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    31 December 2021, Volume 2 Issue 6
    Current contents in Chinese and English#br#
    2021, 2(6):  0. 
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    CO2 based biomanufacturing: from basic research to industrial application
    Jie REN, Anping ZENG
    2021, 2(6):  854-862.  doi:10.12211/2096-8280.2021-086
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    CO2 captures worldwide attention these days because of the urgent need to counteract its ever increasing in the atmosphere and its impact on climate change, with the ultimate goal of carbon neutrality. Recently, large efforts have been made in CO2 capture and utilization, primarily using physical or chemical means. Biotechnological CO2 capture and utilization has been mainly studied using microalgae, but turned out to be economically less competitive. Large-scale biotechnological capture and utilization of CO2 thus urgently needs new concepts and technologies. The teams of Tan Tianwei and Jens Nielsen (Liu et al, 2020) recently reviewed the advances and challenges in biological CO2 fixation in the context of third-generation (3G) biorefineries. The review gives an excellent survey and valuable discussions on sources of CO2, the natural and synthetic CO2 fixation pathways, use of regenerative energy, and 3G-based products. Perspectives of 3G biorefineries are also presented. It is clear that each CO2 fixation pathway has its benefits and drawbacks and the choice depends on the microbial host, the target product(s), and the preferred process and cultivation conditions. In addition to the technical aspects, the authors also emphasized the necessity of further increasing social, political and economic incentives for continued financial support of research and small companies. This commentary briefly introduces the major points of the review of Liu et al. and discusses further aspects on the move from basic research to industrial application of CO2 based biomanufacturing. In particular, we emphasize the following aspects: (1) More fundamental and quantitative studies on the underlying mechanisms of carbon binding and transformation to significantly increase the efficiency of key enzymes and metabolic modules of the different fixation pathways; (2) The interactions of CO2 fixation pathway with metabolic network and their regulation deserve more systems level and quantitative study; (3) Integration of physical, chemical and electrochemical CO2 capture and transformation methods with biological processes in the sense of biorefineries, also by considering the downstream processing of product recovery; (4) From the perspective of industrial application, autotrophic synthesis-based biomanufaturing has several major technological bottlenecks and economic constraints, mixotrophic biosynthesis (using CO2 and mixed carbon sources) seems a practical solution and deserves more attention; (5) Finally, most of the CO2 fixation pathways and their products are still at the proof of concept stage, engineering breakthroughs are urgently needed for moving from “0 to 1” to“1 to 100”and thus to really contribute to carbon neutrality.

    Invited Review
    Research progress of constructing efficient biomanufacturing system based on synthetic biotechnology
    Xiaolong ZHANG, Chenyun WANG, Yanfeng LIU, Jianghua LI, Long LIU, Guocheng DU
    2021, 2(6):  863-875.  doi:10.12211/2096-8280.2021-015
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    Efficient and environmentally friendly biomanufacturing system based on synthetic biotechnology is an important approach to achieve sustainable development. Synthetic biology is expected to bring revolutionary technological breakthroughs in various industries, such as food, pharmacy and chemistry, as well as farming and animal husbandry. In this paper, the latest advances of technologies and strategies in synthetic biology used in the progress of constructing efficient biomanufacturing systems were introduced. Four aspects, namely metabolic regulation of key genes, enzyme engineering, cofactor engineering and fermentation optimization, were discussed. Through these technologies, engineered microorganisms with high robustness and excellent performance were constructed. Secondly, an emphasis was put on the summary of diverse metabolic characteristics of typical model organisms at present. In this part, as many as seven strains were mentioned, such as Escherichia coli, Bacillus subtilis, Bacillus amyloliquefaciens, Bacillus licheniformis, Corynebacterium glutamicum, Pichia pastoris and Saccharomyces cerevisiae. And for each strain, the advantages and disadvantages of different typical model organisms were discussed to clarify the scope of their most suitable products. Escherichia coli is the most intensively studied typical model organism system, making it the preferred expression system for proteins of interest. However, insufficient post-translational processing limits its applications for expressing eukaryotic-derived proteins. Saccharomyces cerevisiae and Pichia pastoris make up for this deficiency. Yeast expression system has significant advantages for the synthesis of natural products from plant, due to the extensive and in-depth research of P450 enzymes, such as the biosynthesis of artemisinin. Lastly, application prospects of synthetic biology in constructing efficient biomanufacturing systems were discussed. With the developments in standard synthetic biology components and data, standard automated work platforms, precise and generally applicable engineering strategies, emerging of machine learning and synthetic biology, it is expected to facilitate efficient biological manufacturing system construction. Precise and various metabolic engineering technology, flexible and convenient enzyme engineering strategies and whole cell microorganism modeling would be the new driving force for efficient biomanufacturing system construction.

    Recent advances in biosynthesis of chemicals by microbial co-culture
    Xianglai LI, Xiaolin SHEN, Jia WANG, Qipeng YUAN, Xinxiao SUN
    2021, 2(6):  876-885.  doi:10.12211/2096-8280.2020-053
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    Biosynthesis has become an important way of green manufacturing of chemicals. Traditionally, microbial synthesis of chemicals mainly uses a single strain. However, mono-culture often has the following problems:(1) Introduction of complex pathways causes heavy metabolic burden, (2) the cell microenvironment cannot fulfil the functional expression of all enzymes in the pathways, and (3) the mutual interference between modules of different pathways. Inspired by the natural symbiosis, researchers have developed co-culture technology. By cultivating two or more different cells in the same system, they can fully simulate the natural symbiosis environment, realizing the exchange of energy, materials and signals between different species and achieving the purpose of division of labor and metabolic compartmentation. This technology shows outstanding advantages in reduction of the metabolic burden and provision of suitable environment for different enzymes. Co-culture strategy can also be applied in the aspect of utilizing complex (e.g. lignocellulose), mixed (e.g. glucose/xylose/arabinose) or nonconventional (methane, CO and CO2) carbon sources. Synthesis of the target products competes with the native metabolism for precursors, energy and other resources. Introduction of long pathways in a single strain may cause severe metabolic burden. Splitting and distributing pathways to different cells can alleviate such burden. In addition, each module can be optimized independently, and the balance between the modules can be achieved by adjusting the proportion of strains. Studies have shown that co-culture can significantly affect microbial metabolism and activate silent biosynthetic pathways. In recent years, hundreds of new compounds including polyketones, macrolides and diterpenes have been discovered through co-culture techniques. As an emerging technology, microbial co-culture still has many challenges in the prediction and control of the proportion of different strains. This review lists recent successful cases of microbial co-culture to produce chemicals, summarizes the research progress on regulating the strain proportion through quorum sensing and predicting the dynamic changes through computer simulation tools. Finally, the prospects and challenges in this emerging technology is also discussed.

    In vitro biosynthesis of chemicals: pathway design, component assembly and applications-a review
    Yichen WAN, Kongliang XU, Renchao ZHENG, Yuguo ZHENG
    2021, 2(6):  886-901.  doi:10.12211/2096-8280.2021-019
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    Chemical industry is one of the pillar industries in the modern society. The demand for refined chemicals with high-quality and diversity is rapidly increasing with the improvement of society and human being's living standards. As a supplement to conventional chemical synthesis, environmental friendly biosynthesis is attracting widespread attention and will be an important step to achieve a sustainable development. In vitro biosynthesis (cell free biosynthesis) is a synthetic method to prepare desired chemicals, which was catalyzed by purified enzymes or cell extracts. With the development of synthetic biotechnology, in vitro biosynthesis has gradually become one of the most important ways for chemicals production, exhibiting the advantages of environmental friendliness, high catalytic efficiency, good atom economy and strong controllability. Pathway design is the key for the construction of in vitro biosynthesis system. In this review, two important principles for designing in vitro biosynthetic pathways have been summarized, including atom economy and energy optimization. As one of the important concepts of green chemistry, principle atom economy means that the synthesis method or process should be designed to convert raw materials into the final product as much as possible. Principle energy optimization means that the ATP-free or ATP minimized process should be designed in the synthesis method. Assembly enzymes into a multi-enzyme complex via biological macromolecules can increase reaction rate and also reduce the side reactions of the in vitro biosynthesis. Thus, three common-used biological macromolecules for enzyme assembly will be introduced here, including peptide linkers, protein scaffolds, DNA. Some recent examples of chemicals produced viain vitro biosynthesis have also been summarized, including glucosamine, glycerol glucoside, pyruvate, α-ketoglutarate, ethanol, 1,3-propanediol, islatravir, azomycin, and etc. Through the introduction of in vitro biosynthesis of bulk chemicals (carbohydrate chemicals, organic acid chemicals, alcohol chemicals, and etc.), this article demonstrates the potentials of in vitro biosynthesis in chemical synthesis. By reviewing the pathway design, enzyme component assembly and chemicals production examples, the future of in vitro biosynthesis of chemicals has been prospected here. With the design improvement of in vitro biosynthesis, the pathway designing will be becoming more and more intelligent and efficient. It is believed that the efficiency of in vitro biosynthesis of chemicals will be gradually increased and the in vitro biosynthesis hopefully can cover all chemicals' production, which is expected to be one of the predominant ways for chemicals production in the future.

    Progress in artificial metabolic pathways for biosynthesis of organic alcohols & acids
    Chenkai CAO, Jialong LI, Kechun ZHANG
    2021, 2(6):  902-919.  doi:10.12211/2096-8280.2021-049
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    Organic acids and alcohols are usually known as commercial feedstock in chemical engineering and pharmaceutical field. It is also known as potential significant biofuel. Compared with traditional fossil fuels, the characteristics of industrial application of organic acids and alcohols can provide great advantages such as renewable feedstock and green production process. They are effective countermeasures to deal with energy crisis and environmental pollution. Traditional metabolic engineering is realized by overexpression specific genes to elevate the corresponding enzyme level. With certain artificial genetic transformation and modification, a microbial cell factory can be constructed for efficient production of certain organic acids and alcohols. However, problems such as limited fermentation raw materials, low metabolic efficiency, and limited product types arise during practical implementation. Constructing artificial metabolic pathways through screening and recombining nonhomologous enzymes can become an effective solution, which is also a cutting-edge general trend in this field. In this paper, recent breakthroughs and progress in constructing artificial metabolic pathway to produce organic acids and alcohols are reviewed. Five novel biosynthetic pathways are expounded with details including C1 compounds assimilation pathway, nonphosphorylation pathway, ketoacid/ammino acid pathway, RBO pathway (reversal of β-oxidation pathway), and PKS pathway (polyketide pathway). Furthermore, the advantages and limitations compared with traditional chemical-produced technique are also discussed. Potential problems such as the tolerance of one carbon compounds, the inefficient conversion of xylose, the inefficient catalysis of key enzyme reactions in ketoacid amino acid pathway, and the lack of diversity of RBO pathway and PKS pathway products are reported in respected to practical application of the technology. In conclusion, the feasibility of establishing new metabolic pathways for production of specific products is analyzed, which has the potential of providing the possibility for the construction of high cost-efficient and diversified organic acid and organic alcohol biosynthesis platform in the future.

    Synthetic biotechnology drives the development of natural eukaryotic lipid cell factories
    Qingzhuo WANG, Ping SONG, He HUANG
    2021, 2(6):  920-941.  doi:10.12211/2096-8280.2020-090
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    Lipids are important industrial raw materials and one of the three major nutrients for human survival. In order to avoid the blockade of external resource imports due to unpredictable international market, we urgently need to build new methods for lipids supply. Biomass is the only kind of physical resource based on carbon among many renewable resources. Therefore, The production of edible and functional lipids with abundant and cheap biomass materials instead of fossil materials is of great significance in ensuring national energy security and food security. Bacteria, yeast, mold, microalgae and some other microorganisms have the potential in using glucose, lignocellulose, starch, glycerol or even one carbon compounds to synthesize fatty acids. Due to the advantages of short production cycle, easy large-scale production , less land occupation, less impact of climate change and abundant raw material sources comparing with lipips from plants and animals, microbial lipid production has attracted much attention from academia and industry in recent years. However, there are still many challenges on how to obtain efficient and robust microbial lipid cell factories, such as limited enabling tools, low lipid production and difficult control of lipid components. The development of synthetic biology then provides new resources, tools and ideas for research in this field in recent years. Thus the research of lipid producing microorganisms has made continuous breakthroughs in the creation of genetic manipulation tools, the engineering of lipip biosynthesis pathways and the development of high value-added products. This review focuses on the genetic elements of natural lipid-producing chassis strains, genetic transformation methods, genome editing tools development, the reconstruction/debugging of the metabolic pathways of lipid cell factories, and the upgrade to high value-added lipid chemicals. By systematically summarized the research progress of synthetic biotechnology in driving the development of lipid cell factories, we very much hope to provide reference for future research in this area.

    Green biomanufacturing of steroids: from biotransformation to de novo synthesis by microorganisms
    Liangbin XIONG, Lu SONG, Yunqiu ZHAO, Kun LIU, Yongjun LIU, Fengqing WANG, Dongzhi WEI
    2021, 2(6):  942-963.  doi:10.12211/2096-8280.2021-061
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    Steroids are widely distributed in natural organisms and play essential roles in growth, breeding, metabolic regulation, and endocrine homeostasis. To date, diverse steroids have been proved to have numerous therapeutic effects on reproductive health, endocrine regulation, inflammation, etc. and can be used as life-saving drugs for some serious diseases, such as cancer, organ transplantation, and serious infection. Therefore, the industrial synthesis of steroid drugs has developed rapidly and steroid drugs have become the second largest drug category just ranked after antibiotics. Due to the complex structure and delicate configuration, steroids are difficult to be produced at an industrial scale by chemical total synthesis. At present, steroids are mainly produced by semi-synthesis ways with natural steroidal sapogenins or sterols as raw materials via the combination of chemical and biological transformations. However, the above routes are long and complex resulting in the low yield and the overuse of toxic reagents and heavy metal catalysts, thus resulting in a large quantity of wastewater and residue as well as high production costs. Therefore, it is necessary to develop green biomanufacturing technologies, such as biocatalysis, biotransformation, and biosynthesis, for the industrial production of steroids. Currently, the production mode of steroids has been profoundly changed due to the successful application of enzyme-catalyzed reactions and microbial transformations in the industrial production of steroids. It is expected that the production of steroids will be changed into a biomanufacturing mode if the robust microbial cell factories for the de novo biosynthesis of steroids can be developed via the biosynthetic way. The de novo synthesis of steroids by microorganisms has been materialized. However, due to the extraordinarily complex metabolic mechanism of steroids in the nature, the efficient production of steroids in engineered hosts still remains a challenge. Here, starting from the evolution of steroidal pharmaceutical industry, we systematically review recent advances in biomanufacturing technologies of steroids, including the identification and application of steroidal biocatalysis and biotransformation, the characterization and modification of the metabolic mechanisms of steroids in certain microorganisms, and the development of de novo biosynthesis pathways of steroids in engineered cell factories. From the above three aspects, this review provides a reasonable summary and prospect for the current status and future development trend of green bio-manufacturing technologies in the steroidal pharmaceutical industry.

    Advances in bioproduction of feed amino acid by Escherichia coli
    Liang GUO, Cong GAO, Yadi LIU, Xiulai CHEN, Liming LIU
    2021, 2(6):  964-981.  doi:10.12211/2096-8280.2021-042
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    With the rapid development of animal husbandry and fishery, it promotes the development of protein for animal feed. Due to the enhancement of people's safety awareness of food additives, there is an urgent need to enhance the supply of sustainable protein for use in animal feed. Because the protein is composed of amino acids, the feed amino acids can replace protein for animal feed, which can provide enough nutrition for the animal's growth. Thus, the feed amino acid is an important product that has attracted a great attention by investigators because of the widely uses in the fields of animal feed supplement and has broad market potential. Using synthetic biology and metabolic engineering strategy, the metabolic pathway and regulatory network of Escherichia coli (E. coli) was designed, engineered, and reconstructed to obtain suitable E. coli cell factories.These factories provide a promising and sustainable alternative for the production of feed amino acid from renewable feedstock, and is attracting great attention as it is an economic and environmentally friendly bioprocessing. E. coli cell factories offer an alternative strategy to replace chemical refinery, animal and plant extraction, reducing the harm to the environment and dependence on natural resources. In the present review, we discussed the biosynthesis pathway of feed amino acid (lysine, methionine, tryptophan, threonine, valine, and arginine) in E. coli. According to the biosynthesis pathway of feed amino acid, the production bottlenecks of feed amino acid in E. coli cell factories were discussed. We also summarized recent studies about in the feed amino acid production from the constructed and optimized of E. coli cell factory. Furthermore, we proposed future research directions to improve the technical level of feed amino acid and enhance the robustness of E. coli cell factories for reducing the cost of bioproduction, simplifying downstream separation, and enhancing industrial productivity.

    Strategies of cell factory construction for the production of aromatic amino acids and their derivatives
    Wei SUN, Dongqin DING, Danyang BAI, Yaru ZHU, Xiaotong XIE, Dawei ZHANG
    2021, 2(6):  982-999.  doi:10.12211/2096-8280.2021-002
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    Aromatic amino acids (including L-tryptophan, L-phenylalanine and L-tyrosine) and their derivatives have been widely used in medicine, food, feed and chemical industry due to their specific physiological properties. The production of aromatic amino acids and their derivatives by recombinant microbial fermentation is an effective way to meet the increasing global demand. By combining metabolic engineering strategy with the developments of synthetic biology, systems biology and bioengineering, remarkable progress has been made in the strains modifications and improvements. However, the metabolic pathways for the synthesis of aromatic amino acids and their derivatives are long and their regulatory mechanisms are complicated, so it is very difficult to significantly improve the yield through simple metabolic pathway-modification. Therefore, many relevant modification methods have emerged in recent years, providing a good reference for overcoming the rate limit problem in the metabolic pathways. In this paper, we review and compare the recent mature technologies and strategies applied in the synthesis of aromatic amino acids and their derivatives, including the commonly used metabolic pathway modification strategies, such as increasing the supply of precursors, removing the feedback inhibition for key enzymes, eliminating the repression of repressor proteins, regulating the transport system and global metabolic network, the coupling of strain's growth and product's production and introducing exogenous related enzymes and so on. Various methods of strain construction are also included, such as the high-throughput screening based on biosensors and optimization of culture medium and culture conditions and so on. Finally, we also discuss the prospect of relevant cutting-edge technologies such as computer design of protein, computer de novo design of enzyme, computer design of metabolic pathway, alphafold algorithm for accurate prediction of protein structure and bifunctional enzyme, and the directed evolution technology for screening high yield strains such as the batch and continuation of the directed evolution.

    Advances and perspective on bioproduction of 5-aminolevulinic acid
    Jiuzhou CHEN, Yu WANG, Wei PU, Ping ZHENG, Jibin SUN
    2021, 2(6):  1000-1016.  doi:10.12211/2096-8280.2021-010
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    As a functional non-proteinogenic amino acid, 5-aminolevulinic acid (5-ALA) is naturally synthesized by microbes, plants, and animals. It is a precursor for biosynthesis of tetrapyrrole compounds, such as heme, porphyrin, chlorophyll, and vitamin B12. Because of the critical roles of tetrapyrrole compounds in cellular metabolism, 5-ALA has gained increasing attention in the fields of medicine, health care, agriculture, and animal husbandry. Methods for chemical synthesis of 5-ALA have been established for decades and are the primary routes for industrial production of 5-ALA. However, the high complexity and relatively low yield of the synthesis process lead to the high price of 5-ALA, which seriously limits the production scale and its widespread applications, especially in the fields of agriculture and animal feed. As an alternative technology, bioproduction of 5-ALA from renewable resources holds great promise to simplify the production process and lower the production cost, and thus has received increasing attentions worldwide. Although some algae and photosynthetic bacteria are capable of synthesizing 5-ALA naturally, the production levels cannot meet the requirement of industrialization and commercialization. Moreover, these microorganisms are usually difficult to engineer due to lack of advanced genome editing tools. With the development of systems biology and synthetic biology approaches, intensive studies have focused on engineering platform microorganisms such as Escherichia coli and Corynebacterium glutamicum for 5-ALA bioproduction. Despite many successes in engineering synthetic 5-ALA producing strains, challenges remain in improving the production indices (titer, yield, and productivity) to levels as high as those for some proteinogenic amino acids, such as lysine and glutamate. In this paper, we review the development history of 5-ALA bioproduction technologies in the last half century and summarize the three key strategies for strain development and improvement, including mutagenesis and screening of natural strains, production by Escherichia coli expressing heterogenous 5-aminolevulinic acid synthases, and microbial cell factories constructed by metabolic engineering strategies. Recent advances on engineering synthetic 5-ALA producers using metabolic engineering and synthetic biotechnology are focused in this review. Furthermore, the bottlenecks of 5-ALA biosynthesis, such as the complex regulation of heme biosynthesis and the combined supply of multiple substrates, are also discussed in this review. Finally, the future development of 5-ALA biosynthesis technology in the era of synthetic biology is prospected from the perspectives of new gene targets, more suitable platform microorganisms and novel technical strategies.

    Progress in construction and applications of methanotrophic cell factory for chemicals biosynthesis
    Shuqi GUO, Ziyue JIAO, Qiang FEI
    2021, 2(6):  1017-1029.  doi:10.12211/2096-8280.2021-011
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    Methane has been considered as a potential carbon source in industrial biotechnology because of its abundance, sustainability, high reducibility, and microbial availability. The biological conversion of methane into chemicals or fuels does not only reduce greenhouse gas emissions, but also substitute food-based substrates used in bio-manufacturing. Methanotrophs are gram-negative bacteria, and most are isolated from methane-plentiful environments. Owing to the presence of the methane monooxygenase, methanotrophs constitute a unique group of microbes. As an important industrially-promising microorganism with the characteristics of robust and anti-contamination ability, methanotrophs capable of growing with methane as the sole energy and carbon source play a significant role in carbon-neutral society by replacing petroleum-based products with biosynthesized products. Therefore, studies on methanotrophs for the biological conversion of methane have attracted extensive attention in recent years. With the rapid development of genetic manipulations tools and strategies for metabolically-engineered methanotrophs construction, including gene editing methods, regulation of metabolic pathways, and bio-elements mining, methanotrophic cell factories have been employed to efficiently convert methane into a variety of bulk chemicals and biofuels. In this review, biosynthetic technologies related to bioconversion of under-utilized methane ranging from fundamental understanding, systematic analysis, metabolic engineering to bio-product production are introduced. The genetic manipulations tools of methanotrophs, the approaches of methanotrophic cell factory construction, and the enhancement of methane assimilation efficiency are summarized from the aspects including the research progress of genetic engineering of methanotrophs, the regulation of methane carbon flux, the overexpression of heterologous pathway genes, and the accumulation of metabolic intermediates. Besides, the applications of genomics, transcriptomics, metabolomics, and metabolic modeling have been also deployed to facilitate the methane metabolism in methanotrophs chassis. Finally, given the strategy of 'waste-to-value' production, the challenges and opportunities for methane bioconversion by methanotrophs are also discussed and prospected based on industrial applications in terms of the research progress in the biosynthesis of methane-based acids, terpenes, alcohols, and other chemicals.

    Research progress in 2-phenylethanol production through biological processes
    Wei YAN, Hao GAO, Yujia JIANG, Xiujuan QIAN, Jie ZHOU, Weiliang DONG, Wenming ZHANG, Fengxue XIN, Min JIANG
    2021, 2(6):  1030-1045.  doi:10.12211/2096-8280.2020-096
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    2-Phenylethanol (2-PE), an important flavor and fragrance compound with a rose-like smell has been widely used in the cosmetics, perfume, and food industries. Furthermore, it is also an important raw material for the derivatives synthesis for the synthesis of other flavors or pharmaceutical compounds, such as 2-phenylethylacetate (2-PEAc) and phenylacetaldehyde (PA). Conventional production of 2-PE is mainly through the extraction from plant materials, such as hyacinths, jasmine and lilies. However, the extraction process is very complicated and the harvest of flowers is greatly influenced by the weather, plant diseases, and trade restrictions, which hinders its further application. On the other hand, 2-PE can also be chemically synthesized through ethylene oxidation of benzene or reduction of styrene oxide. Chemical synthesis processes are generally operated under harsh conditions, such as high temperature, high pressure, and strong acid or alkali environments, producing many undesirable by-products, such as ethylbenzene and styrene. This not only increases the downstream costs, but also seriously debases the grade of 2-PE. The increasing demand for environmental friendly processes and the preference for “natural” products from consumers have considerably stimulated the development of biological production of flavors and fragrances. Biological synthesis of 2-PE has attracted extensive attention because it meets the requirements of environmental friendliness and also satisfies the definition of “natural” products. However, the toxicity of 2-PE to cells is a critical limiting factor for the biosynthesis of 2-PE. In this review, we have comprehensively summarized the current status and perspectives for biological 2-PE production in terms of its advantages over classical chemical synthesis and extraction from natural plants. A comprehensive description of 2-PE synthetic pathways and global regulation mechanisms, strategies to increase 2-PE production, and the utilization of agro-industrial wastes as feedstocks has been systematically discussed. Furthermore, the application of in situ product removal techniques in 2-PE biosynthesis has also been highlighted.

    De novo biosynthesis of 3-phenylpropanol in E. coli
    Hutao GAO, Jia WANG, Xinxiao SUN, Xiaolin SHEN, Qipeng YUAN
    2021, 2(6):  1046-1060.  doi:10.12211/2096-8280.2021-098
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    With the increasing consumption of fossil fuels and growing depravation of environment, development of substitutes for petroleum-derived compounds is becoming more and more important. In the past few years, bio-based production of chemicals, fuels, nutraceuticals and pharmaceuticals from renewable raw materials via metabolic engineering has gained significant attention. As a high-value fragrance with aromatic taste, 3-phenylpropanol has been widely used in the production of food additives, cosmetics and etc. It also acts as the precursor and reactant in pharmaceutical and chemical industries. Because of its high efficiency in promoting bile secretion and mild antispasmodic function, 3-phenylpropanol is widely used in the treatment of cholecystitis, gallstones and biliary surgery syndrome. The current production method mainly relies on plant extraction and chemical synthesis, which, however, are challenging due to the high cost of catalyst, strict reaction condition and low product yield. Recently, engineering microorganisms has become an attractive alternative to efficient production of high-value compounds, such as flavors, fragrances, cosmetics, pharmaceuticals, solvents, biofuels and other chemicals. It is of great significance to construct a microbial cell factory to synthesize 3-phenylpropanol from renewable resources. In this study, we designed and constructed two different artificial 3-phenylpropanol biosynthetic pathways by establishing a connection between the target compound and the microorganism's own metabolic network. Especially, the pathway that relies on carboxylic acid reductase exhibited high efficiency in the production of 3-phenylpropanol. When using glycerol as the sole carbon source, the recombinant strain successfully generated 91 mg/L 3-phenylpropanol in shake flask experiments. By eliminating the rate-limiting steps, increasing the carbon flux towards the shikimate pathway and knocking out the competitive pathways, the titer of 3-phenylpropanol in the shake flask fermentation culture was finally increased to 841 mg/L, representing a 9.2-fold increase compared with the titer generated by the original strain. This work provides a green and sustainable approach for the production of 3-phenylpropanol.