Synthesis of rebaudioside-A by enzymatic transglycosylation of stevioside present in the leaves of Stevia rebaudiana Bertoni

Highly Efficient Biosynthesis of Rebaudioside M8 through Structure-Guided Engineering of Glycosyltransferase UGT94E13

Given the low-calorie, high-sweetness characteristics of steviol glycosides (SGs), developing SGs with improved taste profiles is a key focus. Rebaudioside M8 (Reb M8), a novel non-natural SG derivative obtained through glycosylation at the C-13 position of rebaudioside D (Reb D) using glycosyltransferase UGT94E13, holds promise for further development due to its enhanced sweetness. However, the low catalytic activity of UGT94E13 hampers further research and commercialization. This study aimed to improve the enzymatic activity of UGT94E13 through semirational design, and a variant UGT94E13-F169G/I185G was obtained with the catalytic activity improved by 13.90 times. A cascade reaction involving UGT94E13-F169G/I185G and sucrose synthase AtSuSy was established to recycle uridine diphosphate glucose, resulting in an efficient preparation of Reb M8 with a yield of 98%. Moreover, according to the analysis of the distances between the substrate Reb D and enzymes as well as between Reb D and the glucose donor through molecular dynamics simulations, it is found that the positive effect of shortening the distance on glycosylation reaction activity accounts for the improved catalytic activity of UGT94E13-F169G/I185G. Therefore, this study addresses the bottleneck in the efficient production of Reb M8 and provides a foundation for its widespread application in the food industry.

Efficient Bioconversion of Stevioside and Rebaudioside A to Glucosylated Steviol Glycosides Using an Alkalihalobacillus oshimesis-Derived Cyclodextrin Glucanotransferase

The enzymatic transglycosylation of steviol glycosides can improve the edulcorant quality of steviol glycosides. Cyclodextrin glucanotransferase (CGTase) is one of the most popular glucanotransferases applied in this reaction. Herein, the CGTase-producing strain Alkalihalobacillus oshimensis CGMCC 23164 was isolated from Stevia planting soil. Using mass spectrometry-based secretome profiling, a high-efficiency CGTase that converted steviol glycosides to glucosylated steviol glycosides was identified and termed CGTase-13. CGTase-13 demonstrated optimal transglycosylation activity with 10 g/L steviol glycoside and 50 g/L soluble starch as substrates at <40 °C. Under the above conditions, the conversion rate of stevioside and rebaudioside A, two main components of steviol glycosides, reached 86.1% and 90.8%, respectively. To the best of our knowledge, this is the highest conversion rate reported to date. Compared with Toruzyme^® 3.0 L, the commonly used commercial enzyme blends, glucosylated steviol glycosides produced using CGTase-13 exhibited weaker astringency and unpleasant taste, faster sweetness onset, and stronger sweetness intensity. Thus, CGTase provides a novel option for producing high-quality glucosylated steviol glycoside products and has great potential for industrial applications.

A review of transglycosylated compounds as food additives to enhance the solubility and oral absorption of hydrophobic compounds in nutraceuticals and pharmaceuticals

Transglycosylation has been used to modify the physicochemical properties of original compounds. As a result, transglycosylated compounds can form molecular aggregates in size ranges of a few nanometers in an aqueous medium when their concentrations exceed a specific level. Incorporating these hydrophobic compounds has been observed to enhance the solubility of hydrophobic compounds into aggregate structures. Thus, this review introduces four transglycosylated compounds as food additives that can enhance the solubility and oral absorption of hydrophobic compounds. Here, transglycosylated hesperidin, transglycosylated rutin, transglycosylated naringin, and transglycosylated stevia are the focus as representative substances. Significantly, we observed that amorphous formations containing hydrophobic compounds with transglycosylated compounds improved solubility and oral absorption compared to untreated hydrophobic compounds. Moreover, combining transglycosylated compounds with hydrophilic polymers or surfactants enhanced the solubilizing effects on hydrophobic compounds. Furthermore, the enhanced solubility of hydrophobic compounds improved their oral absorption. Transglycosylated compounds also influenced nanoparticle preparation of hydrophobic compounds as a dispersant. This study demonstrated the benefits of transglycosylated compounds in developing supplements and nutraceuticals of hydrophobic compounds with poor aqueous solubility.

Physicochemical, Microbiological and Microstructural Characteristics of Sucrose-Free Probiotic-Frozen Yogurt during Storage

Frozen yogurt is known as ice cream with some properties of yogurt. Frozen yogurts are a rich source of sucrose levels between 15% and 28% of total ingredients. Consumers suffering from lactose intolerance and metabolic syndrome are looking for sugar-free products. The current study investigates the sugar replacements by using sweeteners (stevia, sucralose and sorbitol) on physicochemical, microbiological, microstructural and sensory characteristics of probiotic-frozen yogurt. Four different treatments of probiotic-frozen yogurts were studied (control probiotic-frozen yogurt with sucrose (F1), probiotic-frozen yogurt with stevia (F2), probiotic-frozen yogurt with sucralose (F3) and probiotic-frozen yogurt with sorbitol (F4)). The chemical properties were not significantly present p > 0.05) during storage in all treatments. In the F1 treatment, sucrose value was higher (14.87%) and not detected in the F2, F3 and F4 treatments. The highest values of overrun, hardness and viscosity (p < 0.05) were detected in the F2, F3 and F3 samples, but the lowest value was detected in the F1 treatment. Total Str. thermophilus and Lb. delbrueckii ssp. bulgaricus counts were gradually decreased (p < 0.05) during storage periods. At 1 day, the Bifidobacteria counts ranged from 7.56 to 7.60 log10 CFU g−1 in all groups and gradually decreased during storage, but these bacterial counts remained viable (>6.00 log10 CFU g−1) during storage periods up to 60 d. During storage periods, the highest scores of total acceptability were detected in the F3, F4 and F2 treatments. Scanning electron microscopy (SEM) micrographs of all probiotic-frozen yogurt treatments illustrated that the microstructures showed a difference with a fine network, size pores and structure between the frozen yogurt with sweeteners (F2, F3 and F3) and control frozen yogurt (F1).

Microstructural, Physicochemical, Microbiological, and Organoleptic Characteristics of Sugar- and Fat-Free Ice Cream from Buffalo Milk

Ice cream is a popular dessert product across the world. Structure, body, taste, and odor properties are created by adding non-milk ingredients and milk ingredients. The main aim of the study is to decrease the caloric value of ice cream by using sugar and fat replacements. Ice cream treatments were investigated based on microstructural, chemical, physical, microbiological, sensory, and calorific values. Four different ice creams were used (control ice cream (SC1), ice cream with stevia (SC2), ice cream with sucralose (SC3), and ice cream with sorbitol (SC4)). The chemical properties in all treatments of ice cream were significantly recorded (p < 0.05). The highest sucrose and fat levels were detected in the SC1 treatment compared with the other treatments (p < 0.05). The lowest fat and sugar amounts were observed in the SC2, SC3, and SC4 treatments (p < 0.05). The highest viscosity, overrun, and hardness values (p < 0.05) were detected in the control ice cream. Total aerobic mesophilic bacterial counts were not significantly recorded between different ice cream treatments (p < 0.05). The sensory scores were not significantly affected by sweeteners and bulk agents in the different treatments. The highest calorific value was calculated in the SC1 samples (p < 0.05). On the other hand, the lowest calorific value was calculated in SC2, followed by the SC3 and SC4 treatments. In scanning electron microscopy (SEM), the gel exhibited a homogeneous structure with a fine network within the SC2, SC3, and SC4 treatments, as it contained a cohesive structure with small-sized pores.

Enrichment of the rebaudioside A concentration in Stevia rebaudiana extract with cyclodextrin glycosyltransferase from Bacillus licheniformis DSM 13

Stevia rebaudiana is a sweet herbaceous perennial plant, which is frequently used in the preparation of plant-based sweeteners. The demand for such sweeteners continues to increase due to purposeful nutrition and modern-day metabolic syndromes. More than 20 types of steviol glycosides provide a sweet taste, which are more than 300 times sweeter than sucrose. They are formed of two main components, namely stevioside and rebaudioside A. Only a handful of studies have dealt with Stevia rebaudiana leaf extracts, the conversion of pure stevioside into the preferred rebaudioside A is more common. The aim of this study was to enrich the rebaudioside A content of Stevia rebaudiana leaf extract using enzymatic bioconversion by applying fermented cyclodextrin glycosyltransferase from Bacillus licheniformis DSM13. Two differently processed plant materials, namely dried and lyophilized Stevia rebaudiana plants, were extracted and compared. Following the bioconversion, the rebaudioside A content was on average doubled. The maximum increase was fivefold with a 70-80% conversion of the stevioside.

Separation and purification of plant terpenoids from biotransformation

The production of plant terpenoids through biotransformation has undoubtedly become one of the research hotspots, and the continuous upgrading of the corresponding downstream technology is also particularly important. Downstream technology is the indispensable technical channel for the industrialization of plant terpenoids. How to efficiently separate high-purity products from complex microbial fermentation broths or enzyme-catalyzed reactions to achieve high separation rates, high returns and environmental friendliness has become the focus of research in recent years. This review mainly introduces the common separation methods of plant terpenoids based on biotransformation from the perspectives of engineering strain construction, unit separation technology, product properties and added value. Then, further attention was paid to the application prospects of intelligent cell factories and control in the separation of plant terpenoids. Finally, some current challenges and prospects are proposed, which provide possible directions and guidance for the separation and purification of terpenoids and even industrialization.

The role of metabolites of steviol glycosides and their glucosylated derivatives against diabetes-related metabolic disorders

Diabetes mellitus (DM), characterized by abnormal carbohydrate, lipid, and protein metabolism, is a metabolic disorder caused by a shortage of insulin secretion or decreased sensitivity of target cells to insulin. In addition to changes in lifestyle, a low-calorie diet is recommended to reduce the development of DM. Steviol glycosides (SGs), as natural sweeteners, have gained attention as sucrose alternatives because of their advantages of high sweetness and being low calorie. Most SGs with multiple bioactivities are beneficial to regulate physiological functions. Though SGs have been widely applied in food industry, there is little data on their glucosylated derivatives that are glucosylated steviol glycosides (GSGs). In this review, we have discussed the metabolic fate of GSGs in contrast to SGs, and the molecular mechanisms of glycoside metabolites against diabetes-related metabolic disorders are also summarized. SGs are generally extracted from the Stevia leaf, while GSGs are mainly manufactured using enzymes that transfer glucose units from a starch source to SGs. Results from this study suggest that SGs and GSGs share same bioactive metabolites, steviol and steviol glucuronide (SVG), which exhibit anti-hyperglycemic effects by activating glucose-induced insulin secretion to enhance pancreatic β-cell function. In addition, steviol and SVG have been found to ameliorate the inflammatory response, lipid imbalance, myocardial fibrosis and renal functions to modulate diabetes-related metabolic disorders. Therefore, both SGs and GSGs may be used as potential sucrose alternatives and/or pharmacological alternatives for preventing and treating metabolic disorders.

Stevia rebaudiana germplasm characterization using microsatellite markers and steviol glycosides quantification by HPLC

Stevia rebaudiana Bertoni, Asteraceae, is an herbaceous perennial plant native to Paraguay. This species is considered since ancient times a medicinal plant with important bioactive compounds and pharmacologic and food properties, namely diterpenes glycosides. The high natural sweetener potential stevioside and rebaudioside A produced by S. rebaudiana plants are suitable sucrose substitutes, and their obtention is influenced by environmental, phytosociological, and genetic factors. The plants’ genetic profile and sweet potential depiction are needed for suitable plant selection for improvement and deployment. Thirty-one S. rebaudiana accessions grown in the same plot where leaves samples were collected in early 2019, were genotyped using six microsatellite markers, including two steviol glycosides biosynthesis functionally involved markers. Additionally, an aqueous extract of each sample was obtained in a water bath and purified by SPE for stevioside and rebaudioside A quantification by normal phase HPLC. Stevioside and rebaudioside A contents varied between 0.53–7.36% (w w⁻¹) and 0.37–3.60% (w w⁻¹), respectively. Two genotypes displayed interesting ratios of rebaudioside A/stevioside (number 3 and 33). The level of genetic similarity between genotypes was tested through a pairwise similarity coefficient, and two groups of individuals had the same fingerprinting. Strong relatedness was found within genotypes, possibly due to cloning, thus, influx of new germplasm ought to be made to prevent mating between relatives, and for further selection and genetic improvement.

Status of the application of exogenous enzyme technology for the development of natural plant resources

Exogenous enzymes are extraneous enzymes that are not intrinsic to the subject. The exogenous enzyme industry has been rapidly developing recently. Successful application of recombinant DNA amplification, high-efficiency expression, and immobilization technology to genetically engineered bacteria provides a rich source of enzymes. Amylase, cellulase, protease, pectinase, glycosidase, tannase, and polyphenol oxidase are among the most widely used such enzymes. Currently, the application of exogenous enzyme technology in the development of natural plant resources mainly focuses on improving the taste and flavor of the product, enriching the active ingredient contents, deriving and transforming the structure of a chosen compound, and enhancing the biological activity and utilization of the functional ingredient. In this review, we discuss the application status of exogenous enzyme technology for the development of natural plant resources using typical natural active ingredients from plant, such as resveratrol, steviosides, catechins, mogrosides, and ginsenosides, as examples, to provide basis for further exploitation and utilization of exogenous enzyme technology.

Efficient production of the anti-aging drug Cycloastragenol: insight from two Glycosidases by enzyme mining

The telomerase activator cycloastragenol (CA) is regarded as a potential anti-aging drug with promising applications in the food and medical industry. However, one remaining challenge is the low efficiency of CA production. Herein, we developed an enzyme-based approach by applying two enzymes (β-xylosidase: Xyl-T; β-glucosidase: Bgcm) for efficient CA production. Both key glycosidases, mined by activity tracking or homology sequence screening, were successfully over-expressed and showed prominent enzymatic activity profiles, including widely pH stability (Xyl-T: pH 3.0-8.0; Bgcm: pH 4.0-10.0), high catalytic efficiency (k_cat/K_m: 0.096 mM^-1s^-1 (Xyl-T) and 3.08 mM^-1s^-1 (Bgcm)), and mesophilic optimum catalytic temperature (50 °C). Besides, the putative catalytic residues (Xyl-T: Asp311/Glu 521; Bgcm: Asp311/Glu 521) and the potential substrate-binding mechanism of Xyl-T and Bgcm were predicted by comprehensive computational analysis, providing valuable insight into the hydrolysis of substrates at the molecular level. Notably, a rationally designed two-step reaction process was introduced to improve the CA yield and increased up to 96.5% in the gram-scale production, providing a potential alternative for the industrial CA bio-production. In essence, the explored enzymes, the developed enzyme-based approach, and the obtained knowledge from catalytic mechanisms empower researchers to further engineer the CA production and might be applied for other chemicals synthesis. KEY POINTS: • A β-xylosidase and a β-glucosidase were mined to hydrolyze ASI into CA. • The two recombinant glycosidases showed prominent catalytic profiles. • Two-step enzymatic catalysis for CA production from ASI was developed. Graphical abstract.

Enhanced Heterologous Production of Glycosyltransferase UGT76G1 by Co-Expression of Endogenous prpD and malK in Escherichia coli and Its Transglycosylation Application in Production of Rebaudioside

Steviol glycosides (SGs) with zero calories and high-intensity sweetness are the best substitutes of sugar for the human diet. Uridine diphosphate dependent glycosyltransferase (UGT) UGT76G1, as a key enzyme for the biosynthesis of SGs with a low heterologous expression level, hinders its application. In this study, a suitable fusion partner, Smt3, was found to enhance the soluble expression of UGT76G1 by 60%. Additionally, a novel strategy to improve the expression of Smt3-UGT76G1 was performed, which co-expressed endogenous genes prpD and malK in Escherichia coli. Notably, this is the first report of constructing an efficient E. coli expression system by regulating prpD and malK expression, which remarkably improved the expression of Smt3-UGT76G1 by 200% as a consequence. Using the high-expression strain E. coli BL21 (DE3) M/P-3-S32U produced 1.97 g/L of Smt3-UGT76G1 with a yield rate of 61.6 mg/L/h by fed-batch fermentation in a 10 L fermenter. The final yield of rebadioside A (Reb A) and rebadioside M (Reb M) reached 4.8 g/L and 1.8 g/L, respectively, when catalyzed by Smt3-UGT76G1 in the practical UDP-glucose regeneration transformation system in vitro. This study not only carried out low-cost biotransformation of SGs but also provided a novel strategy for improving expression of heterologous proteins in E. coli.

RQ3, A Natural Rebaudioside D Isomer, Was Obtained from Glucosylation of Rebaudioside A Catalyzed by the CGTase Toruzyme 3.0 L

In this study, a monoglucosyl rebaudioside A product was isolated from the mixture of glucosylated rebaudioside A obtained from the most reported and industrial used cyclodextrin glycosyl transferase, Toruzyme 3.0 L (CGTase, Toruzyme 3.0 L). The molecular structure of the monoglucosyl rebaudioside A was characterized using LC-MS/MS and methylation analysis combined with 1D and 2D NMR, indicating that it is 13-[(2-O-(3-α-O-D-glucopyranosyl)-β-D-glucopyranosyl-3-O-β-D-glucopyranosyl-β-D-glucopyranosyl)oxy] ent-kaur-16-en-19-oic acid β-D-glucopyranosyl ester (also known as RQ3, which naturally exists in Stevia extract as an isomer of rebaudioside D). This study may help to further understand the reaction mechanism of glucosylation of steviol glycoside assisted by Toruzyme 3.0 L in the aspect of molecule linkage pattern, and also benefit the application of the glucosylated rebaudiosides.

Enzymatic Synthesis and Characterization of a Novel α-1→6-Glucosyl Rebaudioside C Derivative Sweetener

Zero-calorie high-intensity sweeteners from natural sources perform very well in the market place. This has encouraged food scientists to continue the effort to search for novel natural ingredients to satisfy consumer demand. Rebaudioside C (reb C) is the third most prevalent steviol glycoside in the leaves of the Stevia rebaudiana Bertoni plant, but has limited applications in food and beverage products due to its low sweetness and high lingering bitterness compared to other major steviol glycosides, such as rebaudioside A (reb A). Here we present a new enzyme modification strategy to improve the taste profile of reb C by using Cargill's propriety enzyme and sucrose as a glucose donor. A novel α-1→6-glucosyl reb C derivative was produced and its structure was elucidated by mass spectrometry and NMR spectroscopy. Sensory analysis demonstrated that this new reb C derivative has improved sweetness, reduced bitterness, and enhanced solubility in water.

Enzymatic Production of Steviol Glucosides Using β-Glucosidase and Their Applications

Sweet leaf, Stevia rebaudiana Bertoni, is a perennial plant species widely known for its sweet-tastingent-kaurene type diterpenoid glycosides (steviol glucosides). Steviol glucosides include rubusoside (Ru), stevioside (Ste), and rebaudioside (Reb), which have immunomodulatory capability and protective effects against hyperglycemia, hypertension, inflammation, tumors, and diarrhea. In addition, they can enhance the solubility of epotoside, liquiritin, paclitaxel, curcuminoids, quercetin, and wheat bran flavonoids, thus increasing their permeability. The hydrolysis of three glucosyl groups at positions C13 and C19 of Ste will produce steviolbioside, steviol, isosteviol, steviol mono-glucoside, or Ru. S. rebaudiana contains these hydrolyzed products in trace amounts. This chapter describes recent advances in the preparation of various steviol glycosides from Ste by using different β-glycosidases, with particular focus on their potential industrial applications as natural solubilizers of insoluble compounds. Furthermore, the reaction mechanism of β-glycosidases and their kinetic properties are summarized.

Determination of steviol glycosides in commercial extracts of Stevia rebaudiana and sweeteners by ultra-high performance liquid chromatography Orbitrap mass spectrometry

Stevia rebaudiana extracts are used as sweeteners in several countries worldwide. Several extracts of diverse composition are available on the market, and their taste depends on the contents of the various steviol glycosides. This study presents an accurate method for the qualitative and quantitative determination of steviol glycosides in 40 Stevia extracts, 7 sweeteners and 3 Stevia-sweetened beverages by a UHPLC coupled to an Orbitrap mass spectrometer. The sub-2 μm amide column provided the separation of all the target analytes in a run time of 30 min with high resolution. The effect of different eluent compositions on the ionisation efficiency of the steviol glycosides was studied. The optimal ionisation conditions were achieved in negative mode using 0.05% formic acid. Under this condition, adducts were not found, [M-H]^- were the main ions and the spontaneous loss of a glucose residue at C19 was reduced. The %RSD for intra- and inter-day precision for all eleven analytes varied from 2.1 to 4.2% and 3.0-5.1%, respectively. The recoveries from spiked Stevia extract samples were greater than 95% for all analytes. Rebaudioside A was the most abundant, ranging from 23 to 102%. Nine Stevia extracts and one drink were not compliant with the European Regulation. Isosteviol was under the LOD in all samples and steviol was found in four samples in quantities in the range 0.01-0.03%.

A fusion protein strategy for soluble expression of Stevia glycosyltransferase UGT76G1 in Escherichia coli

The UDP-glucosyltransferase UGT76G1 from Stevia rebaudiana converts stevioside to rebaudioside A via a one-step glycosylation reaction, which increases the amount of sweet-tasting rebaudioside A and decreases the amount of stevioside that has a bitter aftertaste. This enzyme could, therefore, conceivably be used to improve the organoleptic properties of steviol glycosides and offer a cost-effective preparation of high-purity rebaudioside A. Producing soluble enzymes by overexpression is a prerequisite for large-scale biocatalysis. However, most of the UGT76G1 overexpressed in Escherichia coli is in inclusion bodies. In this study, three N-terminal fusion partners, 3'-phosphoadenosine-5'-phosphatase (CysQ), 2-keto-3-deoxy-6-phosphogluconate aldolase (EDA) and N-utilisation substance A (NusA), were tested to improve UGT76G1 expression and solubility in E. coli. Compared with the fusion-free protein, the solubility of UGT76G1 was increased 40% by fusion with CysQ, and the glucosyltransferase activity of the crude extract was increased 82%. This successful CysQ fusion strategy could be applied to enhance the expression and solubility of other plant-derived glucosyltransferases and presumably other unrelated proteins in the popular, convenient and cost-effective E. coli host.

Enzymatic Synthesis of a Novel Kaempferol-3-O-β-d-glucopyranosyl-(1→4)-O-α-d-glucopyranoside Using Cyclodextrin Glucanotransferase and Its Inhibitory Effects on Aldose Reductase, Inflammation, and Oxidative Stress

Kaempferol-3-O-β-d-glucopyranoside (astragalin, AS), a major flavonoid that exists in various plants, exerts antioxidant, antitumor, anti-human immunodeficiency virus (HIV), and anti-inflammatory effects. However, the low water solubility of AS limits its use. In this study, we used cyclodextrin glucanotransferase (CGTase) with maltose (G2) as a donor molecule to enzymatically modify AS to improve its water solubility and physiochemical properties. We isolated the glycosylated astragalin (G1-AS) and identified the structure of G1-AS as kaempferol-3-O-β-d-glucopyranosyl-(1→4)-O-α-d-glucopyranoside, where one glucose residue was transferred to AS. G1-AS retained the antioxidative activity of the original AS compound; however, the solubility of G1-AS was 65-fold higher than that of AS. In addition, G1-AS showed enhanced anti-inflammatory effects and aldose reductase inhibitory activity compared to AS when applied to rat lenses.

From Caá-ehé to a commercial sweetener - the diterpenoid glycosides of Stevia rebaudiana

The isolation, structure elucidation, chemistry, biosynthesis and biological activity of the sweet steviol glycosides from Stevia rebaudiana, are reviewed.