Patent Application
20160265013
Glycosyltransferases catalyze the transfer of a glycosyl group from a suitable carbohydrate donor to a suitable carbohydrate acceptor. Recently, a non-Leloir β-1,3(β-1,6)-glucosyltransferase activity has been described for some enzymes in the glycosyl hydrolase family GH17 originating from fungi and bacteria (Gastebois et al., 2010; Hreggvidsson et al., 2011). The catalytic activity of these enzymes can be described as a two-step retaining mechanism. The first step involving the cleavage of a (β1→3) linkage of a β-glucan oligosaccharide or polysaccharide, such as from laminarin, lichenan, curdlan or related polysaccharides or oligosaccharides, and the second step is the formation of a (β1→6) linkage to another laminarin or related polysaccharide or oligosaccharide chain.
β-Glucans, made up of (β1→3)- or (β1→4)-linked glucopyranosyl residues, are important cell wall components of bacteria, fungi, seaweeds, and plants. They can be further classified on the basis of presence/absence of other linkages into various distinct types with different physicochemical properties. Thus, laminarin that can be extracted from seaweed has predominantly (β1→3) linkages with (β1→6)-linked side branching on every 10th glucose subunit on average. On the other hand, β-glucans from barley, oat or wheat have mixed (β1→3) and (β1→4) linkages in the backbone, but no (β1→6) branches, and generally higher molecular weights and viscosities (McIntosh et al., 2005).
A linear glucan chain has two different ends, called reducing (R-end) and non-reducing (NR-end). The glucosyl transfer reaction requires two main steps. 1) The enzyme must bind to a donor substrate molecule and cleave the glucan chain with the R-end part released and the NR-end part covalently attached to the enzyme. 2) The enzyme then must bind an acceptor glucan molecule and transfer the bound glucan chain onto the acceptor. Therefore the donor becomes smaller and the acceptor larger. In glycosyl hydrolases the acceptor is water and no addition to the acceptor occurs. Most enzymes transfer short NR-ends of distinct lengths, i.e. 1, 2, 3 or 4 glucose units. Such enzymes are therefore defined as cutting from the NR-end. Until now, all known bacterial β-glucan transferases are of this type (Hreggvidsson et al., 2011).
Glucosyltransferases acting on β-glucans are rare but have been described from certain yeasts and fungi (Gastebois et al., 2010). These enzymes release laminaribiose from the reducing end of a linear (β1→3)-linked oligosaccharide and transfer the remaining chain to another oligosaccharide acting as acceptor. The transfer occurs at C-6 of the non-reducing end group of the acceptor, creating a kinked (β1→3;β1→6) linear molecule or the transfer takes place at the C-6 of an internal group of the acceptor, forming a (β1→3)-linked branched product with a (β1→6) linkage. The enzyme from Aspergillus fumigatus was shown to make up to 85% of a branched product when incubated for less than 1 h with substrates of larger than five glucose units.
In bacteria, proteins belonging to the GH17 family were proposed, based on mutagenesis studies on the ndvB gene, to be involved in biosynthesis of cyclic (β1→3;β1→6)-glucans from Bradyrhizobium japonicum (Bhagwat et al., 1995), but no enzyme activity has been characterized before A number of bacterial putative transglycosylases belonging to glycosyl hydrolase family GH17 domains were identified, each encoded by a gene also encoding a GT2 glycosyl transferase domain in Gamma-Proteobacteria. The GH17 domains were subsequently cloned separately (from GT2) in E. coli. These recombinant enzymes named Glt1, Glt3 and Glt7, were shown to be non-Leloir β-glucan transferases. They cleave short (β1→3)-linked gluco-oligosaccharides substrates from the non-reducing end. The Glt1 and Glt3 enzymes exhibited mainly (β1→3) elongation activity, but in the case of the Glt7 enzyme also (β1→6) transfer was seen, resulting in a mixture of branched and kinked products (Hreggvidsson et al.). For Glt7 the yield of the (β1→6) transfer products was however small, since the total transfer of both (β1→3) (the major reaction) and (β1→6) was approximately 10% on free laminari-oligosaccharides but much less on alditols with a maximum incubation time of 24 h. Glt7 also had high hydrolysis activity which together with the poor transfer activity makes it inefficient glucantransferase and not suitable for any industrial use.
A further GH17 bacterial domain from the ndvB gene (encoding both a GH17 domain and GT2 domain) was identified in Alpha-Proteobacterium, Bradyrhizobium japonicum USDA110. It was cloned and expressed by the same authors (Jonsson 2010) and the enzyme product, named Glt20, shown to resemble the previously characterized A. fumigatus enzyme in the sense that it specifically cleaves laminaribiose from the reducing end of a linear (β1→3)-glucan and transfers the remainder to an acceptor oligosaccharide with a (β1→6) linkage. A new name B. diazoefficiens has recently been proposed for B. japonicum (Delamuta et al. 2013). Both names are therefore used here as meaning the same species and strains where appropriate. Based on the work of Jonsson (2010) the authors concluded previously that the Glt20 enzyme created a kinked (β1→3;β1→6) linear molecule but not branches.
The present invention provides new methods, processes and products based on surprising features and newly identified and characterised branching activity of the Glt20 enzyme from Bradyrhizobium japonicum, in particular the GH17 domain with MalE attachment removed in the case of expression of the gene with this attachment, and taxonomically related enzymes as defined herein. The inventors have been able to ascertain that in this form the Glt20 has a (β1→6) transferase activity, that cuts by releasing two interlinked glucose units (laminaribiose) from the R-end of (β1→3)-linked gluco-oligosaccharides and therefore transfers a long NR-end to the acceptor, similar as has earlier been described for enzymes from certain fungi and yeasts (Gastebois et al., 2010). However, there are distinct differences between the Glt20 bacterial enzyme and the methods of the present invention versus the fungal enzymes and their described activity. The fungal enzyme makes (β1→6)-linked branches as the major product but also (β1→6)-linked kinks, and it has significant hydrolysis activity as well (Gastebois et al. 2010). The Glt20 bacterial enzyme makes only branches, both single and double (and even triple), and no kinks, even after 48 h incubation when substrate donor oligosaccharide is present. Only after extended incubation of more than 48 h, when the substrate is depleted, are linear ((31>6 kinks) products seen. Such linear (kinked) products are therefore only made from hydrolysis of the non-reducing end of the branched products, but not from direct transfer to the terminal non-reducing end of the acceptor. It has therefore been concluded by the inventors that the Glt20 enzyme is a glucan transferase that specifically transfers the donor oligosaccharide to C-6 of an internal group of the acceptor and makes no transfer to a C-6 on the non-reducing end of the acceptor. This activity is found to be highly efficient. The amount of the formed oligosaccharides (DP>5) from Lam-Glc5 after 48 h, including Pro-Glc8-11-14, was estimated to be about 60% of the total initial substrate amount. When the original substrate (donor) molecule is used up, the Glt20 enzyme shows a limited hydrolysis activity on the branched products and conversion into linear (kinked) products is seen upon extended incubation. The linear kinked products are formed because of the inability of the enzyme to cleave internal 1,6 branch points made by prior transfer events. When the donor substrate is depleted and the reaction goes to completion a substantial amount of 1,6 linkages are found in the products. Using Lam-Glc5 as a substrate donor, the 1,6 linkages increased to 26% in the final products from 0% in the donor, Lam-Glc5. The Glt20 bacterial enzyme according to the present study had no activity on oligosaccharide-alditols, unlike the A. fumigatus fungal enzyme. The Glt20 bacterial enzyme starts branching on residue 3 (counting from the reducing end), but currently it is not known at what residue the branches are introduced by the fungal enzyme.
As the Glt20 enzyme is able to cleave the substrate from the R-end, it should be able to produce cyclic molecules by transferring the new donor R-end to the acceptor NR-end of the same molecule, if the original substrate β-glucan chain is long enough. This would be in agreement with the earlier proposed role of the ndvB gene (Glt20) from mutagenesis studies in Bradyrhizobium japonicum (Bhagwat et al., 1995). However, such cyclization has not been observed in the present work. The reported beta-glucan cycle by Bhagwat et al., (1999) has only 1,3 linkages, which is incompatible with the activity of ndvB GH17 enzymes as reported herein. It would be expected to close such cycles with a 1,6 linkage.
Many β-glucans are bio-active compounds, for example containing immuno-stimulating activity such as, anti-tumoral, anti-infectious, protection against fungi, bacteria and viruses infections and various health benefits to humans. The physiological activity of β-glucans is however affected by the type and degree of branching (Badulescu et al., 2009). Therefore, suitable glucosyltransferase enzyme activities can be used to add carbohydrate side chains onto glucans, and therefore make highly branched molecules that may be useful for many industrial or commercial applications. Enzymes having specific glucosyltransferase activity of the invention may thus have advantageous and useful applications such as for making new bio-active molecules, including bio-active molecules for cosmetic and medical applications, including molecules used for drug design such as molecules used as constituents of drugs.
The present invention therefore shows that when the Glt20 enzyme is produced, without any expressed attachments such as GT2 (native gene organization) or purification tags such as MalE (for purification) it has unexpected and highly useful properties. The specific internal branching activity with no terminal transfer and almost no hydrolysis activity of the recombinant Glt20 as produced and utilized herein, was unexpected. The enzyme is surprisingly an efficient enzyme under in vitro conditions and is expected to be useful and consequently to have large potential for industrial application and commercial use.
The present invention relates to the use of β-glucan branching enzymes in transglycosylation reactions, and more specifically methods for transglycosylating substrate beta-glucan oligo- and polysaccharides to produce certain useful and valuable products.
As such, the invention is based on the use of an isolated glucosyltransferase of a bacterial origin that has glucosyltransferase activity on (β1→3)-glucan oligo- and polysaccharides such that it releases two interlinked glucose units from the reducing end of such a β-glucan oligo/polysaccharide (acting as donor molecule) and transfers the remaining non-reducing end of said β-glucan oligo/polysaccharide to internal glucose units of an additional β-glucan oligo/polysaccharide (acting as acceptor) and not to the terminal glucose unit of said additional β-glucan oligo/polysaccharide, or preferentially to an internal site rather than the terminal site.
The term “isolated” as used herein means that the material is removed from its original environment (e. g. the natural environment where the material is naturally occurring). For example, a polynucleotide or polypeptide while present in a living source organism is not isolated, but the same polynucleotide or polypeptide, which is separated from some or all of the coexisting materials in the natural system, is isolated. Such polynucleotides could for example be part of a vector and/or such polynucleotides or polypeptides could be part of a composition, and still be isolated in that the vector or composition is not part of the natural environment. When referring to a particular enzyme or any polypeptide, the term “isolated” refers to a preparation of the polypeptide outside its natural source and preferably substantially free of contaminants.
Methods of producing replicate copies of the same polynucleotide, such as PCR or gene cloning, are collectively referred to herein as “amplification” or “replication”. For example, single- or double-stranded DNA can be replicated to form another DNA with the same sequence. RNA can be replicated, for example, by RNA directed RNA polymerase, or by reverse transcribing the RNA and then performing a PCR. In the latter case, the amplified copy of the RNA is a DNA with the correlating or homologous sequence.
The polymerase chain reaction (“PCR”) is a reaction in which replicate copies are made of a target polynucleotide using one or more primers, and a catalyst of polymerization, such as a DNA polymerase, and particularly a thermally stable polymerase enzyme. Generally, PCR involves repeatedly performing a “cycle” of three steps: 1) “melting”, in which the temperature is adjusted such that the DNA dissociates to single strands, 2) “annealing”, in which the temperature is adjusted such that oligonucleotide primers are permitted to anneal to their complementary nucleotide sequence to form a duplex at one end of the polynucleotide segment to be amplified; and 3) “extension” or “synthesis”, which can occur at the same or slightly higher and more optimum temperature than annealing, and during which oligonucleotides that have formed a duplex are elongated with a thermostable DNA polymerase. The cycle is then repeated until the desired amount of amplified polynucleotide is obtained. Methods for PCR amplification can be found in U.S. Pat. Nos. 4,683,195 and 4,683,202.
The methods disclosed herein involving the molecular manipulation of nucleic acids are known to those skilled in the art. See generally Ausubel, F. M. et al., “Short Protocols in Molecular Biology,” John Wiley and Sons (1995); and Sambrook, J., et al., “Molecular Cloning, A Laboratory Manual,” 2nd ed., Cold Spring Harbor Laboratory Press (1989).
In one embodiment of the invention, the isolated glucosyltransferase of a bacterial origin has glucosyltransferase activity such that the transferred non-reducing end of one (β1→3)-glucan oligo/polysaccharide molecule is transferred to the third glucose residue from the reducing end of a different (β1→3)-glucan oligo/polysaccharide.
In certain embodiments of the invention, the isolated glycosyltransferase of a bacterial origin is not active on oligosaccharide-alditols as substrate.
The invention also encompasses certain methods for carrying out transglycosylation reactions that involve adding a donor substrate and an acceptor substrate for transglycosylation to a reaction mixture. Then a protein comprising an enzymatically active GH17 protein domain of bacterial origin as described herein can be added to the reaction, and the reaction incubated until the transglycosylation reaction occurs and run for a desired time.
In one embodiment, the invention is a method for transglycosylation of substrate (β1→3)-glucan oligo/polysaccharides using an isolated glucosyltransferase of a bacterial origin that has glucosyltransferase activity on β-glucan oligo/polysaccharides such that it releases two interlinked glucose units from the reducing end of said β-glucan oligo/polysaccharides and transfer the remaining non-reducing end of said β-glucan oligo/polysaccharide to internal glucose units of an additional β-glucan oligo/polysaccharide and not to the terminal glucose unit of said additional oligo/polysaccharide.
As understood from herein, it is particularly advantageous that in useful embodiments the enzyme used in the methods has preferential transferase activity over hydrolase activity in the presence of reactive substrate β-glucan oligo- or polysaccharide that can act as acceptor. ‘Preferential’ as used in this context means that preferably the transferase reaction occurs at least at twofold rate of hydrolysis, and more preferably at fourfold rate, and even more preferably at least at tenfold rate, and yet more preferably at 20-fold rate, or 100-fold rate, over the hydrolysis reaction.
As mentioned, the enzymes used in the methods of the invention are of bacterial origin, and in particular selected from bacterial glycosyltransferases from the GH17 family. A definition of the term GH17 family can be found e.g. on CAZypedia.org (see http://http://www.cazypedia.org/index.php/Glycoside_Hydrolase_Family_17).
In certain embodiments the enzyme used for the transglucosylation reaction is produced recombinantly from a DNA sequence comprising a coding region for GH17 enzyme from the taxonomic family of Bradyrhizobiaceae, such as from the genera of Bradyrhizobium, Rhodopseudomonas, Nitrobacter, and Afipia.
Also, in certain embodiments the GH17 enzymes can be from, B. japonicum, B. diazoefficiens, Nitrobacter hamburgensis, Rhodopseudomonas palustris and Afipia clevelandensis, among other organisms.
The above mentioned enzymes are shown by inventors to share critical sequence similarity in particular in the regions around the substrate binding cleft. Representative embodiments of such enzymes useful in the invention are compared in Example 1.
Accordingly, in some embodiments of the methods, the isolated enzyme comprises a sequence with 75% or higher sequence identity to SEQ ID NO: 1, and more preferably 85% or higher, and even more preferably 90% or higher identity, such as 95% or higher.
Algorithms for sequence comparisons and calculation of “sequence identity” are known in the art as discussed above, such as BLAST, described in Altschul et al. 1990, or the Needleman and Wunsch algorithm (Needleman and Wunsch 1970) Generally, the default settings with respect to e.g. “scoring matrix” and “gap penalty” will be used for alignment. The percentage sequence identity values referred to herein refer to values as calculated with the Needleman and Wunsch algorithm such as implemented in the program Needle (Rice et al. 2000) using the default scoring matrix EBLOSUM62 for protein sequences, (or scoring matrix EDNAFULL for nucleotide sequences) with opening gap penalty set to 10.0 and gap extension penalty set to 0.5. The sequence identity is thus the percentage of identical matches between the two sequences over the aligned region including any gaps in the length. Percentage identity between two sequences in an alignment can also be counted by hand such as the sequence identity in an alignment that has been manually adjusted after automatic alignment.
In one embodiment, the invention is a method for transglycosylation of substrate (β1→3)-glucan oligo/polysaccharides using a glucosyltransferase of a bacterial origin, in particular such as defined above, such that the produced branched polysaccharides have biological activity.
In one embodiment of the invention said biological activity includes immune activity such as anti-tumoral, anti-bacterial, anti-viral or anti-fungal activity. In certain methods the donor substrate in the reaction can be laminarin, lichenan, curdlan, scleroglucan and pustulan or related polysaccharides and the like or their mixtures.
In certain embodiments of the methods the acceptor substrate in the reaction can comprise oligosaccharide from laminarin, cellulose, lichenan, curdlan scleroglucan and pustulan or related polysaccharides and the like or their mixtures.
In certain methods the donor or the acceptor substrates in the reaction can be oligosaccharides or polysaccharides and the like or their mixtures.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the figures.
In certain embodiments the enzyme used for the transglucosylation reaction can be made from a gene where certain DNA sequences have been added, mutated or deleted. In certain embodiments the enzyme used for the transglucosylation reaction according to the invention can be recombinantly expressed in bacteria such as E. coli or any other suitable bacterial expression system known to the skilled person. In certain other embodiments the enzyme used for the transglucosylation reaction can be expressed from eukaryotic organisms such as but not limited to yeasts or fungi. The invention also contemplates certain recombinant vectors for carrying the DNA sequences and transfecting effectively a chosen expression system such that the enzyme is recombinantly expressed.
The enzymes used in the methods of the invention can be partially or substantially purified (e.g., purified to homogeneity), and/or are substantially free of other polypeptides. According to the invention, the amino acid sequence of the enzyme can be that of the naturally occurring enzyme or can comprise alterations therein. Polypeptides comprising alterations are referred to herein as “derivatives” of the native polypeptide. Such alterations include conservative or non-conservative amino acid substitutions, additions and deletions of one or more amino acids; however, such alterations should preserve the transglycosylation activity of the enzyme, i.e., the altered or mutant polypeptides of the invention are active derivatives of the naturally occurring polypeptide having the specific transglycosylation activity. Preferably, the amino acid substitutions are of minor nature, i.e. conservative amino acid substitutions that do not significantly alter the folding or activity of the polypeptide. Deletions are preferably small deletions, typically of one to 30 amino acids. Additions are preferably small amino- or carboxy-terminal extensions, such as amino-terminal methionine residue; a small linker peptide of up to about 25 residues; or a small extension that facilitates purification by changing net charge or another function, such as a poly-histidine tail, an antigenic epitope or a binding domain. The alteration(s) should preserve the three-dimensional configuration of the active site of the native enzyme, such that the activity of the enzyme is preserved.
In certain embodiments of the methods, the MalE fusion protein, His-tag or other suitable recombinant additions are used to enhance expression and/or purification but have preferably been removed from the enzymatically active glucotransferase enzyme when used in the inventive methods.
The methods of the invention can be suitably performed in solution and in any suitable reactor type. The enzyme may in certain embodiments be immobilized on suitable inert media by covalent (e.g. by cross-linking) or non-covalent attachment, such as for continuous operation of the method of the invention. Accordingly, the methods can be operated in e.g moving bed reactors, packed bed reactors, and the like.
The isolated enzyme in the methods of the invention can be used in any concentration suitable in any particular conditions, depending on the nature of substrate, etc. In some embodiments the enzyme is used in a concentration in the reaction medium in the range of about 0.01 to about 20 mg/mL, such as in the range of about 0.05 to about 10 mg/mL, such as in the range of about 0.1 to about 5 mg/mL or in the range of about 0.1 to about 1.0 mg/mL, e.g. about 0.10, about 0.20, about 0.40, about 0.50, about 1.0, or about 2.0 mg/mL. Suitable enzyme concentration may depend on desired reaction time, and amount and concentration of substrate, etc. Substrate concentration can in some embodiments be in the range from about 0.40 to about 50 mg/mL, such as in the range of about 1.0 to about 10 mg/mL.
The methods of the invention can be suitably operated at desired temperature, in certain embodiment the methods are operated at about room temperature (20-25° C.) or can in some embodiment be operated at a temperature in the range from about 10° C. to about 50° C., such as preferably in the range from about 20 to about 40° C., such as in the range from about 20° C. to about 30° C., e.g. at about 25° C. or at about 30° C.
Bioinformatic analysis of DNA sequence databases was used to identify multidomain β-glucan branching enzymes. GH17 enzyme domains were isolated, cloned, and expressed in soluble form in E. coli, to investigate their activity. A phylogenetic tree was made from amino acid alignment of selected GH17 sequences from Gamma-Proteobacteria containing Glt1, GL3 and Glt7 and selected sequences from the family sand the Bradyrhizobiaceae of Alpha-Proteobacteria. Highly discernable differences are seen between the alpha and gamma-proteobacteria sequences and the Bradyrhizobiaceae show high similarity and site specific conservation not found in the GH17 domains from the Gamma-Proteobacteria. Preferred embodiments of the current invention relate to GH17 enzymes from the taxonomic family of Bradyrhizobiaceae. The recombinant enzymes used in the examples were produced with attachments that aid their expression and purification, followed by specific removal of the attachments before assaying them for transglycosyl activity. An assay for transglycosyl activity on (β1→3)-linked gluco-oligosaccharides was developed and activity was studied with several analytical methods.
The methods of the invention use glycosyltransferase enzyme of bacterial origin, the term indicating that the enzyme is encoded by a gene originating from bacteria, which may suitably be isolated and transfected in a suitable expression host as described herein.
This example shows the alignment of amino acid sequences of the GH17 domains including the protein sequences Glt1, Glt3, Glt7 and Glt20. In
The sequences are the following:
1. Glt20-Bradyrhizobium japonicum (USDA-110)
2. Bradyrhizobium diazoefficiens-(USDA-110) gi|27379725|
3. Bradyrhizobium japonicum (USDA-6 gi|384218756|
4. Nitrobacter hamburgensis(X14) gi|92117324|
5. Rhodopseudomonas palustris (BisB5) gi|91977083|
6. Afipia clevelandensis gi|488799329
7. Nitrobacter sp.Nb-311A gi|497485564|
9. Pseudomonas sp.GM79] gi|398239141|
10. Pseudomonas fluorescens-gi|515552181|
11. Pseudomonas sp.45MFCol3.1-gi|518477590|
12. Glt1-Pseudomonas aeruginosa (PAO1)
13. Pseudomonas sp.S9 gi|498172591|
14. Glt3-Pseudomonas putida (KT2440)
15. Glt7-Azotobacter vinelandii (ATCC BAA-1303)
16. Pseudomonas fulva-12-X gi|333901654|
This example demonstrates the production and use of enzymatically active proteins containing the GH17 domain of a Type I enzyme Glt20. An E. coli expression vector from Motej added et al. (2009) was used for the cloning and expression of the glucosyltransferase and to facilitate the purification of the recombinant enzymes. The partial gene encoding the Glt20 protein domain was fused with a gene sequence encoding a maltose binding domain sequence and a His-tag sequence for affinity purification. A Saccharomyces cerevisiae Smt3 domain sequence, optimized for the expression in E. coli, was located between the maltose binding domain sequence and the cloned gene. Expression of the enzyme was induced with L-Rhamnose. For the use of the system it was important to cultivate and purify Ulp1 protease which cleaves the fusion protein at the Smt3 site. High yields of soluble protein were achieved with Glt20 (
After purification the enzyme was routinely tested at 30° C. on -glucan substrates of different length (DP2 to DP10) for 3 days. The reaction progress was monitored by spotting the reaction on TLC. The reaction was as follows: 100 μL substrate DP2 to DP10 (β1→3)-linked oligosaccharides from curdlan (6.25 mg/mL); 30 μL 0.5 M potassium phosphate pH 6.5; 70 μL purified enzyme (1.0 mg/mL) (
The activity-screening results for (β1→3)-linked gluco-oligosaccharides Lam-Glc5 to Lam-Glc10 showed that the Glt20 enzyme always releases laminaribiose (DP2) from the reducing end of substrate and adds the remainder to another substrate molecule. TLC bands lower than the substrate indicate that also larger oligosaccharides are formed. The most intensive transfer products formed after 48 h are: from Lam-Glc5 (substrate)→Pro-Glc8 (product)→Pro-Glc11 (product)→Pro-Glc14 (product); from Lam-Glc6→Pro-Glc10→Pro-Glc14→Pro-Glc18; from Lam-Glc7→Pro-Glc12→Pro-Glc17; from Lam-Glc8→Pro-Glc14→Pro-Glc20; from Lam-Glc9→Pro-Glc16→Pro-Glc23; and from Lam-Glc10→Pro-Glc18→Pro-Glc26. According to MALDI-TOF-MS, the amount of the formed larger oligosaccharides after 48 h, e.g. Pro-Glc8 and Pro-Glc11 in case of Lam-Glc5 was estimated to be about 60% of the total sample amount.
(β1→3)-Gluco-oligosaccharides(-alditols) (Lam-Glc2-Lam-Glc10) were incubated with the Glt20 enzyme in 0.5 M phosphate buffer, pH 6.5, at 30° C. The progress of the reaction (0-72 h) was followed by analyzing aliquots on TLC (Merck Kieselgel 60 F254 sheets; butanol:acetic acid:water=2:1:1; orcinol/sulfuric acid staining). After 24 h, 48 h and 72 h of incubation, the product(s) were isolated by gel-permeation chromatography (GPC) on a Bio-Gel P-2 column (90×1 cm), eluted with 10 mM ammonium bicarbonate, at a flow rate of 12 mL/h. In the high-mass-elution region, fractions of 5 min were collected and analyzed by TLC. Fractions containing oligosaccharides with increased DP (compared to the substrate oligosaccharide) were investigated by MALDI-TOF mass spectrometry and 1D/2D Nuclear Magnetic Resonance (NMR) spectroscopy.
Isolation and Characterization of Reaction Products from the Incubation of Lam-Glc5
The substrate Lam-Glc5 was incubated for 24 h with the cloned Glt20 bacterial enzyme. The TLC-screening showed that Glt20 cleaves (β1→3) linkages in the substrate and releases a biose and a tetraose as the main hydrolysis products (
Based on combined MS, 1D/2D NMR, and enzymatic degradation data, and the hypothesis that Pro-Glc8 and Pro-Glc11 are synthesized by a transfer of the triaose β-
Isolation and Characterization of Reaction Products from the Incubation of Lam-Glc6
The substrate Lam-Glc6 was incubated for 24 h with the cloned Glt20 bacterial enzyme. The TLC-screening showed that Glt20 cleaves (1→3) linkages in the substrate and releases a tetraose, showing biose as the main hydrolysis product (
Oligosaccharide Lam-Glc6 (1000 μL; 6.25 mg/mL) was incubated for 48 h with Glt20 (700 μL; 1.0 mg/mL) in 0.5 M KH2PO4/K2HPO4 buffer (300 μL), pH 6.5, at 30° C. The mixture of oligosaccharides was fractionated on Bio-Gel P-2, yielding fractions I to XII, which were screened by TLC and MALDI-TOF-MS. The MALDI-TOF mass spectrum of fraction IV showed one major sodiated molecular ion [M+Na]+ at m/z 1661.5, corresponding to Pro-Glc10 and very minor peak intensities (<20%) for Pro-Glc9 (m/z 1499.4) and Pro-Glc11 (m/z 1823.7). In the 1D 1H NMR spectrum (
Based on combined MS and 1D/2D NMR data, and the hypothesis that the major products Pro-Glc10 and Pro-Glc14 are synthesized by the transfer of a tetraose, β-
The Glt20 enzyme is not active on Lam-Glc2/3/4. However, Glt20 cleaves a biose from the reducing end of the substrate (DP>4) donor and add the remainder oligosaccharide part again to substrate. The optimal incubation time is about 48 h to get oligosaccharides with DP>DP substrate, together with a minimal amount of hydrolysis products.
For Lam-Glc5 as substrate, the main reaction products are Pro-Glc8 and Pro-Glc11, together with a minor amount of Pro-Glc14, indicating the subsequent transfer of triaose. Initially formed products act again as acceptor.
For Lam-Glc6 as substrate, the main reaction product is Pro-Glc10. Additionally, Pro-Glc14 and Pro-Glc18 are present in minor amounts. Here, there is a subsequent transfer of a tetraose. Pro-Glc5, Pro-Glc7 and Pro-Glc9 are observed as minor hydrolysis products.
The 1H NMR spectra of the isolated high-mass fractions of both substrates (Lam-Glc5 and Lam-Glc6) show that branching takes place up to 48 h of incubation. It should be noted that after 72 h of incubation, when all substrate is consumed, the hydrolysis activity occurs and the branched products are being degraded into linear products (containing internal (β1→6) linkages) (see
The activity of the Glt20 enzyme starting from the reducing end of the substrate, was confirmed by the fact that the enzyme was not active on oligosaccharide-alditols.
It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IS2014/050003 | 3/26/2014 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61805360 | Mar 2013 | US |
