US20200325515A1

US20200325515A1 - Methods for the Enzymatic Modification of Steviol Glycosides, Modified Steviol Glycosides Obtainable Thereby, and the Use Thereof as Sweeteners

Info

Publication number: US20200325515A1
Application number: US16/899,709
Authority: US
Inventors: Evelien Maria te Poele; Lubbert Dijkhuizen; Gerrit Johannes Gerwig; Johannis Paulus Kamerling
Original assignee: Rijksuniversiteit Groningen
Current assignee: Rijksuniversiteit Groningen
Priority date: 2015-03-10
Filing date: 2020-06-12
Publication date: 2020-10-15
Also published as: JP6976173B2; JP2018509902A; EP3268487A1; WO2016144175A1; US20180044708A1; US10731195B2; BR112017019290A2; CN107532189B; CN107532189A

Abstract

The present invention relates generally to the production of steviol glycosides. Provided is a method for enzymatically providing a modified steviol glycoside, comprising incubating a steviol glycoside substrate in the presence of sucrose and the glucansucrase GTF180 of Lactobacillus reuteri strain 180, or a mutant thereof having the desired transglycosylation activity. Also provided are modified steviol glycosides obtainable by a method of the invention, and the use thereof as low-glycemic sweetener.

Description

RELATED APPLICATION DATA

This application is a divisional application which claims priority to U.S. patent application Ser. No. 15/556,846, filed on Sep. 8, 2017, which is a National Stage Application under 35 U.S.C. 371 of co-pending PCT application number PCT/NL2016/050172 designating the United States and filed Mar. 10, 2016; which claims the benefit of EP application number 15158421.6 and filed Mar. 10, 2015 each of which are hereby incorporated by reference in their entireties.
The present invention relates generally to the production of steviol glycosides. Particularly, the invention relates to methods for the enzymatic modification of steviol glycosides to novel steviol glycosides, and use thereof as sweeteners.
Sweeteners are well known as ingredients used most commonly in the food, beverage, or confectionary industries. The sweetener can either be incorporated into a final food product during production or for stand-alone use, e.g. when appropriately diluted or as a tabletop sweetener. Sweeteners include natural sweeteners such as sucrose, high fructose corn syrup, molasses, maple syrup, and honey and artificial sweeteners such as aspartame, saccharine and sucralose.
The leaves of the herb plant Stevia rebaudiana Bertoni, a rhizomatous perennial shrub of the Asteraceae [Compositae] family, contain a high variety of natural sweet compounds, being steviol glycosides (Brandle et al. 1998). Stevioside (5-10% w/w of dried leaves) and Rebaudioside A (2-4% w/w of dried leaves) are the most abundant and they taste about 200-300 times sweeter than sucrose (0.4% water solution). Therefore, they can be considered as “bio” alternatives for sucrose and for artificial (synthetic) sweeteners (Geuns 2003; Goyal et al. 2010; Puri et al. 2011).
In addition to sweetness, some steviol glycosides, in higher doses and regular consumption, possess diverse pharmacological properties, like antioxidant, antibacterial, antifungal, antiviral, antitumor, gastro protective (antidiarrheal), and they seem to have a positive effect on renal function, blood pressure and blood glucose levels (Chatsudthipong and Muanprasat 2009; Madan et al. 2010; Brahmachari et al. 2011; Lemus-Mondaca et al. 2012; Shivanna et al. 2013). They can be of benefit to people suffering from obesity, diabetes mellitus, hypertension, phenylketonuria, heart disease and dental caries (Yadav and Guleria 2012). Steviol glycosides are non-caloric, not carcinogenic, not genotoxic and not associated with any reproductive/developmental toxicity in humans (European Food Safety Authority, 2010).
Structurally, steviol glycosides have an ent-13-hydroxykaur-16-en-19-oic acid as aglycone but differ in carbohydrate composition (see FIG. 1).
The ratio of the number of glucose units at the 13-tert-hydroxyl group to that at 19-carboxyl group seems to have a relationship with the sweetness as well as with the quality of taste of the steviol glycosides (Darise et al. 1984). For instance, Rebaudioside A is less bitter than Stevioside. Enzymatic glucosylation studies of Stevioside show that the glycosidic linkage specificity affects the organoleptic properties of steviol glycosides as well. Fukunaga et al. (1989) found that both mono- and di-(α1→4)-glucosylation of Stevioside at the C-13-position gave products with remarkable improvement in both the intensity and quality of the sweetness. However, both mono- and di-(α1→4)-glycosylation at the C-19 position resulted in an increased bitter aftertaste and a lower sweetness intensity (Fukunaga et al. 1989). On the other hand, attachment of an α-linked glucose to the C-6 hydroxyl group of the glucose unit at the C-19-position led to a remarkable improvement in the quality of taste (Lobov et al. 1991). Apparently, the anomericity of the glycosidic bond does not influence sweet and bitter taste perception to a great extent, since several recent studies show that Rebaudioside D and Rebaudioside M, both Rebaudioside A derivatives with respectively one and two 6-linked glucose units extra at the 19-O-glucosyl moiety, both have a more desirable taste profile than Rebaudioside A and many other steviol glycosides (Hellfritsch et al. 2012; US 2013/00771521A1; WO2014/122227; Prakash et al. 2014). Compared to Rebaudioside A, Rebaudioside D has increased sweetness and decreased bitterness in water and in carbonated beverage base. Rebaudioside M showed reduced bitterness compared to Rebaudioside A but similar sweetness intensity in water solution. In acidified water a reduced bitterness and higher sweetness were perceived compared to Rebaudioside A (Prakash et al. 2014). Moreover, attachment of an (α1→2)- or an (β1→2)-(is Rebaudioside E)-linked glucose to the C-2 hydroxyl group of the glucose unit at the C-19-position of Stevioside improved the organoleptic products of Stevioside, yielding compounds with similar sweetness, but with reduced bitterness (Ye et al. 2013).
The main drawback for successful commercialization of Stevia sweeteners is their slight bitterness and astringency (Stevioside in particular). These undesirable properties can be reduced or eliminated by modifying the glycosyl moieties of the steviol glycosides.
Chemical modifications of steviol glycosides have been performed with the aim of improving the quality of taste of these compounds. For instance, Stevioside and Rebaudioside A could be improved by replacement of the 19-O-glucosyl residues by a (sodiosulfo)propyl [(CH₂)₃SO₃Na] moiety (DuBois et al. 1981; DuBois and Stephenson 1985). Furthermore, several analogs of stevioside have been synthesized by replacing the C-19-O-β-D-glucosyl moiety for another monosaccharide (e.g. β-D-Xyl, α-L-Ara, α-D-Man, or α-L-Rha) or extension of the C-19 β-D-glucosyl moiety with a monosaccharide (α-L-Rha or α-L-Qui). However, it is generally held that the application of chemical methods to modify steviol glycosides is impractical, due to the need of multistep sequences in the selective protection-deprotection synthesis strategies. Furthermore, the use of organic solvents and metallic salts will cause problems for acceptation of the obtained derivatives in the food industry. To overcome these problems, biocatalyst alternatives are more preferred, also with the objectives of “green” chemistry.
A promising procedure is to subject steviol glycosides to the reaction of enzymatic transglycosylation, thereby introducing new monosaccharide residues into the molecule. Depending on the number, position and anomericity of the monosaccharide residues, the taste quality and potency of the compounds will vary.
To improve the taste, enzymatic modifications of the carbohydrate moieties of steviol glycosides have been performed by using different enzyme systems, amongst which UDP-glucosyltransferases (UGTases)(WO2013/176738A9; WO 2014/122227) and cyclodextrin glucanotransferases (CGTases) (Darise et al 1984; Li et al 2013; US2014/0227421A1). UGTases are efficient enzymes with high regio-specificity, catalyzing the transfer of α- or β-linked glucoses at a specific location. However, UGTases require expensive nucleotide activated sugars as glycosyl donor, which makes them less attractive for industrial applications. CGTases catalyze coupling and disproportionation reactions, transferring glucose residues from starch or cyclodextrins to acceptor molecules. The intermolecular transglucosylation reaction is expected to occur exclusively at the C-4-hydroxyl group of the non-reducing-end glucose residues of the steviol glycosides due to the acceptor specificity of the CGTase enzyme. Although often high yields are obtained, CGTase has poor C-13/C-19 regio-specificity producing steviol glycosides that are mostly mixtures of compounds with α-D-glucosyl extensions at C-13 and C-19. Furthermore, the (α1→4)-linkages introduced by CGTase enzymes are rapidly hydrolyzed in the human mouth by the amylolytic enzymes present in saliva, thereby increasing the caloric content of steviol glycosides. Introduction of a-amylase resistant glycosidic linkages, such as the (α1→6) and (α1→3) linkages, is therefore more desirable, since it will answer the consumers demand for low and zero calorie food products.
Accordingly, the present inventors aimed for novel means and methods to provide enzymatically modified steviol glycosides. In particular, they set out to develop an enzymatic method yielding compounds showing a reduced bitterness and/or higher sweetness compared to the unmodified steviol glycoside. Preferably, the method is economically attractive at an industrial scale, and preferably does not require expensive nucleotide activated sugars as glycosyl donor.
To that end, they screened the glucosylation potential of glucansucrase and fructansucrase enzymes of different lactobacilli, of which most members have the generally-recognized-as-safe (GRAS) status, such as Lactobacillus reuteri. Glucansucrases are extracellular enzymes, which are only reported to occur in lactic acid bacteria. They synthesise a-glucan polymers from the cheap donor substrate sucrose. Depending on the glucansucrase enzyme, different (mixtures of) glycosidic linkages are introduced in their glucan products, namely (α1→2)-, (α1-3)-, (α1→4)- and (α1→6)-linkages (Leemhuis et al. 2013). The low cost of the glucosyl donor substrate sucrose used by glucansucrase enzymes is a major advantage for their industrial applications. Most importantly, the (α1→2)-, (α1→3)- and (α1→6)-linkages introduced by glucansucrase enzymes are not hydrolyzed in the mouth by the amylolytic enzymes present in saliva. More than 100 enzymes consisting of wild-type and mutant glucansucrase and fructansucrase enzymes with different product specificity from different Lactobacillus reuteri strains, were screened for their ability to glucosylate the steviol glycoside Rebaudioside A. Rebaudioside A glucosides were isolated by semi-preparative NP-HPLC and their structures were elucidated by MALDI-TOF mass spectrometry and 1D/2D ¹H/¹³C NMR spectroscopy. Sensory evaluations were performed to determine the taste attributes of the novel Rebaudioside A glucosides.
It was surprisingly found that only the glucansucrase GTF180 from Lactobacillus reuteri 180 (GenBank accession number AY697430) was able to glucosylate Rebaudioside A. NMR structural analysis of the Rebaudioside A glucosylation products showed that GTF180 specifically and only glucosylates Rebaudioside A at the C-19 β-linked glucose residue. Interestingly, several GTF180 point mutants displayed much higher transglucosylating activity towards Rebaudioside A. One mutant, Q1140E, even showed ˜96% Rebaudioside A conversion. Similar results were observed with respect to modification of Stevioside.
Accordingly, in one embodiment the invention provides a method for enzymatically providing a modified steviol glycoside, comprising incubating a steviol glycoside substrate in the presence of a glucose donor and the glucansucrase GTF180 of Lactobacillus reuteri strain 180, or a mutant thereof having the desired transglycosylation activity.
To our knowledge there is only one report on glucosylation of stevioside using glucansucrases. Musa et al. reported the enzymatic modification by alternansucrase from Leuconostoc citreum SK24.002 in the biotransformation of stevioside to fully or partially remove the bitter taste of the stevioside. With optimized reaction conditions a maximum transglucosylation yield of 43.7% was achieved with stevioside. Stevioside glycosides with 1 to 3 α-glucose units attached were obtained. In a follow-up study the structure of the product was characterized to be 13-{[α-D-glucopyranosyl-(1→3)-α-D-glucopyranosyl-(1→6)-α-D-glucopyranosyl-(1→3)-8-D-glucopyranosyl-(1→2)-β-D-glucopyranosyl]oxy}ent-kaur-16-en-19-oic acid β-D-glucopyranosyl ester (Musa et al. 2014). Thus, the method of Musa et al. uses a distinct enzyme, shows a lower yield and results in a distinct type of modification, namely α-glucosylation at the C-13 site instead of at the C-19 site.
In one embodiment of the present invention, the steviol glycoside is modified with at least one α-glucose residue at the C-19 β-linked glucose residue. For example, the steviol glycoside is provided with one or more glucoses via an (α1→6), an (a 1→3) glycosidic bond, or a combination thereof. In a specific aspect, the modification involves the addition of one glucose via an (α1→6) glycosidic bond (β-isomaltose) or an (α1→3) glycosidic bond (P-nigerose) (FIG. 5A). In another specific aspect, the modification involves the addition of a glucosyl-glucose unit via an (α1→6) glycosidic bond at the β-linked glucose. Within the unit, the glucose residues can be connected via an (α1→6) glycosidic bond (isomaltose) or an (α1→3) glycosidic bond (nigerose) (FIGS. 5B and 5C).
A steviol glycoside can be modified at multiple positions. For example, modifications can occur at the C-13 and/or the C-19 position(s) of the steviol aglycone. In view of the attractive taste characteristics of Rebaudioside D and Rebaudioside M, the steviol glycoside is preferably at least modified at the C-19 site of the steviol glycoside.
More preferably, the modified steviol glycoside is only modified at the C-19 site of the steviol glycoside. For example, in one embodiment the invention provides a method for the enzymatic production of a modified steviol glycoside, which is only modified with a single glucose residue at the C-19 position of the steviol aglycone. In one embodiment, the modification comprises the addition of a single (α1→6) glucose at the C-19 β-linked glucose residue.
The steviol glycoside substrate can be of any type. For example, it is selected from the group consisting of Stevioside, Rubusoside, Rebaudiosides including Rebaudioside A, Rebaudioside C, Rebaudioside D, and Rebaudioside E, and Dulcoside compounds. In one embodiment, the steviol glycoside substrate has at least one monosaccharide moiety at the C-19 position of the steviol glycoside.
In one specific embodiment, the steviol glycoside substrate is a Rebaudioside. The present inventors hypothesized that α-glucosylation of Rebaudioside A at the C-19 site could thus yield Rebaudioside D and Rebaudioside M anomeric isomers with a similar or even better taste profile than Rebaudioside D and Rebaudioside M. Accordingly, in a preferred embodiment, the invention provides a method for enzymatic modification of Rebaudioside A [13-({β-D-glucopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→3)-]β-D-glucopyranosyl}oxy)ent-kaur-16-en-19-oic acid α-D-glucopyranosyl ester].
In another specific embodiment, the steviol glycoside substrate is Stevioside, the most abundant and one of the bitterest tasting steviol glycosides present in Stevia extracts.
Typically, on a dry weight basis, the four major steviol glycosides found in the leaves of Stevia are Dulcoside A (0.3%), Rebaudioside C (0.6-1.0%), Rebaudioside A (2-4%) and Stevioside (5-10%). Other glycosides identified in reasonable amounts in Stevia extract include Rebaudioside B, D, E, and F, Steviolbioside and Rubusoside. Among these, only Stevioside and Rebaudioside A are currently available on a commercial scale.
Steviol glycosides can be extracted from leaves using methods known in the art, typically involving either water or organic solvent extraction. Supercritical fluid extraction and steam distillation methods have also been described. Methods for the recovery of diterpene sweet glycosides from Stevia rebaudiana using supercritical CO₂, membrane technology, and water or organic solvents, such as methanol and ethanol, may also be used.
US2014/343262 discloses a method for purifying steviol glycosides, comprising the steps of: a. passing a solution of steviol glycosides through a multi-column system including a plurality of columns packed with adsorbent resin, to provide at least one column having adsorbed steviol glycosides; and b. eluting fractions with low Rebaudioside X (was later named as Rebaudioside M (US 2014/0227421; Prakash et al 2014) content from at least one column having adsorbed steviol glycosides to provide an eluted solution comprising steviol glycosides.
Rebaudioside A is generally available at ≤80% purity. The primary impurities comprise Stevioside, Steviolbioside, Rebaudioside B, Rebaudioside C, Rebaudioside D, Dulcoside A, Rebaudioside F, and other steviol glycosides. Many studies focused on the recovery of a high purity of Rebaudioside A in high recovery. U.S. Pat. No. 5,962,678 discloses the re-crystallization of Rebaudioside A using an anhydrous methanol solution to obtain an 80% pure Rebaudioside A. By repeating the re-crystallization with anhydrous methanol numerous times, the purity of rebaudioside A may be increased to over 95%. US 2006/0083838 discloses purification of Rebaudioside A through re-crystallization with a solvent comprising ethanol and between 4 and 15% water. Japanese Patent Application No. 55-23756 discloses a method for purifying rebaudioside A and stevioside by crystallization from aqueous ethanol (>70%) to obtain an 80% pure Rebaudioside A. US 2007/0082103 discloses a method for purifying Rebaudioside A by recrystallization from aqueous ethanol, asserting a two-step recrystallization from crude Rebaudioside (60%) results in the formation of at least 98% pure Rebaudioside A at 97% yield. U.S. Pat. No. 8,791,253 provides a substantially pure Rebaudioside A composition using only a single recrystallization step.
The concentration of steviol glycoside substrate in a method of the invention can vary e.g. depending on type of substrate, desired modification, etc. Typically, the reaction mixture comprises at least 20 mM steviol glycoside to be modified, preferably at least 30 mM, more preferably at least 50 mM, like 60, 70, 80 90 to 100 mM. The maximal concentration is among others dependent on the substrate solubility in an aqueous reaction medium. For example, good results were obtained using 50 to 100 mM Rebaudioside A or Stevioside as substrate.
A method of the invention uses sucrose as glucose donor. Sucrose is cheap and widely available. Good results were obtained when sucrose is used at a concentration of at least 50 mM, preferably at least 100 mM, more preferably at least 500 mM. For example, the reaction mixture comprises at least 500 mM, 600 mM, 700 mM, 800 mM, 900 mM or 1 M sucrose. Even higher concentrations, like up to 2 M or 3 M, may be used. The glucose donor can in its total amount be added at the onset of the reaction. In some embodiment, it is advantageous to add sucrose in a batch wise manner. For example, sucrose is added in a batch wise manner, e.g. at the onset, after 1.5 and 3 hours, to a final amount of at least 1 M, more preferably at least 2 M.
The reaction is typically performed at a temperature of about 20 to 70° C., in the pH range of 3-7. Preferably, a temperature of about 37° C. is used.
The reaction is allowed to proceed until a desirable amount of modified steviol glycoside is produced. Typically, incubations are performed during a period ranging from about 1 hour to overnight.
The skilled person will be able to determine the amount of GTF180 glucansucrase enzyme to be used to obtain a desirable degree of enzymatic modification under the given reaction conditions. For example, 1 to 50 U/mL can be used. Preferably, at least 3 U/mL is used. For economical reasons, it may be advantageous to use up to 35 U/mL. In one embodiment, 5 to 30 U/mL is used. One unit (U) of enzyme is defined as the amount of enzyme required for producing 1 μmol monosaccharide per minute in a reaction mixture containing 25 mM sodium acetate (pH 4.7); 1 mM CaCl₂); and 1 M sucrose at 37° C.
In one embodiment, wild-type GTF180 glucansucrase from Lactobacillus reuteri strain 180 is used (GenBank accession number AY697430). In another embodiment, a mutant GTF180 glucansucrase is used. As used herein, a mutant GTF180 glucansucrase refers to an enzyme comprising one or more amino acid substitutions, amino acid deletions and/or amino acid insertions as compared to the wild type amino acid sequence.
In a preferred embodiment, a GTF180 mutant for enzymatic modification of a steviol glycoside according to the present invention comprises a substitution mutation at positions S1137, Q1140, L981 and/or W1065 (numbering based on the GenBank sequence AY697430). Preferably, the mutation is a non-conservative substitution i.e. a mutation that results in an amino acid change that has different properties than the native amino acid. For example, said mutant has one or more of the following amino acid substitutions: S1137Y, Q1140E, L981A, W1065L/E/Q/F.
In another embodiment, a mutant GTF180 is a deletion mutant or truncated variant, wherein a stretch of at least 10 amino acids is removed from the N- and/or C-terminus. In one aspect, the truncation mutant is GTF180-ΔN comprising residues 742-1772, in which the N-terminal variable domain has been deleted. For example, good results were obtained with GTF180-ΔN which is the 117 kDa N-terminally truncated (741 residues) fragment of the GTF180 full-length wild type protein. GTF180-ΔN is fully active and produces an α-glucan polymer with similar size and linkage distribution as the full length enzyme (Kralj et al. 2004a). NMR structural analysis of the Rebaudioside A glucosylation products showed that GTF180-ΔN specifically and only glucosylates Rebaudioside A at the C-19 8-linked glucose residue. Interestingly, several GTF180-ΔN substitution mutants displayed much higher transglucosylating activity towards Rebaudioside A than GTF180-ΔN. One mutant, Q1140E, even showed 96% Rebaudioside A conversion compared to ˜55% Rebaudioside A conversion by GTF180-ΔN (FIG. 3 and FIG. 4). Accordingly, in a preferred embodiment a method of the invention uses GTF180-ΔN with one or more amino acid substitutions, for example at position Q1140, S1137, L981, and/or W1065. Specific exemplary mutant enzymes include GTF180-ΔNQ1140E, GTF180-ΔNQ1140F, GTF180-ΔNQ1140N, GTF180-ΔNQ1140Y, GTF180-ΔNQ1140R, GTF180-ΔNS1137Y, GTF180-ΔN L981A, GTF180-ΔN W1065L, GTF180-ΔN W1065E, GTF180-ΔN W1065 Q and GTF180-ΔN W1065F.
In another aspect, the truncation mutant is GTF180-ΔNΔV, in which both the N-terminal variable domain and N-terminal domain V fragment (corresponding to the first 793 N-terminal amino acids), and the domain V C-terminal fragment (corresponding to the last 136 C-terminal amino acids) have been deleted (Meng et al. 2015a) to result in a GTF180 mutant consisting of amino acids 794-1636. This GTF180-ΔNΔV truncation mutant, which may be considered as the “catalytic core”, has a ˜50% reduction in size compared to the full length GTF180 wild type, is fully active, creates a similar glycosidic linkage distribution as GTF180 wild type, but is heavily impaired in high-molecular-mass polysaccharide synthesis.
A truncation mutant of the invention may additionally contain substitution mutation(s), e.g. to improve its catalytic properties. In one embodiment, the mutant is GTF180-ΔNΔV, furthermore comprising a substitution mutation at position(s) S1137, Q1140, L981 and/or W1065. For example, said mutant has one or more of the following amino acid substitutions: S1137Y, Q1140E, L981A, W1065L/E/Q/F. Specific exemplary mutant enzymes include GTF180-ΔNΔVQ1140E, GTF180-ΔNΔVQ1140F, GTF180-ΔNΔVQ1140N, GTF180-ΔNΔVQ1140R GTF180-ΔNΔVQ1140Y, GTF180-ΔNΔVS1137Y, GTF180-ΔNΔV L981A, GTF180-ΔNΔV W1065L, GTF180-ΔNΔV W1065E, GTF180-ΔNΔV W1065 Q and GTF180-ΔNΔV W1065F.
Mutant GTF180 glucansucrases have for example been described by Van Leeuwen et al. reporting the mutagenesis of specific amino acid residues of the GTF180-ΔN enzyme, which yielded 12 mutant enzymes that produce modified exopolysaccharides (mEPSs) from sucrose (van Leeuwen et al. 2009). It was found by the present inventors that two of the single mutants, Q1140E and S1137Y of GTF180-ΔN, displayed much higher transglucosylating activity towards Rebaudioside A than GTF180-ΔN, showing respectively ˜96% and ˜73% Rebaudioside A conversion compared to ˜55% Rebaudioside A conversion by GTF180-ΔN (FIG. 3 and FIG. 4). Mutant Q1140E mainly produced mono-α-glucosylated Rebaudioside A, while GTF180-ΔN and mutant S1137Y produced multiple-α-glucosylated forms with DP up to at least 8. NMR structural analysis of the α-glucosylated products showed that GTF180-ΔN and mutants Q1140E and S1137Y specifically and only glucosylate Rebaudioside A at the C-19 site. The three enzymes glucosylated Rebaudioside A exclusively with an (α1→6)-linked glucose at the C-19 β-linked glucose, yielding RebAG1. The di-glucosylated Rebaudioside A products of GTF180-ΔN and mutant S1137Y were both elongations of RebAG1 with an (α1→3)-linked glucose (˜75%) or another (α1-6)-linked glucose (˜25%) coupled at the terminal α-glucose residue. Accordingly, specifically preferred mutants include Q1140E and S1137Y of GTF180-ΔN.
GTF180-ΔN mutants L981A and W1065L/E/Q/F (Meng et al. 2015b) are able to α-glucosylate Rebaudioside A, but show almost no polymerization (i.e. oligosaccharide and glucan formation) activity. This is a clear advantage during downstream processing, the purification of elongated Rebaudioside A products from mono- and disaccharides, oligosaccharides and glucans. By eliminating α-glucan synthesis, the most important side reaction, higher glycosylation yields were obtained for Rebaudioside A. At 200 mM sucrose and 1.5 hour incubation time, these mutants have similar or even higher transglucosylating activity than GTF180-ΔN and mutants Q1140E and S1137Y on Rebaudioside A. As observed with Rebaudioside A as acceptor molecule, mutant Q1140E also converted Stevioside mainly into one mono-α-glucosylated product. Hence, in one embodiment the mutant enzyme comprises the mutation L981A and/or W1065L, W1065E, W1065Q, W1065F.
Also provided herein is the use of glucansucrase GTF180 of Lactobacillus reuteri strain 180 or a mutant thereof having the desired transglycosylation activity to enhance or improve the organoleptic properties of a steviol glycoside, for example to fully enhance the sweetness, partially remove the bitter taste and/or aftertaste of a steviol glycoside, preferably of Rebaudioside A or Stevioside.
In the screening of substitution mutants useful for modifying Rebaudioside A, it was observed that the reaction mixture comprising an inactive mutant or enzyme that is unable to α-glucosylate Rebaudioside A turned cloudy due to the gradual precipitation of Rebaudioside A in time, whereas those comprising an active enzyme able to α-glucosylate Rebaudioside A remained clear. Without wishing to be bound by theory, the addition of glucose moieties to Rebaudioside A increases its solubility. This phenomenon allows the rapid selection of active mutants by evaluating the appearance of the reaction mixture, preferably after ˜6 hours of incubation in case of a final Rebaudioside A concentration at a minimum of 50 mM and ˜16 hours of incubation in case of a final Rebaudioside A concentration at a minimum of 30 mM. For example, when the reactions are performed in a microtiter plate or other type of transparent container, a mere visual inspection can be sufficient to identify one or more mutants for further characterization. Accordingly, the invention also provides a method for identifying a glucansucrase capable of modifying a steviol glycoside, preferably Rebaudioside A or Stevioside, comprising the steps of:
a) generating a panel of mutants of GTF180 of Lactobacillus reuteri strain 180;
b) incubating each mutant with a steviol glycoside in the presence of a glucose donor in an aqueous reaction mixture under conditions allowing for glycosylation of the steviol glycoside; and
c) selecting at least one mutant of GTF180 of Lactobacillus reuteri 180 capable of modifying the steviol glycoside, by determining its capacity to at least partially prevent the reaction mixture from becoming cloudy;
d) optionally further determining the structure of the modified steviol glycoside and selecting a mutant GTF180 of Lactobacillus reuteri 180 capable of modifying the C-19 site of the steviol glycoside.
Preferably, the mutant panel is prepared starting from a truncated GTF180 enzyme, like the truncated variant lacking the N-terminal variable domain (GTF180-ΔN), and/or the N- and C-terminal domain V fragments (GTF180-ΔNΔV).
In one embodiment, the panel of mutants comprises different substitution mutants, preferably non-conservative substitution mutants. For example, the screening method comprises creating a panel of GTF180 (truncation) mutants with different (non-conservative) amino acid substitutions at the Q1140 position and testing all the Q1140 mutants for steviol glycoside α-glucosylation. See FIG. 8 herein below.
A further aspect of the invention relates to a modified steviol glycoside obtainable by a method according to the invention. In one embodiment, the steviol glycoside is modified with at least one glucose residue. In a specific aspect, the modification involves the addition of one glucose via an (α1-6) glycosidic bond (P-isomaltose) (FIG. 5A). In another specific aspect, the modification involves the addition of a glucosyl-glucose unit via an (α1→6) glycosidic bond at the β-linked glucose. Within the unit, the glucose residues can be connected via an (α1→6) glycosidic bond (isomaltose) or an (α1→3) glycosidic bond (nigerose) (FIGS. 5B and 5C).
The invention preferably provides a steviol glycoside modified at the C-19 site of the steviol glycoside. More preferably, the modified steviol glycoside is only modified at the C-19 site of the steviol glycoside. For example, in one embodiment the invention provides a modified steviol glycoside which is only modified with a single α-glucose residue at the C-19 site of the steviol glycoside. In one embodiment, the C-19 site is modified with a single (α1→6) linked glucose.
An exemplary modified steviol glycoside of the invention is selected from the group consisting of

(i) 13-({β-D-glucopyranosyl-(1→2)-[8-D-glucopyranosyl-(1→3)-]β-D-glucopyranosyl}oxy)ent-kaur-16-en-19-oic acid α-D-glucopyranosyl-(1→6)-8-D-glucopyranosyl ester (FIG. 5A)
(ii) 13-({β-D-glucopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→3)-]β-D-glucopyranosyl}oxy)ent-kaur-16-en-19-oic acid α-D-glucopyranosyl-(1-6)-α-D-glucopyranosyl-(1→6)-8-D-glucopyranosyl ester (FIG. 5B)
(iii) 13-({β-D-glucopyranosyl-(1→2)-[8-D-glucopyranosyl-(1→3)-]β-D-glucopyranosyl}oxy)ent-kaur-16-en-19-oic acid α-D-glucopyranosyl-(1→3)-α-D-glucopyranosyl-(1→6)-8-D-glucopyranosyl ester (FIG. 5C)

To determine the effect of (α1→6) glucosylation at the 19-O-glucosyl moiety of Rebaudioside A on sweetness and bitterness of Rebaudioside A, a taste evaluation was performed in which one of the novel Rebaudioside A glucosides, i.e. (i) 13-({f-D-glucopyranosyl-(1→2)-[8-D-glucopyranosyl-(1→3)-]β-D-glucopyranosyl}oxy)ent-kaur-16-en-19-oic acid α-D-glucopyranosyl-(1→6)-8-D-glucopyranosyl ester, was compared to Rebaudioside A. For this, in a blind test, twelve test persons that were able to perceive the bitter aftertaste of steviol glycosides were asked to rate sweetness and bitterness on a scale from 0 to 5, scoring 0 indicating not sweet/not bitter and 5 indicating very sweet/very bitter. A clear trend was observed showing that the novel Rebaudioside A glucoside had an increased and more natural sweetness and reduced bitterness compared to Rebaudioside A (FIG. 7).
Also provided is the use of a modified steviol glycoside according to the invention as low-glycemic sweetener, and a method for sweetening a consumable, comprising including in said consumable an effective amount of the modified steviol glycoside, optionally combined with other edible ingredient, sweetener and/or sweetness enhancer.
A further aspect relates to a sweetening composition comprising at least one modified steviol glycoside as provided herein. In certain embodiments the sweetener composition is a table top sweetener suitable to be used in cooking or to be added by a consumer to a beverage or other food. Such sweetener composition can be packaged and sold in bulk. Alternatively, in certain embodiments the sweetener composition is packaged in single serving packets to be opened at the time of use by the consumer. The at least one other edible ingredient of the sweetener composition, in accordance with certain embodiments, may, for example, be a flavorant, e.g., flavorant below, at or barely above its threshold perception level or in an amount readily perceptible to the consumer, a flowing agent, a coloring agent, a bulking agent to provide ease of handling and/or improved mouthfeel in beverages and other food products in which the sweetener composition is used, and/or other suitable ingredient, or a combination of any two or more of them. In certain embodiments the bulking agent(s) can provide an improved sweetness profile by increasing the up-front sweetness provided by the sweetener composition. In certain embodiments the at least one other edible ingredient is erythritol, D-tagatose, and/or D-psicose, for example a combination of two or more of those ingredients is included in the sweetener composition, such as erythritol and D-tagatose, or erythritol and D-psicose, or D-tagatose and D-psicose.
Also provided is a consumable comprising at least one modified steviol glycoside according to the invention, optionally combined with another sweetener and/or sweetness enhancer. For example, the consumable is selected from the group of beverages, foodstuff, an oral care product, a tobacco product, a pharmaceutical products and nutraceutical products.
Typically, a foodstuff comprises a sweetening amount of a modified steviol glycoside of the invention, and at least one other food ingredient. As used herein, the term “food ingredient” means any edible substance suitable to provide flavor, nutrition, color, bulk, texture or other mouthfeel, stability, acidity, thickening, anti-caking or the like, or a combination of any two or more of these. As further discussed below, exemplary food ingredients suitable for use in the novel food products disclose here include grain components, carbonated or non-carbonated water, other sweeteners, e.g., a sweetening amount of at least one nutritional sweetener, flavorants, acidulants, colorants, bulking agents, etc. In certain exemplary (i.e., non-limiting) embodiments, the food product is packaged in a single serving quantity. The food products of this aspect of the disclosure include, for example, solid foods, gels, beverages, etc.
Examples of suitable sweeteners and sweetness enhancers include sucrose, fructose, glucose, high fructose corn syrup, corn syrup, xylose, arabinose, rhamnose, erythritol, xylitol, mannitol, sorbitol, inositol, acesulfame potassium, aspartame, neotame, sucralose, and saccharine, and mixtures thereof; trilobatin, hesperetin dihydrochalcone glucoside, naringin dihydrochalcone, mogrosides including mogroside V, Luo Han Guo extract, rubusoside, rubus extract, glycyphyllin, isomogroside V, mogroside IV, siamenoside I, neomogroside, mukurozioside lib, (+)-hernandulcin, 4β-hydroxyhemandulcin, baiyunoside, phlomisoside I, bryodulcoside, bryoside bryonoside, abrusosides A-E, cyclocarioside A, cyclocaryoside I, albiziasaponins A-E, glycyrrhizin, araboglycyrrhizin, periandrins I-V, pterocaryosides A and B, osladin, polypodosides A and B, telosmoside A8-18, phyllodulcin, huangqioside E neoastilbin, monatin, 3-acetoxy-5,7-dihydroxy-4′-methoxyflavanone, 2R,3R-(+)-3-Acetoxy-5,7,4′-trihydroxyflavanone, (2R.3R)-dihydroquercetin 3-O-acetate, dihydroquercetin 3-O-acetate 4-methyl ether, brazzein, curculin, mabinlin, monellin, neoculin, pentadin, thaumatin, and combinations thereof. Some of the compounds listed above are known sweetness enhancers as well as sweeteners. When used as sweetness enhancers they are normally used below their sweetness detection thresholds.
The beverages include, for example, juice beverages (e.g., beverages comprising one or more fruit juices and/or one or more vegetable juices), hydration beverages, carbonated soft drinks (CSDs), frozen beverages, frozen carbonated beverages, diet or other reduced calorie beverages, etc. It will be recognized by those skilled in the art that there is overlap between these categories. As used herein, “reduced calorie beverage” means a beverage having at least a 25% reduction in calories per 8 oz. serving of beverage as compared to the full calorie version, typically a previously commercialized full-calorie version (e.g., wherein substantially all of the sweetening comes from a nutritive sweetener, such as sucrose, HFCS or the like). In at least certain embodiments, a reduced calorie beverage has about a 50% reduction in calories per 8 oz. serving as compared to the full calorie version. As used herein, a “low-calorie beverage” has fewer than 40 calories per 8 oz. serving of beverage. As used herein, “zero-calorie” or “diet” means having less than 5 calories per serving, e.g., per 8 oz. for beverages.
According to another aspect, beverage products are provided that comprises water, and acidulant component comprising at least one acid, a flavoring component comprising at least one flavoring ingredient, and a sweetener component comprising a sweetening amount of modified steviol glucoside, and optionally a sweetening amount of one or more other sweeteners. In certain exemplary embodiments of the beverage products according to this aspect, the beverage products are ready-to-drink beverages having a pH higher than 3.0 and lower than 4.0. Such ready-to-drink beverages may, for example, be hydration beverages, also referred to as sports drinks, having added electrolytes. In other exemplary embodiments the ready-to-drink beverages are carbonated soft drinks, for example reduced calorie or diet cola beverages. In certain exemplary embodiments of the beverage products according to this aspect, the beverage products are syrups suitable to be diluted, for example, by a 1-plus-5 throw with carbonated or un-carbonated water to produce a ready-to-drink beverage.
According to certain embodiments, the modified steviol glycoside of the invention provides at least 10% of the total sweetening of the consumable, e.g. a diet cola syrup, a ready-to-drink diet cola beverage, another beverage product, or another food product in accordance with the present disclosure. According to certain embodiments, it provides at least 20% of the total sweetening, or at least 30% of the total sweetening, or at least 40% of the total sweetening, or at least half of the total sweetening, or at least 60% of the total sweetening, or at least 70% of the total sweetening, or at least 80% of the total sweetening, or at least 90% of the total sweetening. Optionally every additional sweetener ingredient is an organic sweetener. Optionally every sweetener ingredient is a natural sweetener.
Optionally every sweetener ingredient is a steviol glycoside. Optionally every ingredient is an organic and/or natural ingredient, such that the reduced calorie (e.g., diet) carbonated cola beverage product is correspondingly an organic and/or natural beverage product.
Preferably, the consumable comprises at least one modified Rebaudioside A selected from those represented in FIG. 5.
For example, a beverage may comprise modified Rebaudioside A in concentrations of about 30 ppm-about 750 ppm (e.g. from about 50 ppm up to 350 ppm). However the amount added mainly depends on the level of sweetness desired and may depend on the presence of other ingredients. For example, fruit juice comprises sugar and thus contributes to the level of sweetness. In one embodiment, the modified Rebaudioside A is the only sweetener added to the flavored beverage. In another embodiment, modified Rebaudioside A may be combined with other sweetener and/or sweetness enhancers. In a preferred embodiment, the modified Rebaudioside A is combined with a mogroside, like mogroside V.

LEGENDS TO THE FIGURES

FIG. 1. Chemical structures of Stevioside, Rebaudioside A and B, and Steviolbioside. [Glc1], etc. denotation of glucose residues for NMR assignment.

FIG. 2A-C. Effect of Rebaudioside A and sucrose concentrations on the transglucosylation (solid bars) and hydrolysis (hatched bars) activity using 0.12 mg/mL purified enzyme: (A) GTF180-ΔN, (B) GTF180-ΔN mutant 51137Y and (C) GTF180-ΔN mutant Q1140E.

FIG. 3. NP-HPLC product profiles of a 4 hour incubation of 10 U/mL GTF180-ΔN(WT), mutant S1137Y and mutant Q1140E with 50 mM Rebaudioside A(*) and 1 M sucrose (**=RebAG1).

FIG. 4A-C. Time course of Rebaudioside A utilization (dashed line) and RebAG1 formation by α-glucosylation (solid line) using 10 U/mL enzyme: (A) GTF180-ΔN, (B) GTF180-ΔN mutant S1137Y and (C) GTF180-ΔN mutant Q1140E, showing 55%, 73% and 96% conversion, respectively.

FIG. 5A-D. Structures of modified Rebaudioside A glucosides produced by GTF180-ΔN and mutants GTF180-ΔN S1137Y and GTF180-ΔN Q1140E: (A) RebAG1=13-({β-D-glucopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→3)-]β-D-glucopyranosyl}oxy)ent-kaur-16-en-19-oic acid α-D-glucopyranosyl-(1→6)-β-D-glucopyranosyl ester (B) 13-({β-D-glucopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→3)-]β-D-glucopyranosyl}oxy)ent-kaur-16-en-19-oic acid α-D-glucopyranosyl-(1-6)-α-D-glucopyranosyl-(1→6)-β-D-glucopyranosyl ester (C) 13-({f-D-glucopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→3)-]β-D-glucopyranosyl}oxy)ent-kaur-16-en-19-oic acid α-D-glucopyranosyl-(1→3)-α-D-glucopyranosyl-(1→6)-β-D-glucopyranosyl ester (D) The 500-MHz ¹H NMR spectra of RebAG1 product of GTF180-ΔN (a) and mutants Q1140E (b) and S1137Y (c) recorded in D₂O at 334K.

FIG. 6. TLC analysis of products obtained in a 1 hour incubation of 50 mM Stevioside and 50 mM Rebaudioside A with 100 mM sucrose with and without 20 U/ml Q1140E, before and after alkaline saponification with 1 M NaOH.

FIG. 7. Sensory evaluation (n=12) of several sweeteners: Stevioside (250 mg/L), Rebaudioside A (300 mg/L), RebAG1 (350 mg/mL), and sucrose (60 g/L). Evaluated taste attributes were sweetness (hatched bars) and bitterness (solid bars). Score 0=not sweet/not bitter, score 5=very sweet/very bitter.

FIG. 8. TLC analysis of products obtained in a 2 hour incubation of 50 mM Rebaudioside A and 200 mM sucrose with GTF180-ΔNΔV (Q), GTF180-ΔNΔV Q1140 amino acid substitution mutants Q1140 G/S/I/V/W/D/M/C/E/L/K/T/P/R/F/N/Y, or GTF180-ΔN Q1140E (E*).

FIG. 9A-C. Amino acid sequence of glucansucrase GTF180 from Lactobacillus reuteri 180. Panel (A) full length protein; panel (B)N-terminally truncated mutant GTF180-ΔN; panel (C)N-terminally truncated and domain V truncated mutant GTF180-ΔNΔV.

EXPERIMENTAL SECTION

The section below exemplifies the advantageous use of glucansucrase GTF180-ΔN of Lactobacillus reuteri strain 180 and its derived single amino acid substitution mutants to α-glucosylate Rebaudioside A. GTF180-ΔN and derived mutant enzymes glucosylate Rebaudioside A specifically at the C-19 site, introducing (α1→6) and (α1→3) glycosidic linkages, which are resistant to hydrolysis by the amylolytic enzymes present in saliva. Several GTF180-ΔN mutants displayed a much higher transglucosylating activity towards Rebaudioside A than GTF180-ΔN. Interestingly, one mutant, Q1140E, showed nearly 100% conversion of Rebaudioside A and attached mostly a single (α1→6)-glucose at the C-19 site of Rebaudioside A.
The produced novel Rebaudioside A glucosides are very interesting, carrying one and two (α1→6)-linked glucose units specifically at the C-19 β-linked glucose. The mono-α-glucosylated Rebaudioside A product, RebAG1, has an increased and more natural sweetness and reduced bitterness compared to Rebaudioside A. These improved novel steviol glycosides of the invention are of great interest as functional food ingredients.

Materials and Methods

Steviol Glycoside Substrates

Rebaudioside A (2) and Stevioside (1) were purchased from Sigma Aldrich.

Glucansucrase Enzymes

All glucansucrase and fructansucrase enzymes were produced as described by Meng et al (2014) and purified as described by Kralj et al (2004b). GTF180-ΔN is the 117 kDa N-terminally truncated (741 residues) fragment of the GTF180 full-length protein (Kralj et al. 2004a). The construction of truncation mutant GTF180-ΔNΔV, consisting of amino acids 794-1636 of the GTF180 enzyme is described in Meng et al. (2015a). GTF180-ΔN mutant enzymes were created by van Leeuwen et al. (2009), Meng et al. (2015a), and Meng et al. (2015b). Amino acid substitutions in truncation mutant GTF180-ΔNΔV were created as described by Meng et al. (2015b)

Enzyme Activity Assays

Enzyme activity assays were performed at 100 mM and 1000 mM sucrose, with and without 50 mM Rebaudioside A in 25 mM sodium acetate (pH 4.7); 1 mM CaCl₂); and 0.12 mg/mL purified GTF180-ΔN enzyme or GTF180-ΔN mutant enzyme at 37° C. Samples of 100 μL were taken every 30 sec for 4 min and the reaction was immediately stopped by incubating with 20 μL 1000 mM NaOH for 30 min. The inactivated samples were diluted two times in deionized water and from 10 μL of the diluted sample the glucose and fructose concentrations were determined enzymatically by monitoring the reduction of NADP with the hexokinase and glucose-6-phosphate dehydrogenase/phosphoglucose isomerase assay (Roche) as described previously (Mayer 1987). Quantitative determination of the release of glucose and fructose from sucrose allowed estimation of the activities of the glucansucrase enzymes (van Geel-Schutten et al. 1999). Fructose release corresponds to the total enzyme activity and glucose release to the hydrolytic activity. The transglycosylation activity can be obtained by subtracting the hydrolytic activity from the total activity. One unit (U) of enzyme is defined as the amount of enzyme required for producing 1 μmol monosaccharide per min in a reaction mixture containing 25 mM sodium acetate (pH 4.7); 1 mM CaCl₂; and 1000 mM sucrose at 37° C.

Enzymatic Glycosylation of Steviol Glycosides

Incubation reactions were performed in 25 mM sodium acetate (pH 4.7), 1 mM CaCl₂), 50 to 1,000 mM sucrose, 50-100 mM steviol glycoside, and 2-30 U/mL purified GTF180-ΔN enzyme or GTF180-ΔN mutant enzyme at 37° C. for 15 min to 24 hours. Reactions were stopped by heat inactivation (100° C. for 15 min). From the inactivated samples 250 uL was mixed with 1000 ul of 10 mM catechol (internal standard) and subsequently purified by solid phase extraction using Strata-X 33u Polymeric Reversed Phase columns (Phenomenex). For HPLC analysis 20 μL of the purified sample was injected on a Luna 10 μm NH2 chromatography column (250 mm×4.6 mm; Phenomenex). Reaction components were separated at a flow-rate of 1 mL/min under gradient elution conditions, starting with a 2 min isocratic step of 70% solvent A followed by a linear gradient from 70 to 55% solvent A over 9 min (solvent A=acetonitrile; solvent B=0.025% acetic acid). Rebaudioside A and the mono-α-glucosylated Rebaudioside A product concentrations were determined with NP-HPLC, using their corresponding calibration curves ranging from 1.56 to 50 mM. All data were normalized with catechol as internal standard. The standard deviation of the response was less than 5%. All NP-HPLC analyses were performed on an UltiMate 3000 chromatography system (ThermoFischer Scientific, Amsterdam, The Netherlands), equipped with an Endurance autosampler (Spark Holland, The Netherlands).

Quantitative Synthesis of α-Glucosylated Rebaudioside A Products

For quantitative synthesis of α-glucosylated Rebaudioside A products using GTF180-ΔN and its derived mutants, incubations were performed in 5 mL 25 mM sodium acetate (pH 4.7), 1 mM CaCl₂), 50 mM steviol glycoside with two batches of 1,000 mM equivalent of sucrose donor (t=0 and 3 h) to a total of 2,000 mM sucrose, using 10 U/mL enzyme at 37° C. for 24 hours. Products were purified from the incubation mixture by solid phase extraction using Strata-X 33u Polymeric Reversed Phase columns (Phenomenex). Products were separated on a Luna 10 μm NH2 semi-preparative chromatography column (250 mm×10 mm, Phenomenex) and were manually collected at a flow-rate of 4.6 mL/min, starting with a 2 min isocratic step of 80% solvent A followed by a linear gradient of 80 to 50% solvent A over 38 min (solvent A=acetonitrile; solvent B=0.025% acetic acid). The solvent of the collected fractions was evaporated under a stream of nitrogen gas and the dried materials were dissolved in deionized water.

Thin-Layer Chromatography

Samples were spotted on TLC sheets (Merck Kieselgel 60 F254, 20×20 cm), which were developed with n-butanol:acetic acid:water=2:1:1. Spots were visualized by orcinol/sulfuric acid staining and compared with a simultaneous run of standard compounds.

Alkaline Saponification

To release the 19-O-linked glycosyl moiety, 4 mg of each steviol glycoside product was dissolved in 1 M NaOH (1 mL) and heated at 80° C. for 2.5 h.

Mass Spectrometry

Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) experiments were performed using an Axima™ mass spectrometer (Shimadzu Kratos Inc., Manchester, UK) equipped with a Nitrogen laser (337 nm, 3 ns pulse width). Positive-ion mode spectra were recorded using the reflector mode at a resolution of 5000 FWHM and delayed extraction (450 ns). Accelerating voltage was 19 kV with a grid voltage of 75.2%. The mirror voltage ratio was 1.12 and the acquisition mass range was 200-6000 Da. Samples were prepared by mixing 1 μL sample solutions with 1 μL 10% 2,5-dihydroxybenzoic acid in 70% ACN as matrix solution.

NMR Spectroscopy

Resolution-enhanced 1D/2D 500-MHz ¹H NMR spectra were recorded in D₂O on a Bruker DRX-500 spectrometer (Bijvoet Center, Department of NMR Spectroscopy, Utrecht University) at a probe temperature of 334K. Data acquisition and processing were done with Bruker Topspin 2.1. Prior to analysis, samples were exchanged twice in D₂O (99.9 atom % D, Cambridge Isotope Laboratories, Inc., Andover, Mass.) with intermediate lyophilization, and then dissolved in 0.6 mL D₂O. Suppression of the deuterated water signal (HOD at 4.40 ppm) was achieved by applying a WEFT (water eliminated Fourier transform) pulse sequence for 1D NMR experiments and by a pre-saturation of 1 s during the relaxation delay in 2D experiments. The 2D TOCSY spectra were recorded using an MLEV-17 (composite pulse devised by Levitt et al (1982)) mixing sequence with spin-lock times of 40-200 ms. The 2D ROESY spectra were recorded using standard Bruker XWINNMR software with mixing time of 200 ms. The carrier frequency was set at the downfield edge of the spectrum in order to minimize TOCSY transfer during spin-locking. Natural abundance 2D ¹³C-¹H HSQC experiments (¹H frequency 500.0821 MHz, ¹³C frequency 125.7552 MHz) were recorded without decoupling during acquisition of the ¹H FID. Resolution enhancement of the spectra was performed by a Lorentzian-to-Gaussian transformation for 1D spectra or by multiplication with a squared-bell function phase shifted by π/(2.3) for 2D spectra, and when necessary, a fifth order polynomial baseline correction was performed. Chemical shifts (δ) are expressed in ppm by reference to internal acetone (δ 2.225 for ¹H and δ 31.07 for ¹³C)

Sensory Evaluation of Novel α-Glucosylated Products of Rebaudioside A

Taste evaluations were performed in which novel α-glucosylated products of Rebaudioside A (350 mg/L) were compared to sucrose (60 g/L), Rebaudioside A (300 mg/L), and stevioside (250 mg/L). In a blind test, twelve test persons that were able to perceive the bitter aftertaste of steviol glycosides were asked to rate sweetness and bitterness on a scale from 0 to 5, with 0 indicating not sweet/not bitter and 5 indicating very sweet/very bitter.

Results

Screening Glucan and Fructansucrase Enzymes for α-Glucosylation of Rebaudioside A

Over a hundred glucan and fructansucrase wild type and mutant enzymes, mostly from Lactobacillus reuteri, were screened for Rebaudioside A α-glucosylation. For this, enzymes were incubated in 50 mM Rebaudioside A (FIG. 1) and 1000 mM sucrose for 3 hours. HPLC and TLC analysis of the reaction mixtures showed that only GTF180-ΔN enzyme and mutant enzymes of GTF180-ΔN were able to glucosylate Rebaudioside A (Table 1). Interestingly, two single GTF180-ΔN mutants S1137Y and Q1140E, which are single amino acid substitutions close to the transition state stabilizing residue D1136 (van Leeuwen et al. 2009) displayed much better transglucosylating activity towards Rebaudioside A than GTF180-ΔN. Also GTF180-ΔN mutants L981A and W1065L (Meng et al. 2015b) were able to a-glucosylate Rebaudioside A (Table 1), but showed almost no polymerization (i.e. oligosaccharide and glucan formation from sucrose) activity (data not shown). This is a clear advantage during downstream processing, the purification of the α-glucosylated Rebaudioside A products from mono- and disaccharides, oligosaccharides and glucans. By eliminating α-glucan synthesis, the most important side reaction, higher glycosylation yields were obtained for Rebaudioside A. At lower sucrose concentration (200 mM), these mutants had similar or even higher transglucosylating activity with Rebaudioside A than GTF180-ΔN and mutants S1137Y and Q1140E (data not shown).

TABLE 1

Overview of the Rebaudioside A α-glucosylation potential of glucansucrase and fructansucrase enzymes from various Lactobacillus strains.

							Activity
Enzyme	Mutation	*Glp (1→	→3)Glp (→	→4)Glp (→	→6)Glp (→	→3,6)Glp (→	on RebA

GTF180-ΔN^a	N-terminal truncated	12	24		52	12	+
	GTF180^a(**AY697430);
	Met-Gly-742-1772-His₆
GTF180-ΔNΔV^b	domain V deletion mutant of	12	23		52	13	+
	GTF180-ΔN; Met-794-1636-His₆
GTF180-ΔN-PNNS^c	triple amino acid mutant	18	10	12	42	18	+
	(V1027P: S1137N: A1139S)
	of GTF180-ΔN
GTF180-ΔN-SNAE^d	single amino acid mutant	12	16	2	52	18	++++++
	(Q1140E) of GTF180-ΔN
GTF180-ΔN-SNAA^d	single amino acid mutant	11	6		69	14	+/−
	(Q1140A) of GTF180-ΔN
GTF180-ΔN-SNAH^d	single amino acid mutant	8	8		76	8	+/−
	(Q1140H) of GTF180-ΔN
GTF180-ΔN-NNA^d	single amino acid mutant	12	26	3	47	12	++
	(S1137N) of GTF180-ΔN
GTF180-ΔN-YDA^d	double amino acid mutant	19	23	7	31	20	++++
	(S1137Y: N1138D)
	of GTF180-ΔN
GTF180-ΔN-YNA^d	single amino acid mutant	18	21	4	39	18	++++
	(S1137Y) of GTF180-ΔN
GTF180-ΔN-SDA^d	single amino acid mutant	10	24		56	10	+
	(N1138D) of GTF180-ΔN
GTF180-ΔN-XM1^e	single amino acid mutant						++
	(L981A) of GTF180-ΔN
GTF180-ΔN-XM2^e	single amino acid mutant						++
	(W1065L) of GTF180-ΔN
GTFA-ΔN^f	N-terminal truncated	9		46	34	12***	−****
	GTFA (AX306822)^g
GTFA-ΔN N1134S^h	single amino acid mutant	8		12	76	4***	−
	(N1134S) of GTFA-ΔN
GTFA-ΔN N1134E^h	single amino acid mutant	8		49	36	7***	−
	(N1134E) of GTFA-ΔN
GTFA-ΔN N1134A^h	single amino acid mutant	13		25	49	13***	−
	(N1134A) of GTFA-ΔN
GTFA-ΔN NEV^h	double amino acid mutant	10		49	29	12***	−
	(N1135E: S1136V)
	of GTFA-ΔN
GTFB^a	wild type (AY697435)						−
GTFMLI^a	N-terminal truncated		47		10	26***	−
	GTFMLI (AY697431)^a
GTFOⁱ	N-terminal truncated			67	13	15	−
	GTFO (AY911856)ⁱ
InuJ^j	N-terminal truncated						−
	InuJ^jfructansucrase
InuGA-RM^k	wild type fructansucrase						−
InuGB-R^k	wild type fructansucrase						−
LevG-R^k	wild type fructansucrase						−

^aKralj et al (2004a);
^bMeng et al (2015a);
^cVan Leeuwen et al (2008);
^dVan Leeuwen et al (2009);
^eMeng et al. (2015b);
^fKralj et al (2004b);
^gKralj et al (2002);
^hKralj et al (2006);
ⁱKralj et al (2005);
^jAnwar et al (2008);
^kAnwar et al (2010);
*linkage distribution;
**Genbank accession number;
***→4,6)Glp (→;
****not active on Rebaudioside A (RebA)

To optimize the reaction conditions towards glucosylation of Rebaudioside A the effect of Rebaudioside A and sucrose on the transglucosylation activity of GTF180-ΔN and mutants S1137Y and Q1140E was determined. For this, enzyme activity assays were performed at 100 mM and 1000 mM sucrose with and without 50 mM Rebaudioside A. All three enzymes were more hydrolytic at low sucrose concentrations (FIG. 2). Both mutant enzymes were almost completely hydrolytic at 100 mM sucrose. However, when 50 mM Rebaudioside A was added to the reaction or when the sucrose concentration was increased to 1000 mM, the transglucosylation activity of all three enzymes was noticeably increased, showing the highest overall activity and highest transglucosylation to hydrolysis ratio at 1 M sucrose and 50 mM Rebaudioside A. These reaction conditions were used to follow the α-glucosylation of Rebaudioside A by GTF180-ΔN and mutants S1137Y and Q1140E in more detail.
When 50 mM Rebaudioside A, 1000 mM sucrose, and 10 U/mL enzyme was used, mutants S1137Y and Q1140E glucosylated respectively ˜73% and ˜96% Rebaudioside A compared to ˜55% by GTF180-ΔN (FIG. 4). To our surprise, mutant Q1140E mainly produced mono-glucosylated Rebaudioside A (RebAG1), yielding ˜35 mM RebAG1 from 50 mM Rebaudioside A (FIG. 3 and FIG. 4). Mutant S1137Y produced a similar amount of RebAG1 as the GTF180-ΔN enzyme (˜13 mM) (FIG. 3), but synthesized a higher amount of multiple glycosides (FIG. 3).
From all the tested glucan- and fructansucrase enzymes, mutant GTF180-ΔN Q1140E showed the highest Rebaudioside A glucosylation activity and displayed mainly mono-glucosylation of Rebaudioside A. Therefore, besides mutations Q1140 E/A/H also additional amino acid substitutions at position Q1140 were created in mutant GTF180-ΔNΔV and tested for Rebaudioside A glucosylation (FIG. 8). For this, 1 mg/ml of the enzymes was incubated for 2 hours at 37° C. with 50 mM Rebaudioside A and 200 mM sucrose in buffer (25 mM sodium acetate (pH 4.7), 1 mM CaCl₂). The incubation mixtures were analyzed by TLC (FIG. 8). Under these conditions, several Q1140 substitution mutants (for e.g. Q1140 F/N/Y) showed even a higher Rebaudioside A glucosylation than mutant Q1140E, although the latter one still had the highest mono-glucosylation yield. Some mutations (for e.g. Q1140D) had a slightly negative effect on Rebaudioside A glucosylation. Interestingly, mutant Q1140R showed almost no activity on sucrose, although Rebaudioside A glucosylation was hardly affected by the mutation. It appears that the sucrose was mainly used for glucosylation of Rebaudioside A and not for side reactions, such as oligosaccharide formation.

Isolation and Characterization of α-Glucosylated Products of Rebaudioside A Glucosides Produced by GTF180-ΔN and GTF180-ΔN Mutants S1137Y and Q1140E

Looking at the molecular structure of Rebaudioside A (FIG. 1), there are four Glcp residues (Glc1, Glc2, Glc3 and Glc4) with a total of 14 free hydroxyl groups, which can act as acceptors for transglucosylation.
GTF180-ΔN converts sucrose into oligo- and polysaccharides, catalyzing the transglucosylation of Glcp residues in (α1→3)- and (α1→6)-linkages (van Leeuwen et al. 2008), there are 3 potential (1→3) sites and 4 potential (1→6) sites present at Rebaudioside A for the first attachment of a Glc residue.
In order to isolate α-glucosylated products of Rebaudioside A glucosides for structural characterization, incubations were done with 10 U/mL enzyme with 50 mM Rebaudioside A and 1000 mM sucrose. After 3 hours 1000 mM extra sucrose was added to the reaction mixtures and incubated for an additional 21 hours. Glucosides were isolated from the reaction mixtures using semi-preparative NP-HPLC. Interestingly, NMR structural analysis and methylation analysis of the mono-α-glucosylated product showed that GTF180-ΔN and mutants S1137Y and Q1140E specifically and only glucosylated Rebaudioside A at the C-19-linked glucose, attaching an (α1→6)-linked glucose (100%), yielding RebAG1 (see also FIG. 5A).
The di-glucosylated Rebaudioside A products of GTF180-ΔN and mutant S1137Y were both linear elongations of the structure of RebAG1 with an (α1→3)-linked glucose (˜75%) (FIG. 5C) or another (α1→6)-linked glucose (˜25%) (FIG. 5B). FIG. 5D panels (a), (b) and (c) show the 500-MHz 1H NMR spectra of RebAG1 produced by, respectively, GTF180-ΔN, GTF180-ΔN Q1140E and GTF180-ΔN S1137Y.
To confirm that introduction of extra Glcp residues via transglucosylation by GTF180-ΔN only occurred on the C-19 β-glucosyl moiety of Rebaudioside A, the isolated fractions were subjected to an alkaline saponification to specifically hydrolyze the 19-carboxyl-glucosyl ester bound (FIG. 1 and FIG. 6). Identical products were obtained from Rebaudioside A and the α-glucosylated products of Rebaudioside A, according to TLC, MALDI-TOF-MS (m/z 826 [M+H]+) and NMR spectroscopy, being Rebaudioside A missing the complete glucosyl moiety at C-19. The resultant structure is known as Rebaudioside B (3) (FIG. 1).

C-19-Site Specific α-Glucosylation of Stevioside by GTF180-ΔN Mutant Q1140E

For commercial purposes it may be desirable to improve sweetness and decrease bitterness of the whole steviol glycoside leave extract. Since Stevioside (˜5-10% w/w of dried leaves) is the most abundant and one of the most bitter tasting steviol glycosides, our aim was also to enhance the taste profile of Stevioside. Therefore, glucosylation reactions were also performed with GTF180-ΔN and substitution mutants derived thereof with Stevioside as the acceptor molecule. All three enzymes were also able to α-glucosylate stevioside. As observed with Rebaudioside A as acceptor molecule, mutant Q1140E converted Stevioside mainly into a single mono-α-glucosylated product (data not shown). In order to determine whether GTF180-ΔN mutant Q1140E glucosylates Stevioside also specifically at the C-19 site, steviol glucosides produced by Q1140E were subjected to alkaline saponification of the 19-carboxyl-glucosyl ester linkage with 1 M NaOH, to specifically remove the C-19 moiety. If alkaline saponification of the Q1140E steviol glucosides yields Steviolbioside (4)(FIG. 1) (i.e. Stevioside minus the C-19 moiety), then glucosylation occurred specifically at the C-19 site of Stevioside. On TLC plates multiple steviol glucosides were visible in the 1 hour incubation of Stevioside with Q1140E (FIG. 6). However, only one spot was observed when the incubation mixture was treated with 1 M NaOH and it migrated at the same height as the product formed after saponification of the Stevioside positive control. With MALDI-TOF analysis a molecular mass of 665.59 was detected, corresponding with the sodium-adduct of Steviolbioside. These results show that GTF180-ΔN Q1140E also specifically α-glucosylates Stevioside at the C-19 β-linked glucosyl moiety.

Taste Evaluation of the Newly Synthesized α-Glucosylated Products of Rebaudioside A

To determine the effect of (α1→6) glucosylation at the 19-O-glucosyl moiety of Rebaudioside A on sweetness and bitterness, a taste evaluation was performed in which one the novel α-glucosylated products of Rebaudioside A, RebAG1, was compared to Rebaudioside A. For this, in a blind test, twelve test persons that were able to perceive the bitter aftertaste of steviol glycosides were asked to rate sweetness and bitterness on a scale from 0 to 5, with 0 indicating not sweet/not bitter and 5 indicating very sweet/very bitter. A clear trend was observed showing that the novel RebAG1 had an increased and a more natural sweetness and reduced bitterness compared to Rebaudioside A (FIG. 7).

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Claims

1-12. (canceled)

13. A modified steviol glycoside obtainable by incubating a steviol glycoside substrate in the presence of sucrose as a glucose donor and the glucansucrase GTF180 of Lactobacillus reuteri strain 180, or a mutant thereof having the desired transglycosylation activity.

14. A modified steviol glycoside selected from the group consisting of

(i) 13-({β-D-glucopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→3)-]β-D-glucopyranosyl}oxy)ent-kaur-16-en-19-oic acid α-D-glucopyranosyl-(1→6)-β-D-glucopyranosyl ester;

(ii) 13-({β-D-glucopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→3)-]β-D-glucopyranosyl}oxy)ent-kaur-16-en-19-oic acid α-D-glucopyranosyl-(1→6)-α-D-glucopyranosyl-(1→6)-β-D-glucopyranosyl ester; and

(iii) 13-({β-D-glucopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→3)-]β-D-glucopyranosyl}oxy)ent-kaur-16-en-19-oic acid α-D-glucopyranosyl-(1→3)-α-D-glucopyranosyl-(1→6)-β-D-glucopyranosyl ester.

15. (canceled)

16. A sweetening composition comprising at least one modified steviol glycoside according to claim 13.

17. A consumable comprising at least one modified steviol glycoside according to claim 13, optionally combined with other sweetener and/or sweetness enhancer.

18. The consumable according to claim 17, selected from the group consisting of beverages, foodstuff, an oral care product, a tobacco product, a pharmaceutical products and nutraceutical products.

19.-22. (canceled)

23. A sweetening composition comprising at least one modified steviol glycoside according to claim 14.

24. A consumable comprising at least one modified steviol glycoside according to claim 14, optionally combined with other sweetener and/or sweetness enhancer.