Co-expression of l-glutamate oxidase and catalase in Escherichia coli to produce α-ketoglutaric acid by whole-cell biocatalyst

Recent Progress on Peroxidase Modification and Application

Peroxdiase is one of the member of oxireductase super family, which has a broad substrate range and a variety of reaction types, including hydroxylation, epoxidation or halogenation of unactivated C-H bonds, and aromatic group or biophenol compounds. Here, we summarized the recently discovered enzymes with peroxidation activity, and focused on the special structures, sites, and corresponding strategies that can change the peroxidase catalytic activity, stability, and substrate range. The comparison of the structural differences between these natural enzymes and the mimic enzymes of binding nanomaterials and polymer materials is helpful to expand the application of peroxidase in industry. In addition, we also reviewed the catalytic application of peroxidase in the synthesis of important organic molecules and the degradation of pollutants.

A chemo-biocatalyst based on glutamate oxidase-integrated biomimetic trimanganese tetraoxide as cascade composite nano-catalyst for synthesis of α‑Ketoglutaric acid

The combination of chemo- and biocatalysts to perform one-pot synthetic route has presented great challenges for decades. Herein, glutamate oxidase (GLOX) and trimanganese tetraoxide (Mn3O4) nanocrystals were combined for the first time by one-step biomineralization to construct a mimic multi-enzyme system (GLOX@Mn3O4) for chemoenzymatic synthesis of α‑ketoglutaric acid (α‑KG). Mn3O4 not only served as a support for the enzyme immobilization, but also contributed its catalytic activity to co-operate with natural enzymes for the cascade reactions. The as-synthesized chemo-enzyme catalysts with directly contacted catalytic sites of the enzyme and inorganic catalyst maximizes the substrate channeling effffects for in situ rapid decomposition of the oxidative intermediate, H2O2, during the enzymatic oxidation of sodium glutamate, thus relieving the inhibition of H2O2 accumulation for GLOX. Benefiting from the excellent stability and reusability of GLOX@Mn3O4, a nearly 100% conversion (99.7%) of l-glutamate to α-KG was achieved, over 4.7 times higher than that of the free GLOX system (21.2%). This work provides a feasibility for constructing a high-performance chemo-enzyme catalyst for cascade catalysis, especially for those reactions with toxic intermediates.

Efficient production of α-ketoglutaric acid using an economical double-strain cultivation and catalysis system

The whole-cell catalysis strategy of alpha-ketoglutaric acid (α-KG) production from L-glutamic acid (L-Glu) using recombinant Escherichia coli, in which L-glutamate oxidase (LGox) was over-expressed, has replaced the traditional chemical synthesis strategy. However, large amounts of toxic by-product, H2O2, should be eliminated through co-expressing catalase (Cat), thus severely increasing burden in cells. To efficiently and economically produce α-KG, here, the genes SpLGox (from Streptomyces platensis NTU3304) and SlCat (from Streptomyces lividans TK24) were inserted into the low-dosage-IPTG (Isopropyl β-D-Thiogalactoside) inducible expression system, constructed in our previous work, in E. coli, respectively. Besides, a double-strain catalysis system was established and optimized to produce α-KG, and the productivity of α-KG was increased 97% compared with that through single strain catalysis. Finally, a double-strain cultivation strategy was designed and employed to simplify the scale-up fermentation. Using the optimized whole-cell biocatalyst conditions (pH 7.0, 35 °C), majority of the L-glutamic acid was transformed into α-KG and the titer reached 95.4 g/L after 6 h with the highest productivity at present. Therefore, this strategy may efficiently and cost-effectively produce α-KG, enhancing its potential for industrial applications. KEY POINTS: • SpLGox and SlCat were over-expressed to catalyze L-Glu to α-KG and eliminate by-product H2O2, respectively. • Double-strain cultivation and catalysis system can efficiently and cost-effectively produce α-KG from L-Glu.

Metabolic engineering and fermentation optimization strategies for producing organic acids of the tricarboxylic acid cycle by microbial cell factories

The organic acids of the tricarboxylic acid (TCA) pathway are important platform compounds and are widely used in many areas. The high-productivity strains and high-efficient and low-cost fermentation are required to satisfy a huge market size. The high metabolic flux of the TCA pathway endows microorganisms potential to produce high titers of these organic acids. Coupled with metabolic engineering and fermentation optimization, the titer of the organic acids has been significantly improved in recent years. Herein, we discuss and compare the recent advances in synthetic pathway engineering, cofactor engineering, transporter engineering, and fermentation optimization strategies to maximize the biosynthesis of organic acids. Such engineering strategies were mainly based on the TCA pathway and glyoxylate pathway. Furthermore, organic-acid-secretion enhancement and renewable-substrate-based fermentation are often performed to assist the biosynthesis of organic acids. Further strategies are also discussed to construct high-productivity and acid-resistant strains for industrial large-scale production.

Semi-rational engineering membrane binding domain of L-amino acid deaminase from Proteus vulgaris for enhanced α-ketoisocaproate

α-Keto acids are important raw materials for pharmaceuticals and functional foods, which could be produced from cheap feed stock by whole cell biocatalysts containing L-amino acid deaminases (L-AADs). However, the production capacity is limited by the low activity of L-AADs. The L-AAD mediated redox reaction employs the electron transport chain to transfer electrons from the reduced FADH₂ to O₂, implying that the interaction between L-AAD and the cell membrane affects its catalytic activity. To improve the catalytic activity of L-AAD from Proteus vulgaris, we redesigned the membrane-bound hydrophobic insertion sequences (INS, residues 325–375) by saturation mutagenesis and high-throughput screening. Mutants D340N and L363N exhibited higher affinity and catalytic efficiency for L-leucine, with half-life 1.62-fold and 1.28-fold longer than that of wild-type L-AAD. D340N catalyzed L-leucine to produce 81.21 g⋅L^–1 α-ketoisocaproate, with a bioconversion rate of 89.06%, which was 17.57% higher than that of the wild-type. It is predicted that the mutations enhanced the interaction between the protein and the cell membrane.

An Artificial Pathway for N-Hydroxy-Pipecolic Acid Production From L-Lysine in Escherichia coli

N-hydroxy-pipecolic acid (NHP) is a hydroxylated product of pipecolic acid and an important systemic acquired resistance signal molecule. However, the biosynthesis of NHP does not have a natural metabolic pathway in microorganisms. Here, we designed and constructed a promising artificial pathway in Escherichia coli for the first time to produce NHP from biomass-derived lysine. This biosynthesis route expands the lysine catabolism pathway and employs six enzymes to sequentially convert lysine into NHP. This artificial route involves six functional enzyme coexpression: lysine α-oxidase from Scomber japonicus (RaiP), glucose dehydrogenase from Bacillus subtilis (GDH), Δ¹-piperideine-2-carboxylase reductase from Pseudomonas putida (DpkA), lysine permease from E. coli (LysP), flavin-dependent monooxygenase (FMO1), and catalase from E. coli (KatE). Moreover, different FMO1s are used to evaluate the performance of the produce NHP. A titer of 111.06 mg/L of NHP was yielded in shake flasks with minimal medium containing 4 g/L of lysine. By this approach, NHP has so far been produced at final titers reaching 326.42 mg/L by 48 h in a 5-L bioreactor. To the best of our knowledge, this is the first NHP process using E. coli and the first process to directly synthesize NHP by microorganisms. This study lays the foundation for the development and utilization of renewable resources to produce NHP in microorganisms.

Occurrence of Z-2-oxo-4-methyl-3-pentene-1,5-dioic acid and its regioisomer 4-methylene-2-oxo-glutaric acid in tulip tissues

Although Z-2-oxo-4-methyl-3-pentene-1,5-dioic acid (Z-OMPD) has been identified as a major dicarboxylic acid in tulip tissues, its biosynthetic pathway has not been elucidated. Herein, Z-OMPD was isolated from tulip leaves and chemically synthesized. Comparisons of these samples revealed that Z-OMPD exists as a tautomeric mixture at physiological pH. As a regioisomer of Z-OMPD, we enzymatically and chemically prepared 4-methylene-2-oxo-glutaric acid (4-MEOG) for the first time. Using these compounds as standards, the occurrence of Z-OMPD and 4-MEOG in various tissues of the tulip cultivar "Murasakizuisho" was evaluated directly and by 2,4-dinitrophenylhydrazone derivatization. Z-OMPD was found to be abundant in the aerial tissues, whereas 4-MEOG was almost absent from all tissues. Stability analyses of Z-OMPD and 4-MEOG revealed that no double bond isomerization occurred at physiological pH, suggesting that enzyme systems are responsible for Z-OMPD biosynthesis in tulip tissues.

Coproduction of 5-Aminovalerate and δ-Valerolactam for the Synthesis of Nylon 5 From L-Lysine in Escherichia coli

The compounds 5-aminovalerate and δ-valerolactam are important building blocks that can be used to synthesize bioplastics. The production of 5-aminovalerate and δ-valerolactam in microorganisms provides an ideal source that reduces the cost. To achieve efficient biobased coproduction of 5-aminovalerate and δ-valerolactam in Escherichia coli, a single biotransformation step from L-lysine was constructed. First, an equilibrium mixture was formed by L-lysine α-oxidase RaiP from Scomber japonicus. In addition, by adjusting the pH and H₂O₂ concentration, the titers of 5-aminovalerate and δ-valerolactam reached 10.24 and 1.82 g/L from 40 g/L L-lysine HCl at pH 5.0 and 10 mM H₂O₂, respectively. With the optimized pH value, the δ-valerolactam titer was improved to 6.88 g/L at pH 9.0 with a molar yield of 0.35 mol/mol lysine. The ratio of 5AVA and δ-valerolactam was obviously affected by pH value. The ratio of 5AVA and δ-valerolactam could be obtained in the range of 5.63:1-0.58:1 at pH 5.0-9.0 from the equilibrium mixture. As a result, the simultaneous synthesis of 5-aminovalerate and δ-valerolactam from L-lysine in Escherichia coli is highly promising. To our knowledge, this result constitutes the highest δ-valerolactam titer reported by biological methods. In summary, a commercially implied bioprocess developed for the coproduction of 5-aminovalerate and δ-valerolactam using engineered Escherichia coli.

A High-Efficiency Artificial Synthetic Pathway for 5-Aminovalerate Production From Biobased L-Lysine in Escherichia coli

Bioproduction of 5-aminovalerate (5AVA) from renewable feedstock can support a sustainable biorefinery process to produce bioplastics, such as nylon 5 and nylon 56. In order to achieve the biobased production of 5AVA, a 2-keto-6-aminocaproate-mediated synthetic pathway was established. Combination of L-Lysine α-oxidase from Scomber japonicus, α-ketoacid decarboxylase from Lactococcus lactis and aldehyde dehydrogenase from Escherichia coli could achieve the biosynthesis of 5AVA from biobased L-Lysine in E. coli. The H₂O₂ produced by L-Lysine α-oxidase was decomposed by the expression of catalase KatE. Finally, 52.24 g/L of 5AVA were obtained through fed-batch biotransformation. Moreover, homology modeling, molecular docking and molecular dynamic simulation analyses were used to identify mutation sites and propose a possible trait-improvement strategy: the expanded catalytic channel of mutant and more hydrogen bonds formed might be beneficial for the substrates stretch. In summary, we have developed a promising artificial pathway for efficient 5AVA synthesis.

Microbial engineering for the production of C2-C6 organic acids

Covering: up to the end of 2020Organic acids, as building block compounds, have been widely used in food, pharmaceutical, plastic, and chemical industries. Until now, chemical synthesis is still the primary method for industrial-scale organic acid production. However, this process encounters some inevitable challenges, such as depletable petroleum resources, harsh reaction conditions and complex downstream processes. To solve these problems, microbial cell factories provide a promising approach for achieving the sustainable production of organic acids. However, some key metabolites in central carbon metabolism are strictly regulated by the network of cellular metabolism, resulting in the low productivity of organic acids. Thus, multiple metabolic engineering strategies have been developed to reprogram microbial cell factories to produce organic acids, including monocarboxylic acids, hydroxy carboxylic acids, amino carboxylic acids, dicarboxylic acids and monomeric units for polymers. These strategies mainly center on improving the catalytic efficiency of the enzymes to increase the conversion rate, balancing the multi-gene biosynthetic pathways to reduce the byproduct formation, strengthening the metabolic flux to promote the product biosynthesis, optimizing the metabolic network to adapt the environmental conditions and enhancing substrate utilization to broaden the substrate spectrum. Here, we describe the recent advances in producing C₂-C₆ organic acids by metabolic engineering strategies. In addition, we provide new insights as to when, what and how these strategies should be taken. Future challenges are also discussed in further advancing microbial engineering and establishing efficient biorefineries.

Pathway engineering of Escherichia coli for α-ketoglutaric acid production

α-Ketoglutaric acid (α-KG) is a multifunctional dicarboxylic acid in the tricarboxylic acid (TCA) cycle, but microbial engineering for α-KG production is not economically efficient, due to the intrinsic inefficiency of its biosynthetic pathway. In this study, pathway engineering was used to improve pathway efficiency for α-KG production in Escherichia coli. First, the TCA cycle was rewired for α-KG production starting from pyruvate, and the engineered strain E. coli W3110Δ4-PCAI produced 15.66 g/L α-KG. Then, the rewired TCA cycle was optimized by designing various strengths of pyruvate carboxylase and isocitrate dehydrogenase expression cassettes, resulting in a large increase in α-KG production (24.66 g/L). Furthermore, acetyl coenzyme A (acetyl-CoA) availability was improved by overexpressing acetyl-CoA synthetase, leading to α-KG production up to 28.54 g/L. Finally, the engineered strain E. coli W3110Δ4-P_(H) CAI_(H) A was able to produce 32.20 g/L α-KG in a 5-L fed-batch bioreactor. This strategy described here paves the way to the development of an efficient pathway for microbial production of α-KG.

Cytochrome P450-mediated N-demethylation of noscapine by whole-cell biotransformation: process limitations and strategies for optimisation

Cytochrome P450 enzymes catalyse reactions of significant industrial interest but are underutilised in large-scale bioprocesses due to enzyme stability, cofactor requirements and the poor aqueous solubility and microbial toxicity of typical substrates and products. In this work, we investigate the potential for preparative-scale N-demethylation of the opium poppy alkaloid noscapine by a P450_BM3 (CYP102A1) mutant enzyme in a whole-cell biotransformation system. We identify and address several common limitations of whole-cell P450 biotransformations using this model N-demethylation process. Mass transfer into Escherichia coli cells was found to be a major limitation of biotransformation rate and an alternative Gram-positive expression host Bacillus megaterium provided a 25-fold improvement in specific initial rate. Two methods were investigated to address poor substrate solubility. First, a biphasic biotransformation system was developed by systematic selection of potentially biocompatible solvents and in silico solubility modelling using Hansen solubility parameters. The best-performing biphasic system gave a 2.3-fold improvement in final product titre compared to a single-phase system but had slower initial rates of biotransformation due to low substrate concentration in the aqueous phase. The second strategy aimed to improve aqueous substrate solubility using cyclodextrin and hydrophilic polymers. This approach provided a fivefold improvement in initial biotransformation rate and allowed a sixfold increase in final product concentration. Enzyme stability and cell viability were identified as the next parameters requiring optimisation to improve productivity. The approaches used are also applicable to the development of other pharmaceutical P450-mediated biotransformations.

Engineering of L-glutamate oxidase as the whole-cell biocatalyst for the improvement of α-ketoglutarate production

L-glutamate oxidase (LGOX) catalyzes the oxidative deamination of l-glutamate to α-ketoglutarate (α-KG) with the formation of ammonia and hydrogen peroxide. Consequently, identifying a novel LGOX with high enzymatic activity is a prime target for industrial biotechnology. In this study, error-prone PCR mutagenesis of Streptomyces mobaraensis LGOX followed by high-throughput screening was performed to yield four single point mutants with improved enzymatic activity, termed F94L, S280T, I282M and H533R. Moreover, site-saturation mutagenesis at these four residues was employed, yielding two additionally improved mutants, termed I282L and H533L. Subsequently, we employed combinatorial mutagenesis of two, three and four point mutants, and the best mutant S280TH533L showed 90 % higher enzymatic activity than the wild-type control. The data also showed that the presence of these point mutations greatly enhanced enzymatic activity, but did not alter its optimum temperature and pH. Furthermore, the S280TH533L mutant had the maximal velocity (V_max) of 231.3 μmol/mg/min and the Michaelis-Menten constant (K_M) of 2.7 mM, which were the highest V_max and lowest K_M values of LGOX reported so far. Finally, we developed a whole-cell biocatalyst for α-KG production by co-expression of both S280TH533L mutant and KatE catalase. Randomized ribosome binding site (RBS) sequences were introduced to generate vectors with varying expression levels of S280TH533L and KatE, and two optimized co-expression strains were obtained after screening. The α-KG production reached a maximum titer of 181.9 g/L after 12 h conversation using the optimized whole-cell biocatalyst, with a molar conversion rate of substrate higher than 86.3 % in the absence of exogenous catalase, while the molar conversion rate of substrate using the wild-type biocatalyst was less than 30 %. Taken together, these data suggest that the engineering of LGOX has great potentials to enhance the industrial production of α-KG.

Study on Transglucosylation Properties of Amylosucrase from Xanthomonas campestris pv. Campestris and Its Application in the Production of α-Arbutin

α-Arbutin (4-hydroquinone-α-D-glucopyranoside), an effective skin-lightening agent due to its considerable inhibitory effect on human tyrosinase activity, is widely used in the pharmaceutical and cosmetic industries. Recently, α-arbutin was prepared through transglucosylation of hydroquinone using microbial glycosyltransferases as catalysts. However, the low yield and prolonged reaction time of the biotransformation process of α-arbutin production limited its industrial application. In this work, an amylosucrase (ASase) from Xanthomonas campestris pv. campestris str. ATCC 33913 (XcAS) was expressed efficiently in Escherichia coli JM109. The catalytic property of the purified XcAS for the synthesis of α-arbutin was tested. The recombinant strain was applied for highly efficient synthesis of α-arbutin using sucrose and hydroquinone as glucosyl donor and acceptor, respectively. By optimizing the biotransformation conditions and applying a fed-batch strategy, the final production yield and conversion rate of α-arbutin reached 60.9 g/L and 95.5%, respectively, which is the highest reported yield by engineered strains. Compared to the highest reported value (<1.4 g/L/h), our productivity (7.6 g/L/h) was improved more than five-fold. This work represents an efficient and rapid method for α-arbutin production with potential industrial applications.

Whole-cell biocatalysts by design

Whole-cell biocatalysts provide unique advantages and have been widely used for the efficient biosynthesis of value-added fine and bulk chemicals, as well as pharmaceutically active ingredients. What is more, advances in synthetic biology and metabolic engineering, together with the rapid development of molecular genetic tools, have brought about a renaissance of whole-cell biocatalysis. These rapid advancements mean that whole-cell biocatalysts can increasingly be rationally designed. Genes of heterologous enzymes or synthetic pathways are increasingly being introduced into microbial hosts, and depending on the complexity of the synthetic pathway or the target products, they can enable the production of value-added chemicals from cheap feedstock. Metabolic engineering and synthetic biology efforts aimed at optimizing the existing microbial cell factories concentrate on improving heterologous pathway flux, precursor supply, and cofactor balance, as well as other aspects of cellular metabolism, to enhance the efficiency of biocatalysts. In the present review, we take a critical look at recent developments in whole-cell biocatalysis, with an emphasis on strategies applied to designing and optimizing the organisms that are increasingly modified for efficient production of chemicals.