly, although hemicellulolytic enzymes must adsorb on the solid surface prior to enzymatic hydrolysis (Kumar and Wyman, 2009e;Sun et al, 1998;Zilliox and Debeire, 1998), the complexity of the hemicellulose structure and the array of enzymes involved have resulted in limited studies of hemicellulolytic enzymes-substrate interactions for real biomass.…”

Structural Characteristics and Distribution of Lignin in Eucalyptus Globulus Pulps Obtained by a Combined Autohydrolsis/Alkaline Extraction Process for Enzymatic Saccharification of Cellulose

“…In previous studies, it was observed that the lignin content is correlated with cellulose accessibility, being a barrier to the enzymatic hydrolysis, by blocking the accessibility of the enzymes to the cellulose microfibrils 12,13 . Lignin also tends to irreversibly bind the enzymes through hydrophobic interactions causing a loss in cellulases activities 14,15,16 .In addition, during the pretreatment step lignin suffer changes in its structure and the effects on the enzymatic hydrolysis of cellulose are not completely understood 17,18 .…”

Continuous Alkali-Cellulase Processing of Corn Stover to Glucose for Bioethanol

“…Minimal cellulase loading that allows sufficient binding to cellulose substrate is a goal that some have begun to study (Klein-Marcuschamer et al 2012). An optimum temperature for adsorption has been explored with conflicting results (Kumar and Wyman 2009). To minimize production of glucose in the enzyme bath, small bath volumes and optimum temperature were evaluated.…”