Chemoenzymatic valorization of agricultural wastes into 4-hydroxyvaleric acid via levulinic acid

Waste to Energy: Steam explosion-based torrefaction process to produce solid biofuel for power generation utilizing various waste biomasses

Currently, humankind is facing a serious environmental and climate crisis, which has accelerated the research on producing bioenergy from waste biomass as a carbon-neutral feedstock. In this study, the aim was to develop an upcycling strategy for waste biomass to solid-type biofuel conversion for power generation. Various types of waste biomass (i.e., waste wood after lumbering, sawdust-type mushroom waste wood, kudzu vine, and empty fruit bunches from palm) were used as sustainable feedstocks for steam explosion-based torrefaction. The reaction conditions were optimized for each waste biomass by controlling the severity index (Ro); the higher heating value increased proportional to the Ro increase. Additionally, component analysis revealed that steam explosion torrefaction mainly degraded hemicellulose, and most of the torrefied waste biomass met the Bio-Solid Refuse Fuel quality standard. The results provide not only a viable waste-to-energy strategy but also insights to address global climate change.

Cell-free chemoenzymatic cascades with bio-based molecules

For the valorization of various bio-based feedstocks, the combination of different catalytic systems with biocatalysis in chemoenzymatic cascades has been shown to have high potential. However, the development of such integrated catalytic systems is often limited by catalyst incompatibility. Therefore, incorporating novel catalytic concepts into the chemoenzymatic valorization of bio-based feedstocks is currently of great interest. This article provides an overview of the methods/approaches used to advance the development of chemoenzymatic cascades for the catalytic upgrading of bio-based feedstocks. It specifically focuses on recent developments in the combination of enzymes with organo- and chemocatalysis. Furthermore, current applications and future perspectives of integrating novel catalytic systems such as photo- and electrocatalysis toward new synthetic routes for the utilization of the often highly functionalized bio-based compounds are reviewed.

Engineering of Yarrowia lipolytica as a platform strain for producing adipic acid from renewable resource

There is an increasing demand for bio-based dicarboxylic acids (DCA) as an eco-friendly alternatives to chemically synthesized DCA. Adipic acid, which is not naturally produced by microorganisms, is an essential DCA with significant industrial importance. This study aimed to develop a platform strain using Yarrowia lipolytica for efficient bioconversion of renewable resources into adipic acid. To prevent the complete oxidation of adipic acid, peroxisomal β-oxidation was engineered by selectively disrupting acyl-CoA oxidases. Furthermore, ω-oxidation activity was improved via introducing an additional copy of cytochrome P450 monooxygenase (ALK5) and reductase (CPR1) with fatty alcohol oxidase (FAO1). The production phase used SP92D medium in a two-stage bioconversion process, during which the engineered strain exhibited the highest production level, achieving a remarkable 9.7-fold increase compared to that of the parental strain. To our knowledge, this is the first report demonstrating that engineered Y. lipolytica can produce adipic acid from fatty acid methyl esters.

Synergetic effect of lytic polysaccharide monooxygenase from Thermobifida fusca on saccharification of agrowastes

Saccharification is one of the most noteworthy processes in biomass-based biorefineries. In particular, the lytic polysaccharide monooxygenase has recently emerged as an oxidative cleavage-recalcitrant polysaccharide; however, there is insufficient information regarding its application to actual biomass. Accordingly, this study focused optimizing the recombinant expression level of a bacterial lytic polysaccharide monooxygenase from Thermobifida fusca (TfLPMO), which was characterized as a cellulolytic enzyme. Finally, the synergistic effect of the lytic polysaccharide monooxygenase and a commercial cellulase cocktail on the saccharification of agrowaste was investigated. TfLPMO functioned on various cellulosic and hemicellulosic substrates, and the combination of TfLPMO with cellulase exhibited a synergistic effect on the saccharification of agrowastes, resulting in a 19.2% and 14.1% increase in reducing sugars from rice straw and corncob, respectively. The results discussed herein can lead to an in-depth understanding of enzymatic saccharification and suggest viable options for valorizing agrowastes as renewable feedstocks in biorefineries.

Ganoderma lucidum bran-derived blue-emissive and green-emissive carbon dots for detection of copper ions

Ganoderma lucidum bran (GB) has a broad application prospect in the preparation of activated carbon, livestock feed, and biogas, but the preparation of carbon dots (CDs) from GB has never been reported. In this work, GB was applied as a carbon source and nitrogen source to prepare both blue fluorescent CDs (BCDs) and green fluorescent CDs (GCDs). The former were prepared at 160 °C for 4 h by a hydrothermal approach, while the latter were acquired at 25 °C for 24 h by chemical oxidation. Two kinds of as-synthesized CDs exhibited unique excitation-dependent fluorescence behavior and high fluorescent chemical stability. Based on the fantastic optical behavior of the CDs, they were utilized as probes for fluorescent determination of copper ions (Cu²⁺). In the range of 1-10 μmol L^-1, the fluorescent intensity of BCDs and GCDs decreased linearly with the increase of Cu²⁺ concentration; the linear correlation coefficient reached 0.9951 and 0.9982, and the limit of detection (LOD) was 0.74 and 1.08 μmol L^-1, respectively. In addition, these CDs remained stable in 0.001-0.1 mmol L^-1 salt solutions; BCDs were more stable in the neutral pH range, but GCDs were more stable in neutral to alkaline conditions. The CDs prepared from GB are not only simple and low-cost, but also can realize the comprehensive utilization of biomass.

Development of bioprocess for corncob-derived levulinic acid production

Levulinic acid is a significant platform chemical obtained from biomass and can potentially be used to produce value-added biofuels, biopolymers, and biopharmaceuticals. This study aims at statistically optimizing levulinic acid production from agrowastes. Based on the total carbohydrate content (71.93 %), corncob was selected as the target feedstock. A Box-Behnken design with four factors, such as feedstock concentration, reaction time, reaction temperature, and catalyst concentration, was used to optimize the hydrothermal conversion of corncob to levulinic acid at 180 °C for 30 min using 1 M H₂SO₄ as the acid catalyst and 120 g/L corncob. The maximum yield of 19.9 % was obtained. Additionally, 8.1 g/L formic acid was co-produced. The results of this study can contribute toward valorization of levulinic acid. Moreover, our results can be useful in developing strategies to utilize agrowastes as a renewable feedstock for recent biorefineries to cope with the climate crisis.

Bioresource Upgrade for Sustainable Energy, Environment, and Biomedicine

We conceptualize bioresource upgrade for sustainable energy, environment, and biomedicine with a focus on circular economy, sustainability, and carbon neutrality using high availability and low utilization biomass (HALUB). We acme energy-efficient technologies for sustainable energy and material recovery and applications. The technologies of thermochemical conversion (TC), biochemical conversion (BC), electrochemical conversion (EC), and photochemical conversion (PTC) are summarized for HALUB. Microalgal biomass could contribute to a biofuel HHV of 35.72 MJ Kg-1 and total benefit of 749 $/ton biomass via TC. Specific surface area of biochar reached 3000 m2 g-1 via pyrolytic carbonization of waste bean dregs. Lignocellulosic biomass can be effectively converted into bio-stimulants and biofertilizers via BC with a high conversion efficiency of more than 90%. Besides, lignocellulosic biomass can contribute to a current density of 672 mA m-2 via EC. Bioresource can be 100% selectively synthesized via electrocatalysis through EC and PTC. Machine learning, techno-economic analysis, and life cycle analysis are essential to various upgrading approaches of HALUB. Sustainable biomaterials, sustainable living materials and technologies for biomedical and multifunctional applications like nano-catalysis, microfluidic and micro/nanomotors beyond are also highlighted. New techniques and systems for the complete conversion and utilization of HALUB for new energy and materials are further discussed.

Rice straw-derived lipid production by HMF/furfural-tolerant oleaginous yeast generated by adaptive laboratory evolution

Research on producing medium- and long-chain hydrocarbons as drop-in biofuels has recently accelerated. In addition, lipids are emerging as precursors for biofuel production, and thus, microbial lipid production utilizing agrowastes is becoming a feasible platform technology. Nonetheless, microorganisms are often inhibited by furan aldehydes in biomass-derived hydrolysates. Accordingly, this study aimed to develop oleaginous yeast strains that can tolerate furan aldehydes for producing lipids as biofuel precursors. Rhodosporidium toruloides was selected as the target for adaptive laboratory evolution. The evolved strain, which was obtained from 16 rounds of subcultures, showed a 2.5-fold higher specific growth rate than the wild-type strain in the presence of furan aldehydes and slightly higher lipid production in rice straw hydrolysate. The results discussed in this study provide insights into the production of lipid production by oleaginous yeast utilizing agrowastes as feedstock to obtain drop-in biofuels and contribute to feasible strategies to address climate crises.

Lytic polysaccharide monooxygenase (LPMO)-derived saccharification of lignocellulosic biomass

Given that traditional biorefineries have been based on microbial fermentation to produce useful fuels, materials, and chemicals as metabolites, saccharification is an important step to obtain fermentable sugars from biomass. It is well-known that glycosidic hydrolases (GHs) are responsible for the saccharification of recalcitrant polysaccharides through hydrolysis, but the discovery of lytic polysaccharide monooxygenase (LPMO), which is a kind of oxidative enzyme involved in cleaving polysaccharides and boosting GH performance, has profoundly changed the understanding of enzyme-based saccharification. This review briefly introduces the classification, structural information, and catalytic mechanism of LPMOs. In addition to recombinant expression strategies, synergistic effects with GH are comprehensively discussed. Challenges and perspectives for LPMO-based saccharification on a large scale are also briefly mentioned. Ultimately, this review can provide insights for constructing an economically viable lignocellulose-based biorefinery system and a closed-carbon loop to cope with climate change.

Substrate-inducible and antibiotic-free high-level 4-hydroxyvaleric acid production in engineered Escherichia coli

In this study, we developed a levulinic acid (LA)-inducible and antibiotic-free plasmid system mediated by HpdR/PhpdH and infA-complementation to produce 4-hydroxyvaleric acid (4-HV) from LA in an engineered Escherichia coli strain. The system was efficiently induced by the addition of the LA substrate and resulted in tight dose-dependent control and fine-tuning of gene expression. By engineering the 5′ untranslated region (UTR) of hpdR mRNA, the gene expression of green fluorescent protein (GFP) increased by at least two-fold under the hpdH promoter. Furthermore, by evaluating the robustness and plasmid stability of the proposed system, the engineered strain, IRV750f, expressing the engineered 3-hydroxybutyrate dehydrogenase (3HBDH∗) and formate dehydrogenase (CbFDH), produced 82 g/L of 4-HV from LA, with a productivity of 3.4 g/L/h and molar conversion of 92% in the fed-batch cultivation (5 L fermenter) without the addition of antibiotics or external inducers. Overall, the reported system was highly beneficial for the large-scale and cost-effective microbial production of value-added products and bulk chemicals from the renewable substrate, LA.

Newly explored formate dehydrogenases from Clostridium species catalyze carbon dioxide to formate

With concerns over global warming and climate change, many efforts have been devoted to mitigate atmospheric CO₂ level. As a CO₂ utilization strategy, formate dehydrogenase (FDH) from Clostridium species were explored to discover O₂-tolerant and efficient FDHs that can catalyze CO₂ to formate (i.e. CO₂ reductase). With FDH from Clostridium ljungdahlii (ClFDH) that plays as a CO₂ reductase previously reported as the reference, FDH from C.autoethanogenum (CaFDH), C. coskatii (CcFDH), and C. ragsdalei (CrFDH) were newly discovered via genome-mining. The FDHs were expressed in Escherichia coli and the recombinant FDHs successfully catalyzed CO₂ reduction with a specific activity of 15 U g^-1-CaFDH, 17 U g^-1-CcFDH, and 8.7 U g^-1-CrFDH. Interestingly, all FDHs newly discovered retain their catalytic activity under aerobic condition, although Clostridium species are strict anaerobe. The results discussed herein can contribute to biocatalytic CO₂ utilization.

Enzymatic synthesis of a novel solid-liquid phase change energy storage material based on levulinic acid and 1,4-butanediol

The current energy crisis has prompted the development and utilization of renewable energy and energy storage material. In this study, levulinic acid (LA) and 1,4-butanediol (BDO) were used to synthesize a novel levulinic acid 1,4-butanediol ester (LBE) by both enzymatic and chemical methods. The enzymatic method exhibited excellent performance during the synthesis process, and resulted in 87.33% of LBE yield, while the chemical method caused more by-products and higher energy consumption. What's more, the thermal properties of the obtained LBE as a phase change material (PCM) were evaluated. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) showed that the melting temperature, latent heat of melting, and pyrolysis temperature were 50.51 °C, 156.1 J/g, and 150-160 °C, respectively. Compared with the traditional paraffin, the prepared PCM has a superior phase transition temperature, a higher latent heat of melting, and better thermal stability. The thermal conductivity could be increased to 0.34 W/m/k after adding expanded graphite (EG). In summary, LBE has great potential in the application of energy storage as a low-temperature phase change energy storage material.

Effect of manganese peroxidase on the decomposition of cellulosic components: Direct cellulolytic activity and synergistic effect with cellulase

Herein, it was unearthed that manganese peroxidase (MnP) from Phanerochaete chrysosporium, a lignin-degrading enzyme, is capable of not only directly decomposing cellulosic components but also boosting cellulase activity. MnP decomposes various cellulosic substrates (carboxymethyl cellulose, cellobiose [CMC], and Avicel®) and produces reducing sugars rather than oxidized sugars such as lactone and ketoaldolase. MnP with Mn^II in acetate buffer evolves the Mn^III-acetate complex functioning as a strong oxidant, and the non-specificity of Mn^III-acetate enables cellulose-decomposition. The catalytic mechanism was proposed by analyzing catalytic products derived from MnP-treated cellopentaose. Notably, MnP also boosts cellulase activity on CMC and Avicel®, even considering the cellulolytic activity of MnP itself. To the best of the authors' knowledge, this is the first report demonstrating a previously unknown fungal MnP activity in cellulose-decomposition in addition to a known delignification activity. Consequently, the results provide a promising insight for further investigation of the versatility of lignin-degrading biocatalysts.