Efficient Biocatalytic Preparation of Rebaudioside KA: Highly Selective Glycosylation Coupled with UDPG Regeneration

Construction of lignan glycosides biosynthetic network in Escherichia coli using mutltienzyme modules

BACKGROUND

Due to the complexity of the metabolic pathway network of active ingredients, precise targeted synthesis of any active ingredient on a synthetic network is a huge challenge. Based on a complete analysis of the active ingredient pathway in a species, this goal can be achieved by elucidating the functional differences of each enzyme in the pathway and achieving this goal through different combinations. Lignans are a class of phytoestrogens that are present abundantly in plants and play a role in various physiological activities of plants due to their structural diversity. In addition, lignans offer various medicinal benefits to humans. Despite their value, the low concentration of lignans in plants limits their extraction and utilization. Recently, synthetic biology approaches have been explored for lignan production, but achieving the synthesis of most lignans, especially the more valuable lignan glycosides, across the entire synthetic network remains incomplete.

RESULTS

By evaluating various gene construction methods and sequences, we determined that the pCDF-Duet-Prx02-PsVAO gene construction was the most effective for the production of (+)-pinoresinol, yielding up to 698.9 mg/L after shake-flask fermentation. Based on the stable production of (+)-pinoresinol, we synthesized downstream metabolites in vivo. By comparing different fermentation methods, including "one-cell, one-pot" and "multicellular one-pot", we determined that the "multicellular one-pot" method was more effective for producing (+)-lariciresinol, (-)-secoisolariciresinol, (-)-matairesinol, and their glycoside products. The "multicellular one-pot" fermentation yielded 434.08 mg/L of (+)-lariciresinol, 96.81 mg/L of (-)-secoisolariciresinol, and 45.14 mg/L of (-)-matairesinol. Subsequently, ultilizing the strict substrate recognition pecificities of UDP-glycosyltransferase (UGT) incorporating the native uridine diphosphate glucose (UDPG) Module for in vivo synthesis of glycoside products resulted in the following yields: (+)-pinoresinol glucoside: 1.71 mg/L, (+)-lariciresinol-4-O-D-glucopyranoside: 1.3 mg/L, (+)-lariciresinol-4'-O-D-glucopyranoside: 836 µg/L, (-)-secoisolariciresinol monoglucoside: 103.77 µg/L, (-)-matairesinol-4-O-D-glucopyranoside: 86.79 µg/L, and (-)-matairesinol-4'-O-D-glucopyranoside: 74.5 µg/L.

CONCLUSIONS

By using various construction and fermentation methods, we successfully synthesized 10 products of the lignan pathway in Isatis indigotica Fort in Escherichia coli, with eugenol as substrate. Additionally, we obtained a diverse range of lignan products by combining different modules, setting a foundation for future high-yield lignan production.

Investigation on production and reaction conditions of sucrose synthase based glucosylation cascade towards flavonoid modification

Enzyme-based glycosylation is of great interest in the context of natural products decoration. Yet, its industrial application is hindered by optimisation difficulties and hard-to-standardise productivities. In this study, five sugar nucleotide-dependent glucosyltransferases from different origins (bacterial, plant and fungal) were coupled with soy sucrose synthase (GmSuSy) to create a set of diverse cascade biocatalysts for flavonoid glucosylation, which evaluation brought new insights into the field. Investigations into co-expression conditions and reaction settings enabled to define optimal induction temperature (25 °C) and uridine diphosphate (UDP) concentration (0.5 mM) for all tested pairs of enzymes. Moreover, the influence of pH and substrate concentration on the monoglucosylated product distribution was detected and analysed. The utilisation of crude protein extracts as a cost-effective source of catalysts unveiled their glycosidase activity against flavonoid glucosides, resulting in decreased productivity, which, to our knowledge, has not previously been discussed in such a context. Additionally, examination of the commercially available EziG immobilisation resins showed that selection of suitable carrier for solid catalyst production can be problematic and not only enzyme's but also reagent's properties have to be considered. Flavonoids, due to their complexation and hydrophobic properties, can adsorb on different types of surfaces, including divalent metal ions required for IMAC based immobilisation, necessitating detailed examination of the resins while the catalysis design.

Design of a chimeric glycosyltransferase OleD for the site-specific O-monoglycosylation of 3-hydroxypyridine in nosiheptide

To identify the potential role of the 3-hydroxyl group of the pyridine ring in nosiheptide (NOS) for its antibacterial activity against Gram-positive pathogens, enzymatic glycosylation was utilized to regio-selectively create a monoglycosyl NOS derivative, NOS-G. For this purpose, we selected OleD, a UDP glycosyltransferase from Streptomyces antibioticus that has a low productivity for NOS-G. Activity of the enzyme was increased by swapping domains derived from OleI, both single and in combination. Activity enhancement was best in mutant OleD-10 that contained four OleI domains. This chimer was engineered by site-directed mutagenesis (single and in combination) to increase its activity further, whereby variants were screened using a newly-established colorimetric assay. OleD-10 with I117F and T118G substitutions (FG) had an increased NOS-G productivity of 56%, approximately 70 times higher than that of wild-type OleD. The reason for improved activity of FG towards NOS was structurally attributed to a closer distance (<3 Å) between NOS/sugar donor and the catalytic amino acid H25. The engineered enzyme allowed sufficient activity to demonstrate that the produced NOS-G had enhanced stability and aqueous solubility compared to NOS. Using a murine MRSA infection model, it was established that NOS-G resulted in partial protection within 20 h of administration and delayed the death of infected mice. We conclude that 3-hydroxypyridine is a promising site for structural modification of NOS, which may pave the way for producing nosiheptide derivatives as a potential antibiotic for application in clinical treatment.

Glycosylation of polyphenolic compounds: Design of a self-sufficient biocatalyst by co-immobilization of a glycosyltransferase, a sucrose synthase and the cofactor UDP

Glycosyltransferases catalyze the regioselective glycosylation of polyphenolic compounds, increasing their solubility without altering their antioxidant properties. Leloir-type glycosyltransferases require UDP-glucose as a cofactor to glycosylate a hydroxyl of the polyphenol, which is expensive and unstable. To simplify these processes for industrial implementation, the preparation of self-sufficient heterogeneous biocatalysts is needed. In this study, a glycosyltransferase and a sucrose synthase (as an UDP-regenerating enzyme) were co-immobilized onto porous agarose-based supports coated with polycationic polymers: polyethylenimine and polyallylamine. In addition, the UDP cofactor was strongly ionically adsorbed and co-immobilized with the enzymes, eliminating the need to add it separately. Thus, the optimal self-sufficient heterogeneous biocatalyst was able to catalyze the glycosylation of three polyphenolic compounds (piceid, phloretin and quercetin) with in situ regeneration of the UDP-glucose, allowing multiple consecutive reaction cycles without the addition of exogenous cofactor. A TTN value of 50 (theoretical maximum) was obtained in the reaction of piceid glycosylation, after 5 reaction cycles, using the self-sufficient biocatalyst based on an improved sucrose synthase variant. This result was 5-fold higher than the obtained using soluble cofactor and the co-immobilized enzymes, and much higher than those reported in the literature for similar processes.

Transglycosylation of Stevioside by a Commercial β-Glucanase with Fungal Extracted β-Glucans as Donors

Abstract

Alternative sweeteners, such as steviol glucosides from the plant Stevia rebaudiana Bertoni, are becoming increasingly popular for the design of next-generation foodstuffs. However, the bitter aftertaste of native steviol glucosides is one of the main reasons behind consumer reluctance towards stevia-containing products. Biocatalysis could be a sustainable solution to this problem, through addition of glucosyl moieties to the molecule. Glycoside hydrolases are enzymes performing transglycosylation reactions, and they can be exploited for such modifications. In the present work, the commercial β-glucanase Finizym 250L® was employed for the transglycosylation of stevioside. After optimization of several reaction parameters, the maximal reaction yield obtained was 19%, with barley β-glucan as the glycosyl donor. With the aim to develop a sustainable process, β-glucan extracts from different fungal sources were prepared. Pulsed Electric Field pretreatment of mycelial biomass resulted in extracts with higher β-glucan content. The extracts were tested as alternative glucosyl donors, reaching up to 15.5% conversion yield, from Pleurotus-extracted β-glucan. Overall, in the present work a novel enzymatic process for the modification of stevioside is proposed, with concomitant valorization of β-glucans extracted from fungal biomass, potentially generated as a byproduct from other applications, in concert with the principles of circular economy.

Graphical Abstract

Highly efficient synthesis of mono-β-1,6-Glucosylated Rebaudioside A derivative catalyzed by glycosyltransferase YjiC

Steviol glycosides have attracted great interest because of their high levels of sweetness and safety, and absence of calories. Improvement of their sensory qualities via glycosylation modification by glycosyltransferase is a research hotspot. In this study, YjiC, a uridine diphosphate-dependent glycosyltransferase from Bacillus subtilis 168, was found with the ability to glycosylate rebaudioside A (Reb A) to produce a novel mono β-1, 6-glycosylated Reb A derivative rebaudioside L2 (Reb L2). It has an improved sweetness compared with Reb A. Next, a cascade reaction was established by combining YjiC with sucrose synthase AtSuSy from Arabidopsis thaliana for scale-up preparation of Reb L2. It shows that Reb L2 (30.94 mg/mL) could be efficiently synthesized with an excellent yield of 91.34% within 12 h. Therefore, this study provides a potential approach for the production and application of new steviol glycoside Reb L2, expanding the scope of steviol glycosides.

Evaluation of double expression system for co-expression and co-immobilization of flavonoid glucosylation cascade

Abstract

Glucosylation cascade consisting of Leloir glycosyltransferase and sucrose synthase with in situ regeneration system of expensive and low available nucleotide sugars is a game-changing strategy for enzyme-based production of glycoconjugates of relevant natural products. We designed a stepwise approach including co-expression and one-step purification and co-immobilization on glass-based EziG resins of sucrose synthase from Glycine max (GmSuSy) with promiscuous glucosyltransferase YjiC from Bacillus licheniformis to produce efficient, robust, and versatile biocatalyst suited for preparative scale flavonoid glucosylation. The undertaken investigations identified optimal reaction conditions (30 °C, pH 7.5, and 10 mM Mg²⁺) and the best-suited carrier (EziG Opal). The prepared catalyst exhibited excellent reusability, retaining up to 96% of initial activity after 12 cycles of reactions. The semi-preparative glucosylation of poorly soluble isoflavone Biochanin A resulted in the production of 73 mg Sissotrin (Biochanin A 7-O-glucoside). Additionally, the evaluation of the designed double-controlled, monocistronic expression system with two independently induced promoters (rhaBAD and trc) brought beneficial information for dual-expression plasmid design.

Key points

• Simultaneous and titratable expression from two independent promoters is possible, although full control over the expression is limited.

• Designed catalyst managed to glucosylate poorly soluble isoflavone.

• The STY of Sissotrin using the designed catalyst reached 0.26 g/L∙h∙g of the resin.

Graphical Abstract

Supplementary information

The online version contains supplementary material available at 10.1007/s00253-022-12259-5.

De novo biosynthesis of rubusoside and rebaudiosides in engineered yeasts

High-sugar diet causes health problems, many of which can be addressed with the use of sugar substitutes. Rubusoside and rebaudiosides are interesting molecules, considered the next generation of sugar substitutes due to their low-calorie, superior sweetness and organoleptic properties. However, their low abundance in nature makes the traditional plant extraction process neither economical nor environmental-friendly. Here we engineer baker's yeast Saccharomyces cerevisiae as a chassis for the de novo production of rubusoside and rebaudiosides. In this process, we identify multiple issues that limit the production, including rate-liming steps, product stress on cellular fitness and unbalanced metabolic networks. We carry out a systematic engineering strategy to solve these issues, which produces rubusoside and rebaudiosides at titers of 1368.6 mg/L and 132.7 mg/L, respectively. The rubusoside chassis strain here constructed paves the way towards a sustainable, large-scale fermentation-based manufacturing of diverse rebaudiosides.

Engineering of a UDP-Glycosyltransferase for the Efficient Whole-Cell Biosynthesis of Siamenoside I in Escherichia coli

The combination of the insufficient availability and the complex structure of siamenoside I (SI), the sweetest glucoside isolated from Siraitia grosvenorii to date, limited its use as a natural sweetener. To solve this problem, an improved biocatalyst, UGT-M2, was semi-rationally created by engineering the uridine diphosphate glycosyltransferase UGT94-289-2 from S. grosvenorii for the monoglucosylation of mogroside IIIE (MG IIIE) to SI. Subsequently, an engineered Escherichia coli cell was constructed, which combined UGT-M2 with a UDP-glucose regeneration system to circumvent the need for expensive UDP-glucose to produce SI. After optimization, high-purity SI (>96.4%) was efficiently prepared from MG IIIE at a 1 L scale with a productivity of 29.78 g/(L day) and a molar yield of 76.5% and without using exogenous UDP-glucose. This study not only developed a whole-cell approach for the preparation of SI but also provided an alternative glycosyltransferase variant for SI biosynthesis with synthetic biology in the future.

Whole-Cell Biocatalyst for Rubusoside Production in Saccharomyces cerevisiae

Rubusoside (Rub) is a highly sweet diterpene glycoside mainly isolated from the leaves of Rubus suavissimus (Rosaceae). It has been used as a low-calorie natural sweetener for decades and was recently found to be a potential drug lead. In this study, we designed a whole-cell biocatalyst to achieve the glycosylation of steviol to Rub in Saccharomyces cerevisiae. The sucrose synthases were applied to construct a uridine diphosphate glucose regeneration system, which were coupled with optimal combinations of different uridine diphosphate (UDP) glycosyltransferases from multiple plant species. After optimization of reaction conditions, the residues in SrUGT74G1 probably influencing glycosylation efficiency were subjected to site-directed mutagenesis. Double mutations of S84A/E87A reduced the accumulation of intermediates, finally glucosylating 1.27 g/L steviol to 0.45 ± 0.06 g/L steviolmonoside (conversion rate = 23.3%) and 1.92 ± 0.17 g/L Rub (conversion rate = 74.9%). A high efficiency of Rub biosynthesis could be achieved without supply of additional UDPG. This work provided the first example of multi-step glycosylation reactions in whole-cell biocatalysis, which laid a foundation of scalable production of the value-added diterpene sweetener in the future.

Effective Glycosylation of Cucurbitacin Mediated by UDP-Glycosyltransferase UGT74AC1 and Molecular Dynamics Exploration of Its Substrate Binding Conformations

Cucurbitacins, a group of diverse tetracyclic triterpenes, display a variety of biological effects. Glycosylation mediated by glycosyltransferases (UGTs) plays a vital role in structural and functional diversity of natural products and influences their biological activities. In this study, GT-SM, a mutant of UGT74AC1 from Siraitia grosvenorii, was chosen as a potential catalyst in glycosylation of cucurbitacins, and its optimal pH, temperature, and divalent metal ions were detected. This enzyme showed high activity (kcat/Km, 120 s−1 µM−1) toward cucurbitacin F 25-O-acetate (CA-F25) and only produced CA-F25 2-O-β-d-glucose which was isolated and confirmed by 1D and 2D nuclear magnetic resonance. A pathway for uridine diphosphate glucose (UDP-Glc) regeneration and cucurbitacin glycoside synthesis was constructed by combing GT-SM and sucrose synthase to cut down the costly UDP-Glc. The molar conversion of CA-F25 was 80.4% in cascade reaction. Molecular docking and dynamics simulations showed that CA-F25 was stabilized by hydrophobic interactions, and the C2-OH of CA-F25 showed more favorable catalytic conformation than that of C3-OH, explaining the high regioselectivity toward the C2-OH rather than the ortho-C3-OH of CA-F25. This work proved the important potential application of UGT74AC1 in cucurbitacins and provided an understanding of glycosylation of cucurbitacins.

Enzymatic Monoglucosylation of Rubusoside and the Structure-Sweetness/Taste Relationship of Monoglucosyl Derivatives

Monoglucosylation of rubusoside not only could increase its structural diversity but may also improve its taste. To biosynthesize the monoglucosyl rubusosides, a series of glycosyltransferases and glycosynthases were screened to identify the enzymes capable of specifically glycosylating the hydroxyl groups of the 13-O-β-d-glucosyl and 19-COO-β-d-glucosyl moieties. After structural characterization, the effect of structure on sweetness and taste was established based on these rubusoside-derived analogues, including two first characterized compounds. β-Monoglucosylation of two 2-hydroxyl groups, as well as α-monoglucosylations of the 4- and 6-hydroxyl groups of the 13-glucosyl moiety, could significantly increase the relative sweetness of rubusoside to 140 while maintaining or improving the taste quality. In contrast, monoglucosylations of other hydroxyl groups in our study usually decreased the taste quality of the rubusoside. Additionally, the possibility of a negative influence of these monoglucosylated derivatives on the function of islets was preliminarily excluded, which should facilitate the development of rubusoside-derived sweeteners.