Direct and simultaneous determination of representative byproducts in a lignocellulosic hydrolysate of corn stover via gas chromatography–mass spectrometry with a Deans switch

Membrane Bioreactors: A Promising Approach to Enhanced Enzymatic Hydrolysis of Cellulose

The depletion of fossil fuel resources and the negative impact of their use on the climate have resulted in the need for alternative sources of clean, sustainable energy. One available alternative, bioethanol, is a potential substitute for, or additive to, petroleum-derived gasoline. In the lignocellulose-to-bioethanol process, the cellulose hydrolysis step represents a major hurdle that hinders commercialization. To achieve economical production of bioethanol from lignocellulosic materials, the rate and yield of the enzymatic hydrolysis of cellulose, which is preferred over other chemically catalyzed processes, must be enhanced. To achieve this, product inhibition and enzyme loss, which are two major challenges, must be overcome. The implementation of membranes, which can permeate molecules selectively based on their size, offers a solution to this problem. Membrane bioreactors (MBRs) can enhance enzymatic hydrolysis yields and lower costs by retaining enzymes for repeated usage while permeating the products. This paper presents a critical discussion of the use of MBRs as a promising approach to the enhanced enzymatic hydrolysis of cellulosic materials. Various MBR configurations and factors that affect their performance are presented.

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.

Comparison of dilute acid and sulfite pretreatments on Acacia confusa for biofuel application and the influence of its extractives

Chemical components of lignocellulosic biomass may impede biofuel processing efficiency. To understand whether the heartwood of Acacia confusa is suitable for biofuel application, extractive-free heartwood of A. confusa was subjected to dilute acid (DA) or sulfite pretreatments. Sugar recoveries were used to evaluate the performance of different pretreatments. Cell wall properties, such as 4-O-alkylated lignin structures, S/G ratios, and xylan contents, of the pretreated samples showed significant correlations with the enzymatic saccharification of glucan. The 4% bisulfite-pretreated samples produced higher total sugar recoveries than DA-treated samples. The highest total sugar recoveries from DA and sulfite pretreatment were 52.0% (170 °C for 20 min) and 65.3% (4% NaHSO3 and 1% H2SO4), respectively. The results also demonstrated that the existence of extractives in the heartwood of A. confusa hindered the sugar recoveries from both the pretreatments and enzymatic saccharification. Total sugar recoveries were reduced 11.7-17.7% in heartwood samples with extractives.

D-Xylose assimilation via the Weimberg pathway by solvent-tolerant Pseudomonas taiwanensis VLB120

The natural ability of Pseudomonas taiwanensis VLB120 to use xylose as sole carbon and energy source offers a high potential for sustainable industrial biotechnology. In general, three xylose assimilation routes are reported for bacteria. To elaborate the metabolic capacity of P. taiwanensis VLB120 and to identify potential targets for metabolic engineering, an in silico/in vivo experiment was designed, allowing for discrimination between these pathways. Kinetics of glucose and xylose degradation in P. taiwanensis VLB120 was determined and the underlying stoichiometry was investigated by genome-based metabolic modelling and tracer studies using stable isotope labelling. Additionally, reverse transcription quantitative polymerase chain reaction experiments have been performed to link physiology to the genomic inventory. Based on in silico experiments, a labelling strategy was developed, ensuring a measurable and unique (13) C-labelling distribution in proteinogenic amino acids for every possible distribution between the different xylose metabolization routes. A comparison with in vivo results allows the conclusion that xylose is metabolized by P. taiwanensis VLB120 via the Weimberg pathway. Transcriptomic and physiological studies point to the biotransformation of xylose to xylonate by glucose dehydrogenase. The kinetics of this enzyme is also responsible for the preference of glucose as carbon source by cells growing in the presence of glucose and xylose.

A time course analysis of the extracellular proteome of Aspergillus nidulans growing on sorghum stover

BACKGROUND

Fungi are important players in the turnover of plant biomass because they produce a broad range of degradative enzymes. Aspergillus nidulans, a well-studied saprophyte and close homologue to industrially important species such as A. niger and A. oryzae, was selected for this study.

RESULTS

A. nidulans was grown on sorghum stover under solid-state culture conditions for 1, 2, 3, 5, 7 and 14 days. Based on analysis of chitin content, A. nidulans grew to be 4-5% of the total biomass in the culture after 2 days and then maintained a steady state of 4% of the total biomass for the next 12 days. A hyphal mat developed on the surface of the sorghum by day one and as seen by scanning electron microscopy the hyphae enmeshed the sorghum particles by day 5. After 14 days hyphae had penetrated the entire sorghum slurry. Analysis (1-D PAGE LC-MS/MS) of the secretome of A. nidulans, and analysis of the breakdown products from the sorghum stover showed a wide range of enzymes secreted. A total of 294 extracellular proteins were identified with hemicellulases, cellulases, polygalacturonases, chitinases, esterases and lipases predominating the secretome. Time course analysis revealed a total of 196, 166, 172 and 182 proteins on day 1, 3, 7 and 14 respectively. The fungus used 20% of the xylan and cellulose by day 7 and 30% by day 14. Cellobiose dehydrogenase, feruloyl esterases, and CAZy family 61 endoglucanases, all of which are thought to reduce the recalcitrance of biomass to hydrolysis, were found in high abundance.

CONCLUSIONS

Our results show that A. nidulans secretes a wide array of enzymes to degrade the major polysaccharides and lipids (but probably not lignin) by 1 day of growth on sorghum. The data suggests simultaneous breakdown of hemicellulose, cellulose and pectin. Despite secretion of most of the enzymes on day 1, changes in the relative abundances of enzymes over the time course indicates that the set of enzymes secreted is tailored to the specific substrates available. Our findings reveal that A. nidulans is capable of degrading the major polysaccharides in sorghum without any chemical pre-treatment.