Strong cellulase inhibitors from the hydrothermal pretreatment of wheat straw

A critical assessment on scalable technologies using high solids loadings in lignocellulose biorefinery: challenges and solutions

The pretreatment and the enzymatic saccharification are the key steps in the extraction of fermentable sugars for further valorization of lignocellulosic biomass (LCB) to biofuels and value-added products via biochemical and/or chemical conversion routes. Due to low density and high-water absorption capacity of LCB, the large volume of water is required for its processing. Integration of pretreatment, saccharification, and co-fermentation has succeeded and well-reported in the literature. However, there are only few reports on extraction of fermentable sugars from LCB with high biomass loading (>10% Total solids-TS) feasible to industrial reality. Furthermore, the development of enzymatic cocktails can overcome technology hurdles with high biomass loading. Hence, a better understanding of constraints involved in the development of technology with high biomass loading can result in an economical and efficient yield of fermentable sugars for the production of biofuels and bio-chemicals with viable titer, rate, and yield (TRY) at industrial scale. The present review aims to provide a critical assessment on the production of fermentable sugars from lignocelluloses with high solid biomass loading. The impact of inhibitors produced during both pretreatment and saccharification has been elucidated. Moreover, the limitations imposed by high solid loading on efficient mass transfer during saccharification process have been elaborated.

Biosynthesis of poly(β-L-malic acid) from rubberwood enzymatic hydrolysates in co-fermentation by Aureobasidium pullulans

Co-fermentation of multiple substrates has emerged as the most effective method to improve the yield of bioproducts. Herein, sustainable rubberwood enzymatic hydrolysates (RWH) were co-fermented by Aureobasidium pullulans to produce poly(β-L-malic acid) (PMA), and RWH + glucose/xylose was also investigated as co-substrates. Owing to low inhibitor concentration and abundant natural nitrogen source content of RWH, a high PMA yield of 0.45 g/g and a productivity of 0.32 g/L/h were obtained by RWH substrate fermentation. After optimization, PMA yields following the fermentation of RWH + glucose and RWH + xylose reached 59.92 g/L and 53.71 g/L, respectively, which were 52 % and 36 % higher than that after the fermentation of RWH. RWH + glucose more significantly affected the correlation between PMA yield and substrate concentration than RWH + xylose. The results demonstrated that the co-fermentation of RWH co-substrate is a promising method for the synthesis of bioproducts.

Enhancing of pretreatment on high solids enzymatic hydrolysis of food waste: Sugar yield, trimming of substrate structure

The development of high solids enzymatic hydrolysis (HSEH) technology is a promising way to improve the efficiency of bioenergy production from solid waste. Pretreatment methods such as ultrasound (USP), freeze-thaw (FTP), hydrothermal (HTP), and dried (DRD) were carried out to evaluate the effect and mechanism of the pretreatment methods on the HSEH of FW. The reducing sugar of HTP and DRD reached 94.75% and 94.92% of the theoretical value. HTP and DRD could reduce the crystallinity of FW. DRD resulted in lower alignment and the occurrence of fractures of the substrate and exposed the α-1,4 glycosidic bond of starch. The high destructive power of HTP and DRD reduced the obstacles caused by the high solid content. Moreover, DRD consumed only 27.62% of the total energy of HTP. DRD could be a promising pretreatment methods for glucose recovery for its high product yield, significant substrate destruction, and economic feasibility.

Advances and perspectives on mass transfer and enzymatic hydrolysis in the enzyme-mediated lignocellulosic biorefinery: A review

Enzymatic hydrolysis is a critical process for the cellulase-mediated lignocellulosic biorefinery to produce sugar syrups that can be converted into a whole range of biofuels and biochemicals. Such a process operating at high-solid loadings (i.e., scarcely any free water or roughly ≥ 15% solids, w/w) is considered more economically feasible, as it can generate a high sugar concentration at low operation and capital costs. However, this approach remains restricted and incurs "high-solid effects", ultimately causing the lower hydrolysis yields with increasing solid loadings. The lack of available water leads to a highly viscous system with impaired mixing that exhibits strong transfer resistance and reaction limitation imposed on enzyme action. Evidently, high-solid enzymatic hydrolysis involves multi-scale mass transfer and multi-phase enzyme reaction, and thus requires a synergistic perspective of transfer and biotransformation to assess the interactions among water, biomass components, and cellulase enzymes. Porous particle characteristics of biomass and its interface properties determine the water form and distribution state surrounding the particles, which are summarized in this review aiming to identify the water-driven multi-scale/multi-phase bioprocesses. Further aided by the cognition of rheological behavior of biomass slurry, solute transfer theories, and enzyme kinetics, the coupling effects of flow-transfer-reaction are revealed under high-solid conditions. Based on the above basic features, this review lucidly explains the causes of high-solid hydrolysis hindrances, highlights the mismatched issues between transfer and reaction, and more importantly, presents the advanced strategies for transfer and reaction enhancements from the viewpoint of process optimization, reactor design, as well as enzyme/auxiliary additive customization.

Alicyclobacillus mali FL18 as a Novel Source of Glycosyl Hydrolases: Characterization of a New Thermophilic β-Xylosidase Tolerant to Monosaccharides

A thermo-acidophilic bacterium, Alicyclobacillus mali FL18, was isolated from a hot spring of Pisciarelli, near Naples, Italy; following genome analysis, a novel putative β-xylosidase, AmβXyl, belonging to the glycosyl hydrolase (GH) family 3 was identified. A synthetic gene was produced, cloned in pET-30a(+), and expressed in Escherichia coli BL21 (DE3) RIL. The purified recombinant protein, which showed a dimeric structure, had optimal catalytic activity at 80 °C and pH 5.6, exhibiting 60% of its activity after 2 h at 50 °C and displaying high stability (more than 80%) at pH 5.0-8.0 after 16 h. AmβXyl is mainly active on both para-nitrophenyl-β-D-xylopyranoside (K_M 0.52 mM, k_cat 1606 s^-1, and k_cat/K_M 3088.46 mM^-1·s^-1) and para-nitrophenyl-α-L-arabinofuranoside (K_M 10.56 mM, k_cat 2395.8 s^-1, and k_cat/K_M 226.87 mM^-1·s^-1). Thin-layer chromatography showed its ability to convert xylooligomers (xylobiose and xylotriose) into xylose, confirming that AmβXyl is a true β-xylosidase. Furthermore, no inhibitory effect on enzymatic activity by metal ions, detergents, or EDTA was observed except for 5 mM Cu²⁺. AmβXyl showed an excellent tolerance to organic solvents; in particular, the enzyme increased its activity at high concentrations (30%) of organic solvents such as ethanol, methanol, and DMSO. Lastly, the enzyme showed not only a good tolerance to inhibition by xylose, arabinose, and glucose, but was activated by 0.75 M xylose and up to 1.5 M by both arabinose and glucose. The high tolerance to organic solvents and monosaccharides together with other characteristics reported above suggests that AmβXyl may have several applications in many industrial fields.

The application of green solvent in a biorefinery using lignocellulosic biomass as a feedstock

The high dependence on crude oil for energy utilization leads to a necessity of finding alternative sustainable resources. Solvents are often employed in valorizing the biomass into bioproducts and other value-added chemicals during treatment stages. Unfortunately, despite the effectiveness of conventional solvents, hindrances such as expensive solvents, unfavourable environmental ramifications, and complicated downstream separation systems often occur. Therefore, the scientific community has been actively investigating more cost-effective, environmentally friendly alternatives and possess the excellent dissolving capability for biomass processing. Generally, 'green' solvents are attractive due to their low toxicity, economic value, and biodegradability. Nonetheless, green solvents are not without disadvantages due to their complicated product recovery, recyclability, and high operational cost. This review summarizes and evaluates the recent contributions, including potential advantages, challenges, and drawbacks of green solvents, namely ionic liquids, deep eutectic solvents, water, biomass-derived solvents and carbon dioxide in transforming the lignocellulosic biomass into high-value products. Moreover, research opportunities for future developments and potential upscale implementation of green solvents are also critically discussed.

Chemical and Enzymatic Synthesis of Biobased Xylo-Oligosaccharides and Fermentable Sugars from Wheat Straw for Food Applications

Xylo-oligosaccharides are sugar oligomers with 2~7 xylose units considered non-digestible fibers that can be produced from biodegradable and low-cost biomass like wheat straw. An integrated approach consisting of hydrothermal pretreatment, alkaline treatment, enzymatic treatment and the combinations thereof was applied to overcome the recalcitrance structure of the wheat straw and allow selective fractioning into fermentable sugars and xylo-oligosaccharides. The hydrolysates and processed solids were chemically characterized by High-performance liquid chromatography and Ion chromatography, and the results were expressed as function of the severity factor and statistically interpreted. The concentration of fermentable sugars (glucose, xylose, arabinose) was the highest after the combination of alkaline and enzymatic treatment with xylanase (18 g/L sugars), while xylo-oligosaccharides (xylotriose and xylotetraose) were released in lower amounts (1.33 g/L) after the same treatment. Refining experiments were carried out to obtain a purified fraction by using anion and cation exchange chromatography. The polymer adsorber resin MN-502 showed efficient removal of salts, phenols and furan derivatives. However, the xylo-oligosaccharides yields were also slightly reduced. Although still requiring further optimization of the treatments to obtain higher purified oligomer yields, the results provide information on the production of xylo-oligosaccharides and fermentable sugars from wheat straw for potential use in food applications.

Identifying the negative cooperation between major inhibitors of cellulase activity and minimizing their inhibitory potential during hydrolysis of acid-pretreated corn stover

Soluble compounds produced during the enzymatic hydrolysis of lignocelluloses hampers cellulose conversion. Cellobiose and vanillin most severely inhibited the effect of cellobiohydrolase I. A concentration-dependent negative cooperative effect was found between cellobiose and vanillin. The combined inhibitory effect was about 83.5% of the cellobiose and 88.1% of the vanillin when their concentration was 20 mg/ml. However, the negative synergy could be eliminated by excessive enzyme loading. Differences in their binding sites on the catalytic domain of cellobiohydrolase I lead to negative synergistic inhibition, which should be considered in devising strategies to alleviate this effect. Combined β-glucosidase and PEG addition at an appropriate dose was feasible to balance cost and hydrolytic efficiency. To achieve efficient hydrolysis, especially at high solid concentrations, it is important to understand the synergistic inhibition between these inhibitors.

Co-production of functional xylo-oligosaccharides and fermentable sugars from corn stover through fast and facile ball mill-assisted alkaline peroxide pretreatment

The aim of this work was to develop a feasible ball mill-assisted alkaline peroxide pretreatment followed by stepwise hydrolysis to improve the yield of xylo-oligosaccharides (XOS) and fermentable sugars. The hydrogen peroxide charge, ball-milling time, and solid-to-liquid ratio affected the compositions, particle sizes, morphology, and crystallinity of the corn stover, directly improving the following hydrolytic efficiency. The optimal pretreatment was with 0.45 g/g (H₂O₂: substrate) and 1:3 solid-to-liquid ratio (w/v) for 1.0 h ball-milling, resulting in 84.29% delignification. Physicochemical properties of the pretreated samples were characterized and their correlations to the enzymatic hydrolysis were revealed. Compared with one-step cellulase hydrolysis, the two-step xylanase-cellulase hydrolysis of the pretreated corn stover showed significant advance in preparing XOS, producing 69.65% (on the base of xylan content in pretreated sample) of XOS, along with 20.55% xylose, 68.94% glucose, and 21.15% gluco-oligosaccharides. The yield of XOS was 2-7 times higher than those in previous studies.

Current understanding of the inhibition factors and their mechanism of action for the lignocellulosic biomass hydrolysis

Biorefining of lignocellulosic biomass is a relatively new concept but it has strong potential to develop and partially replace the fossil derived fuels and myriad of value products to subsequently reduce the greenhouse gas emissions. However, the energy and cost intensive process of releasing the entrapped fermentable sugars is a major challenge for its commercialization. Various factors playing a detrimental role during enzymatic hydrolysis of biomass are inherent recalcitrance of lignocellulosic biomass, expensive enzymes, sub-optimal enzyme composition, lack of synergistic activity and enzyme inhibition caused by various inhibitors. The current study investigated the mechanism of enzyme inhibition during lignocellulosic biomass saccharification especially at high solid loadings. These inhibition factors are categorized into physio-chemical factors, water-soluble and -insoluble enzyme inhibitors, oligomers and enzyme-lignin binding. Furthermore, different approaches are proposed to alleviate the challenges and improve the enzymatic hydrolysis efficiency such as supplementation with surfactants, synergistic catalytic/non-catalytic proteins, and bioprocess modifications.

Coculture with hemicellulose-fermenting microbes reverses inhibition of corn fiber solubilization by Clostridium thermocellum at elevated solids loadings

BACKGROUND

The cellulolytic thermophile Clostridium thermocellum is an important biocatalyst due to its ability to solubilize lignocellulosic feedstocks without the need for pretreatment or exogenous enzyme addition. At low concentrations of substrate, C. thermocellum can solubilize corn fiber > 95% in 5 days, but solubilization declines markedly at substrate concentrations higher than 20 g/L. This differs for model cellulose like Avicel, on which the maximum solubilization rate increases in proportion to substrate concentration. The goal of this study was to examine fermentation at increasing corn fiber concentrations and investigate possible reasons for declining performance.

RESULTS

The rate of growth of C. thermocellum on corn fiber, inferred from CipA scaffoldin levels measured by LC-MS/MS, showed very little increase with increasing solids loading. To test for inhibition, we evaluated the effects of spent broth on growth and cellulase activity. The liquids remaining after corn fiber fermentation were found to be strongly inhibitory to growth on cellobiose, a substrate that does not require cellulose hydrolysis. Additionally, the hydrolytic activity of C. thermocellum cellulase was also reduced to less-than half by adding spent broth. Noting that > 15 g/L hemicellulose oligosaccharides accumulated in the spent broth of a 40 g/L corn fiber fermentation, we tested the effect of various model carbohydrates on growth on cellobiose and Avicel. Some compounds like xylooligosaccharides caused a decline in cellulolytic activity and a reduction in the maximum solubilization rate on Avicel. However, there were no relevant model compounds that could replicate the strong inhibition by spent broth on C. thermocellum growth on cellobiose. Cocultures of C. thermocellum with hemicellulose-consuming partners-Herbinix spp. strain LL1355 and Thermoanaerobacterium thermosaccharolyticum-exhibited lower levels of unfermented hemicellulose hydrolysis products, a doubling of the maximum solubilization rate, and final solubilization increased from 67 to 93%.

CONCLUSIONS

This study documents inhibition of C. thermocellum with increasing corn fiber concentration and demonstrates inhibition of cellulase activity by xylooligosaccharides, but further work is needed to understand why growth on cellobiose was inhibited by corn fiber fermentation broth. Our results support the importance of hemicellulose-utilizing coculture partners to augment C. thermocellum in the fermentation of lignocellulosic feedstocks at high solids loading.

A Whole-Slurry Fermentation Approach to High-Solid Loading for Bioethanol Production from Corn Stover

Corn stover is the most produced byproduct from maize worldwide. Since it is generated as a residue from maize harvesting, it is an inexpensive and interesting crop residue to be used as a feedstock. An ecologically friendly pretreatment such as autohydrolysis was selected for the manufacture of second-generation bioethanol from corn stover via whole-slurry fermentation at high-solid loadings. Temperatures from 200 to 240 °C were set for the autohydrolysis process, and the solid and liquid phases were analyzed. Additionally, the enzymatic susceptibility of the solid phases was assessed to test the suitability of the pretreatment. Afterward, the production of bioethanol from autohydrolyzed corn stover was carried out, mixing the solid with different percentages of the autohydrolysis liquor (25%, 50%, 75%, and 100%) and water (0% of liquor), from a total whole slurry fermentation (saving energy and water in the liquid–solid separation and subsequent washing of the solid phase) to employing water as only liquid medium. In spite of the challenging scenario of using the liquor fraction as liquid phase in the fermentation, values between 32.2 and 41.9 g ethanol/L and ethanol conversions up to 80% were achieved. This work exhibits the feasibility of corn stover for the production of bioethanol via a whole-slurry fermentation process.

Steam Explosion Pretreatment of Beechwood. Part 2: Quantification of Cellulase Inhibitors and Their Effect on Avicel Hydrolysis

Biomass pretreatment is a mandatory step for the biochemical conversion of lignocellulose to chemicals. During pretreatment, soluble compounds are released into the prehydrolyzate that inhibit the enzymatic hydrolysis step. In this work, we investigated how the reaction conditions in steam explosion pretreatment of beechwood (severity: 3.0–5.25; temperature: 160–230 °C) influence the resulting amounts of different inhibitors. Furthermore, we quantified the extent of enzyme inhibition during enzymatic hydrolysis of Avicel in the presence of the prehydrolyzates. The amounts of phenolics, HMF, acetic acid and formic acid increased with increasing pretreatment severities and maximal quantities of 21.6, 8.3, 43.7 and 10.9 mg/gbeechwood, respectively, were measured at the highest severity. In contrast, the furfural concentration peaked at a temperature of 200 °C and a severity of 4.75. The presence of the prehydrolyzates in enzymatic hydrolysis of Avicel lowered the glucose yields by 5–26%. Mainly, the amount of phenolics and xylose and xylooligomers contributed to the reduced yield. As the maximal amounts of these two inhibitors can be found at different conditions, a wide range of pretreatment severities led to severely inhibiting prehydrolyzates. This study may provide guidelines when choosing optimal pretreatment conditions for whole slurry enzymatic hydrolysis.

Biocatalysis of Industrial Kraft Pulps: Similarities and Differences between Hardwood and Softwood Pulps in Hydrolysis by Enzyme Complex of Penicillium verruculosum

Kraft pulp enzymatic hydrolysis is a promising method of woody biomass bioconversion. The influence of composition and structure of kraft fibers on their hydrolysis efficiency was evaluated while using four substrates, unbleached hardwood pulp (UHP), unbleached softwood pulp (USP), bleached hardwood pulp (BHP), and bleached softwood pulp (BSP). Hydrolysis was carried out with Penicillium verruculosum enzyme complex at a dosage of 10 filter paper units (FPU)/g pulp. The changes in fiber morphology and structure were visualized while using optical and electron microscopy. Fiber cutting and swelling and quick xylan destruction were the main processes at the beginning of hydrolysis. The negative effect of lignin content was more pronounced for USP. Drying decreased the sugar yield of dissolved hydrolysis products for all kraft pulps. Fiber morphology, different xylan and mannan content, and hemicelluloses localization in kraft fibers deeply affected the hydrolyzability of bleached pulps. The introduction of additional xylobiase, mannanase, and cellobiohydrolase activities to enzyme mixture will further improve the hydrolysis of bleached pulps. A high efficiency of never-dried bleached pulp bioconversion was shown. At 10% substrate concentration, hydrolysates with more than 50 g/L sugar concentration were obtained. The bioconversion of never-dried BHP and BSP could be integrated into working kraft pulp mills.

Comparison of Calcium Oxide and Calcium Peroxide Pretreatments of Wheat Straw for Improving Biohydrogen Production

Wheat straw was pretreated with either CaO₂ or CaO to improve biohydrogen production. Both CaO and CaO₂ pretreatments improved the biodegradability of the wheat straw. CaO pretreatment raised the H₂ yield by between 48.8 and 163.9% at CaO contents ranging from 2 to 4%. The highest H₂ yield [144 mL/g total solid (TS)] was obtained at 121 °C and 6% CaO. In addition, the highest H₂ yield from wheat straw pretreated at the same temperature and dosage of CaO₂ was 71.8 mL/g TS, which was higher than that of the control group (43.2 mL/g TS), with hot water (121 °C) treatment. Considering pretreatment costs and H₂ production potential, CaO was a better pretreatment agent than CaO₂.

Fed-batch enzymatic hydrolysis of alkaline organosolv-pretreated corn stover facilitating high concentrations and yields of fermentable sugars for microbial lipid production

BACKGROUND

Lignocellulosic biomass has been commonly regarded as a potential feedstock for the production of biofuels and biochemicals. High sugar yields and the complete bioconversion of all the lignocellulosic sugars into valuable products are attractive for the utilization of lignocelluloses. It is essential to pretreat and hydrolyze lignocelluloses at high solids loadings during industrial processes, which is more economical and environmentally friendly as capital cost, energy consumption, and water usage can be reduced. However, oligosaccharides are inevitably released during the high solids loading enzymatic hydrolysis and they are more recalcitrant than monosaccharides for microorganisms.

RESULTS

A fed-batch enzymatic hydrolysis of corn stover pretreated by the sodium hydroxide-methanol solution (SMs) at high solids loading was demonstrated to reach the high concentrations and yields of fermentable sugars. Glucose, xylose, cello-oligosaccharides, and xylo-oligosaccharides achieved 146.7 g/L, 58.7 g/L, 15.6 g/L, and 24.7 g/L, respectively, when the fed-batch hydrolysis was started at 12% (w/v) solids loading, and 7% fresh substrate and a standardized blend of cellulase, β-glucosidase, and hemicellulase were fed consecutively at 3, 6, 24, and 48 h to achieve a final solids loading of 40% (w/v). The total conversion of glucan and xylan reached 89.5% and 88.5%, respectively, when the oligosaccharides were taken into account. Then, a fed-batch culture on the hydrolysates was investigated for lipid production by Cutaneotrichosporon oleaginosum. Biomass, lipid content, and lipid yield were 50.7 g/L, 61.7%, and 0.18 g/g, respectively. The overall consumptions of cello-oligosaccharides and xylo-oligosaccharides reached 74.1% and 68.2%, respectively.

CONCLUSIONS

High sugars concentrations and yields were achieved when the enzyme blend was supplemented simultaneously with the substrate at each time point of feeding during the fed-batch enzymatic hydrolysis. Oligosaccharides were co-utilized with monosaccharides during the fed-batch culture of C. oleaginosum. These results provide a promising strategy to hydrolyze alkaline organosolv-pretreated corn stover into fermentable sugars with high concentrations and yields for microbial lipid production.

Constraints and advances in high-solids enzymatic hydrolysis of lignocellulosic biomass: a critical review

The industrial production of sugar syrups from lignocellulosic materials requires the conduction of the enzymatic hydrolysis step at high-solids loadings (i.e., with over 15% solids [w/w] in the reaction mixture). Such conditions result in sugar syrups with increased concentrations and in improvements in both capital and operational costs, making the process more economically feasible. However, this approach still poses several technical hindrances that impact the process efficiency, known as the "high-solids effect" (i.e., the decrease in glucan conversion yields as solids load increases). The purpose of this review was to present the findings on the main limitations and advances in high-solids enzymatic hydrolysis in an updated and comprehensive manner. The causes for the rheological limitations at the onset of the high-solids operation as well as those influencing the "high-solids effect" will be discussed. The subject of water constraint, which results in a highly viscous system and impairs mixing, and by extension, mass and heat transfer, will be analyzed under the perspective of the limitations imposed to the action of the cellulolytic enzymes. The "high-solids effect" will be further discussed vis-à-vis enzymes end-product inhibition and the inhibitory effect of compounds formed during the biomass pretreatment as well as the enzymes' unproductive adsorption to lignin. This review also presents the scientific and technological advances being introduced to lessen high-solids hydrolysis hindrances, such as the development of more efficient enzyme formulations, biomass and enzyme feeding strategies, reactor and impeller designs as well as process strategies to alleviate the end-product inhibition. We surveyed the academic literature in the form of scientific papers as well as patents to showcase the efforts on technological development and industrial implementation of the use of lignocellulosic materials as renewable feedstocks. Using a critical approach, we expect that this review will aid in the identification of areas with higher demand for scientific and technological efforts.

Biochemical characterization and low-resolution SAXS shape of a novel GH11 exo-1,4-β-xylanase identified in a microbial consortium

Biotechnologies that aim to produce renewable fuels, chemicals, and bioproducts from residual ligno(hemi)cellulosic biomass mostly rely on enzymatic depolymerization of plant cell walls (PCW). This process requires an arsenal of diverse enzymes, including xylanases, which synergistically act on the hemicellulose, reducing the long and complex xylan chains to oligomers and simple sugars. Thus, xylanases play a crucial role in PCW depolymerization. Until recently, the largest xylanase family, glycoside hydrolase family 11 (GH11) has been exclusively represented by endo-catalytic β-1,4- and β-1,3-xylanases. Analysis of a metatranscriptome library from a microbial lignocellulose community resulted in the identification of an unusual exo-acting GH11 β-1,4-xylanase (MetXyn11). Detailed characterization has been performed on recombinant MetXyn11 including determination of its low-resolution small-angle X-ray scattering (SAXS) molecular envelope in solution. Our results reveal that MetXyn11 is a monomeric globular enzyme that liberates xylobiose from heteroxylans as the only product. MetXyn11 has an optimal activity in a pH range from 6 to 9 and an optimal temperature of 50 °C. The enzyme maintained above 65% of its original activity in the pH range 5 to 6 after being incubated for 72 h at 50 °C. Addition of the enzyme to a commercial enzymatic cocktail (CelicCtec3) promoted a significant increase of enzymatic hydrolysis yields of hydrothermally pretreated sugarcane bagasse (16% after 24 h of hydrolysis).

Effect of severity factor on the hydrothermal pretreatment of sugarcane straw

The recalcitrant structures of sugarcane straw and related lignocellulosic biomasses require a pretreatment step to enable a better enzymatic attack during the hydrolysis. Factors like the energy consumption and the formation of inhibitors require the optimization of the pretreatment step. Thus, the influence of different severity factors (SF) on hydrothermal (also called liquid hot water, LHW) pretreatment was evaluated using a factorial design 2² with central point. The obtained results showed that low values of SF (<3.39) did not promote reasonable alteration in the sugarcane straw structures, whereas high SF values (>4.70) resulted in loss of hydrolyzed sugars, generation of inhibitors such as furfural, and formation of pseudo-lignin structures, despite high hemicellulose removal (∼97%). The residence time exhibited low influence on LHW. An optimum condition was found for the process (10 min and 195 °C) with low cellulose solubilization (9.80%) and a reasonable hemicellulose removal (85.45%).

Stepwise enzymatic hydrolysis of alkaline oxidation treated sugarcane bagasse for the co-production of functional xylo-oligosaccharides and fermentable sugars

High chemical input is required for enzymatic production of xylo-oligosaccharides (XOS) using xylan extracted from lignocellulosic biomass. In this study, enzymatic hydrolysis of alkaline oxidation (AO) treated sugarcane bagasse (SCB) directly for the production of XOS was conducted. The effect of AO pretreatment on the chemical compositions and hydrolytic properties of SCB was investigated. The AO pretreatment conditions with low chemical input for the production of XOS were optimized by orthogonal design. Stepwise enzymatic hydrolysis of AO pretreated SCB with xylanase and cellulase produced XOS (1.78 g/L), meanwhile, the cellulose conversion increased from 84.97% to 91.51% compared with directly enzymatic hydrolysis using cellulase. HPLC-UV and MALDI-TOF-MS analysis indicated that the obtained XOS products were mainly composed of xylobiose and xylose with a small amount of arabinose/4-O-methylglucuronic acid substituted xylotriose and xylotetraose. The proposed strategy for the co-production of functional XOS and fermentable sugars from SCB showed potential of industrial application.

The liquid fraction from hydrothermal pretreatment of wheat straw provides lytic polysaccharide monooxygenases with both electrons and H2O2 co-substrate

BACKGROUND

Enzyme-aided valorization of lignocellulose represents a green and sustainable alternative to the traditional chemical industry. The recently discovered lytic polysaccharide monooxygenases (LPMOs) are important components of the state-of-the art enzyme cocktails for cellulose conversion. Yet, these monocopper enzymes are poorly characterized in terms of their kinetics, as exemplified by the growing evidence for that H₂O₂ may be a more efficient co-substrate for LPMOs than O₂. LPMOs need external electron donors and one key question of relevance for bioprocess development is whether the required reducing power may be provided by the lignocellulosic substrate.

RESULTS

Here, we show that the liquid fraction (LF) resulting from hydrothermal pretreatment of wheat straw supports LPMO activity on both chitin and cellulose. The initial, transient activity burst of the LPMO reaction was caused by the H₂O₂ present in the LF before addition of LPMO, while the steady-state rate of LPMO reaction was limited by the LPMO-independent production of H₂O₂ in the LF. H₂O₂ is an intermediate of LF oxidation as evidenced by a slow H₂O₂ accumulation in LF, despite high H₂O₂ production rates. This H₂O₂ scavenging ability of LF is important since high concentrations of H₂O₂ may lead to irreversible inactivation of LPMOs.

CONCLUSIONS

Our results support the growing understanding that fine-tuned control over the rates of H₂O₂ production and consumption in different, enzymatic and non-enzymatic reactions is essential for harnessing the full catalytic potential of LPMOs in lignocellulose valorization.

The effect of alkali-soluble lignin on purified core cellulase and hemicellulase activities during hydrolysis of extractive ammonia-pretreated lignocellulosic biomass

Removing alkali-soluble lignin using extractive ammonia (EA) pretreatment of corn stover (CS) is known to improve biomass conversion efficiency during enzymatic hydrolysis. In this study, we investigated the effect of alkali-soluble lignin on six purified core glycosyl hydrolases and their enzyme synergies, adopting 31 enzyme combinations derived by a five-component simplex centroid model, during EA-CS hydrolysis. Hydrolysis experiment was carried out using EA-CS(-) (approx. 40% lignin removed during EA pretreatment) and EA-CS(+) (where no lignin was extracted). Enzymatic hydrolysis experiments were done at three different enzyme mass loadings (7.5, 15 and 30 mg protein g^-1 glucan), using a previously developed high-throughput microplate-based protocol, and the sugar yields of glucose and xylose were detected. The optimal enzyme combinations (based on % protein mass loading) of six core glycosyl hydrolases for EA-CS(-) and EA-CS(+) were determined that gave high sugar conversion. The inhibition of lignin on optimal enzyme ratios was studied, by adding fixed amount of alkali-soluble lignin fractions to EA-CS(-), and pure Avicel, beechwood xylan and evaluating their sugar conversion. The optimal enzyme ratios that gave higher sugar conversion for EA-CS(-) were CBH I: 27.2-28.2%, CBH II: 18.2-22.2%, EG I: 29.2-34.3%, EX: 9.0-14.1%, βX: 7.2-10.2%, βG: 1.0-5.0% (at 7.5-30 mg g^-1 protein mass loading). Endoglucanase was inhibited to a greater extent than other core cellulases and xylanases by lignin during enzyme hydrolysis. We also found that alkali-soluble lignin inhibits cellulase more strongly than hemicellulase during the course of enzyme hydrolysis.

Integrated bioethanol production to boost low-concentrated cellulosic ethanol without sacrificing ethanol yield

Four integrated designs were proposed to boost cellulosic ethanol titer and yield. Results indicated co-fermentation of corn flour with hydrolysate liquor from saccharified corn stover was the best integration scheme and able to boost ethanol titers from 19.9 to 123.2 g/L with biomass loading of 8% and from 36.8 to 130.2 g/L with biomass loadings of 16%, respectively, while meeting the minimal ethanol distillation requirement of 40 g/L and achieving high ethanol yields of above 90%. These results indicated integration of first and second generation ethanol production could significantly accelerate the commercialization of cellulosic biofuel production. Co-fermentation of starchy substrate with hydrolysate liquor from saccharified biomass is able to significantly enhance ethanol concentration to reduce energy cost for distillation without sacrificing ethanol yields. This novel method could be extended to any pretreatment of biomass from low to high pH pretreatment as demonstrated in this study.

One-step process of hydrothermal and alkaline treatment of wheat straw for improving the enzymatic saccharification

BACKGROUND

To increase the production of bioethanol, a two-step process based on hydrothermal and dilute alkaline treatment was applied to reduce the natural resistance of biomass. However, the process required a large amount of water and a long operation time due to the solid/liquid separation before the alkaline treatment, which led to decrease the pure economic profit for production of bioethanol. Therefore, four one-step processes based on order of hydrothermal and alkaline treatment have been developed to enhance concentration of glucose of wheat straw by enzymatic saccharification. The aim of the present study was to systematically evaluated effect for different one-step processes by analyzing the physicochemical properties (composition, structural change, crystallinity, surface morphology, and BET surface area) and enzymatic saccharification of the treated substrates.

RESULTS

In this study, hemicelluloses and lignins were removed from wheat straw and the morphologic structures were destroyed to various extents during the four one-step processes, which were favorable for cellulase absorption on cellulose. A positive correlation was also observed between the crystallinity and enzymatic saccharification rate of the substrate under the conditions given. The surface area of the substrate was positively related to the concentration of glucose in this study. As compared to the control (3.0 g/L) and treated substrates (11.2-14.6 g/L) obtained by the other three one-step processes, the substrate treated by one-step process based on successively hydrothermal and alkaline treatment had a maximum glucose concentration of 18.6 g/L, which was due to the high cellulose concentration and surface area for the substrate, accompanying with removal of large amounts of lignins and hemicelluloses.

CONCLUSIONS

The present study demonstrated that the order of hydrothermal and alkaline treatment had significant effects on the physicochemical properties and enzymatic saccharification of wheat straw. The one-step process based on successively hydrothermal and alkaline treatment is a simple operating and economical feasible method for the production of glucose, which will be further converted into bioethanol.

The effect of switchgrass loadings on feedstock solubilization and biofuel production by Clostridium thermocellum

BACKGROUND

Efficient deconstruction and bioconversion of solids at high mass loadings is necessary to produce industrially relevant titers of biofuels from lignocellulosic biomass. To date, only a few studies have investigated the effect of solids loadings on microorganisms of interest for consolidated bioprocessing. Here, the effects that various switchgrass loadings have on Clostridium thermocellum solubilization and bioconversion are investigated.

RESULTS

Clostridium thermocellum was grown for 10 days on 10, 25, or 50 g/L switchgrass or Avicel at equivalent glucan loadings. Avicel was completely consumed at all loadings, but total cellulose solubilization decreased from 63 to 37% as switchgrass loadings increased from 10 to 50 g/L. Washed, spent switchgrass could be additionally hydrolyzed and fermented in second-round fermentations suggesting that access to fermentable substrates was not the limiting factor at higher feedstock loadings. Results from fermentations on Avicel or cellobiose using culture medium supplemented with 50% spent fermentation broth demonstrated that compounds present in the supernatants from the 25 or 50 g/L switchgrass loadings were the most inhibitory to continued fermentation.

CONCLUSIONS

Recalcitrance alone cannot fully account for differences in solubilization and end-product formation between switchgrass and Avicel at increased substrate loadings. Experiments aimed at separating metabolic inhibition from inhibition of hydrolysis suggest that C. thermocellum's hydrolytic machinery is more vulnerable to inhibition from switchgrass-derived compounds than its fermentative metabolism.

Biotechnological route for sustainable succinate production utilizing oil palm frond and kenaf as potential carbon sources

Due to the world's dwindling energy supplies, greater thrust has been placed on the utilization of renewable resources for global succinate production. Exploration of such biotechnological route could be seen as an act of counterbalance to the continued fossil fuel dominance. Malaysia being a tropical country stands out among many other nations for its plenty of resources in the form of lignocellulosic biomass. To date, oil palm frond (OPF) contributes to the largest fraction of agricultural residues in Malaysia, while kenaf, a newly introduced fiber crop with relatively high growth rate, holds great potential for developing sustainable succinate production, apart from OPF. Utilization of non-food, inexhaustible, and low-cost derived biomass in the form of OPF and kenaf for bio-based succinate production remains largely untapped. Owing to the richness of carbohydrates in OPF and kenaf, bio-succinate commercialization using these sources appears as an attractive proposition for future sustainable developments. The aim of this paper was to review some research efforts in developing a biorefinery system based on OPF and kenaf as processing inputs. It presents the importance of the current progress in bio-succinate commercialization, in addition to describing the potential use of different succinate production hosts and various pretreatments-saccharifications under development for OPF and kenaf. Evaluations on the feasibility of OPF and kenaf as fermentation substrates are also discussed.

New pentose dimers with bicyclic moieties from pretreated biomass

Dipentoses with bicyclic moieties are proposed as new putative cellulase inhibitory reaction products forming during hydrothermal wheat straw biomass pretreatment.

Anomeric Selectivity and Product Profile of a Processive Cellulase

Cellobiohydrolases (CBHs) make up an important group of enzymes for both natural carbon cycling and industrial deconstruction of lignocellulosic biomass. The consecutive hydrolysis of one cellulose strand relies on an intricate pattern of enzyme-substrate interactions in the long, tunnel-shaped binding site of the CBH. In this work, we have investigated the initial complexation mode with cellulose of the most thoroughly studied CBH, Cel7A from Hypocrea jecorina (HjCel7A). We found that HjCel7A predominantly produces glucose when it initiates a processive run on insoluble microcrystalline cellulose, confirming the validity of an even and odd product ratio as an estimate of processivity. Moreover, the glucose released from cellulose was predominantly α-glucose. A link between the initial binding mode of the enzyme and the reducing end configuration was investigated by inhibition studies with the two anomers of cellobiose. A clear preference for β-cellobiose in product binding site +2 was observed for HjCel7A, but not the homologous endoglucanase, HjCe7B. Possible relationships between this anomeric preference in the product site and the prevalence of odd-numbered initial-cut products are discussed, and a correlation between processivity and anomer selectivity is proposed.

Improving enzymatic hydrolysis of lignocellulosic substrates with pre-hydrolysates by adding cetyltrimethylammonium bromide to neutralize lignosulfonate

Two pretreatment methods to overcome recalcitrance of lignocelluloses, sulfite pretreatment (SPORL) and dilute acid (DA), were conducted to pretreat softwood masson pine and hardwood eucalyptus for enzymatic hydrolysis. In the presence of corresponding pre-hydrolysates, adding moderate cetyltrimethylammonium bromide (CTAB) could enhance the enzymatic hydrolysis of the SPORL-pretreated substrates, but had no enhancement for the DA-pretreated substrates. The results showed that sodium lignosulfonate (SL) in pre-hydrolysates and CTAB together had a strong enhancement on the enzymatic hydrolysis of lignocelluloses. The compound of commercial lignosulfonate SXSL and CTAB (SXSL-CTAB) could enhance the substrate enzymatic digestibility (SED) of SPORL-pretreated masson pine from 27.1% to 71.0%, and that of DA-pretreated eucalyptus from 37.6% to 67.9%. The mechanism that CTAB increased the adsorption of SL on lignin to form more effective steric hindrance and reduced the non-productive adsorption of cellulase on lignin by neutralizing the negative charge of SL was proposed.

Improvement of sugar yields from corn stover using sequential hot water pretreatment and disk milling

Efficient pretreatment is essential for economic conversion of lignocellulosic feedstocks into monosaccharides for biofuel production. To realize high sugar yields with low inhibitor concentrations, hot water or dilute acid pretreatment followed by disk milling is proposed. Corn stover at 20% solids was pretreated with hot water at 160-200°C for 4-8min with and without subsequent milling. Hot water pretreatment and disk milling acted synergistically to improve glucose and xylose yields by 89% and 134%, respectively, compared to hot water pretreatment alone. Hot water pretreated (180°C for 4min) and milled samples had the highest glucose and xylose yields among all hot water pretreated and milled samples, which were comparable to samples pretreated with 0.55% dilute acid at 160°C for 4min. However, samples pretreated with 1% dilute acid at 150°C for 4min and disk milled had the highest observed glucose (87.3%) and xylose yields (83.4%).

Multi-level dissolution and hydrolysis of lignocellulosic waste with a semi-flow hydrothermal system

The hydrothermal process is efficient in lignocellulosic conversion and is beneficial to potential bioethanol production. In batch- and flow-type processes, concurrent dissolution and hydrolysis of lignocellulose result in product loss and inhibitory intermediates. Therefore, multi-level hydrothermal conversion of corn stalks was implemented with a semi-flow system to provide different residence times to undissolved compounds and facilitate dissolution or hydrolysis at respective optimal conditions. First-stage dissolution dissolved amorphous hemicellulose and lignin at 195-200°C. Xylan, acid soluble lignin, and part of Klason lignin were dissolved without affecting glucan. In second-stage dissolution, the crystallinity of the undissolved materials suddenly decreased at 245-250°C. The cellulose dissolution ratio was higher than 75%. Soluble sugars were obtained after the hydrolysis of dissolved cellulose at 280°C. The results provide significant information on the multi-level hydrothermal process and its potential applications for recovering valuable chemicals from lignocellulosic waste.

Characterization of three plant biomass-degrading microbial consortia by metagenomics- and metasecretomics-based approaches

The selection of microbes by enrichment on plant biomass has been proposed as an efficient way to develop new strategies for lignocellulose saccharification. Here, we report an in-depth analysis of soil-derived microbial consortia that were trained to degrade once-used wheat straw (WS1-M), switchgrass (SG-M) and corn stover (CS-M) under aerobic and mesophilic conditions. Molecular fingerprintings, bacterial 16S ribosomal RNA (rRNA) gene amplicon sequencing and metagenomic analyses showed that the three microbial consortia were taxonomically distinct. Based on the taxonomic affiliation of protein-encoding sequences, members of the Bacteroidetes (e.g. Chryseobacterium, Weeksella, Flavobacterium and Sphingobacterium) were preferentially selected on WS1-M, whereas SG-M and CS-M favoured members of the Proteobacteria (e.g. Caulobacter, Brevundimonas, Stenotrophomonas and Xanthomonas). The highest degradation rates of lignin (~59 %) were observed with SG-M, whereas CS-M showed a high consumption of cellulose and hemicellulose. Analyses of the carbohydrate-active enzymes in the three microbial consortia showed the dominance of glycosyl hydrolases (e.g. of families GH3, GH43, GH13, GH10, GH29, GH28, GH16, GH4 and GH92). In addition, proteins of families AA6, AA10 and AA2 were detected. Analysis of secreted protein fractions (metasecretome) for each selected microbial consortium mainly showed the presence of enzymes able to degrade arabinan, arabinoxylan, xylan, β-glucan, galactomannan and rhamnogalacturonan. Notably, these metasecretomes contain enzymes that enable us to produce oligosaccharides directly from wheat straw, sugarcane bagasse and willow. Thus, the underlying microbial consortia constitute valuable resources for the production of enzyme cocktails for the efficient saccharification of plant biomass.

Evaluation of soluble fraction and enzymatic residual fraction of dilute dry acid, ethylenediamine, and steam explosion pretreated corn stover on the enzymatic hydrolysis of cellulose

This study is aimed to examine the inhibition of soluble fraction (SF) and enzymatic residual fraction (ERF) in dry dilute acid (DDA), ethylenediamine (EDA) and steam explosion (SE) pretreated corn stover (CS) on the enzymatic digestibility of cellulose. SF of DDA, EDA and SE pretreated CS has high xylose, soluble lignin and xylo-oligomer content, respectively. SF of EDA pretreated CS leads to the highest inhibition, followed by SE and DDA pretreated CS. Inhibition of ERF of DDA and SE pretreated CS is higher than that of EDA pretreated CS. The inhibition degree (A0/A) of SF is 1.76 and 1.21 times to that of ERF for EDA and SE pretreated CS, respectively. The inhibition degree of ERF is 1.05 times to that of SF in DDA pretreated CS. The quantitative analysis shows that SF of EDA pretreated CS, SF and ERF of SE pretreated CS cause significant inhibition during enzymatic hydrolysis.

Contrasted enzymatic cocktails reveal the importance of cellulases and hemicellulases activity ratios for the hydrolysis of cellulose in presence of xylans

Various enzymatic cocktails were produced from two Trichoderma reesei strains, a cellulase hyperproducer strain and a strain with β-glucosidase activity overexpression. By using various carbon sources (lactose, glucose, xylose, hemicellulosic hydrolysate) for strains growth, contrasted enzymatic activities were obtained. The enzymatic cocktails presented various levels of efficiency for the hydrolysis of cellulose Avicel into glucose, in presence of xylans, or not. These latter were also hydrolyzed with different extents according to cocktails. The most efficient cocktails (TR1 and TR3) on Avicel were richer in filter paper activity (FPU) and presented a low ratio FPU/β-glucosidase activity. Cocktails TR2 and TR5 which were produced on the higher amount of hemicellulosic hydrolysate, possess both high xylanase and β-xylosidase activities, and were the most efficient for xylans hydrolysis. When hydrolysis of Avicel was conducted in presence of xylans, a decrease of glucose release occurred for all cocktails compared to hydrolysis of Avicel alone. Mixing TR1 and TR5 cocktails with two different ratios of proteins (1/1 and 1/4) resulted in a gain of efficiency for glucose release during hydrolysis of Avicel in presence of xylans compared to TR5 alone. Our results demonstrate the importance of combining hemicellulase and cellulase activities to improve the yields of glucose release from Avicel in presence of xylans. In this context, strategies involving enzymes production with carbon sources comprising mixed C5 and C6 sugars or combining different cocktails produced on C5 or on C6 sugars are of interest for processes developed in the context of lignocellulosic biorefinery.

Soluble inhibitors generated during hydrothermal pretreatment of oil palm mesocarp fiber suppressed the catalytic activity of Acremonium cellulase

Oil palm mesocarp fiber was subjected to hydrothermal pretreatment under isothermal and non-isothermal conditions. The pretreated slurries were separated by filtration, pretreated liquids and solids were characterized. An enzymatic digestibility study was performed for both pretreated slurries and solids to understand the effect of soluble inhibitors generated during the pretreatment process. The highest glucose yield obtained from pretreated slurries was 70.1%, and gradually decreased with higher pretreatment severities. The highest glucose yield obtained in pretreated solids was 100%, after pretreatment at 210°C for 20min. In order to study the inhibitory effects of compounds generated during pretreatment with cellulase, technical grade solutions that mimic the pretreated liquid were prepared and their effect on Acremonium cellulase activity was monitored using Avicel. Xylo-oligomers and tannic acid were identified as powerful inhibitors of Acremonium cellulase, and the lowest hydrolysis rate of Avicel of 0.18g/g-glucose released/L/h was obtained from tannic acid.

Efficient ethanol production from dried oil palm trunk treated by hydrothermolysis and subsequent enzymatic hydrolysis

BACKGROUND

Oil palm trunk (OPT) is a valuable bioresource for the biorefinery industry producing biofuels and biochemicals. It has the distinct feature of containing a large amount of starch, which, unlike cellulose, can be easily solubilized by water when heated and hydrolyzed to glucose by amylolytic enzymes without pretreatment for breaking down the biomass recalcitrance. Therefore, it is suggested as beneficial to extract most of the starch from OPT through autoclaving and subsequent amylolytic hydrolysis prior to pretreatment. However, this treatment requires high capital and operational costs, and there could be a high probability of microbial contamination during starch processing. In terms of biochemical conversion of OPT, this study aimed to develop a simple and efficient ethanol conversion process without any chemical use such as acids and bases or detoxification.

RESULTS

For comparison with the proposed efficient ethanol conversion process, OPT was subjected to hydrothermal treatment at 180 °C for 30 min. After enzymatic hydrolysis of PWS, 43.5 g of glucose per 100 g dry biomass was obtained, which corresponds to 81.3 % of the theoretical glucose yield. Through subsequent alcohol fermentation, 81.4 % ethanol yield of the theoretical ethanol yield was achieved. To conduct the proposed new process, starch in OPT was converted to ethanol through enzymatic hydrolysis and subsequent fermentation prior to hydrothermal treatment, and the resulting slurry was subjected to identical processes that were applied to control. Consequently, a high-glucose yield of 96.3 % was achieved, and the resulting ethanol yield was 93.5 %.

CONCLUSIONS

The proposed new process was a simple method for minimizing the loss of starch during biochemical conversion and maximizing ethanol production as well as fermentable sugars from OPT. In addition, this methodology offers the advantage of reducing operational and capital costs due to minimizing the process for ethanol production by excluding expensive processes related to detoxification prior to enzymatic hydrolysis and fermentation such as washing/conditioning and solid-liquid separation of pretreated slurry. The potential future use of xylose-digestible microorganisms could further increase the ethanol yield from the proposed process, thereby increasing its effectiveness for the conversion of OPT into biofuels and biochemicals.

Fermentative l-lactic acid production from pretreated whole slurry of oil palm trunk treated by hydrothermolysis and subsequent enzymatic hydrolysis

A simple and cost-effective biochemical conversion process consisting of hydrothermal treatment, enzymatic hydrolysis and fermentation of pretreated whole slurry (PWS) was developed for producing l-lactic acid (L-LA) from oil palm trunk (OPT). When OPT was hydrothermally treated at optimal condition capable of achieving maximum yield of hemicellulosic sugars after enzymatic hydrolysis, the enzymatic digestibility of the PWS afforded a yield of 81.4% of the theoretical glucose yield (TGY). However, glucose yield from washed pretreated solid (WPS) was only 43.5% of TGY. The use of two hydrolysates from PWS and WPS for fermentation by Lactobacillus paracasei engineered to selectively produce L-LA afforded yields of 89.5% and 45.8% of the theoretical LA yield (TLY), respectively. This study confirmed the inevitable extensive sugar loss during washing of pretreated slurry due to loss of soluble starch. Alternatively, the proposed design process is considered suitable for converting OPT to L-LA without such starch loss.

Structural insights into the inhibition of cellobiohydrolase Cel7A by xylo-oligosaccharides

The filamentous fungus Hypocrea jecorina (anamorph of Trichoderma reesei) is the predominant source of enzymes for industrial saccharification of lignocellulose biomass. The major enzyme, cellobiohydrolase Cel7A, constitutes nearly half of the total protein in the secretome. The performance of such enzymes is susceptible to inhibition by compounds liberated by physico-chemical pre-treatment if the biomass is kept unwashed. Xylan and xylo-oligosaccharides (XOS) have been proposed to play a key role in inhibition of cellobiohydrolases of glycoside hydrolase family 7. To elucidate the mechanism behind this inhibition at a molecular level, we used X-ray crystallography to determine structures of H. jecorina Cel7A in complex with XOS. Structures with xylotriose, xylotetraose and xylopentaose revealed a predominant binding mode at the entrance of the substrate-binding tunnel of the enzyme, in which each xylose residue is shifted ~ 2.4 Å towards the catalytic center compared with binding of cello-oligosaccharides. Furthermore, partial occupancy of two consecutive xylose residues at subsites -2 and -1 suggests an alternative binding mode for XOS in the vicinity of the catalytic center. Interestingly, the -1 xylosyl unit exhibits an open aldehyde conformation in one of the structures and a ring-closed pyranoside in another complex. Complementary inhibition studies with p-nitrophenyl lactoside as substrate indicate mixed inhibition rather than pure competitive inhibition.

DATABASE

The atomic coordinates and structure factors are available in the Protein Data Bank under accession number 4D5I (H. jecorina Cel7A E212Q variant, complex with xylotriose), 4D5J (H. jecorina Cel7A E217Q variant, complex with xylotriose), 4D5O (H. jecorina Cel7A E212Q variant, complex with xylopentaose), 4D5P (H. jecorina Cel7A E217Q variant, complex with xylopentaose), 4D5Q (wild-type H. jecorina Cel7A, complex with xylopentaose) and 4D5V (H. jecorina Cel7A E217Q variant, complex with xylotetraose).