Combination of multi-enzyme expression fine-tuning and co-substrates addition improves phenyllactic acid production with an Escherichia coli whole-cell biocatalyst

A novel high-level phenyllactic acid fungal producer, Kodamaea ohmeri w5 screened from fermented broad bean-chili-paste

Phenyllactic acid (PLA) is a broad-spectrum and efficient antimicrobial phenolic acid with potential applications in the food industry. Previous studies have demonstrated that fungi may be ideal producers of PLA. In this study, 15 fungi screened from Doubanjiang with the ability to produce PLA were first reported, including Wickerhamomyces anomalus, Candida etchellsii, Candida parasitosis, Pichia kudriavzevii, Pichia membranifaciens and Kodamaea ohmeri. Among them, K. ohmeri w5 had the highest PLA yield, producing up to 7160 mg/L PLA in shake flask fermentation with phenylalanine as substrate, which was more than ten times higher than the PLA produced by wild-type LAB under the similar conditions. In addition, K. ohmeri w5 was able to grow under extreme hypertonic conditions of 20 % NaCl (w/v) and 50 % glucose (w/v) as well as produce 57.12 ± 0.42 and 1609.22 ± 36.26 mg/L of PLA, respectively. Furthermore, the fermentation supernatant of K. ohmeri w5 demonstrated direct inhibitory effects against foodborne pathogenic microorganisms, Aspergillus flavus and Bacillus cereus. However, the inhibitory effect was weaker than that of the PLA standard at the same concentration. Further, 12,497,932 bp of w5 genome-wide information was obtained by sequencing and assembling. And its gene model was predicted based on transcriptomic evidence, which showed that a total of 7 genes related to PLA synthesis were identified in the w5 genome. Based on qRT-PCR, structure prediction, and molecular docking, a potentially key genetic resource from K. ohmeri w5 for PLA production was uncovered. The results will provide novel producers of PLA and its potential genetic resources.

Highly efficient bio-production of putrescine from L-arginine with arginase and L-ornithine decarboxylase in engineered Escherichia coli

To achieve industrial-scale putrescine production, a high efficient bio-synthesis of putrescine involving arginase (ARG, EC 3.5.3.1) and L-ornithine decarboxylase was evaluated here. Among the four arginases tested, ARGBT from Bos Taurus showed the highest activity (1966 U/mg). Compared to the L-arginine decarboxylase (ADC) pathway, the strain expressing ARGBT and L-ornithine decarboxylase (SpeC) produced 28.7 g/L putrescine, a 38.6 % increase. Two pyridoxal phosphate (PLP) salvage pathways were evaluated, and the strain BL-PTac-PdxK co-expressed pyridoxal kinase (PdxK) performed better. D-Glucose was used as the co-substrate to improve the putrescine titer further. Under optimal conditions, 43.6 g/L putrescine was produced from 87.1 g/L L-arginine, and 76 g/L putrescine was synthesized on a 0.5 L scale. Using L-arginine fermentation broth (60 g/L) as the substrate, a titer of 30 g/L putrescine was achieved. This efficient biotransformation process presented here enables feasible industrial-scale putrescine production.

One-pot enantioselective synthesis of chiral phenyllactic acids by combining stereocomplementary d- and l-lactate dehydrogenases with multi-enzyme expression fine-tuning

Chiral phenyllactic acid (PLA) is a new type of antiseptic agent and a valuable precursor for active ingredients in pharmaceuticals and agrochemicals. In this study, we designed a multi-enzyme cascade that combined stereocomplementary d- and l-lactate dehydrogenases with threonine aldolase, phenylserine dehydratase, and formate dehydrogenase for the one-pot conversion of achiral glycine and benzaldehyde to synthesize d-PLA and l-PLA. To overcome the imbalance of multi-enzymes in a single cell, two enzyme modules, overexpressing four enzymes, were assembled in Escherichia coli cells to construct whole-cell catalysis systems (WCCSs). Furthermore, by optimizing reaction conditions and components, recombinant E. coli (WCCS 26) was able to produce 100 mM d-PLA with >99 % ee using a fed-batch strategy, while E. coli (WCCS 60) produced 47.2 mM l-PLA with >99 % ee. This study presents a sustainable and efficient method for synthesizing chiral PLAs from food-grade achiral starting materials.

Development of whole cell biocatalytic system for asymmetric synthesis of esomeprazole with enhancing coenzyme biosynthesis pathway

Esomeprazole is the most popular proton pump inhibitor for treating gastroesophageal reflux disease. Previously, a phenylacetone monooxygenase mutant LnPAMOmu15 (LM15) was obtained by protein engineering for asymmetric synthesis of esomeprazole using pyrmetazole as substrate. To scale up the whole cell asymmetric synthesis of esomeprazole and reduce the cost, in this work, an Escherichia coli whole-cell catalyst harboring LM15 and formate dehydrogenase from Burkholderia stabilis 15516 (BstFDH) were constructed through optimized gene assembly patterns. CRISPR/Cas9 mediated insertion of Ptrc promoter in genome was done to enhance the expression of key genes to increase the cellular NADP supply in the whole cell catalyst, by which the amount of externally added NADP+ for the asymmetric synthesis of esomeprazole decreased to 0.05 mM from 0.3 mM for reducing the cost. After the optimization of reaction conditions in the reactor, the scalable synthesis of esomeprazole was performed using the efficient LM15-BstFDH whole-cell as catalyst, which showed the highest reported space-time yield of 3.28 g/L/h with 50 mM of pyrmetazole loading. Isolation procedure was conducted to obtain esomeprazole sodium of 99.55 % purity and > 99.9 % ee with 90.1 % isolation yield. This work provides the basis for production of enantio-pure esomeprazole via cost-effective whole cell biocatalysis.

Multienzymatic Cascade for Synthesis of Hydroxytyrosol via Two-Stage Biocatalysis

Hydroxytyrosol, a naturally occurring compound with antioxidant and antiviral activity, is widely applied in the cosmetic, food, and nutraceutical industries. The development of a biocatalytic approach for producing hydroxytyrosol from simple and readily accessible substrates remains a challenge. Here, we designed and implemented an effective biocatalytic cascade to obtain hydroxytyrosol from 3,4-dihydroxybenzaldehyde and l-threonine via a four-step enzymatic cascade composed of seven enzymes. To prevent cross-reactions and protein expression burden caused by multiple enzymes expressed in a single cell, the designed enzymatic cascade was divided into two modules and catalyzed in a stepwise manner. The first module (FM) assisted the assembly of 3,4-dihydroxybenzaldehyde and l-threonine into (2S,3R)-2-amino-3-(3,4-dihydroxyphenyl)-3-hydroxypropanoic acid, and the second module (SM) entailed converting (2S,3R)-2-amino-3-(3,4-dihydroxyphenyl)-3-hydroxypropanoic acid into hydroxytyrosol. Each module was cloned into Escherichia coli BL21 (DE3) and engineered in parallel by fine-tuning enzyme expression, resulting in two engineered whole-cell catalyst modules, BL21(FM01) and BL21(SM13), capable of converting 30 mM 3,4-dihydroxybenzaldehyde to 28.7 mM hydroxytyrosol with a high space-time yield (0.88 g/L/h). To summarize, the current study proposes a simple and effective approach for biosynthesizing hydroxytyrosol from low-cost substrates and thus has great potential for industrial applications.

Enhanced synthesis of chloramphenicol intermediate L-threo-p-nitrophenylserine using engineered L-threonine transaldolase and by-product elimination

L-threo-p-nitrophenylserine (component 2) is an important intermediate during synthesis of chloramphenicol. However, its biosynthesis is limited by enzyme activity and stereoselectivity. In this study, we achieved a breakthrough in the high-efficiency production of 2 by employing engineered Chitiniphilus shinanonensis L-threonine transaldolase (ChLTTA) in conjunction with a by-product elimination system within a one-pot reaction. Notably, a novel visual stepwise high-throughput screening method was developed for the directed evolution of ChLTTA, leveraging its characteristic color. The engineered mutant F70D/F59A (Mu6 variant) emerged as a star performer, exhibiting a remarkable 2.6-fold increase in catalytic efficiency over the wild-type ChLTTA, coupled with an outstanding 91.5 % diastereoisomer excess (de). Molecular dynamics (MD) simulations unraveled the mechanism responsible for the enhanced catalytic performance observed in the Mu6 variant. Meanwhile, the Mu6 variant was coupled with Saccharomyces cerevisiae ethanol dehydrogenase (ScADH) and Candida boidinii formate dehydrogenase (CbFDH) to create a high-efficiency cascade system (E.coli/pRSF-Mu6-ScADH-CbFDH). Under optimized conditions, this cascade system demonstrated unparalleled performance, yielding 201.5 mM of 2 with an impressive conversion of 95.9 % and a de value of 94.5 %. This achievement represents the highest reported yield to date. This study offers a novel insight into the sustainable and efficient production of chloramphenicol intermediate.

An artificial biocatalytic cascade for efficient synthesis of norepinephrine by combination of engineered L-threonine transaldolase with multi-enzyme expression fine-tuning

Norepinephrine, a kind of β-adrenergic receptor agonist, is commonly used for treating shocks and hypotension caused by a variety of symptoms. The development of a straightforward, efficient and environmentally friendly biocatalytic route for manufacturing norepinephrine remains a challenge. Here, we designed and realized an artificial biocatalytic cascade to access norepinephrine starting from 3, 4-dihydroxybenzaldehyde and L-threonine mediated by a tailored-made L-threonine transaldolase PsLTTA-Mu1 and a newly screened tyrosine decarboxylase ErTDC. To overcome the imbalance of multi-enzymes in a single cell, engineering of PsLTTA for improved activity and fine-tuning expression mode of multi-enzymes in single E.coli cells were combined, leading to a robust whole cell biocatalyst ES07 that could produce 100 mM norepinephrine with 99% conversion, delivering a highest time-space yield (3.38 g/L/h) ever reported. To summarized, the current study proposed an effective biocatalytic approach for the synthesis of norepinephrine from low-cost substrates, paving the way for industrial applications of enzymatic norepinephrine production.

Advances in succinic acid production: the enhancement of CO2 fixation for the carbon sequestration benefits

Succinic acid (SA), one of the 12 top platform chemicals produced from biomass, is a precursor of various high value-added derivatives. Specially, 1 mol CO2 is assimilated in 1 mol SA biosynthetic route under anaerobic conditions, which helps to achieve carbon reduction goals. In this review, methods for enhanced CO2 fixation in SA production and utilization of waste biomass for SA production are reviewed. Bioelectrochemical and bioreactor coupling systems constructed with off-gas reutilization to capture CO2 more efficiently were highlighted. In addition, the techno-economic analysis and carbon sequestration benefits for the synthesis of bio-based SA from CO2 and waste biomass are analyzed. Finally, a droplet microfluidics-based high-throughput screening technique applied to the future bioproduction of SA is proposed as a promising approach.

Modelling and optimization of biodiesel production from waste fish oil using nano immobilized rPichiapastoris whole cell biocatalyst with response surface methodology and hybrid artificial neural network based approach

In this study, zinc oxide (ZnO) nano particle immobilized recombinant whole cell biocatalyst (rWCB) was used for bioconversion of waste fish oil in to biodiesel in a lab scale packed bed reactor (PBR). Central composite design and hybrid artificial neural network (ANN) models were explored to optimize the production of biodiesel. Developed rWCB exhibited maximum lipase activity at 15 % (v/v) of glutaraldehyde concentration and 6 % (w/v) of ZnO nanoparticles at pH of 7. Maximum biodiesel yield reached about 91.54 ± 1.86 % after 43 h in PBR using hybrid ANN model predicted process conditions of 13.2 % (w/v) of nano immobilized rWCB concentration and 4.7:1 of methanol to oil ratio at 33 °C. Importantly, developed nano immobilized rWCB was adequately stable for commercialization. Thus, production of biodiesel from waste fish oil using ZnO nano immobilized rWCB could become potential candidate for commercialization.

A Novel Strategy for Whole-Cell Biotransformation Enabling Simultaneous l-Phenyllactic Acid Production and Coenzyme Regeneration

l-Phenyllactic acid (l-PLA) is a small molecular organic acid that exhibits a powerful capacity for inhibition against foodborne pathogens. In this work, we developed a new cost-effective and environmentally friendly process for the biosynthesis of l-PLA. This strategy designed a novel whole-cell biotransformation system employing two heterologous enzymes, namely, phenylalanine dehydrogenase (PheDH) and l-hydroxyisocaproate dehydrogenase (l-HicDH). The novelty of this strategy lies in the first-time utilization of these two enzymes, which not only enables cascade catalysis for the production of l-PLA but also facilitates the regeneration of the coenzymes (NAD+/NADH) using only two enzymes rather than introducing more heterologous enzymes to the system. Consequently, this strategy can effectively simplify the biosynthesis process of l-PLA and minimize production costs. The initial l-PLA yield using this process achieved 2.53 ± 0.07 g/L. Furthermore, through meticulous optimization of the parameters for inducible enzyme expression and l-PLA biosynthesis, the l-PLA yield was successfully increased to 4.68 ± 0.04 g/L with a yield rate of 64.54 ± 0.29%. Moreover, this novel strategy is versatile in the biosynthesis of other organic acids, which can be achieved by easily modulating the combinations of substrates and enzymes.

Efficient synthesis of 2'-deoxyguanosine in one-pot cascade by employing an engineered purine nucleoside phosphorylase from Brevibacterium acetylicum

2'-deoxyguanosine is a key medicinal intermediate that could be used to synthesize anti-cancer drug and biomarker in type 2 diabetes. In this study, an enzymatic cascade using thymidine phosphorylase from Escherichia coli (EcTP) and purine nucleoside phosphorylase from Brevibacterium acetylicum (BaPNP) in a one-pot whole cell catalysis was proposed for the efficient synthesis of 2'-deoxyguanosine. BaPNP was semi-rationally designed to improve its activity, yielding the best triple variant BaPNP-Mu3 (E57A/T189S/L243I), with a 5.6-fold higher production of 2'-deoxyguanosine than that of wild-type BaPNP (BaPNP-Mu0). Molecular dynamics simulation revealed that the engineering of BaPNP-Mu3 resulted in a larger and more flexible substrate entrance channel, which might contribute to its catalytic efficiency. Furthermore, by coordinating the expression of BaPNP-Mu3 and EcTP, a robust whole cell catalyst W05 was created, capable of producing 14.8 mM 2'-deoxyguanosine (74.0% conversion rate) with a high time-space yield (1.32 g/L/h) and therefore being very competitive for industrial applications.

Co-production of 7-chloro-tryptophan and indole pyruvic acid based on an efficient FAD/FADH2 regeneration system

Efficient FAD/FADH₂ regeneration is vital for enzymatic biocatalysis and metabolic pathway optimization. Here, we constructed an efficient and simple FAD/FADH₂ regeneration system through a combination of L-amino acid deaminase (L-AAD) and halogenase (CombiAADHa), which was applied for catalyzing the conversion of an L-amino acid to halide and an α-keto acid. For cell-free biotransformation, the optimal activity ratio of L-AAD and halogenase was set between 1:50 and 1:60. Within 6 h, 170 mg/L of 7-chloro-tryptophan (7-Cl-Trp) and 193 mg/L of indole pyruvic acid (IPA) were synthesized in the selected mono-amino acid system. For whole-cell biotransformation, 7-Cl-Trp and IPA synthesis was enhanced by 15% (from 96 to 110 mg/L) and 12% (from 115 to 129 mg/L), respectively, through expression fine-tuning and the strengthening of FAD/FADH₂ supply. Finally, ultrasound treatment was applied to improve membrane permeability and adjust the activity ratio, resulting in 1.6-and 1.4-fold higher 7-Cl-Trp and IPA yields. The products were then purified. This system could also be applied to the synthesis of other halides and α-keto acids. KEY POINTS: • In this study, a whole cell FAD/FADH₂ regeneration system co-expressing l-AAD and halogenase was constructed • This study found that the activity and ratio of enzyme and the concentration of cofactors had a significant effect on the catalytic process for the efficient co-production of 7-chlorotryptophan and indole pyruvate.

Dissecting LuxS/AI-2 quorum sensing system-mediated phenyllactic acid production mechanisms of Lactiplantibacillus plantarum L3

The phenyllactic acid (PLA) produced by lactic acid bacteria (LAB) inhibits fungi and facilitates the quality control of fermented milk. A strain of Lactiplantibacillus plantarum L3 (L. plantarum L3) with high PLA production was screened in the pre-laboratory, but the mechanism of its PLA formation is unclear. The amount of autoinducer-2 (AI-2) increased with increasing culture time, as did cell density and PLA. The results in this study suggest that PLA production in L. plantarum L3 may be regulated by the LuxS/AI-2 Quorum Sensing (QS) system. Tandem mass tag (TMT) quantitative proteomics analysis showed that a total of 1291 differentially expressed proteins (DEPs) were quantified in the incubated for 24 h compared with the incubated for 2 h, of which 516 DEPs were up-regulated and 775 DEPs were down-regulated. Among them, S-ribosomal homocysteine lyase (luxS), aminotransferase (araT), and lactate dehydrogenase (ldh) are the key proteins for PLA formation. The DEPs were mainly involved in the QS pathway and the core pathway of PLA synthesis. Furanone effectively inhibited the production of L. plantarum L3 PLA. In addition, Western blot analysis demonstrated that luxS, araT, and ldh were the key proteins regulating PLA production. This study reveals the regulatory mechanism of PLA based on the LuxS/AI-2 QS system, which provides a theoretical basis for the efficient and large-scale production of PLA in industries in the future.

Recent development of phenyllactic acid: physicochemical properties, biotechnological production strategies and applications

Phenyllactic acid (PLA) is capable of inhibiting the growth of many microorganisms, showing a broad-spectrum antimicrobial property, which allows it to hold vast applications in the: food, feed, pharmaceutical, and cosmetic industries, especially in the field of food safety. Recently, the production of PLA has garnered considerable attention due to the increasing awareness of food safety from the public. Accordingly, this review mainly updates the recent development for the production of PLA through microbial fermentation and whole-cell catalysis (expression single-, double-, and triple-enzyme) strategies. Firstly, the: physicochemical properties, existing sources, and measurement methods of PLA are systematically covered. Then, the inhibition spectrum of PLA is summarized, and synchronously, the antimicrobial and anti-biofilm mechanisms of PLA on commonly pathogenic microorganisms in foods are described in detail, thereby clarifying the reason for extending the shelf life of foods. Additionally, the factors affecting the production of PLA are summarized from the biosynthesis and catabolism pathway of PLA in microorganisms, as well as external environmental parameters insights. Finally, the downstream treatment process and applications of PLA are discussed and outlined. In the future, clinical data should be supplemented with the metabolic kinetics of PLA in humans and to evaluate animal toxicology, to enable regulatory use of PLA as a food additive. A food-grade host, such as Bacillus subtilis and Lactococcus lactis, should also be developed as a cell vector expressing enzymes for PLA production from a food safety perspective.

Coenzyme self-sufficiency system-recent advances in microbial production of high-value chemical phenyllactic acid

Phenyllactic acid (PLA), a natural antimicrobial substance, has many potential applications in the food, animal feed, pharmaceutical and cosmetic industries. However, its production is limited by the complex reaction steps involved in its chemical synthesis. Through advances in metabolic engineering and synthetic biology strategies, enzymatic or whole-cell catalysis was developed as an alternative method for PLA production. Herein, we review recent developments in metabolic engineering and synthetic biology strategies that promote the microbial production of high-value PLA. Specially, the advantages and disadvantages of the using of the three kinds of substrates, which includes phenylpyruvate, phenylalanine and glucose as starting materials by natural or engineered microbes is summarized. Notably, the bio-conversion of PLA often requires the consumption of expensive coenzyme NADH. To overcome the issues of NADH regeneration, efficiently internal cofactor regeneration systems constructed by co-expressing different enzyme combinations composed of lactate dehydrogenase with others for enhancing the PLA production, as well as their possible improvements, are discussed. In particular, the construction of fusion proteins with different linkers can achieve higher PLA yield and more efficient cofactor regeneration than that of multi-enzyme co-expression. Overall, this review provides a comprehensive overview of PLA biosynthesis pathways and strategies for increasing PLA yield through biotechnology, providing future directions for the large-scale commercial production of PLA and the expansion of downstream applications.

Rational Design of Artificial Biofilms as Sustainable Supports for Whole-Cell Catalysis Through Integrating Extra- and Intracellular Catalysis

Biofilms are promising candidates for sustainable bioprocessing applications. This work presents a rational design of biofilm catalysts by integrating extra- and intracellular catalysis systems with optimized substrate channeling to realize efficient multistep biosynthesis. An assembly of four enzymes in a "three-in-one" structure was achieved by rationally placing the enzymes on curli nanofibers, the cell surface, and inside cells. The catalytic efficiency of the biofilm catalysts was over 2.8 folds higher than that of the control whole-cell catalysis when the substrate benzaldehyde was fed at 100 mm. The highest yield of d-phenyllactic acid catalyzed by biofilm catalysts under optimized conditions was 102.19 mm, also much higher than that of the control catalysis test (52.29 mm). The results demonstrate that engineered biofilms are greatly promising in integrating extra- and intracellular catalysis, illustrating great potentials of rational design in constructing biofilm catalysts as sustainable supports for whole-cell catalysis.

Lactate dehydrogenase encapsulated in a metal-organic framework: A novel stable and reusable biocatalyst for the synthesis of D-phenyllactic acid

In this study, we synthesized a novel biocatalyst by encapsulating lactate dehydrogenase (LDH) in the metal-organic framework ZIF-90 by one-pot embedding. It showed strong biological activity for efficient synthesis of D-phenyllactic acid (D-PLA). The morphology and structure of LDH@ZIF-90 was systematically characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, confocal laser scanning microscopy (CLSM) and gas sorption. According to thermogravimetric analysis (TGA), the enzyme loading of the biocatalyst was 3 %. The Michaelis-Menten constant (K_m) and maximal reaction rate (V_max) of LDH@ZIF-90 were similar to those of free LDH, which proved that ZIF-90 had good biocompatibility to encapsulate LDH. At the same time, LDH@ZIF-90 exhibited enhanced tolerance to temperature, pH and organic solvents, and its reusability was greatly improved with 68 % of initial enzyme activity remaining after 7 rounds of recylcing. Overall, LDH encapsulated in ZIF-90 may be an economically competitive and environmentally friendly novel biocatalyst for the synthesis of D-PLA.

Enantioselective Biosynthesis of L-Phenyllactic Acid From Phenylpyruvic Acid In Vitro by L-Lactate Dehydrogenase Coupling With Glucose Dehydrogenase

As a valuable versatile building block, L-phenyllactic acid (L-PLA) has numerous applications in the fields of agriculture, pharmaceuticals, and biodegradable plastics. However, both normally chemically synthesized and naturally occurring PLA are racemic, and the production titer of L-PLA is not satisfactory. To improve L-PLA production and reduce the high cost of NADH, an in vitro coenzyme regeneration system of NADH was achieved using the glucose dehydrogenase variant LsGDHD255C and introduced into the L-PLA production process. Here an NADH-dependent L-lactate dehydrogenase-encoding variant gene (L-Lcldh1Q88A/I229A) was expressed in Pichia pastoris GS115. The specific activity of L-LcLDH1Q88A/I229A (Pp) was as high as 447.6 U/mg at the optimum temperature and pH of 40°C and 5.0, which was 38.26-fold higher than that of wild-type L-LcLDH1 (Pp). The catalytic efficiency (kcat/Km) of L-LcLDH1Q88A/I229A (Pp) was 94.3 mM−1 s−1, which was 67.4- and 25.5-fold higher than that of L-LcLDH1(Pp) and L-LcLDH1Q88A/I229A (Ec) expressed in Escherichia coli, respectively. Optimum reactions of L-PLA production by dual-enzyme catalysis were at 40°C and pH 5.0 with 10.0 U/ml L-LcLDH1Q88A/I229A (Pp) and 4.0 U/ml LsGDHD255C. Using 0.1 mM NAD+, 400 mM (65.66 g/L) phenylpyruvic acid was completely hydrolyzed by fed-batch process within 6 h, affording L-PLA with 90.0% yield and over 99.9% eep. This work would be a promising technical strategy for the preparation of L-PLA at an industrial scale.

Easily biodegradable substrates are crucial for enhancing antibiotic risk reduction: Low-carbon discharging policies need to be more specified

Governments have formulated stricter wastewater treatment plant (WWTP) discharge standards to address water pollution; however, with the cost of aggravating the refractory of the discharges. These policies are not in line with the classic co-metabolism theory; thus, we evaluated the effects of an easily biodegradable substrate on the removal efficiency of antibiotics and antibiotic resistance genes (ARGs) in the receiving water. In this study, reactor with 8 d of hydraulic retention time (HRT) was constructed to simulate a receiving river, and several antibiotics (0.30 mg/L each) were continuously discharged to the reactor (tetracycline, ciprofloxacin, amoxicillin, chloramphenicol, and sulfamethoxazole). Sodium acetate (NaAc) was used as a representative easily biodegradable substrate, and treatment protocols with and without a co-substrate were compared. The attenuation of the antibiotics in the simulated river and the production and dissemination of ARGs were analyzed. The results showed that 50 mg/L NaAc activated non-specific enzymes (a log2-fold change of 3.1-8.8 compared with 0 mg/L NaAc). The removal rate of the antibiotics was increased by 4-32%, and the toxicity of the downstream water was reduced by 35%. The upregulation of antioxidant enzymes caused the intracellular reactive oxygen species (ROSs) decreased by up to 47%, inhibiting horizontal gene transfer and reducing mobile genetic element-mediated ARGs (mARGs) by 18-56%. Furthermore, NaAc also increased the alpha diversity of the microbial community by 5-15% (Shannon-Wiener Index) and reduced the abundance of human bacterial pathogens by 22-36%. In summary, easily biodegradable substrates in the receiving water are crucial for reducing antibiotic risk.

Constructing a methanol-dependent Bacillus subtilis by engineering the methanol metabolism

Methanol is a promising green feedstock for producing fuels and chemicals because it is inexpensive, clean, environmentally friendly, and easily prepared. Thus, many studies have been devoted to engineering non-native methylotrophic platform microorganisms to utilize methanol. This study adopted a series of strategies to develop a synthetic methylotrophic Bacillus subtilis that can use methanol as the carbon source, including the heterologous expression of methanol dehydrogenase (Mdh), enhancement of the expressions of 3-hexulose-6-phosphate synthase (Hps) and 6-phospho-3-hexuloisomerase (Phi), regulation of the expressions of key enzymes at both the translational and transcriptional levels, stabilization of the key enzyme expression through a dual-system for expressing the target genes on both the plasmid and genome, and improvement of the catalytic activity of Mdh with a recycling strategy for NAD⁺. As a result, the methanol consumption of the synthetic methylotrophic B. subtilis reached 4.09 g/L, with the maximum OD₆₀₀ showing a 2.21-fold increase compared with the wild-type B. subtilis, which cannot use methanol. We further deleted the phosphoglucose isomerase (Pgi) and added co-substrates to increase the supply of ribulose-5-phosphate (Ru-5-P), and the specific methanol consumption rate increased by an additional 27.54%. Finally, we successfully constructed two strains that cannot grow in M9 medium with xylose or ribose unless methanol is utilized. The strategies used in this study are generally applicable to other studies on synthetic methylotrophy.

Using Unnatural Protein Fusions to Engineer a Coenzyme Self-Sufficiency System for D-Phenyllactic Acid Biosynthesis in Escherichia coli

The biosynthetic production of D-penyllactic acid (D-PLA) is often affected by insufficient supply and regeneration of cofactors, leading to high production cost, and difficulty in industrialization. In this study, a D-lactate dehydrogenase (D-LDH) and glycerol dehydrogenase (GlyDH) co-expression system was constructed to achieve coenzyme NADH self-sufficiency and sustainable production of D-PLA. Using glycerol and sodium phenylpyruvate (PPA) as co-substrate, the E. coli BL21 (DE3) harboring a plasmid to co-express LfD-LDH and BmGlyDH produced 3.95 g/L D-PLA with a yield of 0.78 g/g PPA, similar to previous studies. Then, flexible linkers were used to construct fusion proteins composing of D-LDH and GlyDH. Under the optimal conditions, 5.87 g/L D-PLA was produced by expressing LfD-LDH-l3-BmGlyDH with a yield of 0.97 g/g PPA, which was 59.3% increased compared to expression of LfD-LDH. In a scaled-up reaction, a productivity of 5.83 g/L/h was reached. In this study, improving the bio-catalytic efficiency by artificial redox self-equilibrium system with a bifunctional fusion protein could reduce the bio-production cost of D-PLA, making this bio-production of D-PLA a more promising industrial technology.

Efficient Production of 3′-Sialyllactose by Single Whole-Cell in One-Pot Biosynthesis

Sialyllactose (SL) is one of the most important acidic oligosaccharides in human milk, which plays an important role in the health of infants. In this work, an efficient multi-enzyme cascade was developed in a single whole cell to produce 3′-SL. We constructed two compatible plasmids with double cloning sites to co-express four genes. Different combinations were assessed to verify the optimal catalytic ability. Then, the conversion temperature, pH, and stability under the optimal temperature and pH were investigated. Moreover, the optimal conversion conditions and surfactant concentration were determined. By using the optimal conditions (35 °C, pH 7.0, 20 mM polyphosphate, 10 mM cytidine monophosphate (CMP), 20 mM MgCl2), 25 mL and 4 L conversion systems were carried out to produce 3′-SL. Similar results were obtained between different volume conversion reactions, which led the maximum production of 3′-SL to reach 53 mM from 54.2 mM of sialic acid (SA) in the 25 mL system and 52.8 mM of 3′-SL from 53.8 mM of SA in the 4 L system. These encouraging results demonstrate that the developed single whole-cell multi-enzyme system exhibits great potential and economic competitiveness for the manufacture of 3′-SL.

Programming Integrative Multienzyme Systems and Ionic Strength For Recyclable Synthesis of Glutathione

In the enzymatic cascade catalysis, it is a big challenge to construct a stable and reusable catalyst with targeted enzymes. The artificial multienzyme reactor has attracted great attention due to its potential for facilitating the performance of enzyme catalysis. In this study, we set up a reliable system that could assemble polyphosphate kinase (PPK) with bifunctional glutathione synthetase (GshF) via SpyCatcher/SpyTag to form multienzyme systems (MESs). Furthermore, MESs could assemble into nanoaggregates by altering the ionic strength, and the larger nanoaggregates could be applied in robust and reusable synthesis of glutathione (GSH). To enhance MES levels in vivo, gene duplication and different coexpression modes were performed. Finally, the optimized production of GSH and oxidized glutathione (GSSG) reached 102.6 and 6.7 mM within 2 h. Compared with the first round, the total yield only decreased by 9.4% after five continuous rounds of biocatalysis.

Efficient single whole-cell biotransformation for L-2-aminobutyric acid production through engineering of leucine dehydrogenase combined with expression regulation

Leucine dehydrogenase (LDH) is widely used in the preparation of L-2-aminobutyric acid (L-2-ABA), however its wide application is limited by 2-ketobutyric acid (2-OBA) inhibition. Firstly, a novel high-throughput screening method of LDH was established, specific enzyme activity and 2-OBA tolerance of Lys72Ala mutant were 33.3% higher than those of the wild type. Subsequently, we constructed a single cell comprised of ivlA, Esldh_K72A, fdh and optimized expression through fine-tuning RBS intensity, so that the yield of E. coli BL21/pET28a-R3ivlA-Esldh_K72A-fdh was 2.6 times higher than that of the original strain. As a result, 150 g L-threonine was transformed to 121 g L-2-ABA in 5 L fermenter with 95% molar conversion rate, and a productivity of 5.04 g·L^-1·h^-1, which is the highest productivity of L-2-ABA currently reported by single-cell biotransformation. In summary, our research provided a green synthesis for L-2-ABA, which has potential for industrial production of drug precursors.

Recent advances in artificial enzyme cascades for the production of value-added chemicals

Enzyme cascades are efficient tools to perform multi-step synthesis in one-pot in a green and sustainable manner, enabling non-natural synthesis of valuable chemicals from easily available substrates by artificially combining two or more enzymes. Bioproduction of many high-value chemicals such as chiral and highly functionalised molecules have been achieved by developing new enzyme cascades. This review summarizes recent advances on engineering and application of enzyme cascades to produce high-value chemicals (alcohols, aldehydes, ketones, amines, carboxylic acids, etc) from simple starting materials. While 2-step enzyme cascades are developed for versatile enantioselective synthesis, multi-step enzyme cascades are engineered to functionalise basic chemicals, such as styrenes, cyclic alkanes, and aromatic compounds. New cascade reactions have also been developed for producing valuable chemicals from bio-based substrates, such as ʟ-phenylalanine, and renewable feedstocks such as glucose and glycerol. The challenges in current process and future outlooks in the development of enzyme cascades are also addressed.

Whole-cell biosynthesis of d-tagatose from maltodextrin by engineered Escherichia coli with multi-enzyme co-expression system

d-tagatose is a functional sweetener that occurs in small quantity in nature. It is mainly produced through the isomerization of d-galactose by l-arabinose isomerase (l-AI; EC 5.3.1.4). However, the cost of d-galactose is much higher than those commonly used for the production of functional sweeteners such as glucose, maltodextrin, or starch. Here, a multi-enzyme catalytic system consists of five enzymes that utilizes maltodextrin as substrate to synthesize d-tagatose were co-expressed in E. coli, resulting in recombinant cells harboring the plasmids pETDuet-αgp-pgm and pCDFDuet-pgi-gatz-pgp. The activity of this whole-cell catalyst was optimal at 60 °C and pH 7.5, and 1 mM Mg²⁺ and 50 mM phosphate were the optimal cofactors for activity. Under the optimal reaction conditions, 2.08 and 3.2 g L^-1d-tagatose were produced by using 10 and 20 g L^-1 maltodextrin as substrates with recombinant cells for 24 h. This co-expression system provides a one-pot synthesis approach for the production of d-tagatose using inexpensive substrate, avoiding enzymes purification steps and supplementation of expensive cofactors.

Highly selective synthesis of D-amino acids via stereoinversion of corresponding counterpart by an in vivo cascade cell factory

BACKGROUND

D-Amino acids are increasingly used as building blocks to produce pharmaceuticals and fine chemicals. However, establishing a universal biocatalyst for the general synthesis of D-amino acids from cheap and readily available precursors with few by-products is challenging. In this study, we developed an efficient in vivo biocatalysis system for the synthesis of D-amino acids from L-amino acids by the co-expression of membrane-associated L-amino acid deaminase obtained from Proteus mirabilis (LAAD), meso-diaminopimelate dehydrogenases obtained from Symbiobacterium thermophilum (DAPDH), and formate dehydrogenase obtained from Burkholderia stabilis (FDH), in recombinant Escherichia coli.

RESULTS

To generate the in vivo cascade system, three strategies were evaluated to regulate enzyme expression levels, including single-plasmid co-expression, double-plasmid co-expression, and double-plasmid MBP-fused co-expression. The double-plasmid MBP-fused co-expression strain Escherichia coli pET-21b-MBP-laad/pET-28a-dapdh-fdh, exhibiting high catalytic efficiency, was selected. Under optimal conditions, 75 mg/mL of E. coli pET-21b-MBP-laad/pET-28a-dapdh-fdh whole-cell biocatalyst asymmetrically catalyzed the stereoinversion of 150 mM L-Phe to D-Phe, with quantitative yields of over 99% ee in 24 h, by the addition of 15 mM NADP+ and 300 mM ammonium formate. In addition, the whole-cell biocatalyst was used to successfully stereoinvert a variety of aromatic and aliphatic L-amino acids to their corresponding D-amino acids.

CONCLUSIONS

The newly constructed in vivo cascade biocatalysis system was effective for the highly selective synthesis of D-amino acids via stereoinversion.

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.

Optimization and Multiomic Basis of Phenyllactic Acid Overproduction by Lactobacillus plantarum

The goal of this study was to explore the regulatory mechanisms of phenyllactic acid (PLA) overaccumulation in Lactobacillus plantarum. The dynamics of PLA production revealed that 24 h was a suitable fermentation time, at which one of the largest differences in PLA content between strains S1 and YM-4-3 was 22.42 mg/L. Additionally, an optimization experiment showed that PLA production under the optimal condition (sample YM-4-3y) was up to 400.74 mg/L, 7.61-13.26 times as those of YM-4-3 and S1. Subsequently, an integrated analysis of genomic, transcriptomic and metabolomic data revealed that, YM-4-3 and YM-4-3y, compared with S1, although lacking a complete de novo biosynthetic pathway, increased PLA production by strengthening the core pathway and central carbon metabolism, and weakening the biosynthesis pathway of amino acids and their derivatives. These changes can provide sufficient precursors and compensate for or balance the energy consumed by the reinforced core pathway.