Enzyme adsorption and recycling during hydrolysis of wheat straw lignocellulose

Adsorption of β-glucosidases in two commercial preparations onto pretreated biomass and lignin

BACKGROUND

Enzyme recycling is a method to reduce the production costs for advanced bioethanol by lowering the overall use of enzymes. Commercial cellulase preparations consist of many different enzymes that are important for efficient and complete cellulose (and hemicellulose) hydrolysis. This abundance of different activities complicates enzyme recycling since the individual enzymes behave differently in the process. Previously, the general perception was that β-glucosidases could easily be recycled via the liquid phase, as they have mostly been observed not to adsorb to pretreated biomass or only adsorb to a minor extent.

RESULTS

The results from this study with Cellic® CTec2 revealed that the vast majority of the β-glucosidase activity was lost from the liquid phase and was adsorbed to the residual biomass during hydrolysis and fermentation. Adsorption studies with β-glucosidases in two commercial preparations (Novozym 188 and Cellic® CTec2) to substrates mimicking the components in pretreated wheat straw revealed that the Aspergillus niger β-glucosidase in Novozym 188 did not adsorb significantly to any of the components in pretreated wheat straw, whereas the β-glucosidase in Cellic® CTec2 adsorbed strongly to lignin.The extent of adsorption of β-glucosidase from Cellic® CTec2 was affected by both type of biomass and pretreatment method. With approximately 65% of the β-glucosidases from Cellic® CTec2 adsorbed onto lignin from pretreated wheat straw, the activity of the β-glucosidases in the slurry decreased by only 15%. This demonstrated that some enzyme remained active despite being bound. It was possible to reduce the adsorption of Cellic® CTec2 β-glucosidase to lignin from pretreated wheat straw by addition of bovine serum albumin or poly(ethylene glycol).

CONCLUSIONS

Contrary to the β-glucosidases in Novozym 188, the β-glucosidases in Cellic® CTec2 adsorb significantly to lignin. The lignin adsorption observed for Cellic® CTec2 is usually not a problem during hydrolysis and fermentation since most of the catalytic activity is retained. However, adsorption of β-glucosidases to lignin may prove to be a problem when trying to recycle enzymes in the production of advanced bioethanol.

Quantification of bound and free enzymes during enzymatic hydrolysis and their reactivities on cellulose and lignocellulose

Enzymatic hydrolysis of insoluble biomass is a surface reaction. Part of the enzyme adsorb on the surface of biomass, whereas the others stay in the liquid phase. In this study, three substrates (Avicel cellulose, bleached hardwood pulp, and green-liquor pretreated hardwood pulp) were used to study the reactivity of bound and free enzyme. In a continuous enzymatic hydrolysis, 35-65% initially added enzymes became bound enzymes, which were primarily responsible for enzymatic hydrolysis. The contribution from free enzymes became insignificant after a certain period of reaction time. SDS-PAGE analysis showed that CBH I was significantly decreased in the free enzyme, which might be the reason for the low digestibility of free enzymes due to the loss of synergistic effect. When Tween 80 was added during enzymatic hydrolysis, the digestibility of free enzyme on Avicel was greatly enhanced. However, the benefit of surfactant was not noticeable for lignocellulosic pulps, comparing to Avicel.

Recycling cellulases for cellulosic ethanol production at industrial relevant conditions: potential and temperature dependency at high solid processes

Different versions of two commercial cellulases were tested for their recyclability of enzymatic activity at high dry matter processes (12% or 25% DM). Recyclability was assessed by measuring remaining enzyme activity in fermentation broth and the ability of enzymes to hydrolyse fresh, pretreated wheat straw. Industrial conditions were used to study the impact of hydrolysis temperature (40 or 50°C) and residence time on recyclability. Enzyme recycling at 12% DM indicated that hydrolysis at 50°C, though ideal for ethanol yield, should be kept short or carried out at lower temperature to preserve enzymatic activity. Best results for enzyme recycling at 25% DM was 59% and 41% of original enzyme load for a Celluclast:Novozyme188 mixture and a modern cellulase preparation, respectively. However, issues with stability of enzymes and their strong adsorption to residual solids still pose a challenge for applicable methods in enzyme recycling.

Enzymatic hydrolysis, adsorption, and recycling during hydrolysis of bagasse sulfite pulp

The high costs of enzymatic hydrolysis along with the high enzyme dosage are often considered as the major bottlenecks in lignocellulosic bioconversion. This study investigated the hydrolysis efficiency, cellulase adsorption and enzyme recycling during the hydrolysis of bagasse sulfite pulp (BSP). After 48 h of hydrolysis, more than 70% of the cellulose was hydrolyzed, while the protein concentration and cellulase activity in solution remained 31% and 17% of the initial value, respectively. The cellulase adsorption on the fresh BSP was better fitted by a Sips model, suggesting the occurrence of a multilayer adsorption at low cellulase concentration and monolayer adsorption at high concentration on the BSP surfaces. Desorption profile studies showed that the optimum desorption condition was at pH 4.8 and 40 °C. Moreover, considering the limited ability to desorption, directly empolying the bound enzyme with residual substrate is more effective method to recover cellulase during the hydrolysis of BSP.

The challenging measurement of protein in complex biomass-derived samples

Measurement of the protein content in samples from production of lignocellulosic bioethanol is an important tool when studying the adsorption of cellulases. Several methods have been used for this, and after reviewing the literature, we concluded that one of the most promising assays for simple and fast protein measurement on this type of samples was the ninhydrin assay. This method has also been used widely for this purpose, but with two different methods for protein hydrolysis prior to the assay-alkaline or acidic hydrolysis. In samples containing glucose or ethanol, there was significant interference from these compounds when using acid hydrolysis, which was not the case when using the alkaline hydrolysis. We evaluated the interference from glucose, cellulose, xylose, xylan, lignin and ethanol on protein determination of BSA, Accellerase(®) 1500 and Cellic(®) CTec2. The experiments demonstrated that the presence of cellulose, lignin and glucose (above 50 g/kg) could significantly affect the results of the assay. Comparison of analyses performed with the ninhydrin assay and with a CN analyser revealed that there was good agreement between these two analytical methods, but care has to be taken when applying the ninhydrin assay. If used correctly, the ninhydrin assay can be used as a fast method to evaluate the adsorption of cellulases to lignin.

Recycling cellulase from enzymatic hydrolyzate of acid treated wheat straw by electroultrafiltration

This work explores the feasibility of recycling cellulase by electroultrafiltration (EUF), an ultrafiltration process enhanced by an electric field, to reduce the cost of enzymatic transformation of cellulose. The effect of electric field under different operating conditions (buffer concentration, acid treated wheat straw concentration, current and temperature) on flux during EUF was examined. The results showed that EUF was effective to reduce concentration polarization (CP) and enhance filtration flux in recycling cellulase. The flux improvement by the electric field could be strengthened at low buffer concentration (5 mM) and relatively low temperature (room temperature) and high current (150 mA). The flux for 2% (substrate concentration, w/v) lignocellulosic hydrolyzate increased by a factor of 4.4 at 836 V/m and room temperature, compared to that without electric field. This work shows that under appropriate operating conditions EUF can efficiently recycle cellulase from lignocellulosic hydrolyzate and thus substantially reduce hydrolysis cost.

Lignin isolated from steam-exploded eucalyptus wood chips by phase separation and its affinity to Trichoderma reesei cellulase

Steam-exploded eucalyptus wood chips were treated with p-cresol and 72% sulfuric acid at ambient temperature. Steam-exploded lignin was isolated as acetone-soluble and diethyl ether-insoluble compounds from the cresol layer. The lignin extraction yield was only 47%, and the amount of cresol grafted to lignin was much less than that in the case of eucalyptus lignin without steam explosion. Clearly, the steam explosion process depolymerized native lignin, and simultaneously, promoted polymerization via labile benzyl positions. The steam-exploded eucalyptus lignin adsorbed more Trichoderma reesei cellulase; however, its enzymatic activity was less than that of eucalyptus lignin that did not undergo steam explosion. It is evident that pretreatment potentially affects the affinity between lignin and cellulase and the resultant saccharification efficiency.

Influence of high temperature and ethanol on thermostable lignocellulolytic enzymes

Lignocellulolytic enzymes are among the most costly part in production of bioethanol. Therefore, recycling of enzymes is interesting as a concept for reduction of process costs. However, stability of the enzymes during the process is critical. In this work, focus has been on investigating the influence of temperature and ethanol on enzyme activity and stability in the distillation step, where most enzymes are inactivated due to high temperatures. Two enzyme mixtures, a mesophilic and a thermostable mixture, were exposed to typical process conditions [temperatures from 55 to 65 °C and up to 5 % ethanol (w/v)] followed by specific enzyme activity analyses and SDS-PAGE. The thermostable and mesophilic mixture remained active at up to 65 and 55 °C, respectively. When the enzyme mixtures reached their maximum temperature limit, ethanol had a remarkable influence on enzyme activity, e.g., the more ethanol, the faster the inactivation. The reason could be the hydrophobic interaction of ethanol on the tertiary structure of the enzyme protein. The thermostable mixture was more tolerant to temperature and ethanol and could therefore be a potential candidate for recycling after distillation.

Behavior of lignin-binding cellulase in the presence of fresh cellulosic substrate

A model lignin-binding cellulase was prepared from Trichoderma reesei cellulase and lignocresol, which was synthesized from softwood or hardwood lignin. Filter paper was incubated with the lignocresol-cellulase complex, and it was observed that only a limited amount of cellulase migrated to the filter paper. The cellulase adsorption isotherms for the lignocresols and filter paper were fitted to a Langmuir absorption model, and the determined Langmuir constants were as follows: softwood lignocresol>hardwood lignocresol>>filter paper. The calculations demonstrated that lignin-binding cellulase can potentially be recovered by the addition of a sufficient quantity of cellulosic substrate. As a result, the lignocresol-binding cellulase is highly stable and lignocresol can potentially be used for immobilizing cellulase in the active state.

Enzyme affinity to cell types in wheat straw (Triticum aestivum L.) before and after hydrothermal pretreatment

BACKGROUND

Wheat straw used for bioethanol production varies in enzymatic digestibility according to chemical structure and composition of cell walls and tissues. In this work, the two biologically different wheat straw organs, leaves and stems, are described together with the effects of hydrothermal pretreatment on chemical composition, tissue structure, enzyme adhesion and digestion. To highlight the importance of inherent cell wall characteristics and the diverse effects of mechanical disruption and biochemical degradation, separate leaves and stems were pretreated on lab-scale and their tissue structures maintained mostly intact for image analysis. Finally, samples were enzymatically hydrolysed to correlate digestibility to chemical composition, removal of polymers, tissue composition and disruption, particle size and enzyme adhesion as a result of pretreatment and wax removal. For comparison, industrially pretreated wheat straw from Inbicon A/S was included in all the experiments.

RESULTS

Within the same range of pretreatment severities, industrial pretreatment resulted in most hemicellulose and epicuticular wax/cutin removal compared to lab-scale pretreated leaves and stems but also in most re-deposition of lignin on the surface. Tissues were furthermore degraded from tissues into individual cells while lab-scale pretreated samples were structurally almost intact. In both raw leaves and stems, endoglucanase and exoglucanase adhered most to parenchyma cells; after pretreatment, to epidermal cells in all the samples. Despite heavy tissue disruption, industrially pretreated samples were not as susceptible to enzymatic digestion as lab-scale pretreated leaves while lab-scale pretreated stems were the least digestible.

CONCLUSIONS

Despite preferential enzyme adhesion to epidermal cells after hydrothermal pretreatment, our results suggest that the single most important factor determining wheat straw digestibility is the fraction of parenchyma cells rather than effective tissue disruption.

Novel cellulase recycling method using a combination of Clostridium thermocellum cellulosomes and Thermoanaerobacter brockii β-glucosidase

This report describes a novel recycling method utilizing a combination of Clostridium thermocellum cellulosomes and Thermoanaerobacter brockii β-glucosidase (CglT). To recover cellulosomes and CglT through re-binding to additional cellulose, a chimeric CBM3-CglT was created by fusing carbohydrate binding module (CBM3) from the scaffolding protein CipA into the N-terminal region of CglT. When a recycling test using cellulosomes and CBM3-CglT was performed on microcrystalline cellulose, the process was capable of 4 rounds of recycling (1%w/vcellulose/round). Although irreversible absorption of cellulosomes and CBM3-CglT into the residues was observed when ammonia-pretreated rice straw and delignified rice straw was used as substrates, a maximum of 2 and 4 recycling rounds (1%w/vglucan/round) were achieved, respectively, consistent with a 70% saccharification rate. This novel recycling method using cellulosomes and CBM3-CglT has great potential as an effective lignocellulose degradation system.

Enzymatic lignocellulose hydrolysis: Improved cellulase productivity by insoluble solids recycling

BACKGROUND

It is necessary to develop efficient methods to produce renewable fuels from lignocellulosic biomass. One of the main challenges to the industrialization of lignocellulose conversion processes is the large amount of cellulase enzymes used for the hydrolysis of cellulose. One method for decreasing the amount of enzyme used is to recycle the enzymes. In this study, the recycle of enzymes associated with the insoluble solid fraction after the enzymatic hydrolysis of cellulose was investigated for pretreated corn stover under a variety of recycling conditions.

RESULTS

It was found that a significant amount of cellulase activity could be recovered by recycling the insoluble biomass fraction, and the enzyme dosage could be decreased by 30% to achieve the same glucose yields under the most favorable conditions. Enzyme productivity (g glucose produced/g enzyme applied) increased between 30 and 50% by the recycling, depending on the reaction conditions. While increasing the amount of solids recycled increased process performance, the methods applicability was limited by its positive correlation with increasing total solids concentrations, reaction volumes, and lignin content of the insoluble residue. However, increasing amounts of lignin rich residue during the recycle did not negatively impact glucose yields.

CONCLUSIONS

To take advantage of this effect, the amount of solids recycled should be maximized, based on a given processes ability to deal with higher solids concentrations and volumes. Recycling of enzymes by recycling the insoluble solids fraction was thus shown to be an effective method to decrease enzyme usage, and research should be continued for its industrial application.

Improving the performance of enzymes in hydrolysis of high solids paper pulp derived from MSW

BACKGROUND

The research aimed to improve the overall conversion efficiency of the CTec® family of enzymes by identifying factors that lead to inhibition and seeking methods to overcome these through process modification and manipulation. The starting material was pulp derived from municipal solid waste and processed in an industrial-scale washing plant.

RESULTS

Analysis of the pulp by acid hydrolysis showed a ratio of 55 : 12 : 6 : 24 : 3 of glucan : xylan : araban/galactan/mannan : lignin : ash. At high total solids content (>18.5% TS) single-stage enzyme hydrolysis gave a maximum glucan conversion of 68%. It was found that two-stage hydrolysis could give higher conversion if sugar inhibition was removed by an intermediate fermentation step between hydrolysis stages. This, however, was not as effective as direct removal of the sugar products, including xylose, by washing of the residual pulp at pH 5. This improved the water availability and allowed reactivation of the pulp-bound enzymes. Inhibition of enzyme activity could further be alleviated by replenishment of β-glucosidase which was shown to be removed during the wash step.

CONCLUSIONS

The two-stage hydrolysis process developed could give an overall glucan conversion of 88%, with an average glucose concentration close to 8% in 4 days, thus providing an ideal starting point for ethanol fermentation with a likely yield of 4 wt%. This is a significant improvement over a single-step process. This hydrolysis configuration also provides the potential to recover the sugars associated with residual solids which are diluted when washing hydrolysed pulp.

Bioenergy grass feedstock: current options and prospects for trait improvement using emerging genetic, genomic, and systems biology toolkits

For lignocellulosic bioenergy to become a viable alternative to traditional energy production methods, rapid increases in conversion efficiency and biomass yield must be achieved. Increased productivity in bioenergy production can be achieved through concomitant gains in processing efficiency as well as genetic improvement of feedstock that have the potential for bioenergy production at an industrial scale. The purpose of this review is to explore the genetic and genomic resource landscape for the improvement of a specific bioenergy feedstock group, the C4 bioenergy grasses. First, bioenergy grass feedstock traits relevant to biochemical conversion are examined. Then we outline genetic resources available bioenergy grasses for mapping bioenergy traits to DNA markers and genes. This is followed by a discussion of genomic tools and how they can be applied to understanding bioenergy grass feedstock trait genetic mechanisms leading to further improvement opportunities.

Recycling of cellulases in lignocellulosic hydrolysates using alkaline elution

The recovery of cellulases from lignin, lignocellulosic hydrolysates and cellulose by alkaline washes at pH 9 and 10 was examined. The effect of the pH on the structural stability of purified Cel7A was analyzed by circular dichroism. Purified Cel7A showed conformational changes at pH 9 and 10 that were reversible at pH 4.8. Temperature influenced the enzymatic hydrolysis of wheat straw and may be critical for the efficiency of cellulase recycling from wheat straw hydrolysates. Operation at moderate temperatures (37°C) resulted in a rate of saccharification 19% higher than that obtained at 50° C, improving cellulase recycling by 49%. Over 60% of the enzyme activity on the synthetic substrate 4-methylumbelliferyl-β-d-cellobioside (MUC) may be recovered by using a simple alkaline wash. This is thus a promising strategy for enzyme recycling that is simple to implement at industrial scale, economical and effective.

A review of lignocellulose bioconversion using enzymatic hydrolysis and synergistic cooperation between enzymes--factors affecting enzymes, conversion and synergy

Lignocellulose is a complex substrate which requires a variety of enzymes, acting in synergy, for its complete hydrolysis. These synergistic interactions between different enzymes have been investigated in order to design optimal combinations and ratios of enzymes for different lignocellulosic substrates that have been subjected to different pretreatments. This review examines the enzymes required to degrade various components of lignocellulose and the impact of pretreatments on the lignocellulose components and the enzymes required for degradation. Many factors affect the enzymes and the optimisation of the hydrolysis process, such as enzyme ratios, substrate loadings, enzyme loadings, inhibitors, adsorption and surfactants. Consideration is also given to the calculation of degrees of synergy and yield. A model is further proposed for the optimisation of enzyme combinations based on a selection of individual or commercial enzyme mixtures. The main area for further study is the effect of and interaction between different hemicellulases on complex substrates.

Application of ultrafiltration and nanofiltration for recycling cellulase and concentrating glucose from enzymatic hydrolyzate of steam exploded wheat straw

Application of combined ultrafiltration (UF) and nanofiltration (NF) was examined to recycle cellulase and concentrate glucose present in lignocellulosic hydrolyzate. With PES10 membrane operated at 25.6 l/m(2) h, 73.9% of cellulase protein present in the hydrolyzate suspension could be recovered while allowing free transmission of glucose. The permeate obtained from UF was then concentrated by NF. With NF270 membrane operated at 13.3 l/m(2) h, the glucose concentration in the ultrafiltered hydrolyzate increased from 30.2 to 110.2 g/l. Recycling cellulase by UF could reduce the hydrolysis cost of lignocellulosic feedstock, while concentrating glucose by NF could improve the fermentation efficiency of lignocellulosic hydrolyzate and lower the separation and purification cost of fermentative product. Therefore, the use of UF and NF for treating lignocellulosic hydrolyzate could be a promising approach in fermentative production of bioproducts and biofuels using lignocellulosic feedstock as substrate.