Reaction of furfural and methylfurfural with DNA: use of single-strand-specific nucleases

Investigation of the formation of furfural compounds in apple products treated with pasteurization and high pressure processing

The thermal treatment carried out in the processing of apple products is very likely to induce Maillard reaction to produce furfurals, which have raised toxicological concerns. This study aimed to elucidate the formation of furfural compounds in apple products treated with pasteurization and high pressure processing (HPP). The method for simultaneous determination of five furfural compounds including 5-hydroxymethyl-2-furfural (5-HMF), furfural (F), 4-hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF), 2-acetylfuran (FMC), and 5-Methyl-2-furfural (MF) using high performance liquid chromatography equipped with diode array detector (HPLC-DAD) was successfully developed and validated. All five furfurals exhibited an increasing trend after the pasteurization treatment of apple clear juice, cloudy juice, and puree. 5-HMF, F, FMC, and MF were increased significantly during the precooking of apple puree. Whereas there was no significant change in the furfurals formation after apple products treated with high pressure processing (HPP) with 300 MPa and 15 min. Based on the variation of the fructose, glucose and sucrose detected in apple products after thermal treatment, it revealed that the saccharides and thermal treatment have great effect on the furfural compounds formation. The commercial fruit juice samples with different treatments and fruit puree samples treated with pasteurization were also analyzed. Five furfurals were detected more frequently in the fruit juice samples treated with pasteurization or ultra-high temperature instantaneous sterilization (UHT) than those treated with HPP. 5-HMF and FMC were detected in all fruit puree samples treated with pasteurization, followed by F, MF, and HDMF with the detection rate of 79.31 %, 72.41 %, and 51.72 %. The results could provide a reference for risk assessment of furfural compounds and dietary guidance of fruit products for human, especially for infants and young children. Moreover, moderate HPP treatment with 300 MPa and 15 min would be a worthwhile alternative processing technology in the fruit juice and puree production to reduce the formation of furfural compounds.

Overexpression of Oxidoreductase YghA Confers Tolerance of Furfural in Ethanologenic Escherichia coli Strain SSK42

Furfural is a common furan inhibitor formed due to dehydration of pentose sugars, like xylose, and acts as an inhibitor of microbial metabolism. Overexpression of NADH-specific FucO and deletion of NADPH-specific YqhD had been a successful strategy in the past in conferring tolerance against furfural in Escherichia coli, which highlights the importance of oxidoreductases in conferring tolerance against furfural. In a screen consisting of various oxidoreductases, dehydrogenases, and reductases, we identified the yghA gene as an overexpression target to confer tolerance against furfural. YghA preferably used NADH as a cofactor and had an apparent Km value of 0.03 mM against furfural. In the presence of 1 g liter-1 furfural and 10% xylose (wt/vol), yghA overexpression in an ethanologenic E. coli strain SSK42 resulted in an ethanol efficiency of ∼97%, with a 5.3-fold increase in ethanol titers compared to the control. YghA also exhibited activity against the less toxic inhibitor 5-hydroxymethyl furfural, which is formed due to dehydration of hexose sugars, and thus is a formidable target for overexpression in ethanologenic strain for fermentation of sugars in biomass hydrolysate. IMPORTANCE Lignocellulosic biomass represents an inexhaustible source of carbon for second-generation biofuels. Thermo-acidic pretreatment of biomass is performed to loosen the lignocellulosic fibers and make the carbon bioavailable for microbial metabolism. The pretreatment process also results in the formation of inhibitors that inhibit microbial metabolism and increase production costs. Furfural is a potent furan inhibitor that increases the toxicity of other inhibitors present in the hydrolysate. Thus, it is desirable to engineer furfural tolerance in E. coli for efficient fermentation of hydrolysate sugars.

Bioprospecting of Native Efflux Pumps To Enhance Furfural Tolerance in Ethanologenic Escherichia coli

Efficient microbial conversion of lignocellulose into valuable products is often hindered by the presence of furfural, a dehydration product of pentoses in hemicellulose sugar syrups derived from woody biomass. For a cost-effective lignocellulose microbial conversion, robust biocatalysts are needed that can tolerate toxic inhibitors while maintaining optimal metabolic activities. A comprehensive plasmid-based library encoding native multidrug resistance (MDR) efflux pumps, porins, and select exporters from Escherichia coli was screened for furfural tolerance in an ethanologenic E. coli strain. Small multidrug resistance (SMR) pumps, such as SugE and MdtJI, as well as a lactate/glycolate:H⁺ symporter, LldP, conferred furfural tolerance in liquid culture tests. Expression of the SMR pump potentially increased furfural efflux and cellular viability upon furfural assault, suggesting novel activities for SMR pumps as furfural efflux proteins. Furthermore, induced expression of mdtJI enhanced ethanol fermentative production of LY180 in the presence of furfural or 5-hydroxymethylfurfural, further demonstrating the applications of SMR pumps. This work describes an effective approach to identify useful efflux systems with desired activities for nonnative toxic chemicals and provides a platform to further enhance furfural efflux by protein engineering and mutagenesis.IMPORTANCE Lignocellulosic biomass, especially agricultural residues, represents an important potential feedstock for microbial production of renewable fuels and chemicals. During the deconstruction of hemicellulose by thermochemical processes, side products that inhibit cell growth and production, such as furan aldehydes, are generated, limiting cost-effective lignocellulose conversion. Here, we developed a new approach to increase cellular tolerance by expressing multidrug resistance (MDR) pumps with putative efflux activities for furan aldehydes. The developed plasmid library and screening methods may facilitate new discoveries of MDR pumps for diverse toxic chemicals important for microbial conversion.

Toxicological challenges to microbial bioethanol production and strategies for improved tolerance

Bioethanol production output has increased steadily over the last two decades and is now beginning to become competitive with traditional liquid transportation fuels due to advances in engineering, the identification of new production host organisms, and the development of novel biodesign strategies. A significant portion of these efforts has been dedicated to mitigating the toxicological challenges encountered across the bioethanol production process. From the release of potentially cytotoxic or inhibitory compounds from input feedstocks, through the metabolic co-synthesis of ethanol and potentially detrimental byproducts, and to the potential cytotoxicity of ethanol itself, each stage of bioethanol production requires the application of genetic or engineering controls that ensure the host organisms remain healthy and productive to meet the necessary economies required for large scale production. In addition, as production levels continue to increase, there is an escalating focus on the detoxification of the resulting waste streams to minimize their environmental impact. This review will present the major toxicological challenges encountered throughout each stage of the bioethanol production process and the commonly employed strategies for reducing or eliminating potential toxic effects.

Genome-wide mapping of furfural tolerance genes in Escherichia coli

Advances in genomics have improved the ability to map complex genotype-to-phenotype relationships, like those required for engineering chemical tolerance. Here, we have applied the multiSCale Analysis of Library Enrichments (SCALEs; Lynch et al. (2007) Nat. Method.) approach to map, in parallel, the effect of increased dosage for >10(5) different fragments of the Escherichia coli genome onto furfural tolerance (furfural is a key toxin of lignocellulosic hydrolysate). Only 268 of >4,000 E. coli genes (∼ 6%) were enriched after growth selections in the presence of furfural. Several of the enriched genes were cloned and tested individually for their effect on furfural tolerance. Overexpression of thyA, lpcA, or groESL individually increased growth in the presence of furfural. Overexpression of lpcA, but not groESL or thyA, resulted in increased furfural reduction rate, a previously identified mechanism underlying furfural tolerance. We additionally show that plasmid-based expression of functional LpcA or GroESL is required to confer furfural tolerance. This study identifies new furfural tolerant genes, which can be applied in future strain design efforts focused on the production of fuels and chemicals from lignocellulosic hydrolysate.

Molecular adaptation mechanisms employed by ethanologenic bacteria in response to lignocellulose-derived inhibitory compounds

Current international interest in finding alternative sources of energy to the diminishing supplies of fossil fuels has encouraged research efforts in improving biofuel production technologies. In countries which lack sufficient food, the use of sustainable lignocellulosic feedstocks, for the production of bioethanol, is an attractive option. In the pre-treatment of lignocellulosic feedstocks for ethanol production, various chemicals and/or enzymatic processes are employed. These methods generally result in a range of fermentable sugars, which are subjected to microbial fermentation and distillation to produce bioethanol. However, these methods also produce compounds that are inhibitory to the microbial fermentation process. These compounds include products of sugar dehydration and lignin depolymerisation, such as organic acids, derivatised furaldehydes and phenolic acids. These compounds are known to have a severe negative impact on the ethanologenic microorganisms involved in the fermentation process by compromising the integrity of their cell membranes, inhibiting essential enzymes and negatively interact with their DNA/RNA. It is therefore important to understand the molecular mechanisms of these inhibitions, and the mechanisms by which these microorganisms show increased adaptation to such inhibitors. Presented here is a concise overview of the molecular adaptation mechanisms of ethanologenic bacteria in response to lignocellulose-derived inhibitory compounds. These include general stress response and tolerance mechanisms, which are typically those that maintain intracellular pH homeostasis and cell membrane integrity, activation/regulation of global stress responses and inhibitor substrate-specific degradation pathways. We anticipate that understanding these adaptation responses will be essential in the design of 'intelligent' metabolic engineering strategies for the generation of hyper-tolerant fermentation bacteria strains.

Cellulosic hydrolysate toxicity and tolerance mechanisms in Escherichia coli

The sustainable production of biofuels will require the efficient utilization of lignocellulosic biomass. A key barrier involves the creation of growth-inhibitory compounds by chemical pretreatment steps, which ultimately reduce the efficiency of fermentative microbial biocatalysts. The primary toxins include organic acids, furan derivatives, and phenolic compounds. Weak acids enter the cell and dissociate, resulting in a drop in intracellular pH as well as various anion-specific effects on metabolism. Furan derivatives, dehydration products of hexose and pentose sugars, have been shown to hinder fermentative enzyme function. Phenolic compounds, formed from lignin, can disrupt membranes and are hypothesized to interfere with the function of intracellular hydrophobic targets. This review covers mechanisms of toxicity and tolerance for these compounds with a specific focus on the important industrial organism Escherichia coli. Recent efforts to engineer E. coli for improved tolerance to these toxins are also discussed.