Metabolic Engineering of Escherichia coli for Production of 2-Phenylethanol and 2-Phenylethyl Acetate from Glucose

A novel toolbox for precise regulation of gene expression and metabolic engineering in Bacillus licheniformis

Despite industrial bio-manufacturing progress using Bacillus licheniformis, the absence of a well-characterized toolbox allowing precise regulation of multiple genes limits its expansion for basic research and application. Here, a novel gene expression toolbox (GET) was developed for precise regulation of gene expression and high-level production of 2-phenylethanol. Firstly, we established a novel promoter core region mosaic combination model to combine, characterize and analyze different core regions. Characterization and orthogonal design of promoter ribbons allowed convenient construction of an adaptable and robust GET, gene gfp expression intensity was 0.64%-16755.77%, with a dynamic range of 2.61 × 104 times, which is the largest regulatory range of GET in Bacillus based on modification of promoter P43. Then we verified the protein and species universality of GET using different proteins expressed in B. licheniformis and Bacillus subtilis. Finally, the GET for 2-phenylethanol metabolic breeding, resulting in a plasmid-free strain producing 6.95 g/L 2-phenylethanol with a yield and productivity of 0.15 g/g glucose and 0.14 g/L/h, respectively, the highest de novo synthesis yield of 2-phenylethanol reported. Taken together, this is the first report elucidating the impact of mosaic combination and tandem of multiple core regions to initiate transcription and improve the output of proteins and metabolites, which provides strong support for gene regulation and diversified product production in Bacillus.

Alcohol acyltransferases for the biosynthesis of esters

Esters are widely used in food, energy, spices, chemical industry, etc., becoming an indispensable part of life. However, their production heavily relies on the fossil energy industry, which presents significant challenges associated with energy shortages and environmental pollution. Consequently, there is an urgent need to identify alternative green methods for ester production. One promising solution is biosynthesis, which offers sustainable and environmentally friendly processes. In ester biosynthesis, alcohol acyltransferases (AATs) catalyze the condensation of acyl-CoAs and alcohols to form esters, enabling the biosynthesis of nearly 100 different kinds of esters, such as ethyl acetate, hexyl acetate, ethyl crotonate, isoamyl acetate, and butyl butyrate. However, low catalytic efficiency and low selectivity of AATs represent the major bottlenecks for the biosynthesis of certain specific esters, which should be addressed with protein molecular engineering approaches before practical biotechnological applications. This review provides an overview of AAT enzymes, including their sequences, structures, active sites, catalytic mechanisms, and metabolic engineering applications. Furthermore, considering the critical role of AATs in determining the final ester products, the current research progresses of AAT modification using protein molecular engineering are also discussed. This review summarized the major challenges and prospects of AAT enzymes in ester biosynthesis.

Metabolic Engineering of Escherichia coli for the Biosynthesis of 3-Phenylpropionic Acid and 3-Phenylpropyl Acetate

3-Phenylpropionic acid (3PPA) and its derivative 3-phenylpropyl acetate (3PPAAc) are important aromatic compounds with broad applications in the cosmetics and food industries. In this study, we constructed a plasmid-free 3PPA-producing Escherichia coli strain and designed a novel 3PPAAc biosynthetic pathway. A module containing tyrosine ammonia lyase and enoate reductase, evaluated under the control of different promoters, was combined with phenylalanine-overproducing strain E. coli ATCC31884, enabling the plasmid-free de novo production of 218.16 ± 43.62 mg L^-1 3PPA. The feasibility of the pathway was proved by screening four heterologous alcohol acetyltransferases, which catalyzed the transformation of 3-phenylpropyl alcohol into 3PPAAc. Afterward, 94.59 ± 16.25 mg L^-1 3PPAAc was achieved in the engineered E. coli strain. Overall, we have not only demonstrated the potential of de novo synthesis of 3PPAAc in microbes for the first time but also provided a platform for the future of biosynthesis of other aromatic compounds.

Systematic Adaptation of Bacillus licheniformis to 2-Phenylethanol Stress

The compound 2-phenylethanol (2-PE) is a bulk flavor and fragrance with a rose-like aroma that can be produced by microbial cell factories, but its cellular toxicity inhibits cellular growth and limits strain performance. Specifically, the microbe Bacillus licheniformis has shown a strong tolerance to 2-PE. Understanding these tolerance mechanisms is crucial for achieving the hyperproduction of 2-PE. In this report, the mechanisms of B. licheniformis DW2 resistance to 2-PE were studied by multi-omics technology coupled with physiological and molecular biological approaches. 2-PE induced reactive oxygen species formation and affected nucleic acid, ribosome, and cell wall synthesis. To manage 2-PE stress, the antioxidant and global stress response systems were activated; the repair system of proteins and homeostasis of the ion and osmotic were initiated. Furthermore, the tricarboxylic acid cycle and NADPH synthesis pathways were upregulated; correspondingly, scanning electron microscopy revealed that cell morphology was changed. These results provide deeper insights into the adaptive mechanisms of B. licheniformis to 2-PE and highlight the potential targets for genetic manipulation to enhance 2-PE resistance. IMPORTANCE The ability to tolerate organic solvents is essential for bacteria producing these chemicals with high titer, yield, and productivity. As exemplified by 2-PE, bioproduction of 2-PE represents a promising alternative to chemical synthesis and plant extraction approaches, but its toxicity hinders successful large-scale microbial production. Here, a multi-omics approach is employed to systematically study the mechanisms of B. licheniformis DW2 resistance to 2-PE. As a 2-PE-tolerant strain, B. licheniformis displays multifactorial mechanisms of 2-PE tolerance, including activating global stress response and repair systems, increasing NADPH supply, changing cell morphology and membrane composition, and remodeling metabolic pathways. The current work yields novel insights into the mechanisms of B. licheniformis resistance to 2-PE. This knowledge can also be used as a clue for improving bacterial performances to achieve industrial-scale production of 2-PE and potentially applied to the production of other relevant organic solvents, such as tyrosol and hydroxytyrosol.

De Novo Biosynthesis of Anisyl Alcohol and Anisyl Acetate in Engineered Escherichia coli

Anisyl alcohol and its ester anisyl acetate are both important fragrance compounds and have a wide range of applications in the cosmetics, perfumery, and food industries. The currently commercially available anisyl alcohol and anisyl acetate are based on chemical synthesis. However, consumers increasingly prefer natural fragrance compounds. Therefore, it is of great significance to construct microbial cell factories to produce anisyl alcohol and anisyl acetate. In this study, we first established a biosynthetic pathway in engineered Escherichia coli MG1655 for the production of anisyl alcohol from simple carbon sources. We further increased the anisyl alcohol production to 355 mg/L by the increasing availability of erythrose-4-phosphate and phosphoenolpyruvate. Finally, we further demonstrated the production of anisyl acetate by overexpressing alcohol acetyltransferase ATF1 for the subsequent acetylation of anisyl alcohol to produce anisyl acetate. To our knowledge, this is the first report on the biosynthesis of anisyl alcohol and anisyl acetate directly from a renewable carbon source.

De Novo Biosynthesis of Methyl Cinnamate in Engineered Escherichia coli

Methyl cinnamate with a fruity balsamic odor is an important fragrance ingredient in perfumes and cosmetics. Chemical processes are currently the only means of producing methyl cinnamate. But consumers prefer natural flavors. Therefore, it is necessary to design and develop microbial cell factories for the production of methyl cinnamate. In this study, we established for the first time a biosynthetic pathway in engineered Escherichia coli for production of methyl cinnamate from glucose. We further increased the methyl cinnamate production to 302 mg/L by increasing the availability of the metabolic precursors. Finally, the titer was increased to 458 mg/L in a two-phase culture system.

Bioproduction of Natural Phenethyl Acetate, Phenylacetic Acid, Ethyl Phenylacetate, and Phenethyl Phenylacetate from Renewable Feedstock

Natural phenethyl acetate (PEA), phenylacetic acid (PAA), ethyl phenylacetate (Et-PA), and phenethyl phenylacetate (PE-PA) are highly desirable aroma chemicals, but with limited availability and high price. Here, green, sustainable, and efficient bioproduction of these chemicals as natural products from renewable feedstocks was developed. PEA and PAA were synthesized from l-phenylalanine (l-Phe) via novel six- and five-enzyme cascades, respectively. Whole-cell-based cascade biotransformation of 100 mm l-Phe in a two-phase system (aqueous/organic: 1 : 0.5 v/v) containing ethyl oleate or biodiesel as green solvent gave 13.6 g L^-1 PEA (83.1 % conv.) and 11.6 g L^-1 PAA (87.1 % conv.), respectively. Coupled fermentation and biotransformation approach produced 10.4 g L^-1 PEA and 9.2 g L^-1 PAA from glucose or glycerol, respectively. The biosynthesized PAA was converted to natural Et-PA and PE-PA by esterification using lipases with ethanol or 2-phenylethanol derived from sugar, affording 2.7 g L^-1 Et-PA (83.1 % conv.) and 4.6 g L^-1 PE-PA (96.3 % conv.), respectively.

Multilevel metabolic engineering of Bacillus licheniformis for de novo biosynthesis of 2-phenylethanol

Due to its pleasant rose-like scent, 2-phenylethanol (2-PE) has been widely used in the fields of cosmetics and food. Microbial production of 2-PE offers a natural and sustainable production process. However, the current bioprocesses for de novo production of 2-PE suffer from low titer, yield, and productivity. In this work, a multilevel metabolic engineering strategy was employed for the high-level production of 2-PE. Firstly, the native alcohol dehydrogenase YugJ was identified and characterized for 2-PE production via genome mining and gene function analysis. Subsequently, the redirection of carbon flux into 2-PE biosynthesis by combining optimization of Ehrlich pathway, central metabolic pathway, and phenylpyruvate pathway enabled the production of 2-PE to a titer of 1.81 g/L. Specifically, AroK and AroD were identified as the rate-limiting enzymes of 2-PE production through transcription and metabolite analyses, and overexpression of aroK and aroD efficiently boosted 2-PE synthesis. The precursor competing pathways were blocked by eliminating byproduct formation pathways and modulating the glucose transport system. Under the optimal condition, the engineered strain PE23 produced 6.24 g/L of 2-PE with a yield and productivity of 0.14 g/g glucose and 0.13 g/L/h, respectively, using a complex medium in shake flasks. This work achieves the highest titer, yield, and productivity of 2-PE from glucose via the phenylpyruvate pathway. This study provides a promising platform that might be widely useful for improving the production of aromatic-derived chemicals.

Bioproduction of 2-Phenylethanol through Yeast Fermentation on Synthetic Media and on Agro-Industrial Waste and By-Products: A Review

Due to its pleasant rosy scent, the aromatic alcohol 2-phenylethanol (2-PE) has a huge market demand. Since this valuable compound is used in food, cosmetics and pharmaceuticals, consumers and safety regulations tend to prefer natural methods for its production rather than the synthetic ones. Natural 2-PE can be either produced through the extraction of essential oils from various flowers, including roses, hyacinths and jasmine, or through biotechnological routes. In fact, the rarity of natural 2-PE in flowers has led to the inability to satisfy the large market demand and to a high selling price. Hence, there is a need to develop a more efficient, economic, and environmentally friendly biotechnological approach as an alternative to the conventional industrial one. The most promising method is through microbial fermentation, particularly using yeasts. Numerous yeasts have the ability to produce 2-PE using l-Phe as precursor. Some agro-industrial waste and by-products have the particularity of a high nutritional value, making them suitable media for microbial growth, including the production of 2-PE through yeast fermentation. This review summarizes the biotechnological production of 2-PE through the fermentation of different yeasts on synthetic media and on various agro-industrial waste and by-products.

Biosynthesis of actarit using engineered Escherichia coli

Actarit is widely regarded as a safe and effective drug for the treatment of rheumatoid arthritis. There is no report on the bioproductin of actarit so far. In this study, we demonstrated for the first time the development of an artificial actarit biosynthetic pathway in Escherichia coli. First, 4-aminophenylacetic acid is selected as precursor substrates for the production of actarit. Second, an N-acetyltransferase that can efficiently catalyse the esterification of acetyl-CoA and 4-aminophenylacetic acid to form actarit was discovered. Subsequently, an engineered E. coli that allows production of actarit from simple carbon sources was established. Finally, we further increased the production of actarit to 206 ± 16.9 mg/L by overexpression of shikimate dehydrogenase ydiB and shikimate kinase aroK.

De novo biosynthesis of tyrosol acetate and hydroxytyrosol acetate from glucose in engineered Escherichia coli

Tyrosol and hydroxytyrosol derived from virgin olive oil and olives extract, have wide applications both as functional food components and as nutraceuticals. However, they have low bioavailability due to their low absorption and high metabolism in human liver and small intestine. Acetylation of tyrosol and hydroxytyrosol can effectively improve their bioavailability and thus increase their potential use in the food and cosmeceutical industries. There is no report on the bioproductin of tyrosol acetate and hydroxytyrosol acetate so far. Thus, it is of great significance to develop microbial cell factories for achieving tyrosol acetate or hydroxytyrosol acetate biosynthesis. In this study, a de novo biosynthetic pathway for the production of tyrosol acetate and hydroxytyrosol acetate was constructed in Escherichia coli. First, an engineered E. coli that allows production of tyrosol from simple carbon sources was established. Four aldehyde reductases were compared, and it was found that yeaE is the best aldehyde reductase for tyrosol accumulation. Subsequently, the pathway was extended for tyrosol acetate production by further overexpression of alcohol acetyltransferase ATF1 for the conversion of tyrosol to tyrosol acetate. Finally, the pathway was further extended for hydroxytyrosol acetate production by overexpression of 4-hydroxyphenylacetate 3-hydroxylase HpaBC.

Development of an artificial biosynthetic pathway for biosynthesis of (S)-reticuline based on HpaBC in engineered Escherichia coli

Benzylisoquinoline alkaloids (BIAs) are an important class of plant secondary metabolites with a variety of pharmacological activities. Although they are widely used, traditionally these compounds are extracted from natural sources because their structure is too complicated to achieve economically feasible chemical synthesis. Thus, microbial biosynthesis of BIAs is expected to reduce dependence on natural extracts. (S)-Reticuline is an important precursor for BIAs biosynthesis. Therefore, it is an attractive engineering target. In this study, we reported the development of a novel (S)-reticuline biosynthetic pathway based on 4-hydroxyphenylacetate 3-hydroxylase (HpaBC) in Escherichia coli. Then, we further improved the (S)-reticuline production to 307 ± 26.8 mg/L by increasing the availability of the precursor 3, 4-dihydroxyphenylacetaldehyde. The E. coli cell factory developed in this study can be used as a potential platform for further efficient biosynthesis of BIAs derivatives.

Brewing rich 2-phenylethanol beer from cassava and its producing metabolisms in yeast

BACKGROUND

Cassava is rich in nutrition and has high edible value, but the development of the cassava industry is limited by the traditional low added value processing and utilization mode. In this study, cassava tuber was used as beer adjunct to develop a complete set of fermentation technology for manufacturing cassava beer.

RESULTS

The activities of transaminase, phenylpyruvate decarboxylase and dehydrogenase in 2-phenylethanol Ehrlich biosynthesis pathway of Saccharomyces cerevisiae were higher in cassava beer than that of malt beer. Aminotransferase ARO9 gene and phenylpyruvate decarboxylase ARO10 gene were up-regulated in the late stage of fermentation, which indicated that they were the main regulated genes of 2-phenylethanol Ehrlich pathway with phenylalanine as substrate in cassava beer preparation.

CONCLUSIONS

Compared with traditional wheat beer, cassava beer was similar in the content of nutrition elements, diacetyl, total acid, alcohol and carbon dioxide, but has the characteristics of fresh fragrance and better taste. The hydrocyanic acid contained in cassava root tubes was catabolized during fermentation and compliant with the safety standard of beverage. Further study found that the content of 2-phenylethanol in cassava beer increased significantly, which gave cassava beer a unique elegant and delicate rose flavor. © 2020 Society of Chemical Industry.

Engineering promiscuity of chloramphenicol acetyltransferase for microbial designer ester biosynthesis

Robust and efficient enzymes are essential modules for metabolic engineering and synthetic biology strategies across biological systems to engineer whole-cell biocatalysts. By condensing an acyl-CoA and an alcohol, alcohol acyltransferases (AATs) can serve as interchangeable metabolic modules for microbial biosynthesis of a diverse class of ester molecules with broad applications as flavors, fragrances, solvents, and drop-in biofuels. However, the current lack of robust and efficient AATs significantly limits their compatibility with heterologous precursor pathways and microbial hosts. Through bioprospecting and rational protein engineering, we identified and engineered promiscuity of chloramphenicol acetyltransferases (CATs) from mesophilic prokaryotes to function as robust and efficient AATs compatible with at least 21 alcohol and 8 acyl-CoA substrates for microbial biosynthesis of linear, branched, saturated, unsaturated and/or aromatic esters. By plugging the best engineered CAT (CATec3 Y20F) into the gram-negative mesophilic bacterium Escherichia coli, we demonstrated that the recombinant strain could effectively convert various alcohols into desirable esters, for instance, achieving a titer of 13.9 g/L isoamyl acetate with 95% conversion by fed-batch fermentation. The recombinant E. coli was also capable of simulating the ester profile of roses with high conversion (>97%) and titer (>1 g/L) from fermentable sugars at 37 °C. Likewise, a recombinant gram-positive, cellulolytic, thermophilic bacterium Clostridium thermocellum harboring CATec3 Y20F could produce many of these esters from recalcitrant cellulosic biomass at elevated temperatures (>50 °C) due to the engineered enzyme's remarkable thermostability. Overall, the engineered CATs can serve as a robust and efficient platform for designer ester biosynthesis from renewable and sustainable feedstocks.

Construction of recombinant Escherichia coli for production of L-phenylalanine-derived compounds

L-phenylalanine is an important amino acid that is widely used in the fields of food flavors and pharmaceuticals. Apart from L-phenylalanine itself, various commercially valuable chemical compounds can also be generated via the L-phenylalanine biosynthesis pathway. Compared with direct extraction from plants or synthesis by chemical reaction, microbial production of L-phenylalanine -derived compounds can overcome the drawbacks of environmental pollution, low yield, and mixtures of stereoisomeric products. Accordingly, increasing intracellular levels of precursors, deregulating feedback inhibition and transcription repression, engineering global regulators and other effective strategies have been implemented to produce different L-phenylalanine -derived compounds in the excellent chassis host Escherichia coli. Finally, this review highlights principal strategies for improving the production of L-phenylalanine and/or its derivatives in E. coli, and discusses the future outlook for further enhancing the titer and yields of these compounds.

CRISPR-mediated multigene integration enables Shikimate pathway refactoring for enhanced 2-phenylethanol biosynthesis in Kluyveromyces marxianus

BACKGROUND

2-phenylethanol (2-PE) is a rose-scented flavor and fragrance compound that is used in food, beverages, and personal care products. Compatibility with gasoline also makes it a potential biofuel or fuel additive. A biochemical process converting glucose or other fermentable sugars to 2-PE can potentially provide a more sustainable and economical production route than current methods that use chemical synthesis and/or isolation from plant material.

RESULTS

We work toward this goal by engineering the Shikimate and Ehrlich pathways in the stress-tolerant yeast Kluyveromyces marxianus. First, we develop a multigene integration tool that uses CRISPR-Cas9 induced breaks on the genome as a selection for the one-step integration of an insert that encodes one, two, or three gene expression cassettes. Integration of a 5-kbp insert containing three overexpression cassettes successfully occurs with an efficiency of 51 ± 9% at the ABZ1 locus and was used to create a library of K. marxianus CBS 6556 strains with refactored Shikimate pathway genes. The 3³-factorial library includes all combinations of KmARO4, KmARO7, and KmPHA2, each driven by three different promoters that span a wide expression range. Analysis of the refactored pathway library reveals that high expression of the tyrosine-deregulated KmARO4^K221L and native KmPHA2, with the medium expression of feedback insensitive KmARO7^G141S, results in the highest increase in 2-PE biosynthesis, producing 684 ± 73 mg/L. Ehrlich pathway engineering by overexpression of KmARO10 and disruption of KmEAT1 further increases 2-PE production to 766 ± 6 mg/L. The best strain achieves 1943 ± 63 mg/L 2-PE after 120 h fed-batch operation in shake flask cultures.

CONCLUSIONS

The CRISPR-mediated multigene integration system expands the genome-editing toolset for K. marxianus, a promising multi-stress tolerant host for the biosynthesis of 2-PE and other aromatic compounds derived from the Shikimate pathway.

Enhancement of 2-phenylethanol production by a wild-type Wickerhamomyces anomalus strain isolated from rice wine

2-Phenylethanol (2-PE) is an important high-grade aromatic alcohol, which is widely used in the cosmetics, perfumery and food industries. However, 2-PE is mainly synthesized using a chemical route, which produces environmental pollution and harmful by-products. Screening of high-yielding wild-type strains has become an important goal for the future biosynthesis of 2-PE. In this study, a wild-type Wickerhamomyces anomalus was isolated from rice wine fermented mash. By optimizing the initial glucose and l-phenylalanine concentrations, 2630.7 mg/L of 2-PE was obtained in shaking flasks. The conditions of initial glucose and l-phenylalanine concentration, pH, and inoculation amount were optimized for 2-PE production with W. anomalus. Finally, based on the optimal conditions, the 2-PE titer reached 4,727.3 mg/L by a single-dose fed-batch strategy in a 5-L bioreactor. The results showed that the ability was expanded to harness the Ehrlich pathway for the production of high-value aromatics in aroma-producing yeast species.

Efficient Synthesis of Phenylacetate and 2-Phenylethanol by Modular Cascade Biocatalysis

The green and sustainable synthesis of chemicals from renewable feedstocks by a biotransformation approach has gained increasing attention in recent years. In this work, we developed enzymatic cascades to efficiently convert l-phenylalanine into 2-phenylethanol (2-PE) and phenylacetic acid (PAA), l-tyrosine into tyrosol (p-hydroxyphenylethanol, p-HPE) and p-hydroxyphenylacetic acid (p-HPAA). The enzymatic cascade was cast into an aromatic aldehyde formation module, followed by an aldehyde reduction module, or aldehyde oxidation module, to achieve one-pot biotransformation by using recombinant Escherichia coli. Biotransformation of 50 mM l-Phe produced 6.76 g/L PAA with more than 99 % conversion and 5.95 g/L of 2-PE with 97 % conversion. The bioconversion efficiencies of p-HPAA and p-HPE from l-Tyr reached to 88 and 94 %, respectively. In addition, m-fluoro-phenylalanine was further employed as an unnatural aromatic amino acid substrate to obtain m-fluoro-phenylacetic acid; >96 % conversion was achieved. Our results thus demonstrated high-yielding and potential industrial synthesis of above aromatic compounds by one-pot cascade biocatalysis.

Unveiling the Multipath Biosynthesis Mechanism of 2-Phenylethanol in Proteus mirabilis

Proteus mirabilis could convert l-phenylalanine into 2-phenylethanol (2-PE) via the Ehrlich pathway, the amino acid deaminase pathway, and the aromatic amino acid decarboxylase pathway. The aromatic amino acid decarboxylase pathway was proved for the first time in P. mirabilis. In this pathway, l-aromatic amino acid transferase demonstrated a unique catalytic property, transforming 2-penylethylamine into phenylacetaldehyde. Eleven enzymes were supposed to involve in 2-phenylethanol synthesis. The mRNA expression levels of 11 genes were assessed over time by reverse transcription-quantitative polymerase chain reaction (RT-qPCR) in vivo. As a result, the expression of 11 genes was significantly increased, suggesting that P. mirabilis could transform l-phenylalanine into 2-phenylethanol via three pathways under aerobic conditions; nine genes were significantly overexpressed, suggesting that P. mirabilis could synthesize 2-phenylethanol via the Ehrlich pathway under anaerobic conditions. This study reveals the multipath synthetic metabolism for 2-phenylethanol in P. mirabilis and will enrich the new ideas for natural (2-PE) synthesis.

Structural Basis for Broad Substrate Selectivity of Alcohol Dehydrogenase YjgB from Escherichia coli

In metabolic engineering and synthetic biology fields, there have been efforts to produce variable bioalcohol fuels, such as isobutanol and 2-phenylethanol, in order to meet industrial demands. YjgB is an aldehyde dehydrogenase from Escherichia coli that shows nicotinamide adenine dinucleotide phosphate (NADP)-dependent broad selectivity for aldehyde derivatives with an aromatic ring or small aliphatic chain. This could contribute to the design of industrial synthetic pathways. We determined the crystal structures of YjgB for both its apo-form and NADP-complexed form at resolutions of 1.55 and 2.00 Å, respectively, in order to understand the mechanism of broad substrate selectivity. The hydrophobic pocket of the active site and the nicotinamide ring of NADP(H) are both involved in conferring its broad specificity toward aldehyde substrates. In addition, based on docking-simulation data, we inferred that π–π stacking between substrates and aromatic side chains might play a crucial role in recognizing substrates. Our structural analysis of YjgB might provide insights into establishing frameworks to understand its broad substrate specificity and develop engineered enzymes for industrial biofuel synthesis.

Recent Advances in Metabolically Engineered Microorganisms for the Production of Aromatic Chemicals Derived From Aromatic Amino Acids

Aromatic compounds derived from aromatic amino acids are an important class of diverse chemicals with a wide range of industrial and commercial applications. They are currently produced via petrochemical processes, which are not sustainable and eco-friendly. In the past decades, significant progress has been made in the construction of microbial cell factories capable of effectively converting renewable carbon sources into value-added aromatics. Here, we systematically and comprehensively review the recent advancements in metabolic engineering and synthetic biology in the microbial production of aromatic amino acid derivatives, stilbenes, and benzylisoquinoline alkaloids. The future outlook concerning the engineering of microbial cell factories for the production of aromatic compounds is also discussed.

Bioprocess Optimization for the Production of Aromatic Compounds With Metabolically Engineered Hosts: Recent Developments and Future Challenges

The most common route to produce aromatic chemicals - organic compounds containing at least one benzene ring in their structure - is chemical synthesis. These processes, usually starting from an extracted fossil oil molecule such as benzene, toluene, or xylene, are highly environmentally unfriendly due to the use of non-renewable raw materials, high energy consumption and the usual production of toxic by-products. An alternative way to produce aromatic compounds is extraction from plants. These extractions typically have a low yield and a high purification cost. This motivates the search for alternative platforms to produce aromatic compounds through low-cost and environmentally friendly processes. Microorganisms are able to synthesize aromatic amino acids through the shikimate pathway. The construction of microbial cell factories able to produce the desired molecule from renewable feedstock becomes a promising alternative. This review article focuses on the recent advances in microbial production of aromatic products, with a special emphasis on metabolic engineering strategies, as well as bioprocess optimization. The recent combination of these two techniques has resulted in the development of several alternative processes to produce phenylpropanoids, aromatic alcohols, phenolic aldehydes, and others. Chemical species that were unavailable for human consumption due to the high cost and/or high environmental impact of their production, have now become accessible.

Towards renewable flavors, fragrances, and beyond

Esters constitute a large space of unique molecules with broad range of applications as flavors, fragrances, pharmaceuticals, cosmetics, green solvents, and advanced biofuels. Global demand of natural esters in food, household cleaner, personal care, and perfume industries is increasing while the ester supply from natural sources has been limited. Development of novel microbial cell factories for ester production from renewable feedstocks can potentially provide an alternative and sustainable source of natural esters and hence help fulfill growing demand. Here, we highlight recent advances in microbial production of esters and provide perspectives for improving its economic feasibility. As the field matures, microbial ester production platforms will enable renewable and sustainable production of flavors and fragrances, and open new market opportunities beyond what nature can offer.

The draft genome sequence of Meyerozyma guilliermondii strain YLG18, a yeast capable of producing and tolerating high concentration of 2-phenylethanol

The draft genome of a wild-type Meyerozyma guilliermondii strain YLG18, which could convert l-phenylalanine (l-phe) to 2-phenylethanol (2-PE) and tolerate high concentration of 2-PE was sequenced and analyzed. 18S rDNA analysis indicated that strain YLG18 is closely related to M. guilliermondii. The assembled draft genome of strain YLG18 is 12.8 Mb, containing 5275 encoded protein sequences with G + C content of 43.75%. Among these annotated genes, two aminotransferases, one phenylpyruvate decarboxylase and two bifunctional alcohol dehydrogenases (adh) play key roles in the achievement of 2-PE production from l-phe via Ehrlich pathway. In addition, membrane protein insertase (YidC), heat shock protein (Hsp90) and chaperons (SGT1) were identified, which may contribute to the increased tolerance to 2-PE.

Microbial biosynthesis of lactate esters

BACKGROUND

Green organic solvents such as lactate esters have broad industrial applications and favorable environmental profiles. Thus, manufacturing and use of these biodegradable solvents from renewable feedstocks help benefit the environment. However, to date, the direct microbial biosynthesis of lactate esters from fermentable sugars has not yet been demonstrated.

RESULTS

In this study, we present a microbial conversion platform for direct biosynthesis of lactate esters from fermentable sugars. First, we designed a pyruvate-to-lactate ester module, consisting of a lactate dehydrogenase (ldhA) to convert pyruvate to lactate, a propionate CoA-transferase (pct) to convert lactate to lactyl-CoA, and an alcohol acyltransferase (AAT) to condense lactyl-CoA and alcohol(s) to make lactate ester(s). By generating a library of five pyruvate-to-lactate ester modules with divergent AATs, we screened for the best module(s) capable of producing a wide range of linear, branched, and aromatic lactate esters with an external alcohol supply. By co-introducing a pyruvate-to-lactate ester module and an alcohol (i.e., ethanol, isobutanol) module into a modular Escherichia coli (chassis) cell, we demonstrated for the first time the microbial biosynthesis of ethyl and isobutyl lactate esters directly from glucose. In an attempt to enhance ethyl lactate production as a proof-of-study, we re-modularized the pathway into (1) the upstream module to generate the ethanol and lactate precursors and (2) the downstream module to generate lactyl-CoA and condense it with ethanol to produce the target ethyl lactate. By manipulating the metabolic fluxes of the upstream and downstream modules through plasmid copy numbers, promoters, ribosome binding sites, and environmental perturbation, we were able to probe and alleviate the metabolic bottlenecks by improving ethyl lactate production by 4.96-fold. We found that AAT is the most rate-limiting step in biosynthesis of lactate esters likely due to its low activity and specificity toward the non-natural substrate lactyl-CoA and alcohols.

CONCLUSIONS

We have successfully established the biosynthesis pathway of lactate esters from fermentable sugars and demonstrated for the first time the direct fermentative production of lactate esters from glucose using an E. coli modular cell. This study defines a cornerstone for the microbial production of lactate esters as green solvents from renewable resources with novel industrial applications.

One-Pot Conversion of Cinnamaldehyde to 2-Phenylethanol via a Biosynthetic Cascade Reaction

A novel biosynthetic pathway for the production of natural 2-phenylethanol from cinnamaldehyde is reported. An ene-reductase (OYE)-mediated selective hydrogenation of cinnamaldehyde to hydrocinnamaldehyde is followed by a regioselective Baeyer-Villiger oxidation (BVMO) to produce the corresponding formate ester that either spontaneously hydrolyzes to 2-phenylethanol in water or is assisted by a formate dehydrogenase (FDH). This cascade reaction is performed in a one-pot fashion at ambient temperature and pressure. High selectivity and complete conversion were achieved.

De Novo Biosynthesis of Indole-3-acetic Acid in Engineered Escherichia coli

Indole-3-acetic acid (IAA) is considered the most common and important naturally occurring auxin in plants and a major regulator of plant growth and development. In this study, an aldehyde dehydrogenase AldH from Escherichia coli was found to convert indole-3-acetylaldehyde into IAA. Then we established an artificial pathway in engineered E. coli for microbial production of IAA from glucose. The overall pathway includes the upstream pathway from glucose to L-tryptophan and the downstream pathway from L-tryptophan to IAA. To our knowledge, this is the first report on the biosynthesis of IAA directly from a renewable carbon source. The study described here shows the way for the development of a beneficial microbe for biosynthesis of auxin and promoting plant growth in the future.

Fed-Batch and Sequential-Batch Approaches To Enhance the Bioproduction of 2-Phenylethanol and 2-Phenethyl Acetate in Solid-State Fermentation Residue-Based Systems

This study describes the use of alternative operational strategies in the solid-state fermentation of the agro-industrial leftover sugar cane bagasse (SCB) supplemented with l-phenylalanine, for bioproducing natural 2-phenylethanol (2-PE) and 2-phenethyl acetate (2-PEA) using K. marxianus. Here, fed-batch and sequential-batch have been assessed at two scales (1.6 and 22 L) as tools to increase the production, as well as to enhance the sustainability of this residue-based process. While in the reference batch strategy a maximum of 17 mg of 2-PE+2-PEA per gram of added SCB was reached at both scales, the implementation of fed-batch mode induced a production increase of 11.6% and 12.5%, respectively. Also, the production was increased by 16.9% and 2.4% as compared to the batch when a sequential-batch mode was used. Furthermore, the use of these strategies was accompanied by lower consumption of key resources like the inoculum, air, and time, promoting savings between 22% and 76% at both scales.

Different performance of Escherichia coli mutants with defects in the phosphoenolpyruvate: carbohydrate phosphotransferase system under low glucose condition

In Escherichia coli, the transport and phosphorylation of glucose is mainly accomplished by the phosphoenolpyruvate-dependent glucose-specific phosphotransferase system (PTS^Glc), which is, therefore, frequently selected as a target for engineering to increase the intracellular level of phosphoenolpyruvate. Here we characterized the effects of a low glucose concentration on the growth, glucose consumption, and acetate secretion of individual strains with a single PTS^Glc mutation. We found that most mutants accumulated similar amounts of biomass, consumed glucose at lower rates, and secreted less acetate compared with the wild-type parental strain. The exception was the growth-impaired strain MG1655I harboring a ptsI deletion. In summary, the fermentation performance of mutant strains under 5 g/L glucose was obviously different with those strains under 20 g/L glucose. This study is a good complement to the knowledge of PTS^Glc in E. coli and indicates that engineering the components of PTS^Glc should be carefully optimized, particularly during fermentation in the presence of low concentrations of glucose.

Biosynthesis of advanced biofuel farnesyl acetate using engineered Escherichia coli

Diminishing petroleum reserves and the rapid accumulation of greenhouse gases lead to increasing interest in microbial biofuels. In this study, a heterologous farnesyl acetate biosynthesis pathway was constructed in Escherichia coli for the first time. Firstly, the AtoB, ERG13, tHMG1, ERG12, ERG8, MVD1, Idi, IspA and PgpB were expressed to accumulate farnesol in the E. coli cells. Then the alcohol acetyltransferase (ATF1) was heterologous overexpressed for the subsequent esterification farnesol to farnesyl acetate. The engineered strain DG 106 accumulated 128 ± 10.5 mg/L of farnesyl acetate. Finally, the isopentenyl-diphosphate isomerase was further overexpressed, and the recombinant strain DG107 produced 201 ± 11.7 mg/L of farnesyl acetate. This study shows the novel method for the biosynthesis of the advanced biofuel farnesyl acetate directly from glucose and highlight the enormous designing strategies for metabolic engineering of bioproducts.