A Study on the Efficient Preparation of α-Ketoglutarate with L-Glutamate Oxidase

Alpha-ketoglutaric acid (α-KG), as an intermediate product of the tricarboxylic acid cycle, plays a crucial role in peptide and amino acid synthesis. In order to reduce costs and improve efficiency in the oxidative production of α-ketoglutaric acid, this study successfully synthesized and expressed L-glutamate oxidase (LGOXStr) from Streptomyces viridosporus R111 and catalase (KatGEsc) from Escherichia coli H736. Two immobilization methods and the conditions for one-step whole-cell catalysis of α-ketoglutaric acid were investigated. α-Ketoglutaric acid has broad applications in the pharmaceutical, food, and chemical industries. The specific research results are as follows: (1) By fusing the sfGFP tag, L-glutamate oxidase (LGOXStr r) and catalase (KatGEsc) were successfully anchored to the outer membrane of Escherichia coli cells, achieving one-step whole-cell catalysis of α-ketoglutaric acid with a conversion efficiency of up to 75%. (2) Through the co-immobilization of LGOXStr and KatGEsc, optimization of the preparation parameters of immobilized cells, and exploration of the immobilization method using E.coli@ZIF-8, immobilized cells with conversion rates of over 60% were obtained even after 10 cycles of reuse. Under the optimal conditions, the production rate of α-ketoglutaric acid reached 96.7% in a 12 h reaction, which is 1.1 times that of E. coli@SA and 1.29 times that of free cells.

Combining directed evolution with high cell permeability for high-level cadaverine production in engineered Escherichia coli

The biosynthesis of cadaverine from lysine is an environmentally promising technology, that could contribute to a more sustainable approach to manufacturing bio-nylon 5X. However, the titer of biosynthesized cadaverine has still not reached a sufficient level for industrial production. A powerful green cell factory was developed to enhance cadaverine production by regulating lipopolysaccharide (LPS) genes and improving membrane permeability. Firstly, 10 LPS mutant strains were constructed and the effect on the growth was investigated. Then, the lysine decarboxylase (CadA) was overexpressed in 10 LPS mutant strains of Escherichia coli MG1655 and the ability to produce cadaverine was compared. Using 20.0 g L-1 of L-lysine hydrochloride (L-lysine-HCl) as the substrate for the biotransformation reaction, Cad02 and Cad06 strains exhibited high production levels of cadaverine, with 8.95 g L-1 and 7.55 g L-1 respectively while the control strain Cad00 only 4.92 g L-1 . Directed evolution of CadA was also used to improve its stability under alkaline conditions. The cadaverine production of the Cad02-M mutant stain increased by 1.86 times at pH 8.0. Finally, the production process was scaled up using recombinant whole cells as catalysts, achieving a high titer of 211 g L-1 cadaverine (96.8%) by fed-batch bioconversion. This study demonstrates the potential role of LPS in enhancing the efficiency of mass transfer between substrate and enzymes in vivo by increasing cell permeability. The results indicate that the argumentation of cell permeability could not only significantly enhance the biotransformation efficiency of cadaverine, but also provide a universally applicable, straightforward, environment-friendly, and cost-effective method for the biosynthesis of other high-value chemicals.

A sustainable strategy for biosynthesis of Rebaudioside D using a novel glycosyltransferase of Solanum tuberosum

Bioconversion of Rebaudioside D faces high-cost obstacles. Herein, a novel glycosyltransferase StUGT converting Rebaudioside A to Rebaudioside D was screened and characterized, which exhibits stronger affinity and substrate specificity for Rebaudioside A than previously reported enzymes. A whole-cell catalytic system was thus developed using the StUGT strain. The production of Rebaudioside D was enhanced significantly by enhancing cell permeability, and the maximum production of 6.12 g/L and the highest yield of 98.08% by cell catalyst was obtained by statistical-based optimization. A new cascade process utilizing this recombinant strain and E. coli expressing sucrose synthase was further established to reduce cost through replacing expensive UDPG with sucrose. A StUGT-GsSUS1 system exhibited high catalytic capability, and 5.27 g L-1 Rebaudioside D was achieved finally without UDPG addition by systematic optimization. This is the best performance reported in cell-cascaded biosynthesis, which paves a new cost-effective strategy for sustainable synthesis of scarce premium sweeteners from biomass.

Characterizing Biogenic MnOx Produced by Pseudomonas putida MnB1 and Its Catalytic Activity towards Water Oxidation

Mn-oxidizing microorganisms oxidize environmental Mn(II), producing Mn(IV) oxides. Pseudomonas putida MnB1 is a widely studied organism for the oxidation of manganese(II) to manganese(IV) by a multi-copper oxidase. The biogenic manganese oxides (BMOs) produced by MnB1 and similar organisms have unique properties compared to non-biological manganese oxides. Along with an amorphous, poorly crystalline structure, previous studies have indicated that BMOs have high surface areas and high reactivities. It is also known that abiotic Mn oxides promote oxidation of organics and have been studied for their water oxidation catalytic function. MnB1 was grown and maintained and subsequently transferred to culturing media containing manganese(II) salts to observe the oxidation of manganese(II) to manganese(IV). The structures and compositions of these manganese(IV) oxides were characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy, inductively coupled plasma optical emission spectroscopy, and powder X-ray diffraction, and their properties were assessed regarding catalytic functionality towards water oxidation in comparison to abiotic acid birnessite. Water oxidation was accomplished through the whole-cell catalysis of MnB1, the results for which compare favorably to the water-oxidizing ability of abiotic Mn(IV) oxides.

Biosynthesis of 10-Hydroxy-2-decenoic Acid through a One-Step Whole-Cell Catalysis

10-Hydroxy-2-decenoic acid (10-HDA) is an important component of royal jelly, known for its antimicrobial, anti-inflammatory, blood pressure-lowering, and antiradiation effects. Currently, 10-HDA biosynthesis is limited by the substrate selectivity of acyl-coenzyme A dehydrogenase, which restricts the technique to a two-step process. This study aimed to develop an efficient and simplified method for synthesizing 10-HDA. In this study, ACOX from Candida tropicalis 1798, which catalyzes 10-hydroxydecanoyl coenzyme A desaturation for 10-HDA synthesis, was isolated and heterologously coexpressed with FadE, Macs, YdiI, and CYP in Escherichia coli/SK after knocking out FadB, FadJ, and FadR genes. The engineered E. coli/AKS strain achieved a 49.8% conversion of decanoic acid to 10-HDA. CYP expression was improved through ultraviolet mutagenesis and high-throughput screening, increased substrate conversion to 75.6%, and the synthesis of 10-HDA was increased to 0.628 g/L in 10 h. This is the highest conversion rate and product concentration achieved in the shortest time to date. This study provides a simple and efficient method for 10-HDA biosynthesis and offers an effective method for developing strains with high product yields.

Production of Gibberellins via a Non-Natural Pathway Using Steviol as a Substrate

Gibberellins (GAs) are plant hormones widely used in agriculture. At present, GAs are produced by fermentation of the fungus Fusarium fujikuroi. However, fungal growth is too slow, resulting in slow fungal fermentation and a low yield. Here, to develop an alternative production source of GAs, an artificial pathway was engineered in Escherichia coli. By selecting and combining enzymes derived from plants and bacteria, a novel 4-enzyme pathway was successfully constructed to produce GAs using steviol, a readily available and less valuable byproduct during enzymatic refining of rebaudioside A, as a feedstock. Whole-cell biotransformation with E. coli strain expressing the novel pathway produced 71.17 ± 2.00 mg/L GA1 and a trace amount of GA3 from steviol in 48 h. This report presents a significant advancement in the fast production of GAs and establishes a method for the metabolism of terpenoids to produce target products in microbial hosts.

High-Performance Production of N-Acetyl-d-Neuraminic Acid with Whole Cells of Fast-Growing Vibrio natriegens via a Thermal Strategy

High performance is the core objective that biotechnologists pursue, of which low efficiency, low titer, and side products are the chief obstacles. Here, a thermal strategy is proposed for simultaneously addressing the obstacles of whole-cell catalysis that is widely applied in the food industry. The strategy, by combining fast-growing Vibrio natriegens, thermophilic enzymes, and high-temperature whole-cell catalysis, was successfully applied for the high-performance production of N-acetyl-d-neuraminic acid (Neu5Ac) that plays essential roles in the fields of food (infant formulas), healthcare, and medicine. By using this strategy, we realized the highest Neu5Ac titer and productivity of 126.1 g/L and up to 71.6 g/(L h), respectively, 7.2-fold higher than the productivity of Escherichia coli. The major byproduct acetic acid was also eliminated via quenching complex metabolic side reactions enabled by temperature elevation. This study offers a broadly applicable strategy for producing chemicals relevant to the food industry, providing insights for its future development.

Biosynthesis of 3-Hydroxyphloretin Using Rational Design of 4-Hydroxyphenylacetate 3-Monooxygenase

The compound 3-hydroxyphloretin is a typical dihydrochalcone that can be obtained in plants by the 3-hydroxylation of phloretin. Here, the flavin-dependent two-component monooxygenase (HpaBC) derived from Pseudomonas aeruginosa was used to convert phloretin into 3-hydroxyphloretin. Following molecular docking and sequence alignment, modifications to the substrate pocket and loop of PaHpaBC were rationally designed, and mutant residues were selected. The results showed that the mutant Q212G/F292A/Q376N gave the best yield of 3-hydroxyphloretin and showed improved catalytic efficiency. Under optimal reaction condition, 2.03 g/L of 3-hydroxyphloretin was produced in the whole-cell catalysis experiment. Molecular docking and molecular dynamics simulations were used to analyze mutants and elucidate the potential mechanism. It was found that the increase in 3-hydroxyphloretin yield was due to the improvement in the flexibility of the loop and the expansion of its substrate pocket. This strategy based on loop and substrate pocket modification has significance in the engineering of PaHpaB.

Optimization of multi-enzyme cascade process for the biosynthesis of benzylamine

Benzylamine is a valuable intermediate in the synthesis of organic compounds such as curing agents and antifungal drugs. To improve the efficiency of benzylamine biosynthesis, we identified the enzymes involved in the multi-enzyme cascade, regulated the expression strength by using RBS engineering in Escherichia coli, and established a regeneration-recycling system for alanine. This is a cosubstrate, coupled to cascade reactions, which resulted in E. coli RARE-TP and can synthesize benzylamine using phenylalanine as a precursor. By optimizing the supply of cosubstrates alanine and ammonia, the yield of benzylamine produced by whole-cell catalysis was increased by 1.5-fold and 2.7-fold, respectively, and the final concentration reached 6.21 mM. In conclusion, we achieved conversion from l-phenylalanine to benzylamine and increased the yield through enzyme screening, expression regulation, and whole-cell catalytic system optimization. This demonstrated a green and sustainable benzylamine synthesis method, which provides a reference and additional information for benzylamine biosynthesis research.

Insights into the Unusual Activity of a Novel Homospermidine Synthase with a Promising Application to Produce Spermidine

Spermidine is a naturally occurring polyamine with multiple biological activities and potential food and agricultural applications. However, sustainable and scalable spermidine production has not yet been attained. In this study, a homospermidine synthase (HSS) from Pseudomonas frederiksbergensis (PfHSS) capable of catalyzing the synthesis of spermidine from 1,3-diaminopropane and putrescine was identified based on multiple sequence alignment using Blastochloris viridis HSS (BvHSS) as a template. The optimal reaction pH and temperature for purified PfHSS were determined to be 8.5 and 45 °C, respectively, and K+ was able to promote the enzyme activity. Further analysis of the structural and functional relationships through molecular docking and molecular dynamics simulation indicates that glutamic acid at position 359 is the essential residue for the enzyme-catalyzed synthesis of spermidine. The whole-cell catalytic reaction yielded 1321.4 mg/L spermidine and 678.2 mg/L of homospermidine. This study presents a novel, promising, and sustainable biological method for producing spermidine.

[Synthesis of cello-oligosaccharides which promotes the growth of intestinal probiotics by multi-enzyme cascade reaction]

Soluble cello-oligosaccharide with 2-6 oligosaccharide units is a kind of oligosaccharide with various biological functions, which can promote the proliferation of intestinal probiotics such as Bifidobacteria and Lactobacillus paracei. Therefore, it has a regulatory effect on human intestinal microbiota. In this study, a Cc 01 strain was constructed by expressing cellodextrin phosphorylase (CDP) in Escherichia coli. By combining with a previously constructed COS 01 strain, a three-enzyme cascade reaction system based on strains COS 01 and Cc 01 was developed, which can convert glucose and sucrose into cello-oligosaccharide. After optimization, the final titer of soluble cello-oligosaccharides with 2-6 oligosaccharide units reached 97 g/L, with a purity of about 97%. It contained cellobiose (16.8 wt%), cellotriose (49.8 wt%), cellotetrose (16.4 wt%), cellopentaose (11.5 wt%) and cellohexose (5.5 wt%). When using inulin, xylo-oligosaccharide and fructooligosaccharide as the control substrate, the biomass (OD600) of Lactobacillus casei (WSH 004), Lactobacillus paracei (WSH 005) and Lactobacillus acidophilus (WSH 006) on cello-oligosaccharides was about 2 folds higher than that of the control. This study demonstrated the efficient synthesis of cello-oligosaccharides by a three-enzyme cascade reaction and demonstrated that the synthesized cello-oligosaccharides was capable of promoting intestinal microbial proliferation.

Improvement of biodegradation of PET microplastics with whole-cell biocatalyst by interface activation reinforcement

Polyethylene terephthalate (PET) is an important basic polymer, which was used widely in variety of fields. Due to its high crystallinity, compact structure and strong surface hydrophobicity, PET has prominent resistance to biodegradation. In recent years, microplastics, especially polyethylene terephthalate (PET) microplastics, was considered as serious threaten to ecosystems. In this study, alkali-resistant bacteria were used as whole-cell catalysts to try to improve the biodegradation of PET microplastics by increasing the bio-interfacial activity of the polymer substrate. Surfactants were applicated to enhance interfacial activation of enzyme and PET interactions. And an integrated strategy was constructed based on alkali resistant bacteria to catalysis the hydrolysis of PET. The results showed that Tween 20 had the most obvious promoting effect among the four interfacial biocatalysts on biological-chemical combined hydrolysis of PET microplastics with whole-cell biocatalysts in alkaline environment. Obvious etching and fracture were observed on the PET fibre surface after biodegradation in presence of surfactant. The weight loss rate of PET substrate can reach 11.04% after 5 days of biodegradation. Thus, this research provides a promising method for efficient degradation of PET microplastics.

Conversion of Similar Xenochemicals to Dissimilar Products: Exploiting Competing Reactions in Whole-Cell Catalysis

Many enzymes have latent activities that can be used in the conversion of non-natural reactants for novel organic conversions. A classic example is the conversion of benzaldehyde to a phenylacetyl carbinol, a precursor for ephedrine manufacture. It is often tacitly assumed that purified enzymes are more promising catalysts than whole cells, despite the lower cost and easier maintenance of the latter. Competing substrates inside the cell have been known to elicit currently hard-to-predict selectivities that are not easily measured inside the living cell. We employ NMR spectroscopic assays to rationally combine isomers for selective reactions in commercial S. cerevisiae. This approach uses internal competition between alternative pathways of aldehyde clearance in yeast, leading to altered selectivities compared to catalysis with the purified enzyme. In this manner, 4-fluorobenzyl alcohol and 2-fluorophenylacetyl carbinol can be formed with selectivities in the order of 90%. Modification of the cellular redox state can be used to tune product composition further. Hyperpolarized NMR shows that the cellular reaction and pathway usage are affected by the xenochemical. Overall, we find that the rational construction of ternary or more complex substrate mixtures can be used for in-cell NMR spectroscopy to optimize the upgrading of similar xenochemicals to dissimilar products with cheap whole-cell catalysts.

A hybrid system for the overproduction of complex ergot alkaloid chanoclavine

Synthetic biology-based methods (Sbio) and chemical synthesis (Csyn) are two independent approaches that are both widely used for synthesizing biomolecules. In the current study, two systems were combined for the overproduction of chanoclavine (CC), a structurally complex ergot alkaloid. The whole synthetic pathway for CC was split into three sections: enzymatic synthesis of 4-Br-Trp (4-Bromo-trptophan) using cell-lysate catalysis (CLC), chemical synthesis of prechanoclavine (PCC) from 4-Br-Trp, and overproduction CC from PCC using a whole-cell catalysis (WCC) platform. The final titer of the CC is over 3 g/L in this Sbio-Csyn hybrid system, the highest yield reported so far, to the best of our knowledge. The development of such a combined route could potentially avoid the limitations of both Sbio and Csyn systems and boost the overproduction of complex natural products.

Light-Driven [FeFe] Hydrogenase Based H2 Production in E. coli: A Model Reaction for Exploring E. coli Based Semiartificial Photosynthetic Systems

Biohybrid technologies like semiartificial photosynthesis are attracting increased attention, as they enable the combination of highly efficient synthetic light-harvesters with the self-healing and outstanding performance of biocatalysis. However, such systems are intrinsically complex, with multiple interacting components. Herein, we explore a whole-cell photocatalytic system for hydrogen (H₂) gas production as a model system for semiartificial photosynthesis. The employed whole-cell photocatalytic system is based on Escherichia coli cells heterologously expressing a highly efficient, but oxygen-sensitive, [FeFe] hydrogenase. The system is driven by the organic photosensitizer eosin Y under broad-spectrum white light illumination. The direct involvement of the [FeFe] hydrogenase in the catalytic reaction is verified spectroscopically. We also observe that E. coli provides protection against O₂ damage, underscoring the suitability of this host organism for oxygen-sensitive enzymes in the development of (photo) catalytic biohybrid systems. Moreover, the study shows how factorial experimental design combined with analysis of variance (ANOVA) can be employed to identify relevant variables, as well as their interconnectivity, on both overall catalytic performance and O₂ tolerance.

Producing high value unsaturated fatty acid by whole-cell catalysis using microalga: A case study with Tribonema minus

High value unsaturated fatty acids can be produced by de novo synthesis in microalgal cells, especially via heterotrophic cultivation. Unfortunately, the lipid accumulation of heterotrophic microalgae cannot be improved efficiently in conventional ways. Here we reported heterotrophic Tribonema minus, a promising resource for the production of palmitoleic acid which has increasing demands in health service for patients with metabolic syndrome, as whole-cell biocatalyst to develop a novel way of shifting low value exogenous saturated fatty acids to high value ones. Results showed that myristic acid is the best precursor for whole-cell catalysis; it elevated the lipid content of T. minus to 42.2%, the highest among the tried precursors. The influences of cultivation condition on the utilization of extrinsic myristic acid and lipid accumulation were also determined. Under the optimized condition, the lipid content reached as high as 48.9%. In addition, our findings showed that ~13.0% of C16:1 in T. minus is derived from extrinsic myristic acid, and 30.1% of metabolized precursor is converted into heterologous fatty acids. Thus, a feasible approach for both increasing the value of low value saturated fatty acid by bioconversion and enhancing the lipid accumulation in microalgae is proposed by supplementing extrinsic myristic acid.

Whole-cell biosynthesis of cytarabine by an unnecessary protein-reduced Escherichia coli that coexpresses purine and uracil phosphorylase

Currently, whole-cell catalysts face challenges due to the complexity of reaction systems, although they have a cost advantage over pure enzymes. In this work, cytarabine was synthesized by purified purine phosphorylase 1 (PNP1) and uracil phosphorylase (UP), and the conversion of cytarabine from adenine arabinoside reached 72.3±4.3%. However, the synthesis was unsuccessful by whole-cell catalysis due to interference from unnecessary proteins (UNPs) in cells. Thus, we carried out a large-scale gene editing involving 377 genes in the genome of Escherichia coli to reduce the negative effect of UNPs on substrate conversion and cytarabine production. Finally, the PNP1 and UP activities of the obtained mutant were increased significantly compared with the parental strain, and more importantly, the conversion rate of cytarabine by whole-cell catalysis reached 67.4±2.5%. The lack of 148 proteins and down-regulation of 783 proteins caused by gene editing were equivalent to partial purification of the enzymes within cells, and thus, we provided inspiration to solve the problem caused by UNP interference, which is ubiquitous in the field of whole-cell catalysis. This article is protected by copyright. All rights reserved.

De Novo Biosynthesis and Whole-Cell Catalytic Production of 2-Acetamidophenol in Escherichia coli

2-Acetamidophenol (AAP) is an aromatic product with promising activities in agricultural applications and medical research. At present, AAP is synthesized by chemical methods from nonrenewable fossil fuel resources, which cause environmental pollution and the reaction conditions are harsh. In this study, we constructed the artificial biosynthetic pathway of AAP with five different expressed proteins in Escherichia coli for the first time. By introducing the hydrogen peroxide degrading enzyme catalase and improving cell tolerance to toxic intermediates or products, the yield of AAP reached 33.54 mg/L using shaking-flask culture. The best-engineered strain could produce 568.57 mg/L AAP by fed-batch fermentation from glucose and precursor (2-aminophenol, 2-AP) addition. Furthermore, a one-pot whole-cell cascade biocatalytic pathway to AAP and analogues was developed and optimized. This method can efficiently produce 1.8 g/L AAP using the methyl anthranilate hydrolysis product as the substrate. This study provides not only the de novo artificial biosynthetic pathway of AAP in E. coli but also a promising sustainable and efficient strategy to enable the synthesis of AAP on a gram scale.

Segmented Flow Processes to Overcome Hurdles of Whole-Cell Biocatalysis in the Presence of Organic Solvents

In modern process development, it is imperative to consider biocatalysis, and whole‐cell catalysts often represent a favored form of such catalysts. However, the application of whole‐cell catalysis in typical organic batch two‐phase synthesis often struggles due to mass transfer limitations, emulsion formation, tedious work‐up and, thus, low yields. Herein, we demonstrate that utilizing segmented flow tools enables the conduction of whole‐cell biocatalysis efficiently in biphasic media. Exemplified for three different biotransformations, the power of such segmented flow processes is shown. For example, a 3‐fold increase of conversion from 34 % to >99 % and a dramatic simplified work‐up leading to a 1.5‐fold higher yield from 44 % to 65 % compared to the analogous batch process was achieved in such a flow process.

Enhanced Production of (S)-2-arylpropionic Acids by Protein Engineering and Whole-Cell Catalysis

Esterases are important biocatalysts for chemical synthesis. Several bHSL family esterases have been used to prepare (S)-2-arylpropionic acids with stronger anti-inflammatory effects via kinetic resolution. Here, we presented the discovery of key residues that controlled the enantioselectivity of bHSL family esterases to ethyl 2-arylpropionates, through careful analysis of the structural information and molecular docking. A new bHSL family esterase, Est924, was identified as a promising catalyst for kinetic resolution of racemic ethyl 2-arylpropionates with slight (R)-stereopreference. Using Est924 as the starting enzyme, protein engineering was conducted at hotspots, and the substitution of A203 was proved to enhance the enantioselectivity. The stereopreference of the mutant M1 (A203W) was inverted to ethyl (S)-2-arylpropionates, and this stereopreference was further improved in variant M3 (I202F/A203W/G208F). In addition, the optimal variant, M3, was also suitable for the resolution of ibuprofen ethyl ester and ketoprofen ethyl ester, and their efficient (S)-isomers were synthesized. Next, the whole-cell catalyst harboring M3 was used to prepare (S)-ketoprofen. (S)-ketoprofen with 86%ee was produced by whole-cell catalyst with a single freeze-thaw cycle, and the cells could be reused for at least five cycles. Our results suggested that Est924 variants could kinetically resolve economically important racemates for industrial production and further offer the opportunity for the rational design of enzyme enantioselectivity. Moreover, it is an economical process to prepare optically pure (S)-ketoprofen and (S)-naproxen by using an engineered strain harboring M3 as the catalyst.

Wasteful Azo Dyes as a Source of Biologically Active Building Blocks

In this work, an environment-friendly enzymatic strategy was developed for the valorisation of dye-containing wastewaters. We set up biocatalytic processes for the conversion of azo dyes representative of the main classes used in the textile industry into valuable aromatic compounds: aromatic amines, phenoxazinones, phenazines, and naphthoquinones. First, purified preparations of PpAzoR azoreductase efficiently reduced mordant, acid, reactive, and direct azo dyes into aromatic amines, and CotA-laccase oxidised these compounds into phenazines, phenoxazinones, and naphthoquinones. Second, whole cells containing the overproduced enzymes were utilised in the two-step enzymatic conversion of the model mordant black 9 dye into sodium 2-amino-3-oxo-3H-phenoxazine-8-sulphonate, allowing to overcome the drawbacks associated with the use of expensive purified enzymes, co-factors, or exquisite reaction conditions. Third, cells immobilised in sodium alginate allowed recycling the biocatalysts and achieving very good to excellent final phenoxazine product yields (up to 80%) in water and with less impurities in the final reaction mixtures. Finally, one-pot systems using recycled immobilised cells co-producing both enzymes resulted in the highest phenoxazinone yields (90%) through the sequential use of static and stirring conditions, controlling the oxygenation of reaction mixtures and the successive activity of azoreductase (anaerobic) and laccase (aerobic).

Preparation of Sulforaphene from Radish Seed Extracts with Recombinant Food-Grade Yarrowia lipolytica Harboring High Myrosinase Activity

Sulforaphene prepared from glucoraphenin by myrosinase is one of the main active ingredients of radish, which has various biological activities and brilliant potential for food and pharmaceutical applications. In this paper, a recombinant food-grade yeast transformant 20-8 with high-level myrosinase activity was constructed by over-expressing a myrosinase gene from Arabidopsis thaliana in Yarrowia lipolytica. The highest myrosinase activity produced by the transformant 20-8 reached 44.84 U/g dry cell weight when it was cultivated in a 10 L fermentor within 108 h. Under the optimal reaction conditions, 6.1 mg of sulforaphene was yielded from 1 g of radish seeds under the catalysis of the crude myrosinase preparation (4.95 U) at room temperature within 1.5 h. What is more is that when the whole yeast cells harboring myrosinase activity were reused 10 times, the sulforaphene yield still reached 92.53% of the initial level. Therefore, this efficient approach has broad application prospects in recyclable and large-scale preparation of sulforaphene.

Designed whole-cell-catalysis-assisted synthesis of 9,11-secosterols

A method for the synthesis of 9,11-secosteroids starting from the natural corticosteroid cortisol is described. There are two key steps in this approach, combining chemistry and synthetic biology. Stereo- and regioselective hydroxylation at C9 (steroid numbering) is carried out using whole-cell biocatalysis, followed by the chemical cleavage of the C-C bond of the vicinal diol. The two-step method features mild reaction conditions and completely excludes the use of toxic oxidants.

High-Level Production of Indole-3-acetic Acid in the Metabolically Engineered Escherichia coli

Indole-3-acetic acid (IAA) is a critical plant hormone that regulates cell division, development, and metabolism. IAA synthesis in plants and plant-associated microorganisms cannot fulfill the requirement for large-scale agricultural production. Here, two novel IAA biosynthesis pathways, tryptamine (TAM) and indole-3-acetamide (IAM), were developed for IAA production by whole-cell catalysis and de novo biosynthesis in an engineered Escherichia coli MG1655. When 10 g/L l-tryptophan was used as a substrate, an MIA-6 strain containing a heterologous IAM pathway had the highest IAA titer of 7.10 g/L (1.34 × 10³ mg/g DCW), which was 98.4 times more than MTAI-5 containing the TAM pathway by whole-cell catalysis. De novo IAA biosynthesis was optimized by improving NAD(P)H availability, resulting in an increased IAA titer of 906 mg/L obtained by the MGΔadhE::icd strain, which is 29.7% higher than the control. These strategies exhibit the potential for IAA production in engineered E. coli and possible industrial applications.

Photosynthetic Reduction of Xylose to Xylitol Using Cyanobacteria

Photosynthetic generation of reducing power makes cyanobacteria an attractive host for biochemical reduction compared to cell-free and heterotrophic systems, which require burning of additional resources for the supply of reducing equivalent. Here, using xylitol synthesis as an example, efficient uptake and reduction of xylose photoautotrophically in Synechococcus elongatus PCC7942 are demonstrated upon introduction of an effective xylose transporter from Escherichia coli (Ec-XylE) and the NADPH-dependent xylose reductase from Candida boidinii (Cb-XR). Simultaneous activation of xylose uptake and matching of cofactor specificity enabled an average xylitol yield of 0.9 g g^-1 xylose and a maximum productivity of about 0.15 g L^-1 day^-1 OD^-1 with increased level of xylose supply. While long-term cellular maintenance still appears challenging, high-density conversion of xylose to xylitol using concentrated resting cell further pushes the titer of xylitol formation to 33 g L^-1 in six days with 85% of maximum theoretical yield. While the results show that the unknown dissipation of xylose can be minimized when coupled to a strong reaction outlet, it remains to be the major hurdle hampering the yield despite the reported inability of cyanobacteria to metabolize xylose.

Highly Efficient Biosynthesis of Heliotropin by Engineered Escherichia coli Coexpressing Trans-Anethole Oxygenase and Formate Dehydrogenase

Heliotropin, a compound with important roles in the spice and fragrance industries and broad application prospects, is mainly produced through chemical methods. Here, we established a novel process for the synthesis of heliotropin by Escherichia coli whole cells through biotransformation of isosafrole. Directed evolution and high-throughput screening based on 2,4-dinitrophenylhydrazine were used to improve the activity of trans-anethole oxygenase toward isosafrole, and a mutant (TAO_3G2) was obtained that had a high ability to oxidize isosafrole. Formate dehydrogenase (FDH) and TAO_3G2 were coexpressed in E. coli, significantly increasing the catalytic efficiency by regenerating more NADH to promote isosafrole oxidation. Furthermore, after optimizing the molar ratio of isosafrole to the auxiliary substrate, the final concentration of heliotropin was increased from 9.15 to 19.45 g/L, and the maximum yield and space-time yield reached 96.02% and 3.89 g/L/h, respectively. These results suggest that the biosynthesis of heliotropin should have excellent industrial application value.

One-Pot Biosynthesis of l-Aspartate from Maleate via an Engineered Strain Containing a Dual-Enzyme System

l-Aspartate has been widely used in medicine and the food and chemical industries. In this study, Serratia marcescens maleate cis-trans isomerase (MaiA) and Escherichia coli aspartase (AspA) were coupled and coexpressed in an engineered E. coli strain in which the byproduct metabolic pathway was inactivated. The engineered E. coli strain containing the dual-enzyme system (pMA) was employed to bioproduce l-aspartate from maleate with a conversion of 98%. We optimized the activity ratio of double enzymes through ribosome binding site (RBS) regulation and molecular modification of MaiA, resulting in an engineered strain: pMA-RBS4-G27A/G171A. The conversion of l-aspartate biotransformed from maleate using the pMA-RBS4-G27A/G171A strain was almost 100%. It required 40 min to complete the whole-cell catalysis, without the intermediate product and byproduct, compared to 120 min before optimization. The induction timing and the amount of inducer in a 5-liter fermentor were optimized for scale-up of the production of l-aspartate. The amount of produced l-aspartate using the cells obtained by fermentation reached 419.8 g/liter (3.15 M), and the conversion was 98.4%. Our study demonstrated an environmentally responsible and efficient method to bioproduce l-aspartate from maleate and provided an available pathway for the industrial production of l-aspartate. This work should greatly improve the economic benefits of l-aspartate, which can now be simply produced from maleate by the engineered strain constructed based on dual-enzyme coupling.IMPORTANCE l-Aspartate is currently produced from fumarate by biological methods, and fumarate is synthesized from maleate by chemical methods in industry. We established a biosynthesis method to produce l-aspartate from maleate that is environmentally responsible, convenient, and efficient. Compared to conventional l-aspartate production, no separation and purification of intermediate products is required, which could greatly improve production efficiency and reduce costs. As environmental issues are attracting increasing attention, conventional chemical methods gradually will be replaced by biological methods. Our results lay an important foundation for the industrialization of l-aspartate biosynthesis from maleate.

Bioproduction of High-Concentration 4-Vinylguaiacol Using Whole-Cell Catalysis Harboring an Organic Solvent-Tolerant Phenolic Acid Decarboxylase From Bacillus atrophaeus

The compound 4-vinyl guaiacol (4-VG) is highly valued and widely applied in the pharmaceutical, cosmetic, and food industries. The bioproduction of 4-VG from ferulic acid (FA) by non-oxidative decarboxylation using phenolic acid decarboxylases is promising but has been hampered by low conversion yields and final product concentrations due to the toxicities of 4-VG and FA. In the current study, a new phenolic acid decarboxylase (BaPAD) was characterized from Bacillus atrophaeus. The BaPAD possessed excellent catalytic activity and stability in various organic solvents. Whole Escherichia coli cells harboring intracellular BaPAD exhibited greater tolerances to FA and 4-VG than those of free BaPAD. A highly efficient aqueous-organic biphasic system was established using 1-octanol as the optimal organic phase for whole-cell catalysis. In this system, a very high concentration (1580 mM, 237.3 g/L) of 4-VG was achieved in a 2 L working volume bioreactor, and the molar conversion yield and productivity reached 98.9% and 18.3 g/L/h in 13 h, respectively.

Efficient Synthesis of Hydroxytyrosol from l-3,4-Dihydroxyphenylalanine Using Engineered Escherichia coli Whole Cells

Hydroxytyrosol is a high-value-added compound with a variety of biological and pharmacological activities. In this study, a whole-cell catalytic method for the synthesis of hydroxytyrosol was developed: aromatic amino acid aminotransferase (TyrB), l-glutamate dehydrogenase (GDH), α-keto acid decarboxylase (PmKDC), and aldehyde reductase (YahK) were co-expressed in Escherichia coli to catalyze the synthesis of hydroxytyrosol from l-3,4-dihydroxyphenylalanine (l-DOPA). The plasmids with different copy numbers were used to balance the expression of the four enzymes, and the most appropriate strain (pRSF- yahK- tyrB and pCDF- gdh- Pmkdc) was identified. After determination of the optimum temperature (35 °C) and pH (7.5) for whole-cell catalysis, the yield of hydroxytyrosol reached 36.33 mM (5.59 g/L) and the space-time yield reached 0.70 g L^-1 h^-1.

Enzymatic Synthesis of Bioactive O-Glucuronides Using Plant Glucuronosyltransferases

Many O-glucuronides exhibiting various pharmacological activities have been found in nature and in drug metabolism. The glucuronidation of bioactive natural products or drugs to generate glucuronides with better activity and druggability is important in drug discovery and research. In this study, by using two uridine diphosphate (UDP)-dependent glucuronosyltransferases (GATs, UGT88D4 and UGT88D7) from plants, we developed two glucuronidation approaches, pure enzyme catalysis in vitro and recombinant whole-cell catalysis in vivo, to efficiently synthesize bioactive O-glucuronides by the glucuronidation of natural products. In total, 14 O-glucuronides with different structures, including flavonoids, anthraquinones, coumarins, and lignans, were obtained, 7 of which were new compounds. Furthermore, one of the biosynthesized O-glucuronides, kaempferol-7- O-β-d-glucuronide (3a), potently inhibited protein tyrosine phosphatase (PTP) 1B with an IC₅₀ value of 8.02 × 10^-6 M. Some of the biosynthesized O-glucuronides also exhibited significant antioxidant activities.

Transcriptomic and Ectoine Analysis of Halotolerant Nocardiopsis gilva YIM 90087T Under Salt Stress

The genus Nocardiopsis is an unique actinobacterial group that widely distributed in hypersaline environments. In this study, we investigated the growth conditions, transcriptome analysis, production and accumulation of ectoine by Nocardiopsis gilva YIM 90087^T under salt stress. The colony color of N. gilva YIM 90087^T changed from yellow to white under salt stress conditions. Accumulation of ectoine and hydroxyectoine in cells was an efficient way to regulate osmotic pressure. The ectoine synthesis was studied by transferring the related genes (ectA, ectB, and ectC) to Escherichia coli. Transcriptomic analysis showed that the pathways of ABC transporters (ko02010) and glycine, serine, and threonine metabolism (ko00260) played a vital role under salt stress environment. The ectABC from N. gilva YIM 90087^T was activated under the salt stress. Addition of exogenous ectoine and hydroxyectoine were helpful to protect N. gilva YIM 90087^T from salt stress.

[Synthesis of vitamin K2 by isopentenyl transferase NovA in Pichia pastoris Gpn12]

The effect of methanol addition on the heterologous expression of isoprenyl transferase NovQ was studied in Pichia pastoris Gpn12, with menadione and isopentenol as precursors to catalyze vitamin K2 (MK-3) synthesis. The expression of NovQ increased by 36% when 2% methanol was added every 24 h. The influence of initial pH, temperature, methanol addition, precursors (menadione, isopentenol) addition, catalytic time and cetyltrimethyl-ammonium bromide (CTAB) addition were explored in the P. pastoris whole-cell catalytic synthesis process of MK-3 in shaking flask. Three significant factors were then studied by response surface method. The optimal catalytic conditions obtained were as follows: catalytic temperature 31.56 ℃, menadione 295.54 mg/L, catalytic time 15.87 h. Consistent with the response surface prediction results, the optimized yield of MK-3 reached 98.47 mg/L in shaking flask, 35% higher than that of the control group. On this basis, the production in a 30-L fermenter reached 189.67 mg/L when the cell catalyst of 220 g/L (dry weight) was used to catalyze the synthesis for 24 h. This method laid the foundation for the large-scale production of MK-3 by P. pastoris Gpn12.

Semirational Protein Engineering of CYP153AM.aq. -CPRBM3 for Efficient Terminal Hydroxylation of Short- to Long-Chain Fatty Acids

The regioselective terminal hydroxylation of alkanes and fatty acids is of great interest in a variety of industrial applications, such as in cosmetics, in fine chemicals, and in the fragrance industry. The chemically challenging activation and oxidation of non-activated C-H bonds can be achieved with cytochrome P450 enzymes. CYP153AM.aq. -CPRBM3 is an artificial fusion construct consisting of the heme domain from Marinobacter aquaeolei and the reductase domain of CYP102A1 from Bacillus megaterium. It has the ability to hydroxylate medium- and long-chain fatty acids selectively at their terminal positions. However, the activity of this interesting P450 construct needs to be improved for applications in industrial processes. For this purpose, the design of mutant libraries including two consecutive steps of mutagenesis is demonstrated. Targeted positions and residues chosen for substitution were based on semi-rational protein design after creation of a homology model of the heme domain of CYP153AM.aq. , sequence alignments, and docking studies. Site-directed mutagenesis was the preferred method employed to address positions within the binding pocket, whereas diversity was created with the aid of a degenerate codon for amino acids located at the substrate entrance channel. Combining the successful variants led to the identification of a double variant-G307A/S233G-that showed alterations of one position within the binding pocket and one position located in the substrate access channel. This double variant showed twofold increased activity relative to the wild type for the terminal hydroxylation of medium-chain-length fatty acids. This variant furthermore showed improved activity towards short- and long-chain fatty acids and enhanced stability in the presence of higher concentrations of fatty acids.

Applications and mechanisms of ionic liquids in whole-cell biotransformation

Ionic liquids (ILs), entirely composed of cations and anions, are liquid solvents at room temperature. They are interesting due to their low vapor pressure, high polarity and thermostability, and also for the possibility to fine-tune their physicochemical properties through modification of the chemical structures of their cations or anions. In recent years, ILs have been widely used in biotechnological fields involving whole-cell biotransformations of biodiesel or biomass, and organic compound synthesis with cells. Research studies in these fields have increased from the past decades and compared to the typical solvents, ILs are the most promising alternative solvents for cell biotransformations. However, there are increasing limitations and new challenges in whole-cell biotransformations with ILs. There is little understanding of the mechanisms of ILs' interactions with cells, and much remains to be clarified. Further investigations are required to overcome the drawbacks of their applications and to broaden their application spectrum. This work mainly reviews the applications of ILs in whole-cell biotransformations, and the possible mechanisms of ILs in microbial cell biotransformation are proposed and discussed.