Production and applications of pullulan from lignocellulosic biomass: Challenges and perspectives

Pullulan is an exopolysaccharide produced by Aureobasidium pullulans, with interesting characteristics which lead to its application in industries such as pharmaceuticals, cosmetics, food, and others. To reduce production costs for industrial applications, cheaper raw materials such as lignocellulosic biomass can be utilized as a carbon and nutrient source for the microbial process. In this study, a comprehensive and critical review was conducted, encompassing the pullulan production process and the key influential variables. The main properties of the biopolymer were presented, and different applications were discussed. Subsequently, the utilization of lignocellulosics for pullulan production within the framework of a biorefinery concept was explored, considering the main published works that deal with materials such as sugarcane bagasse, rice husk, corn straw, and corn cob. Next, the main challenges and future prospects in this research area were highlighted, indicating the key strategies to favor the industrial production of pullulan from lignocellulosic biomasses.

KMnO4-oxidized whole pine needle based adsorbent for selective and efficient removal of cationic dyes

In the present study, we report the chemical modification of the dried and fallen pine needles (PNs) via a simple protocol using KMnO4 oxidation. The oxidized PNs (OPNs) were evaluated as adsorbents using some cationic and anionic dyes. The successful synthesis of OPNs adsorbent was characterized by various techniques to ascertain its structural attributes. The adsorbent showed selectivity for the cationic dyes with 96.11% removal (Pr) for malachite green (MG) and 89.68% Pr for methylene blue (MB) in 120 min. Kinetic models namely, pseudo-first order, pseudo-second order, and Elovich were applied to have insight into adsorption. Additionally, three adsorption isotherms, i.e., Langmuir, Freundlich, and Temkin were also applied. The dye adsorption followed a pseudo-second-order kinetic model with R2 > 0.99912 for MG and R2 > 0.9998 for MB. The adsorbent followed the Langmuir model with a maximum adsorption capacity (qm) of 223.2 mg/g and 156.9 mg/g for MG and MB, respectively. Furthermore, the OPNs showed remarkable regeneration and recyclability up to nine adsorption-desorption cycles with appreciable adsorption for both the dyes. The use of OPNs as an adsorbent for the removal of dyes from wastewater, therefore, provides an ecologically benign, low-cost, and sustainable solution.

Fungal strain improvement for efficient cellulase production and lignocellulosic biorefinery: Current status and future prospects

Lignocellulosic biomass (LCB) has been recognized as a valuable carbon source for the sustainable production of biofuels and value-added biochemicals. Crude enzymes produced by fungal cell factories benefit economic LCB degradation. However, high enzyme production cost remains a great challenge. Filamentous fungi have been widely used to produce cellulolytic enzymes. Metabolic engineering of fungi contributes to efficient cellulase production for LCB biorefinery. Here the latest progress in utilizing fungal cell factories for cellulase production was summarized, including developing genome engineering tools to improve the efficiency of fungal cell factories, manipulating promoters, and modulating transcription factors. Multi-omics analysis of fungi contributes to identifying novel genetic elements for enhancing cellulase production. Furthermore, the importance of translation regulation of cellulase production are emphasized. Efficient development of fungal cell factories based on integrative strain engineering would benefit the overall bioconversion efficacy of LCB for sustainable bioproduction.

An insight into omics analysis and metabolic pathway engineering of lignin-degrading enzymes for enhanced lignin valorization

Lignin, a highly heterogeneous polymer of lignocellulosic biomass, is intricately associated with cellulose and hemicellulose, responsible for its strength and rigidity. Lignin decomposition is carried out through certain enzymes derived from microorganisms to promote the hydrolysis of lignin. Analyzing multi-omics data helps to emphasize the probable value of fungal-produced enzymes to degrade the lignocellulosic material, which provides them an advantage in their ecological niches. This review focuses on lignin biodegrading microorganisms and associated ligninolytic enzymes, including lignin peroxidase, manganese peroxidase, versatile peroxidase, laccase, and dye-decolorizing peroxidase. Further, enzymatic catalysis, lignin biodegradation mechanisms, vital factors responsible for lignin modification and degradation, and the design and selection of practical metabolic pathways are also discussed. Highlights were made on metabolic pathway engineering, different aspects of omics analyses, and its scope and applications to ligninase enzymes. Finally, the advantages and essential steps of successfully applying metabolic engineering and its path forward have been addressed.

Environment friendly emerging techniques for the treatment of waste biomass: a focus on microwave and ultrasonication processes

In the recent past, an increasing interest is mostly observed in using microwave and ultrasonic irradiation to aid the biological conversion of waste materials into value-added products. This study is focused on various individual impacts of microwaves and ultrasonic waves for the treatment of biomass before the synthesis of value-added products. Following, a comprehensive review of the mechanisms governing microwaves and ultrasonication as the treatment methods, their effects on biomass disruption, solubilization of organic matter, modification of the crystalline structure, enzymatic hydrolysis and production of reducing sugars was performed. However, based on the lab-scale experiments evaluated, microwaves and ultrasonication were studied to be economically and energetically ineffective despite their beneficial effects on the waste biomass. This article reviews some of the difficulties associated with using microwaves and ultrasonic irradiation for the efficient processing of waste biomasses and identified some potential directions for future study.

Bioprocess development for the production of xylooligosaccharide prebiotics from agro-industrial lignocellulosic waste

The development of sustainable biorefineries and bioeconomy has been the mandate of most of the governments with major focus on restricting the climate change concerns and finding new strategies to maintain the global food supply chain. Xylooligosaccharides (XOS) are short-chain oligomers which due to their excellent prebiotic potential in the nutraceutical sector has attracted intense research focus in the recent years. The agro-industrial crop and food waste can be utilized for the production of XOS which are derived from hemicellulose fraction (xylan) of the lignocellulosic materials. The extraction of xylan, is traditionally achieved by acidic and alkaline pretreatments which, however, have limited industrial applications. The inclusion of cutting-edge and environmentally beneficial pretreatment methods and technologies such as deep eutectic solvents and green catalysts are preferred. Moreover, the extraction of xylans from biomass using combinatorial pretreatment approaches may help in economizing the whole bioprocess. The current review outlines the factors involved in the xylan extraction and depolymerization processes from different lignocellulosic biomass and the subsequent enzymatic hydrolysis for XOS production. The different types of oligosaccharides and their prebiotic potential for the growth of healthy gut bacteria have also been explained. The introduction of modern molecular technologies has also made it possible to identify enzymes and microorganisms with the desired characteristics for usage in XOS industrial production processes.

Significance of glycans in cellulolytic enzymes for lignocellulosic biorefinery - A review

Lignocellulosic (LC) biomass is the most abundant renewable resource for mankind gravitating society towards sustainable solution for energy that can reduce the carbon footprint. The economic feasibility of 'biomass biorefinery' depends upon the efficiency cellulolytic enzymes which is the main crux. Its high production cost and low efficiencies are the major limitations, that need to be resolved. As the complexity of the genome increases, so does the complexity of the proteome, further facilitated by protein post-translational modifications (PTMs). Glycosylation is regarded the major PTMs and hardly any recent work is focused on importance of glycosylation in cellulase. By modifying protein side chains and glycans, superior cellulases with improved stability and efficiency can be obtained. Functional proteomics relies heavily on PTMs because they regulate activity, localization, and interactions with protein, lipid, nucleic acid, and cofactor molecules. O- and N- glycosylation in cellulases influences its characteristics adding positive attributes to the enzymes.

Cellulose esterification with carboxylic acid in deep eutectic solvent pretreatment inhibits enzymatic hydrolysis

Avicel cellulose was pretreated using two commonly used carboxylic acid-based deep eutectic solvents, i.e., choline chloride-lactic acid and choline chloride-formic acid. The pretreatment process resulted in the formation of cellulose esters with lactic acid and formic acid, which was confirmed by infrared and nuclear magnetic resonance spectra. Surprisingly, the esterified cellulose led to a significant decrease in the 48-h enzymatic glucose yield (≥75%) compared to raw Avicel cellulose. Analysis of changes in cellulose properties caused by pretreatment, including crystallinity, degree of polymerization, particle size and cellulose accessibility, contradicted the observed decline in enzymatic cellulose hydrolysis. However, removing the ester groups through saponification largely recovered the reduction in cellulose conversion. The decreased enzymatic cellulose hydrolysis by esterification may be attributed to changes in the interaction between cellulose-binding domain of cellulase and cellulose. These findings provide valuable insights into improving the saccharification of lignocellulosic biomass pretreated by carboxylic acid-based DESs.

Lignocellulosic biorefineries: A multiscale approach for resource exploitation

Biomass can become the source for chemicals towards a sustainable production system. However, the challenges it presents such as the variety of species, their widespread and sparse availability, and the expensive transportation claims for an integrated approach to design the novel production system. Multiscale approaches have not been properly extended to biorefineryes design and deployment, due to the comprehensive experimental and modelling work they require. A systems perspective provides the systematic framework to analyze the availability and composition of raw materials across regions, how that affects process design, the portfolio of products that can be obtained by evaluating the strong link between the biomass features and the process design. The use of lignocellulosic materials requires for a multidisciplinary work, that must lead to new process engineers with technical competences in biology, biotechnology but also process engineering, mathematics, computer science and social sciences towards a sustainable process/chemical industry.

Triggering photo fermentative biohydrogen production through NiFe2O4 photo nanocatalysts with various excitation sources

The triggering effects of nickel ferrite (NiFe₂O₄) photo nanocatalysts on photo fermentative hydrogen production (PFHP), and metabolic pathways under various excitation sources (incandescent lamp, Xenon lamp, and 532 laser) have been investigated. Compare to the control group (CG) highest cumulative hydrogen volume (CHV) and the maximum hydrogen production rate (HPR) of 568.8 mL and 9.17 mL/h, respectively were achieved at a loading centration of 150 mg/L excited with an incandescent lamp. The change in metabolites with NiFe₂O₄ incorporation suggests that bacterial activity is significantly affected by photo nanocatalysts. Triggering of NiFe₂O₄ by laser excitation showed the highest HPR of 7.83 mL /h within 24 h, which greatly reduces the lag time. The microbial community investigation showed that the addition of NiFe₂O₄ photo nanocatalysts and the change of light source effectively improved the microbial community structure and increased the abundance of hydrogen-producing bacteria (HPB) which leads to enhanced hydrogen production.

Metagenomic analysis reveals the efficient digestion mechanism of corn stover in Angus bull rumen: Microbial community succession, CAZyme composition and functional gene expression

Ruminant rumen is a biological fermentation system that can efficiently degrade lignocellulosic biomass. The knowledge about mechanisms of efficient lignocellulose degradation with rumen microorganisms is still limited. In this study, composition and succession of bacteria and fungi, carbohydrate-active enzymes (CAZymes), and functional genes involved in hydrolysis and acidogenesis were revealed during fermentation in Angus bull rumen via metagenomic sequencing. Results showed that degradation efficiency of hemicellulose and cellulose reached 61.2% and 50.4% at 72 h fermentation, respectively. Main bacterial genera were composed of Prevotella, Butyrivibrio, Ruminococcus, Eubacterium, and Fibrobacter, and main fungal genera were composed of Piromyces, Neocallimastix, Anaeromyces, Aspergillus, and Orpinomyces. Principal coordinates analysis indicated that community structure of bacteria and fungi dynamically changed during 72 h fermentation. Bacterial networks with higher complexity had stronger stability than fungal networks. Most CAZyme families showed a significant decrease trend after 48 h fermentation. Functional genes related to hydrolysis decreased at 72 h, while functional genes involved in acidogenesis did not change significantly. These findings provide a in-depth understanding of mechanisms of lignocellulose degradation in Angus bull rumen, and may guide the construction and enrichment of rumen microorganisms in anaerobic fermentation of waste biomass.

Different pre-treatments and kinetic models for bioethanol production from lignocellulosic biomass: A review

Lignocellulosic biomass is the generally explored substrate to produce bioethanol for environmental sustainability due to its availability in abundance. However, the complex network of cellulose-hemicellulose-lignin present in it makes its hydrolysis as a challenging task. To boost the effectiveness of conversion, biomass is pre-treated before enzymatic hydrolysis to alter or destroy its original composition. Enzymes like Cellulases are widely used for breaking down cellulose into fermentable sugars. Enzymatic hydrolysis is a complex process involving many influencing factors such as pH, temperature, substrate concentration. This review presents major four pre-treatment methods used for hydrolysing different substrates under varied reaction conditions along with their mechanism and limitations. A relative comparison of data analysis for most widely studied 10 kinetic models is briefly explained in terms of substrates used to get the brief insight about hydrolysis rates. The summary of pre-treatment methods and hydrolysis rates including cellulase enzyme kinetics will be the value addition for upcoming researchers for optimising the hydrolysis process.

Simultaneous and rate-coordinated conversion of lignocellulose derived glucose, xylose, arabinose, mannose, and galactose into D-lactic acid production facilitates D-lactide synthesis

D-lactide is the precursor of poly(D-lactide) (PDLA) or stereo-complex with poly(L-lactide) (PLLA). Lignocellulosic biomass provides the essential feedstock option to synthesize D-lactic acid and D-lactide. The residual sugars in D-lactic acid fermentation broth significantly blocks the D-lactide synthesis. This study showed a simultaneous and rate-coordinated conversion of lignocellulose derived glucose, xylose, arabinose, mannose, and galactose into D-lactic acid by adaptively evolved Pediococcus acidilactici ZY271 by simultaneous saccharification and co-fermentation (SSCF) of wheat straw. The produced D-lactic acid achieved minimum residual sugars (∼1.7 g/L), high chirality (∼99.1%) and high titer (∼128 g/L). A dry acid pretreatment eliminated the wastewater stream generation and the biodetoxification by fungus Amorphotheca resinae ZN1 removed the inhibitors from the pretreatment. The removal of the sugar residues and inhibitor impurities in D-lactic acid production from lignocellulose strongly facilitated the D-lactide synthesis. This study filled the gap in cellulosic D-lactide production from lignocellulose-derived D-lactic acid.

Enzymatic hydrolysis cocktail optimization for the intensification of sugar extraction from sugarcane bagasse

Lignocellulosic biomasses have a very important role as a raw material to produce biofuels and biochemicals. However, a sustainable, efficient, and economically competitive process for the release of sugars from such materials has still not been achieved. In this work, the optimization of the enzymatic hydrolysis cocktail was evaluated as an approach to maximize sugar extraction from mildly pretreated sugarcane bagasse. Different additives and enzymes, including hydrogen peroxide (H₂O₂), laccase, hemicellulase and the surfactants Tween 80 and PEG4000 were added to a cellulolytic cocktail with the aim of improving biomass hydrolysis. An increase of 39 % and 46 % of glucose and xylose concentrations, respectively, compared to the control (when only the cellulolytic cocktail (20 or 35 FPU g^-1 dry mass), was obtained when H₂O₂ (0.24 mM) was added at the beginning of the hydrolysis. On the other hand, the addition of hemicellulase (81-162 μL g^-1 DM) increased the production of glucose up to 38 % and xylose up to 50 %. The findings of this study reveal that it is possible to increase the extraction of sugars from mildly pretreated lignocellulosic biomass by using an appropriate enzymatic cocktail supplemented with additives. This opens up new opportunities for the development of a more sustainable, efficient, and economically competitive process for biomass fractionation.

Bioupgrading of the aqueous phase of pyrolysis oil from lignocellulosic biomass: a platform for renewable chemicals and fuels from the whole fraction of biomass

Pyrolysis, a thermal decomposition without oxygen, is a promising technology for transportable liquids from whole fractions of lignocellulosic biomass. However, due to the hydrophilic products of pyrolysis, the liquid oils have undesirable physicochemical characteristics, thus requiring an additional upgrading process. Biological upgrading methods could address the drawbacks of pyrolysis by utilizing various hydrophilic compounds as carbon sources under mild conditions with low carbon footprints. Versatile chemicals, such as lipids, ethanol, and organic acids, could be produced through microbial assimilation of anhydrous sugars, organic acids, aldehydes, and phenolics in the hydrophilic fractions. The presence of various toxic compounds and the complex composition of the aqueous phase are the main challenges. In this review, the potential of bioconversion routes for upgrading the aqueous phase of pyrolysis oil is investigated with critical challenges and perspectives.

Machine Learning Approach for Determining and Optimizing Influential Factors of Biogas Production from Lignocellulosic Biomass

Machine learning (ML) was used to predict specific methane yields (SMY) with a dataset of 14 features from lignocellulosic biomass (LB) characteristics and operating conditions of completely mixed reactors under continuous feeding mode. The random forest (RF) model was best suited for predicting SMY with a coefficient of determination (R²) of 0.85 and root mean square error (RMSE) of 0.06. Biomass compositions greatly influenced SMYs from LB, and cellulose prevailed over lignin and biomass ratio as the most important feature. Impact of LB to manure ratio was assessed to optimize biogas production with the RF model. Under typical organic loading rates (OLR), optimum LB to manure ratio of 1:1 was identified. Experimental results confirmed influential factors revealed by the RF model and provided the highest SMY of 79.2% of the predicted value. Successful applications of ML for anaerobic digestion modelling and optimization specifically for LB were revealed in this work.

Valorization of lignocellulosic biomass into sustainable materials for adsorption and photocatalytic applications in water and air remediation

An exponential rise in global pollution and industrialization has led to significant economic and environmental problems due to the insufficient application of green technology for the chemical industry and energy production. Nowadays, the scientific and environmental/industrial communities push to apply new sustainable ways and/or materials for energy/environmental applications through the so-called circular (bio)economy. One of today's hottest topics is primarily valorizing available lignocellulosic biomass wastes into valuable materials for energy or environmentally related applications. This review aims to discuss, from both the chemistry and mechanistic points of view, the recent finding reported on the valorization of biomass wastes into valuable carbon materials. The sorption mechanisms using carbon materials prepared from biomass wastes by emphasizing the relationship between the synthesis route or/and surface modification and the retention performance were discussed towards the removal of organic and heavy metal pollutants from water or air (NO_x, CO₂, VOCs, SO₂, and Hg⁰). Photocatalytic nanoparticle-coated biomass-based carbon materials have proved to be successful composites for water remediation. The review discusses and simplifies the most raised interfacial, photonic, and physical mechanisms that might take place on the surface of these composites under light irradiation. Finally, the review examines the economic benefits and circular bioeconomy and the challenges of transferring this technology to more comprehensive applications.

Selective saccharification of crude glycerol pretreated sugarcane bagasse via fast pyrolysis: reaction kinetics and life cycle assessment

Saccharification is a pivotal step in the conversion of lignocellulose to biofuels and chemicals. In this study, crude glycerol derived from biodiesel production was used in pretreatment to facilitate efficient and clean pyrolytic saccharification of sugarcane bagasse. Delignification, demineralization, destruction of lignin-carbohydrate complex structure, and cellulose crystallinity improvement in crude glycerol pretreated biomass could enhance levoglucosan producing reactions against competitive reactions, and therefore facilitate a kinetically controlled pyrolysis with apparent activation energy increased by 2-fold. Accordingly, selective levoglucosan production (44.4%) was promoted by 6-fold whilst light oxygenates and lignin monomers were limited to <25% in bio-oil. Owing to the high-efficiency saccharification, life cycle assessment suggested that the environmental impacts of the integrated process were less than those of typical acid pretreatment and petroleum-based processes, especially on the acidification (8-fold less) and global warming potential. This study provides an environmentally benign approach to efficient biorefinery and waste management.

Advances in machine learning technology for sustainable advanced biofuel production systems in lignocellulosic biorefineries

In view of the global climate change concerns, the society is approaching towards the development of 'green' and renewable energies for sustainable future. The non-renewable fossil fuels may be largely replaced by renewable energy sources, which could facilitate sustainable growth, energy development and lessen the reliance on conventional energy sources. The traditional methods employed in biorefineries to estimate the data values for the biofuel production systems are often complicated, time-consuming and labour-intensive. Modern machine learning (ML) technologies hold enormous potential in managing high-dimensional complex scientific tasks and improving decision-making in energy distribution networks and systems. The data-driven probabilistic ML algorithms could be applied to smart biofuel systems and networks that may reduce the cost of experimental research while providing accurate estimates of product yields. The current review demonstrates a thorough understanding of the application of different ML models to regulate and monitor the production of biofuels from waste biomass through prediction, optimization and real-time monitoring. The in-depth analysis of the most recent advancements in ML-assisted biofuel production methods, including thermochemical and biochemical processes is provided. Moreover, the ML models in addressing the issues of biofuel supply chains, case studies, scientific challenges and future direction in ML applications are also summarized.

Engineering Corynebacterium glutamicum for de novo production of 2-phenylethanol from lignocellulosic biomass hydrolysate

BACKGROUND

2-Phenylethanol is a specific aromatic alcohol with a rose-like smell, which has been widely used in the cosmetic and food industries. At present, 2-phenylethanol is mainly produced by chemical synthesis. The preference of consumers for "natural" products and the demand for environmental-friendly processes have promoted biotechnological processes for 2-phenylethanol production. Yet, high 2-phenylethanol cytotoxicity remains an issue during the bioproduction process.

RESULTS

Corynebacterium glutamicum with inherent tolerance to aromatic compounds was modified for the production of 2-phenylethanol from glucose and xylose. The sensitivity of C. glutamicum to 2-phenylethanol toxicity revealed that this host was more tolerant than Escherichia coli. Introduction of a heterologous Ehrlich pathway into the evolved phenylalanine-producing C. glutamicum CALE1 achieved 2-phenylethanol production, while combined expression of the aro10. Encoding 2-ketoisovalerate decarboxylase originating from Saccharomyces cerevisiae and the yahK encoding alcohol dehydrogenase originating from E. coli was shown to be the most efficient. Furthermore, overexpression of key genes (aroG^fbr, pheA^fbr, aroA, ppsA and tkt) involved in the phenylpyruvate pathway increased 2-phenylethanol titer to 3.23 g/L with a yield of 0.05 g/g glucose. After introducing a xylose assimilation pathway from Xanthomonas campestris and a xylose transporter from E. coli, 3.55 g/L 2-phenylethanol was produced by the engineered strain CGPE15 with a yield of 0.06 g/g xylose, which was 10% higher than that with glucose. This engineered strain CGPE15 also accumulated 3.28 g/L 2-phenylethanol from stalk hydrolysate.

CONCLUSIONS

In this study, we established and validated an efficient C. glutamicum strain for the de novo production of 2-phenylethanol from corn stalk hydrolysate. This work supplied a promising route for commodity 2-phenylethanol bioproduction from nonfood lignocellulosic feedstock.

Molecular mechanism of engineered Zymomonas mobilis to furfural and acetic acid stress

Acetic acid and furfural (AF) are two major inhibitors of microorganisms during lignocellulosic ethanol production. In our previous study, we successfully engineered Zymomonas mobilis 532 (ZM532) strain by genome shuffling, but the molecular mechanisms of tolerance to inhibitors were still unknown. Therefore, this study investigated the responses of ZM532 and its wild-type Z. mobilis (ZM4) to AF using multi-omics approaches (transcriptomics, genomics, and label free quantitative proteomics). Based on RNA-Seq data, two differentially expressed genes, ZMO_RS02740 (up-regulated) and ZMO_RS06525 (down-regulated) were knocked out and over-expressed through CRISPR-Cas technology to investigate their roles in AF tolerance. Overall, we identified 1865 and 14 novel DEGs in ZM532 and wild-type ZM4. In contrast, 1532 proteins were identified in ZM532 and wild-type ZM4. Among these, we found 96 important genes in ZM532 involving acid resistance mechanisms and survival rates against stressors. Furthermore, our knockout results demonstrated that growth activity and glucose consumption of mutant strains ZM532∆ZMO_RS02740 and ZM4∆ZMO_RS02740 decreased with increased fermentation time from 42 to 55 h and ethanol production up to 58% in ZM532 than that in ZM532∆ZMO_RS02740. Hence, these findings suggest ZMO_RS02740 as a protective strategy for ZM ethanol production under stressful conditions.

Biologically derived copper oxide-based nanocatalyst using Moringa oleifera leaves and its applications in hydrolytic enzymes and biohydrogen production

Due to the limited availability of fossil fuels, pollution causing serious environmental issues, and their continuously rising price, the development of low-cost efficient enzymes and their implementation in biomass-based bioenergy industries are highly demanded. In the present work, phytogenic fabrication of copper oxide based nanocatalyst has been performed using moringa leaves and has been characterized using different techniques. Herein, the impact of different dosages of as-prepared nanocatalyst on fungal co-cultured cellulolytic enzyme production under co-substrate fermentation using wheat straw and sugarcane bagasse in 4:2 ratios in solid state fermentation (SSF) has been investigated. An optimal concentration of 25 ppm of nanocatalyst influenced the production of 32 IU/gds of enzyme, which showed thermal stability at 70 °C for 15 h. Additionally, enzymatic bioconversion of rice husk at 70 °C librated 41 g/L of total reducing sugars, which led to the production of 2390 mL/L of cumulative H₂ in 120 h.

Selective degradation of hemicellulose and lignin for improving enzymolysis efficiency via pretreatment using deep eutectic solvents

Deep eutectic solvents (DESs) with different acidity and alkalinity were applied for biomass pretreatment, and the conditions were optimized by response surface methodology. The results showed that lactic acid/betaine hydrochloride had the optimal pretreatment efficiency, where the removal rates of hemicellulose and lignin came up to 89% and 73%, and the enzymolysis efficiency was as high as 92%. Furthermore, eight types of chloride salts with different valence states were introduced into the DESs as the third component. The chloride salts could improve the pretreatment efficiency and positively correlated with the metal valence state. Specifically, AlCl₃ was significantly superior in improving the pretreatment efficiency, where the enzymolysis efficiency reached 96% due to the destruction of crystalline region and the esterification of partial cellulose. Therefore, it is proposed that adding highly valent metal salts to acidic DESs has higher pretreatment and enzymatic efficiency.

Recombinant Production and Biochemical Characterization of Thermostable Arabinofuranosidase from Acidothermophilic Alicyclobacillus Acidocaldarius

The complete enzymatic degradation of lignocellulosic biomass requires the cooperative action of cellulosic, hemicellulosic, and lignolytic enzymes such as cellulase, xylanase, laccase, galactosidase, and arabinofuranosidase. Arabinofuranosidases (E.C 3.2.1.55), which belong to the glycoside hydrolase family of enzymes, hydrolyze the 1,3- and 1,5-α-arabinosyl bonds in L-arabinose- containing molecules. L-arabinoses are present in hemicellulosic part of lignocellulosic biomass. Arabinofuranosidases also play an important role in the complete hydrolysis of arabinoxylans. Analysis of the genome project and CAZY database revealed two putative arabinofuranosidase genes in the A. acidocaldarius genome. The aim of the study was cloning, heterologous expression, purification and biochemical characterization of the arabinofuranosidase enzyme encoded in A. acidocaldarius genome. For this purpose, the AbfA gene of the arabinofuranosidase protein was cloned into the pQE-40 vector, heterologously expressed in E. coli BL21 GOLD (DE3) and successfully purified using His-Tag. Biochemical characterization of the purified enzyme revealed that A. acidocaldarius arabinofuranosidase exhibited activity over a wide pH and temperature range with optimum activity at 45 ºC and pH 6.5 in phosphate buffer towards 4-nitrophenyl-α-L-arabinofuranoside as the substrate. In addition, the enzyme is highly stable over wide range of temperature and maintaining 60% of its activity after 90 min of incubation at 80 ºC. Through the bioinformatics studies, the homology model of A. acidocaldarius arabinofuranosidase was generated and the substrate binding site and residues located in this site were identified. Further molecular docking analysis revealed that the substrate located in the catalytically active pose and, residues N174, E175, and E294 have direct interaction with 4-nitrophenyl-α-L-arabinofuranoside. Moreover, based on phylogenetic analysis, A. acidocaldarius arabinofuranosidase exists in the sub-group of intracellular arabinofuranosidases, and G. stearothermophilus and B.subtilis arabinofuranosidases are close relatives of A. acidocaldarius arabinofuranosidase. This is the first study to report the gene cloning, recombinant expression and biochemical and bioinformatic characterization of an auxiliary GH51 arabinofuranosidase from an acidothermophilic bacterium A. acidocaldarius.

Enhanced production of itaconic acid from enzymatic hydrolysate of lignocellulosic biomass by recombinant Corynebacteriumglutamicum

Itaconic acid (IA) is a value-added chemical currently produced by Aspergillus terreus from edible glucose and starch but not from inedible lignocellulosic biomass owing to the high sensitivity to fermentation inhibitors present in the hydrolysate of lignocellulosic biomass. To produce IA from lignocellulosic biomass, a gram-positive bacterium, Corynebacterium glutamicum, with a high tolerance to fermentation inhibitors was metabolically engineered to express a fusion protein composed of cis-aconitate decarboxylase from A. terreus responsible for IA formation from cis-aconitate and a maltose-binding protein (malE) from Escherichia coli. The codon-optimized cadA_malE gene was expressed in C. glutamicum ATCC 13032, and the resulting recombinant strain produced IA from glucose. IA concentration increased 4.7-fold by the deletion of the ldh gene encoding lactate dehydrogenase. With the Δldh strain HKC2029, an 18-fold higher IA production was observed from enzymatic hydrolysate of kraft pulp as a model lignocellulosic biomass than from glucose (6.15 and 0.34 g/L, respectively). The enzymatic hydrolysate of kraft pulp contained various potential fermentation inhibitors involved in furan aldehydes, benzaldehydes, benzoic acids, cinnamic acid derivatives, and aliphatic acid. Whereas cinnamic acid derivatives severely inhibited IA production, furan aldehydes, benzoic acids, and aliphatic acid improved IA production at low concentrations. The present study suggests that lignocellulosic hydrolysate contains various potential fermentation inhibitors; however, some of them can serve as enhancers for microbial fermentation likely due to the changing of redox balance in the cell.