Synthesis of Three Bioactive Aromatic Compounds by Introducing Polyketide Synthase Genes into Engineered Escherichia coli

Production of (hydroxy)benzoate-derived polyketides by engineered Pseudomonas with in situ extraction

Polyketides from (hydroxy)benzoates are an interesting group of plant polyphenolic compounds, whose biotechnological production is so far underrepresented due to their challenging heterologous biosynthesis. Efficient heterologous production of 2,4,6-tri- and 2,3',4,6-tetrahydroxybenzophenone, 3,5-dihydroxybiphenyl, and 4-hydroxycoumarin by whole-cell biocatalysis in combination with in situ product extraction with an organic solvent was demonstrated. Production was highly dependent on the used CoA ligase and polyketide synthase type III. Therefore, different combinations of polyketide synthases and benzoate-CoA ligases were evaluated for their biosynthesis performance in the solvent-tolerant Pseudomonas taiwanensis VLB120. A solvent screening yielded 2-undecanone as biocompatible, extraction-efficient solvent with good phase separation. In aqueous-organic two-phase cultivations, this solvent extraction circumvents product instability in the aqueous cultivation medium, and it increases yields by reducing inhibitory effects. Complete de novo synthesis from glucose of all (hydroxy)benzoate-derived polyketides was achieved in two-phase cultivations with metabolically engineered strains. Additionally, mutasynthesis was applied to obtain fluorinated benzophenone derivatives.

Citric acid treatment directly on anaerobic digestor sludge alleviates the inhibitory effect of in-situ generated humic acids by their deconstruction and redistribution

Humic acids were known to inhibit sewage sludge anaerobic digestion (AD) and many studies have been investigated to deal with the inhibitory effect of humic acids (HA) in raw sludge feeding the digestor. However, HA could also be in-situ produced during the sludge AD process with a significant amount, but the relieving of the inhibitory effect posed by the in-situ produced HA has been historically ignored. Thus, this study attempted to use citric acid (CA), an easily biodegradable polyatomic acid, to directly treat sludge in the anaerobic digestor instead of the feeding sludge, for alleviating inhibition caused by the in-situ produced HA. Results showed that methane production of AD batches with CA pretreatment at 0.06∼0.10 g/g TS on digestor sludge were effectively enhanced, with the highest production being 2.89 times of that observed in control AD without CA treatment and 2.08 times of that achieved by contrast AD with CA pretreatment on raw sludge. However, the AD batches with CA treatment at low dosages of 0.02∼0.04 g/g TS had lower methane production, during which sludge solubilization and hydrolysis were enhanced initially associated with released HA surfactant and hydrolytic enzymes; then later inhibited since higher structurally sound HA in SP easily rebind proteinaceous substrates and enzymes again. Whilst CA treatment at 0.06∼0.10 g/g TS could promote sludge solubilization and hydrolysis throughout the sludge AD process. This dosage-dependent effect of CA treatment can be caused by its discriminating impact on HA structure and distribution. At low dosages, complexes of HA-proteins (including hydrolases) can be decrosslinked and transferred to the soluble phase (SP), thus enhancing solubilization and hydrolysis initially. However, the released HA in SP could rebind more organics and hydrolases during AD, resulting in augmented inhibition. While the dosage increased, HA were drastically deconstructed with aromatic cores in SP migrating to extracellular polymeric substances, preventing binding effects on organics and hydrolases. Accordingly, a smart mutex-switch operational strategy of CA treatment was proposed. The results suggested, to alleviate the inhibitory effect of HA on sludge AD, more attention should be given to HA produced and accumulated in AD rather than raw sludge so that the CA treatment directly on sludge in the digestor can be a promising method.

Combinatorial biosynthesis: playing chess with the metabolism

Secondary metabolites are a group of natural products that produced by bacteria, fungi and plants. Many applications of these compounds from medicine to industry have been discovered. However, some changes in their structure and biosynthesis mechanism are necessary for their properties to be more suitable and also for their production to be profitable. The main and most useful method to achieve this goal is combinatorial biosynthesis. This technique uses the multi-unit essence of the secondary metabolites biosynthetic enzymes to make changes in their order, structure and also the organism that produces them.

Engineered biosynthesis of plant polyketides by type III polyketide synthases in microorganisms

Plant specialized metabolites occupy unique therapeutic niches in human medicine. A large family of plant specialized metabolites, namely plant polyketides, exhibit diverse and remarkable pharmaceutical properties and thereby great biomanufacturing potential. A growing body of studies has focused on plant polyketide synthesis using plant type III polyketide synthases (PKSs), such as flavonoids, stilbenes, benzalacetones, curcuminoids, chromones, acridones, xanthones, and pyrones. Microbial expression of plant type III PKSs and related biosynthetic pathways in workhorse microorganisms, such as Saccharomyces cerevisiae, Escherichia coli, and Yarrowia lipolytica, have led to the complete biosynthesis of multiple plant polyketides, such as flavonoids and stilbenes, from simple carbohydrates using different metabolic engineering approaches. Additionally, advanced biosynthesis techniques led to the biosynthesis of novel and complex plant polyketides synthesized by diversified type III PKSs. This review will summarize efforts in the past 10 years in type III PKS-catalyzed natural product biosynthesis in microorganisms, especially the complete biosynthesis strategies and achievements.

Thermal Hydrolysis Pretreatment-Anaerobic Digestion Promotes Plant-Growth Biostimulants Production from Sewage Sludge by Upregulating Aromatic Amino Acids Transformation and Quinones Supply

Micromolecular plant-growth biostimulants (micro-PBs) production from sewage sludge is attracting increasing interest, as it is expected to enhance the fertilizing effect of sludge for land application. This study attempted to promote effective micro-PBs production from sewage sludge through thermal hydrolysis pretreatment-anaerobic digestion (THP-AD) and explore the underpinning regulation mechanisms. Results showed that the highest effective micro-PB production in digested sludge was achieved in THP(160 °C)-AD by day 12, with 80.73 mg/kg volatile solid (VS) of phytohormones and 417.75 mg/kg VS of allelochemicals, and these effective micro-PBs all originated from aromatic amino acids (AAAs). The metabolomic and metagenomic results revealed that, as compared with THP(120 °C)-AD and AD without THP, THP(160°C)-AD uniquely upregulated AAAs biosynthesis and consequently improved AAAs metabolism toward effective micro-PBs production. Further exploration of related microbial pathways and metabolites suggested that the upregulated AAAs biosynthesis in THP(160 °C)-AD in the early stage was partially attributed to the enhanced carbohydrate release. More importantly, the results showed that the amount of quinones, which probably facilitate energy generation via acting as electron-transfer mediators, was significantly positively correlated with the abundance of AAAs biosynthesis genes (R² = 0.93). Hence, the improved initial release and biosynthesis of quinones are critical in enhancing the AAAs biosynthesis in THP(160 °C)-AD. Moreover, the enhanced quinones supply and the consequent active AAAs transformation in THP(160 °C)-AD reinforced the humification process, highly supporting effective micro-PBs stabilization. The important roles of quinones in effective micro-PBs production and stabilization in sludge anaerobic digestion should be considered in technology development for micro-PBs recovery.

Synthesis of chlorogenic acid and p-coumaroyl shikimate by expressing shikimate gene modules in Escherichia coli

AIM

Chlorogenic acid and p-coumaroyl shikimate are hydroxycinnamic acid derivatives. These compounds are nutraceutical supplements due to their biological activities including prevention of cardiovascular disease and cancers. These two compounds were synthesized in Escherichia coli through two-culture system using two mutants, which are biochemically interdependent. The aim of this work was to improve the titres of their production in a single E. coli mutant in which all necessary genes were introduced. This was done by testing various shikimate gene combinations to determine the optimal gene combination for the synthesis of chlorogenic acid and p-coumaroyl shikimate.

METHODS AND RESULTS

A series of gene modules harbouring shikimate pathway genes were constructs. Six gene module constructs for chlorogenic acid synthesis and eight constructs for p-coumaric acid synthesis were tested in order to find the best one. Chlorogenic acid synthesis showed highest with the gene module construct containing ydiB, aroB, aroG^f , ppsA and tktA. Using the E. coli strain, 109.7 mg L^-1 chlorogenic acid was synthesized. The best gene module construct for the p-coumaroyl shikimate synthesis contained aroD and aroG^f . In addition, we used two E. coli deletion mutant strains (ΔaroK and ΔaroL) to increase the final titre. The E. coli ΔaroK mutant harbouring this gene module construct synthesized 713.4 mg L^-1 of p-coumaroyl shikimate.

CONCLUSION

The chlorogenic acid synthesis using the current system was approximately 35.4% higher of the titre than titres obtained with an alternative method that depends on co-cultivation of two mutants. At the same time, production of p-coumaroyl shikimate increased 5.8 times.

SIGNIFICANCE AND IMPACT OF THE STUDY

The current study's findings indicate that our selection of the shikimate gene module contributed to increases in the levels of the substrates and could be applied to synthesize other compounds whose synthesis requires intermediates of the shikimate pathway.

Synthesis of 4-Hydroxybenzoic Acid Derivatives in Escherichia coli

Hydroxybenzoic acids (HBAs) such as 4-hydroxybenzoic acid (4-HBA) and 3,4-dihydroxybenzoic acid (DHB; protocatechuic acid) and its ester with methanol (methylparaben [MP]) are known to have various functional biological properties, including antibacterial, anticancer, antidiabetic, antiaging, antiviral, and anti-inflammatory activities. Since these compounds are widely used in cosmetic, food, and pharmaceutical industries, the use of renewable feedstocks for the production of HBAs is an area of growing interest. In this study, we used Escherichia coli to synthesize these three hydroxybenzoic acid derivatives (4-HBA, DHB, and MP). We overexpressed ubiC in E. coli to synthesize 4-HBA from chorismate, a substrate that is produced by the shikimate pathway in E. coli. For the synthesis of DHB, an additional gene (pobA) was introduced, while hbad and EHT1 were co-expressed to synthesize MP. To supply more chorismate, we introduced the shikimate gene module construct and selected the best construct for increased yields. Using this approach, 723.5 mg/L 4-HBA, 942.0 mg/L DHB, and 347.7 mg/L MP were synthesized. Our study showed that the shikimate gene module constructs can be applicable to increase the yields of HBA derivatives in HBA-tolerant microorganisms.

Fermentative N-Methylanthranilate Production by Engineered Corynebacterium glutamicum

The N-functionalized amino acid N-methylanthranilate is an important precursor for bioactive compounds such as anticancer acridone alkaloids, the antinociceptive alkaloid O-isopropyl N-methylanthranilate, the flavor compound O-methyl-N-methylanthranilate, and as a building block for peptide-based drugs. Current chemical and biocatalytic synthetic routes to N-alkylated amino acids are often unprofitable and restricted to low yields or high costs through cofactor regeneration systems. Amino acid fermentation processes using the Gram-positive bacterium Corynebacterium glutamicum are operated industrially at the million tons per annum scale. Fermentative processes using C. glutamicum for N-alkylated amino acids based on an imine reductase have been developed, while N-alkylation of the aromatic amino acid anthranilate with S-adenosyl methionine as methyl-donor has not been described for this bacterium. After metabolic engineering for enhanced supply of anthranilate by channeling carbon flux into the shikimate pathway, preventing by-product formation and enhancing sugar uptake, heterologous expression of the gene anmt encoding anthranilate N-methyltransferase from Ruta graveolens resulted in production of N-methylanthranilate (NMA), which accumulated in the culture medium. Increased SAM regeneration by coexpression of the homologous adenosylhomocysteinase gene sahH improved N-methylanthranilate production. In a test bioreactor culture, the metabolically engineered C. glutamicum C1* strain produced NMA to a final titer of 0.5 g·L⁻¹ with a volumetric productivity of 0.01 g·L⁻¹·h⁻¹ and a yield of 4.8 mg·g⁻¹ glucose.

Microbial Engineering for Production of N-Functionalized Amino Acids and Amines

N-functionalized amines play important roles in nature and occur, for example, in the antibiotic vancomycin, the immunosuppressant cyclosporine, the cytostatic actinomycin, the siderophore aerobactin, the cyanogenic glucoside linamarin, and the polyamine spermidine. In the pharmaceutical and fine-chemical industries N-functionalized amines are used as building blocks for the preparation of bioactive molecules. Processes based on fermentation and on enzyme catalysis have been developed to provide sustainable manufacturing routes to N-alkylated, N-hydroxylated, N-acylated, or other N-functionalized amines including polyamines. Metabolic engineering for provision of precursor metabolites is combined with heterologous N-functionalizing enzymes such as imine or ketimine reductases, opine or amino acid dehydrogenases, N-hydroxylases, N-acyltransferase, or polyamine synthetases. Recent progress and applications of fermentative processes using metabolically engineered bacteria and yeasts along with the employed enzymes are reviewed and the perspectives on developing new fermentative processes based on insight from enzyme catalysis are discussed.

Synthesis of acridone derivatives via heterologous expression of a plant type III polyketide synthase in Escherichia coli

BACKGROUND

Acridone alkaloids are heterocyclic compounds that exhibit a broad-range of pharmaceutical and chemotherapeutic activities, including anticancer, antiviral, anti-inflammatory, antimalarial, and antimicrobial effects. Certain plant species such as Citrus microcarpa, Ruta graveolens, and Toddaliopsis bremekampii synthesize acridone alkaloids from anthranilate and malonyl-CoA.

RESULTS

We synthesized two acridones in Escherichia coli. Acridone synthase (ACS) and anthraniloyl-CoA ligase genes were transformed into E. coli, and the synthesis of acridone was examined. To increase the levels of endogenous anthranilate, we tested several constructs expressing proteins involved in the shikimate pathway and selected the best construct. To boost the supply of malonyl-CoA, genes coding for acetyl-coenzyme A carboxylase (ACC) from Photorhabdus luminescens were overexpressed in E. coli. For the synthesis of 1,3-dihydroxy-10-methylacridone, we utilized an N-methyltransferase gene (NMT) to supply N-methylanthranilate and a new N-methylanthraniloyl-CoA ligase. After selecting the best combination of genes, approximately 17.3 mg/L of 1,3-dihydroxy-9(10H)-acridone (DHA) and 26.0 mg/L of 1,3-dihydroxy-10-methylacridone (NMA) were synthesized.

CONCLUSIONS

Two bioactive acridone derivatives were synthesized by expressing type III plant polyketide synthases and other genes in E. coli, which increased the supplement of substrates. This study showed that is possible to synthesize diverse polyketides in E. coli using plant polyketide synthases.