Improved cell growth and biosynthesis of glycolic acid by overexpression of membrane-bound pyridine nucleotide transhydrogenase

Cofactor engineering for improved production of 2,4-dihydroxybutyric acid via the synthetic homoserine pathway

(L)-2,4-dihydroxybutyrate (DHB) is a versatile compound that can serve as a precursor for the synthesis of the methionine analog 2-hydroxy-4-(methylthio)butyrate and new advanced polymers. We previously implemented in Escherichia coli an artificial biosynthetic pathway for the aerobic production of DHB from glucose, which relies on the deamination of (L)-homoserine followed by the reduction of 2-oxo-4-hydroxybutyrate (OHB) and yields DHB by an enzyme-bearing NADH-dependent OHB reductase activity. Under aerobic conditions, using NADPH as a cofactor is more favorable for reduction processes. We report the construction of an NADPH-dependent OHB reductase and increased intracellular NADPH supply by metabolic engineering to improve DHB production. Key cofactor discriminating positions were identified in the previously engineered NADH-dependent OHB reductase (E. coli malate dehydrogenase I12V:R81A:M85Q:D86S:G179D) and tested by mutational scanning. The two point mutations D34G:I35R were found to increase the specificity for NADPH by more than three orders of magnitude. Using the new OHB reductase enzyme, replacing the homoserine transaminase with the improved variant Ec.AlaC A142P:Y275D and increasing the NADPH supply by overexpressing the pntAB gene encoding the membrane-bound transhydrogenase yielded a strain that produced DHB from glucose at a yield of 0.25 molDHB molGlucose -1 in shake-flask experiments, which corresponds to a 50% increase compared to previous producer strains. Upon 24 h of batch cultivation of the most advanced DHB producer strain constructed in this work, a volumetric productivity of 0.83 mmolDHB L-1 h-1 was reached.

Biosensor-Assisted Engineering for Diverse Microbial Cellular Physiologies

Recent advancements in biosensor technology have revolutionized the field of microbial engineering, enabling efficient and precise optimization of strains for the production of valuable chemicals. This review comprehensively explores the innovative integration of biosensors to enhance microbial cell factories, with a particular emphasis on the crucial role of high-throughput biosensor-assisted screening. Biosensor-assisted approaches have enabled the identification of novel transporters, the elucidation of underlying transport mechanisms, and the fine-tuning of metabolic pathways for enhanced production. Furthermore, this review illustrates the utilization of biosensors for manipulating cellular behaviors, including interactions with environmental factors, and the reduction of nongenetic cell-to-cell variations. This review highlights the indispensable role of biosensors in advancing the field of microbial engineering through the modulation and exploitation of diverse cellular physiological processes.

Exploring yeast biodiversity and process conditions for optimizing ethylene glycol conversion into glycolic acid

Plastics have become an indispensable material in many fields of human activities, with production increasing every year; however, most of the plastic waste is still incinerated or landfilled, and only 10% of the new plastic is recycled even once. Among all plastics, polyethylene terephthalate (PET) is the most produced polyester worldwide; ethylene glycol (EG) is one of the two monomers released by the biorecycling of PET. While most research focuses on bacterial EG metabolism, this work reports the ability of Saccharomyces cerevisiae and nine other common laboratory yeast species not only to consume EG, but also to produce glycolic acid (GA) as the main by-product. A two-step bioconversion of EG to GA by S. cerevisiae was optimized by a design of experiment approach, obtaining 4.51 ± 0.12 g l-1 of GA with a conversion of 94.25 ± 1.74% from 6.21 ± 0.04 g l-1 EG. To improve the titer, screening of yeast biodiversity identified Scheffersomyces stipitis as the best GA producer, obtaining 23.79 ± 1.19 g l-1 of GA (yield 76.68%) in bioreactor fermentation, with a single-step bioprocess. Our findings contribute in laying the ground for EG upcycling strategies with yeasts.

Metabolic engineering for the utilization of carbohydrate portions of lignocellulosic biomass

The petrochemical industry has grown to meet the need for massive production of energy and commodities along with an explosive population growth; however, serious side effects such as greenhouse gas emissions and global warming have negatively impacted the environment. Lignocellulosic biomass with myriad quantities on Earth is an attractive resource for the production of carbon-neutral fuels and chemicals through environmentally friendly processes of microbial fermentation. This review discusses metabolic engineering efforts to achieve economically feasible industrial production of fuels and chemicals from microbial cell factories using the carbohydrate portion of lignocellulosic biomass as substrates. The combined knowledge of systems biology and metabolic engineering has been applied to construct robust platform microorganisms with maximum conversion of monomeric sugars, such as glucose and xylose, derived from lignocellulosic biomass. By comprehensively revisiting carbon conversion pathways, we provide a rationale for engineering strategies, as well as their features, feasibility, and recent representative studies. In addition, we briefly discuss how tools in systems biology can be applied in the field of metabolic engineering to accelerate the development of microbial cell factories that convert lignocellulosic biomass into carbon-neutral fuels and chemicals with economic feasibility.

Ferulic acid production by metabolically engineered Escherichia coli

Ferulic acid (p-hydroxy-3-methoxycinnamic acid, FA) is a natural active substance present in plant cell walls, with antioxidant, anticancer, antithrombotic and other properties; it is widely used in medicine, food, and cosmetics. Production of FA by eco-friendly bioprocess is of great potential. In this study, FA was biosynthesized by metabolically engineered Escherichia coli. As the first step, the genes tal (encoding tyrosine ammonia-lyase, RsTAL) from Rhodobacter sphaeroides, sam5 (encoding p-coumarate 3-hydroxylase, SeSAM5) from Saccharothrix espanaensis and comt (encoding Caffeic acid O-methytransferase, TaCM) from Triticum aestivum were cloned in an operon on the pET plasmid backbone, E. coli strain containing this construction was proved to produce FA from L-tyrosine successfully, and confirmed the function of TaCM as caffeic acid O-methytransferase. Fermentation result revealed JM109(DE3) as a more suitable host cell for FA production than BL21(DE3). After that the genes expression strength of FA pathway were optimized by tuning of promoter strength (T7 promoter or T5 promoter) and copy number (pBR322 or p15A), and the combination p15a-T5 works best. To further improve FA production, E. coli native pntAB, encoding pyridine nucleotide transhydrogenase, was selected from five NADPH regeneration genes to supplement redox cofactor NADPH for converting p-coumaric acid into caffeic acid in FA biosynthesis process. Sequentially, to further convert caffeic acid into FA, a non-native methionine kinase (MetK from Streptomyces spectabilis) was also overexpressed. Based on the flask fermentation data which show that the engineered E. coli strain produced 212 mg/L of FA with 11.8 mg/L caffeic acid residue, it could be concluded that it is the highest yield of FA achieved by E. coli K-12 strains reported to the best of our knowledge.

Understanding D-xylonic acid accumulation: a cornerstone for better metabolic engineering approaches

The xylose oxidative pathway (XOP) has been engineered in microorganisms for the production of a wide range of industrially relevant compounds. However, the performance of metabolically engineered XOP-utilizing microorganisms is typically hindered by D-xylonic acid accumulation. It acidifies the media and perturbs cell growth due to toxicity, thus curtailing enzymatic activity and target product formation. Fortunately, from the growing portfolio of genetic tools, several strategies that can be adapted for the generation of efficient microbial cell factories have been implemented to address D-xylonic acid accumulation. This review centers its discussion on the causes of D-xylonic acid accumulation and how to address it through different engineering and synthetic biology techniques with emphasis given on bacterial strains. In the first part of this review, the ability of certain microorganisms to produce and tolerate D-xylonic acid is also tackled as an important aspect in developing efficient microbial cell factories. Overall, this review could shed some insights and clarity to those working on XOP in bacteria and its engineering for the development of industrially applicable product-specialist strains. KEY POINTS: D-Xylonic acid accumulation is attributed to the overexpression of xylose dehydrogenase concomitant with basal or inefficient expression of enzymes involved in D-xylonic acid assimilation. Redox imbalance and insufficient cofactors contribute to D-xylonic acid accumulation. Overcoming D-xylonic acid accumulation can increase product formation among engineered strains. Engineering strategies involving enzyme engineering, evolutionary engineering, coutilization of different sugar substrates, and synergy of different pathways could potentially address D-xylonic acid accumulation.

Engineering of xylose metabolism in Escherichia coli for the production of valuable compounds

The lignocellulosic sugar d-xylose has recently gained prominence as an inexpensive alternative substrate for the production of value-added compounds using genetically modified organisms. Among the prokaryotes, Escherichia coli has become the de facto host for the development of engineered microbial cell factories. The favored status of E. coli resulted from a century of scientific explorations leading to a deep understanding of its systems. However, there are limited literature reviews that discuss engineered E. coli as a platform for the conversion of d-xylose to any target compounds. Additionally, available critical review articles tend to focus on products rather than the host itself. This review aims to provide relevant and current information about significant advances in the metabolic engineering of d-xylose metabolism in E. coli. This focusses on unconventional and synthetic d-xylose metabolic pathways as several review articles have already discussed the engineering of native d-xylose metabolism. This paper, in particular, is essential to those who are working on engineering of d-xylose metabolism using E. coli as the host.

Enhanced glycolic acid yield through xylose and cellobiose utilization by metabolically engineered Escherichia coli

Microbial biorefinery is a promising route toward sustainable production of glycolic acid (GA), a valuable raw material for various industries. However, inherent microbial GA production has limited substrate consumption using either D-xylose or D-glucose as carbon catabolite repression (CCR) averts their co-utilization. To bypass CCR, a GA-producing strain using D-xylose via Dahms pathway was engineered to allow cellobiose uptake. Unlike glucose, cellobiose was assimilated and intracellularly degraded without repressing D-xylose uptake. The final GA-producing E. coli strain (CLGA8) has an overexpressed cellobiose phosphorylase (cep94A) from Saccharophagus degradans 2-40 and an activated glyoxylate shunt pathway. Expression of cep94A improved GA production reaching the maximum theoretical yield (0.51 g GA g^-1 xylose), whereas activation of glyoxylate shunt pathway enabled GA production from cellobiose, which further increased the GA titer (2.25 g GA L^-1). To date, this is the highest reported GA yield from D-xylose through Dahms pathway in an engineered E. coli with cellobiose as co-substrate.

Ethylene glycol and glycolic acid production by wild-type Escherichia coli

Ethylene glycol and glycolic acid are bulk chemicals with a broad range of applications. The ethylene glycol and glycolic acid biosynthesis pathways have been produced by microorganisms and used as a biological route for their production. Unlike the methods that use xylose or glucose as carbon sources, xylonic acid was used as a carbon source to produce ethylene glycol and glycolic acid in this study. Amounts of 4.2 g/L of ethylene glycol and 0.7 g/L of glycolic acid were produced by a wild-type Escherichia coli W3110 within 10 H of cultivation with a substrate conversion ratio of 0.5 mol/mol. Furthermore, E. coli strains that produce solely ethylene glycol or glycolic acid were constructed. 10.3 g/L of glycolic acid was produced by E. coli ΔyqhD+aldA, and the achieved conversion ratio was 0.56 mol/mol. Similarly, the E. coli ΔaldA+yqhD produced 8.0 g/L of ethylene glycol with a conversion ratio of 0.71 mol/mol. Ethylene glycol and glycolic acid production by E. coli on xylonic acid as a carbon source provides new information on the biosynthesis pathway of these products and opens a novel way of biomass utilization.

Metabolic engineering of E. coli for improving mevalonate production to promote NADPH regeneration and enhance acetyl-CoA supply

Microbial production of mevalonate from renewable feedstock is a promising and sustainable approach for the production of value-added chemicals. We describe the metabolic engineering of Escherichia coli to enhance mevalonate production from glucose and cellobiose. First, the mevalonate-producing pathway was introduced into E. coli and the expression of the gene atoB, which encodes the gene for acetoacetyl-CoA synthetase, was increased. Then, the deletion of the pgi gene, which encodes phosphoglucose isomerase, increased the NADPH/NADP⁺ ratio in the cells but did not improve mevalonate production. Alternatively, to reduce flux toward the tricarboxylic acid cycle, gltA, which encodes citrate synthetase, was disrupted. The resultant strain, MGΔgltA-MV, increased levels of intracellular acetyl-CoA up to sevenfold higher than the wild-type strain. This strain produced 8.0 g/L of mevalonate from 20 g/L of glucose. We also engineered the sugar supply by displaying β-glucosidase (BGL) on the cell surface. When cellobiose was used as carbon source, the strain lacking gnd displaying BGL efficiently consumed cellobiose and produced mevalonate at 5.7 g/L. The yield of mevalonate was 0.25 g/g glucose (1 g of cellobiose corresponds to 1.1 g of glucose). These results demonstrate the feasibility of producing mevalonate from cellobiose or cellooligosaccharides using an engineered E. coli strain.

A pH-responsive genetic sensor for the dynamic regulation of D-xylonic acid accumulation in Escherichia coli

The xylose oxidative pathway (XOP) is continuously gaining prominence as an alternative for the traditional pentose assimilative pathways in prokaryotes. It begins with the oxidation of D-xylose to D-xylonic acid, which is further converted to α-ketoglutarate or pyruvate + glycolaldehyde through a series of enzyme reactions. The persistent drawback of XOP is the accumulation of D-xylonic acid intermediate that causes culture media acidification. This study addresses this issue through the development of a novel pH-responsive synthetic genetic controller that uses a modified transmembrane transcription factor called CadCΔ. This genetic circuit was tested for its ability to detect extracellular pH and to control the buildup of D-xylonic acid in the culture media. Results showed that the pH-responsive genetic sensor confers dynamic regulation of D-xylonic acid accumulation, which adjusts with the perturbation of culture media pH. This is the first report demonstrating the use of a pH-responsive transmembrane transcription factor as a transducer in a synthetic genetic circuit that was designed for XOP. This may serve as a benchmark for the development of other genetic controllers for similar pathways that involve acidic intermediates.