Efficient Bioconversion of Sucrose to High-Value-Added Glucaric Acid by In Vitro Metabolic Engineering

Production of D-glucaric acid with phosphoglucose isomerase-deficient Saccharomyces cerevisiae

D-Glucaric acid is a potential biobased platform chemical. Previously mainly Escherichia coli, but also the yeast Saccharomyces cerevisiae, and Pichia pastoris, have been engineered for conversion of D-glucose to D-glucaric acid via myo-inositol. One reason for low yields from the yeast strains is the strong flux towards glycolysis. Thus, to decrease the flux of D-glucose to biomass, and to increase D-glucaric acid yield, the four step D-glucaric acid pathway was introduced into a phosphoglucose isomerase deficient (Pgi1p-deficient) Saccharomyces cerevisiae strain. High D-glucose concentrations are toxic to the Pgi1p-deficient strains, so various feeding strategies and use of polymeric substrates were studied. Uniformly labelled 13C-glucose confirmed conversion of D-glucose to D-glucaric acid. In batch bioreactor cultures with pulsed D-fructose and ethanol provision 1.3 g D-glucaric acid L-1 was produced. The D-glucaric acid titer (0.71 g D-glucaric acid L-1) was lower in nitrogen limited conditions, but the yield, 0.23 g D-glucaric acid [g D-glucose consumed]-1, was among the highest that has so far been reported from yeast. Accumulation of myo-inositol indicated that myo-inositol oxygenase activity was limiting, and that there would be potential to even higher yield. The Pgi1p-deficiency in S. cerevisiae provides an approach that in combination with other reported modifications and bioprocess strategies would promote the development of high yield D-glucaric acid yeast strains.

Self-assembled multienzyme complex facilitates synthesis of glucosylglycerol from maltodextrin and glycerol

BACKGROUND

Compound 2-O-α-d-glucosylglycerol (2αGG) naturally serves as a compatible osmolyte in acclimation to environmental stresses, such as high osmolarity, dryness, and extreme temperature. It presents several bioactivities and has been used in the food, agriculture, and cosmetics areas.

RESULTS

In the present study, we attempted to synthesize the 2αGG from low-cost maltodextrin and glycerol by constructing an in vitro multi-enzyme system. The system contained two core enzymes, namely glucan phosphorylases (GPs) and glucosylglycerol phosphorylases (GGPs), and two auxiliary enzymes, namely isoamylase and 4-α-glucanotransferase. Several new GGPs from different organisms were characterized with the function of converting α-G1P and glycerol to sole stereo-configuration product 2αGG. Then, polypeptide SpyTag-SpyCatcher was employed to construct a self-assembled multienzyme complex, and different combinations between enzymes and peptides were constructed and tested. The best self-assembled multienzyme complex exhibited three-fold higher productivity compared to that of free enzyme. This reaction system also produced 240 mm (61 g L-1 ) 2αGG under high substrate concentration, with a conversion yield of 86%.

CONCLUSION

The present study provides an efficient approach for producing 2αGG. It also demonstrates that the SpyTag-SpyCatcher system could be applied to construct other multienzyme complexes for increased productivity and product titer. © 2023 Society of Chemical Industry.

Efficient Production of Glucaric Acid by Engineered Saccharomyces cerevisiae

Glucaric acid is a valuable chemical with applications in the detergent, polymer, pharmaceutical and food industries. In this study, two key enzymes for glucaric acid biosynthesis, MIOX4 (myo-inositol oxygenase) and Udh (uronate dehydrogenase), were fused and expressed with different peptide linkers. It was found that a strain harboring the fusion protein MIOX4-Udh linked by the peptide (EA3K)3 produced the highest glucaric acid titer and thereby resulted in glucaric acid production that was 5.7-fold higher than that of the free enzymes. Next, the fusion protein MIOX4-Udh linked by (EA3K)3 was integrated into delta sequence sites of the Saccharomyces cerevisiae opi1 mutant, and a strain, GA16, that produced a glucaric acid titer of 4.9 g/L in a shake flask fermentation was identified by a high-throughput screening method using an Escherichia coli glucaric acid biosensor. Strain improvement by further engineering was performed to regulate the metabolic flux of myo-inositol to increase the supply of glucaric acid precursors. The downregulation of ZWF1 and the overexpression of INM1 and ITR1 increased glucaric acid production significantly, and glucaric acid production was increased to 8.49 g/L in the final strain GA-ZII in a shake flask fermentation. Finally, in a 5-L bioreactor, GA-ZII produced a glucaric acid titer of 15.6 g/L through fed-batch fermentation. IMPORTANCE Glucaric acid is a value-added dicarboxylic acid that was synthesized mainly through the oxidation of glucose chemically. Due to the problems of the low selectivity, by-products, and highly polluting waste of this process, producing glucaric acid biologically has attracted great attention. The activity of key enzymes and the intracellular myo-inositol level were both rate-limiting factors for glucaric acid biosynthesis. To increase glucaric acid production, this work improved the activity of the key enzymes in the glucaric acid biosynthetic pathway through the expression of a fusion of Arabidopsis thaliana MIOX4 and Pseudomonas syringae Udh as well as a delta sequence-based integration. Furthermore, intracellular myo-inositol flux was optimized by a series of metabolic strategies to increase the myo-inositol supply, which improved glucaric acid production to a higher level. This study provided a way for constructing a glucaric acid-producing strain with good synthetic performance, making glucaric acid production biologically in yeast cells much more competitive.

Biosynthesis of monoterpenoid and sesquiterpenoid as natural flavors and fragrances

Terpenoids are a large class of plant-derived compounds, that constitute the main components of essential oils and are widely used as natural flavors and fragrances. The biosynthesis approach presents a promising alternative route in terpenoid production compared to plant extraction or chemical synthesis. In the past decade, the production of terpenoids using biotechnology has attracted broad attention from both academia and the industry. With the growing market of flavor and fragrance, the production of terpenoids directed by synthetic biology shows great potential in promoting future market prospects. Here, we reviewed the latest advances in terpenoid biosynthesis. The engineering strategies for biosynthetic terpenoids were systematically summarized from the enzyme, metabolic, and cellular dimensions. Additionally, we analyzed the key challenges from laboratory production to scalable production, such as key enzyme improvement, terpenoid toxicity, and volatility loss. To provide comprehensive technical guidance, we collected milestone examples of biosynthetic mono- and sesquiterpenoids, compared the current application status of chemical synthesis and biosynthesis in terpenoid production, and discussed the cost drivers based on the data of techno-economic assessment. It is expected to provide critical insights into developing translational research of terpenoid biomanufacturing.

Biomanufacturing by In Vitro Biotransformation (ivBT) Using Purified Cascade Multi-enzymes

In vitro biotransformation (ivBT) refers to the use of an artificial biological reaction system that employs purified enzymes for the one-pot conversion of low-cost materials into biocommodities such as ethanol, organic acids, and amino acids. Unshackled from cell growth and metabolism, ivBT exhibits distinct advantages compared with metabolic engineering, including but not limited to high engineering flexibility, ease of operation, fast reaction rate, high product yields, and good scalability. These characteristics position ivBT as a promising next-generation biomanufacturing platform. Nevertheless, challenges persist in the enhancement of bulk enzyme preparation methods, the acquisition of enzymes with superior catalytic properties, and the development of sophisticated approaches for pathway design and system optimization. In alignment with the workflow of ivBT development, this chapter presents a systematic introduction to pathway design, enzyme mining and engineering, system construction, and system optimization. The chapter also proffers perspectives on ivBT development.

Alternative design strategies to help build the enzymatic retrosynthesis toolbox

Most of the complex molecules found in nature still cannot be synthesized by current organic chemistry methods. Given the number of enzymes that exist in nature and the incredible potential of directed evolution, the field of synthetic biology contains perhaps all the necessary building blocks to bring about the realization of applied enzymatic retrosynthesis. Current thinking anticipates that enzymatic retrosynthesis will be implemented using conventional cell-based synthetic biology approaches where requisite native, heterologous, designer, and evolved enzymes making up a given multi-enzyme pathway are hosted by chassis organisms to carry out designer synthesis. In this perspective, we suggest that such an effort should not be limited by solely exploiting living cells and enzyme evolution and describe some useful yet less intensive complementary approaches that may prove especially productive in this grand scheme. By decoupling reactions from the environment of a living cell, a significantly larger portion of potential synthetic chemical space becomes available for exploration; most of this area is currently unavailable to cell-based approaches due to toxicity issues. In contrast, in a cell-free reaction a variety of classical enzymatic approaches can be exploited to improve performance and explore and understand a given enzyme's substrate specificity and catalytic profile towards non-natural substrates. We expect these studies will reveal unique enzymatic capabilities that are not accessible in living cells.

Comprehensive utilization of sucrose resources via chemical and biotechnological processes: A review

Sucrose, one of the most widespread disaccharides in nature, has been available in daily human life for many centuries. As an abundant and cheap sweetener, sucrose plays an essential role in our diet and the food industry. However, it has been determined that many diseases, such as obesity, diabetes, hyperlipidemia, etc., directly relate to the overconsumption of sucrose. It arouses many explorations for the conversion of sucrose to high-value chemicals. Production of valuable substances from sucrose by chemical methods has been studied since a half-century ago. Compared to chemical processes, biotechnological conversion approaches of sucrose are more environmentally friendly. Many enzymes can use sucrose as the substrate to generate functional sugars, especially those from GH68, GH70, GH13, and GH32 families. In this review, enzymatic catalysis and whole-cell fermentation of sucrose for the production of valuable chemicals were reviewed. The multienzyme cascade catalysis and metabolic engineering strategies were addressed.

Cell-free Biosynthesis of Chlorogenic Acid Using a Mixture of Chassis Cell Extracts and Purified Spy-Cyclized Enzymes

A novel cell-free biosynthesis system based on a mixture of chassis cell extracts and purified Spy-cyclized enzymes (CFBS-mixture) was developed. As a demonstration, the CFBS-mixture was applied to chlorogenic acid (CGA) biosynthesis. The mix-and-match and Plackett-Burman experiments demonstrated that Lonicera japonica hydroxycinnamate-CoA quinate transferase and p-hydroxyphenylacetate 3-hydroxylase were the key enzymes for the production of CGA. After optimization of the concentrations of the biosynthetic enzymes in the CFBS-mixture reaction using the Plackett-Burman experimental design and the path of the steepest ascent, 711.26 ± 15.63 mg/L CGA was produced after 16 h, which is 71.1-fold the yield obtained using the conventional crude extract-based CFBS and 9.1-fold the reported yield obtained using the living cells. Based on the CFBS-mixture results, the production of CGA was further enhanced in engineered Escherichia coli. The CFBS-mixture strategy is highly effective and will be useful for high-level CFBS of natural products.

Effect of magnesium ions on glucaric acid production in the engineered Saccharomyces cerevisiae

Glucaric acid has been successfully produced in Escherichia coli and fungus. Here, we first analyzed the effects of different metal ions on glucaric acid production in the engineered Saccharomyces cerevisiae Bga-3 strain harboring the glucaric acid synthesis pathway. We found that magnesium ions could promote the growth rate of yeast cells, and thus, increase the glucaric acid production by elevating the glucose and myo-inositol utilization of Bga-3 strain. RNA-Seq transcriptome analysis results showed that the upregulation of genes involved in the gluconeogenesis pathway, as well as the downregulation of genes associated with the glycolysis pathway and pentose phosphate pathway in response to MgCl₂ were all benefit for the enhancement of the glucose-6-phosphate flux, which was the precursor for myo-inositol and glucaric acid. In addition, we found that MgCl₂ could also increase the activity of MIOX4, which was also crucial for glucaric acid synthesis. At last, a final glucaric acid titer of 10.6 g/L, the highest reported titer, was achieved in the fed-batch fermentation using a 5-L bioreactor by adding 100 mM MgCl₂. Our findings will provide a new way of promoting the production of other chemicals in the engineered yeast cells.

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.

Cell-based and cell-free biocatalysis for the production of D-glucaric acid

D-Glucaric acid (GA) is a value-added chemical produced from biomass, and has potential applications as a versatile platform chemical, food additive, metal sequestering agent, and therapeutic agent. Marketed GA is currently produced chemically, but increasing demand is driving the search for eco-friendlier and more efficient production approaches. Cell-based production of GA represents an alternative strategy for GA production. A series of synthetic pathways for GA have been ported into Escherichia coli, Saccharomyces cerevisiae and Pichia pastoris, respectively, and these engineered cells show the ability to synthesize GA de novo. Optimization of the GA metabolic pathways in host cells has leapt forward, and the titer and yield have increased rapidly. Meanwhile, cell-free multi-enzyme catalysis, in which the desired pathway is constructed in vitro from enzymes and cofactors involved in GA biosynthesis, has also realized efficient GA bioconversion. This review presents an overview of studies of the development of cell-based GA production, followed by a brief discussion of potential applications of biosensors that respond to GA in these biosynthesis routes.

Combinatorial synthetic pathway fine-tuning and cofactor regeneration for metabolic engineering of Escherichia coli significantly improve production of D-glucaric acid

D-glucaric acid (GA) has been identified as among promising biotechnological alternatives to oil-based chemicals. GA and its derivatives are widely used in food additives, dietary supplements, drugs, detergents, corrosion inhibitors and biodegradable materials. The increasing availability of a GA market is improving the cost-effectiveness and efficiency of various biosynthetic pathways. In this study, an engineered Escherichia coli strain GA10 was constructed by systematic metabolic engineering. This involved redirecting metabolic flux into the GA biosynthetic pathways, blocking the conversion pathways of d-glucuronic acid (GlcA) and GA into by-products, introducing an in situ NAD⁺ regeneration system and fine-tuning the activity of the key enzyme, myo-inositol oxygenase (Miox). Subsequently, the culture medium was optimized to achieve the best performance of the GA10 strain. GA was produced at 5.35 g/L (extracellular and intracellular), with a maximized yield of ∼0.46 mol/mol on d-glucose and glycerol, by batch fermentation. This work demonstrates efficient biosynthetic pathways of GA in E. coli by metabolic engineering and should accelerate the application of GA biosynthetic pathways in industrial processes.

Efficient Biosynthesis of 2'-Fucosyllactose Using an In Vitro Multienzyme Cascade

2'-Fucosyllactose (2-FL) is a fucose-containing oligosaccharide that is found in humans and is believed to have potential nutraceutical and pharmaceutical uses. Here, a promising in vitro multienzyme cascade catalysis system (MECCS) was designed to convert L-fucose and lactose to 2-FL. The cascade comprises L-fucokinase/GDP-L-fucose phosphorylase (FKP), α-1,2-fucosyltransferase (FucT), and pyruvate kinase (PK). This MECCS was able to efficiently regenerate ATP or GTP with 5.67-fold improvement of GDP-L-fucose. To address the rate-limiting step in the MECCS, various FucT orthologues were screened, and HpFucT from Helicobacter pylori showed the highest catalytic efficiency, with a (kcat/KM) of 39.28 min-1 mM-1, while TeFucT from Thermosynechococcus elongatus showed the highest thermostability, with a melting temperature (Tm) of 48 °C. The dissociation constant (KD) of TeFucT (1.34 ± 0.41 μM) was 15-fold lower than that of HpFucT (20.24 ± 1.81 μM), suggesting that TeFucT had much higher affinity for GDP. Structural analysis of HpFucT indicated that Arg169 is part of a unique substrate-binding site that interacts with two oxygen atoms from the phosphate group of GDP-L-fucose. The 2-FL productivities of the MECCS in fed-batch reached 0.67 and 0.73 g/L/h with TeFucT and HpFucT, respectively. This research provides an alternative pathway for efficient production of 2-FL.

Sequence-based bioprospecting of myo-inositol oxygenase (Miox) reveals new homologues that increase glucaric acid production in Saccharomyces cerevisiae

myo-Inositol oxygenase (Miox) is a rate-limiting enzyme for glucaric acid production via microbial fermentation. The enzyme converts myo-inositol to glucuronate, which is further converted to glucaric acid, a natural compound with industrial uses that range from detergents to pharmaceutical synthesis to polymeric materials. More than 2,000 Miox sequences are available in the Uniprot database but only thirteen are classified as reviewed in Swiss-Prot (August 2019). In this study, sequence similarity networks were used to identify new homologues to be expressed in Saccharomyces cerevisiae for glucaric acid production. The expression of four homologues did not lead to product formation. Some of these enzymes may have a defective "dynamic lid" - a structural feature important to close the reaction site - which might explain the lack of activity. Thirty-one selected Miox sequences did allow for product formation, of which twenty-five were characterized for the first time. Expression of Talaromyces marneffei Miox led to the accumulation of 1.76 ± 0.33 g glucaric acid/L from 20 g glucose/L and 10 g/L myo-inositol. Specific glucaric acid titer with TmMiox increased 44 % compared to the often-used Arabidopsis thaliana variant AtMiox4 (0.258 vs. 0.179 g glucaric acid/g biomass). AtMiox4 activity decreased from 12.47 to 0.40 nmol/min/mg protein when cells exited exponential phase during growth on glucose, highlighting the importance of future research on Miox stability in order to further improve microbial production of glucaric acid.

Sucrose Phosphorylase and Related Enzymes in Glycoside Hydrolase Family 13: Discovery, Application and Engineering

Sucrose phosphorylases are carbohydrate-active enzymes with outstanding potential for the biocatalytic conversion of common table sugar into products with attractive properties. They belong to the glycoside hydrolase family GH13, where they are found in subfamily 18. In bacteria, these enzymes catalyse the phosphorolysis of sucrose to yield α-glucose 1-phosphate and fructose. However, sucrose phosphorylases can also be applied as versatile transglucosylases for the synthesis of valuable glycosides and sugars because their broad promiscuity allows them to transfer the glucosyl group of sucrose to a diverse collection of compounds other than phosphate. Numerous process and enzyme engineering studies have expanded the range of possible applications of sucrose phosphorylases ever further. Moreover, it has recently been discovered that family GH13 also contains a few novel phosphorylases that are specialised in the phosphorolysis of sucrose 6F-phosphate, glucosylglycerol or glucosylglycerate. In this review, we provide an overview of the progress that has been made in our understanding and exploitation of sucrose phosphorylases and related enzymes over the past ten years.

Enhanced Production of Pinene by Using a Cell-Free System with Modular Cocatalysis

α-Pinene is an important monoterpene that is widely used as a pharmaceutical product, biofuel, and so forth. We first established a cell-free system with modular cocatalysis for the production of pinene from glucose. After optimization of the compositions of the cell-free reaction mixture using the Plackett-Burman experimental design and the path of steepest ascent, the production of pinene increased by 57%. It was found that ammonium acetate, NAD⁺, and NADPH are the three most important parameters for the production of pinene. Mix-and-match experiments showed that the simultaneous addition of the lysate of Escherichia coli overexpressing native 4-hydroxy-3-methylbut-2-enyl diphosphate reductase, SufBCD Fe-S cluster assembly protein, isopentenyl-diphosphate isomerase, and Pinus taeda pinene synthase improved the production of pinene. Increasing the enzyme concentration of the extract further enhanced the production of pinene to 1256.31 ± 46.12 mg/L with a productivity of 104.7 mg/L h, almost 1.2-fold faster than any system reported thus far. This study demonstrates that a cell-free system is a powerful and robust platform for biomanufacture.

Enzymatic production of 4-O-methyl d-glucaric acid from hardwood xylan

BACKGROUND

Dicarboxylic acids offer several applications in detergent builder and biopolymer fields. One of these acids, 4-O-methyl d-glucaric acid, could potentially be produced from glucuronoxylans, which are a comparatively underused fraction of wood and agricultural biorefineries.

RESULTS

Accordingly, an enzymatic pathway was developed that combines AxyAgu115A, a GH115 α-glucuronidase from Amphibacillus xylanus, and GOOX, an AA7 gluco-oligosaccharide oxidase from Sarocladium strictum, to produce this bio-based chemical from glucuronoxylan. AxyAgu115A was able to release almost all 4-O-methyl d-glucuronic acid from glucuronoxylan while a GOOX variant, GOOX-Y300A, could convert 4-O-methyl d-glucuronic acid to the corresponding glucaric acid at a yield of 62%. Both enzymes worked effectively at alkaline conditions that increase xylan solubility. Given the sensitivity of AxyAgu115A to hydrogen peroxide and optimal performance of GOOX-Y300A at substrate concentrations above 20 mM, the two-step enzyme pathway was demonstrated as a sequential, one-pot reaction. Additionally, the resulting xylan was easily recovered from the one-pot reaction, and it was enzymatically hydrolysable.

CONCLUSIONS

The pathway in this study requires only two enzymes while avoiding a supplementation of costly cofactors. This cell-free approach provides a new strategy to make use of the underutilized hemicellulose stream from wood and agricultural biorefineries.