Isomaltulose synthase from Klebsiella sp. strain LX3: gene cloning and characterization and engineering of thermostability

A Critical Review on Immobilized Sucrose Isomerase and Cells for Producing Isomaltulose

Isomaltulose is a novel sweetener and is considered healthier than the common sugars, such as sucrose or glucose. It has been internationally recognized as a safe food product and holds vast potential in pharmaceutical and food industries. Sucrose isomerase is commonly used to produce isomaltulose from the substrate sucrose in vitro and in vivo. However, free cells/enzymes were often mixed with the product, making recycling difficult and leading to a significant increase in production costs. Immobilized cells/enzymes have the following advantages including easy separation from products, high stability, and reusability, which can significantly reduce production costs. They are more suitable than free ones for industrial production. Recently, immobilized cells/enzymes have been encapsulated using composite materials to enhance their mechanical strength and reusability and reduce leakage. This review summarizes the advancements made in immobilized cells/enzymes for isomaltulose production in terms of refining traditional approaches and innovating in materials and methods. Moreover, innovations in immobilized enzyme methods include cross-linked enzyme aggregates, nanoflowers, inclusion bodies, and directed affinity immobilization. Material innovations involve nanomaterials, graphene oxide, and so on. These innovations circumvent challenges like the utilization of toxic cross-linking agents and enzyme leakage encountered in traditional methods, thus contributing to enhanced enzyme stability.

Analyzing Current Trends and Possible Strategies to Improve Sucrose Isomerases' Thermostability

Due to their ability to produce isomaltulose, sucrose isomerases are enzymes that have caught the attention of researchers and entrepreneurs since the 1950s. However, their low activity and stability at temperatures above 40 °C have been a bottleneck for their industrial application. Specifically, the instability of these enzymes has been a challenge when it comes to their use for the synthesis and manufacturing of chemicals on a practical scale. This is because industrial processes often require biocatalysts that can withstand harsh reaction conditions, like high temperatures. Since the 1980s, there have been significant advancements in the thermal stabilization engineering of enzymes. Based on the literature from the past few decades and the latest achievements in protein engineering, this article systematically describes the strategies used to enhance the thermal stability of sucrose isomerases. Additionally, from a theoretical perspective, we discuss other potential mechanisms that could be used for this purpose.

Thermostability improvement of sucrose isomerase PalI NX-5: a comprehensive strategy

OBJECTIVE

To increase the thermal stability of sucrose isomerase from Erwinia rhapontici NX-5, we designed a comprehensive strategy that combines different thermostabilizing elements.

RESULTS

We identified 19 high B value amino acid residues for site-directed mutagenesis. An in silico evaluation of the influence of post-translational modifications on the thermostability was also carried out. The sucrose isomerase variants were expressed in Pichia pastoris X33. Thus, for the first time, we report the expression and characterization of glycosylated sucrose isomerases. The designed mutants K174Q, L202E and K174Q/L202E, showed an increase in their optimal temperature of 5 °C, while their half-lives increased 2.21, 1.73 and 2.89 times, respectively. The mutants showed an increase in activity of 20.3% up to 25.3%. The Km values for the K174Q, L202E, and K174Q/L202E mutants decreased by 5.1%, 7.9%, and 9.4%, respectively; furthermore, the catalytic efficiency increased by up to 16%.

CONCLUSIONS

With the comprehensive strategy followed, we successfully obtain engineered mutants more suitable for industrial applications than their counterparts: native (this research) and wild-type from E. rhapontici NX-5, without compromising the catalytic activity of the molecule.

Enhanced Extracellular Production and Characterization of Sucrose Isomerase in Bacillus subtilis with Optimized Signal Peptides

Sucrose isomerase (SIase) catalyzes the hydrolysis and isomerization of sucrose into isomaltulose, which is an important functional sugar widely used in the food industry. However, the lack of safe and efficient expression systems for recombinant SIase has impeded its production and application. In this study, enhanced expression of a SIase from Klebsiella sp. LX3 (referred to as KsLX3-SIase) was achieved in Bacillus subtilis WB800N, by optimizing the signal peptides. First, 13 candidate signal peptides were selected using a semi-rational approach, and their effects on KsLX3-SIase secretion were compared. The signal peptide WapA was most efficient in directing the secretion of KsLX3-SIase into the culture medium, producing a specific activity of 23.0 U/mL, as demonstrated by shake flask culture. Using a fed-batch strategy, the activity of KsLX3-SIase in the culture medium was increased to 125.0 U/mL in a 5-L fermentor. Finally, the expressed KsLX3-SIase was purified and was found to have maximum activity at 45 °C and pH 5.5. Its K_m for sucrose was 267.6 ± 18.6 mmol/L, and its k_cat/K_m was 10.1 ± 0.2 s⁻¹mM⁻¹. These findings demonstrated an efficient expression of SIase in B. subtilis, and this is thought to be the highest level of SIase produced in a food-grade bacteria to date.

Tuning the catalytic performances of a sucrose isomerase for production of isomaltulose with high concentration

Obtaining a sucrose isomerase (SIase) with high catalytic performance is of great importance in industrial production of isomaltulose (a reducing sugar). In order to obtain such SIase mutant, a high-throughput screening system in microtiter plate format was developed based on a widely used 2,4-dinitrosalicylic acid (DNS) method for determination of reducing sugar. An SIase from Erwinia sp. Ejp617 (ErSIase) was selected to improve its catalytic efficiency. After screening of ~ 8000 mutants from a random mutagenesis library, Q209 and R456 were identified as beneficial positions. Saturation mutagenesis of the two positions resulted in a double-site mutant ErSIase_Q209S-R456H that showed the highest catalytic efficiency, and its specific activity reached 684 U/mg that is 17.5-fold higher than that of the wild-type ErSIase. By employing the lyophilized Escherichia coli (E. coli) cells harboring ErSIase_Q209S-R456H, a high space-time yield (STY = 3.9 kg/(L·d)) was achieved toward 600 g/L sucrose. Furthermore, the in silico analysis suggested that the hydrogen bond network was improved and steric hindrance was reduced due to the beneficial substitutions.Key points• A sucrose isomerase mutant with high catalytic efficiency was obtained.• The highest space-time yield was achieved toward high-concentration sucrose.• The optimized H-bond network contributed to the enhanced catalytic efficiency.

Immobilization of Sucrose Isomerase from Erwinia sp. with Graphene Oxide and Its Application in Synthesizing Isomaltulose

Sucrose isomerase (SIase) is a key enzyme used for the production of isomaltulose from sucrose. In this study, an SIase gene from Erwinia sp. Ejp617 (ErSIase) was heterologously expressed in Escherichia coli BL21(DE3), and the recombinant ErSIase was served as biocatalyst combined with the graphene oxide (GO) as carrier for ErSIase immobilization. The Fourier transform infrared spectroscopy, transmission electron microscope, and confocal laser microscopy analyses showed that ErSIase was successfully immobilized on the surface of GO to form ErSIase-GO. The loading capacity of ErSIase on GO reached up to 460 mg/g with a specific activity of 727.04 U/mg protein when the optimal immobilization time of 12 h and the ErSIase/GO ratio of 7.4:4 (w/w) were applied. A high conversion rate of 95.3% was reached from sucrose to isomaltulose using ErSIase-GO as biocatalyst with 600 g/L sucrose as substrate, after 180 min at 40 °C and pH 6.0. Moreover, stabilities of the immobilized ErSIase-GO in the aspects of thermal, pH, and storage were improved, and its activity after 10 batches still remained around 80% under the optimal conditions. The Km value of ErSIase-GO was 29.32 mM, and the kcat/Km was increased to 27.34 s-1 mM-1 when 0.1% (w/v) detergent NP40 was added. These results indicated that the ErSIase was well immobilized onto GO, and the ErSIase-GO is a promising biocatalyst with high operational stability and catalytic activity for industrial production of isomaltulose.

Simple and efficient preparation of high-purity trehalulose from the waste syrup of isomaltulose production using solid-phase extraction followed by hydrophilic interaction chromatography

A simple and efficient method was developed for the preparation of high-purity trehalulose from the waste syrup of isomaltulose production. The waste syrup was pre-treated with C18 solid-phase extraction, where 98% decolorization and 97% reducing sugar recovery were obtained, followed by hydrophilic interaction liquid chromatography separation on a cysteine-bonded zwitterionic column. Under optimized conditions, trehalulose was separated from isomaltulose isomer and prepared on a semi-preparative scale with >99% purity. The structure of the prepared trehalulose was subsequently confirmed by nuclear magnetic resonance, and three tautomers of trehalulose (α-D-glucosylpyranosyl-1,1-β-D-fructopyranose, α-D-glucosylpyranosyl-1,1-β-D-fructofuranose, and α-D-glucosylpyranosyl-1,1-α-D-fructofuranose) were detected and completely characterized by ¹³ C NMR spectroscopy for the first time in this study. The tautomerization of α-D and β-D type transition was observed by hydrophilic interaction liquid chromatography on an AdvanceBio Glycan Mapping column, with smaller particle size (2.7 μm). Furthermore, the prepared trehalulose was applied as a standard for trehalulose quantification during the sucrose conversion by Klebsiella sp. LX3. The combination of solid-phase extraction and hydrophilic interaction liquid chromatography offers a new avenue for the preparation of sugar isomers from complex natural or fermentation products.

Characterization of a recombinant sucrose isomerase and its application to enzymatic production of isomaltulose

OBJECTIVE

To characterize a recombinant isomerase that can catalyze the isomerization of sucrose into isomaltulose and investigate its application for the enzymatic production of isomaltulose.

RESULTS

A sucrose isomerase gene from Erwinia sp. Ejp617 was synthesized and expressed in Escherichia coli BL21(DE3). The enzymatic characterization revealed that the optimal pH and temperature of the purified sucrose isomerase were 6.0 and 40 °C, respectively. The enzyme activity was slightly activated by Mn²⁺and Mg²⁺, but partially inhibited by Ca²⁺, Ba²⁺, Cu²⁺, Zn²⁺ and EDTA. The kinetic parameters of K_m and V_max for sucrose were 69.28 mM and 118.87 U/mg, respectively. The time course showed that 240.9 g/L of isomaltulose was produced from 300 g/L of sucrose, and the yield reached 80.3% after bioreaction for 180 min.

CONCLUSIONS

This recombinant enzyme showed excellent capability for biotransforming sucrose to isomaltulose at the substrate concentration of 300 g/L. Further investigations should be carried out focusing on selection of suitable heterologous expression system with the aim to improve its expression level.

Economical production of isomaltulose from agricultural residues in a system with sucrose isomerase displayed on Bacillus subtilis spores

A safe, efficient, environmentally friendly process for producing isomaltulose is needed. Here, the biocatalyst, sucrose isomerase (SIase) from Erwinia rhapontici NX-5, displayed on the surface of Bacillus subtilis 168 spores (food-grade strain) was applied for isomaltulose production. The anchored SIase showed relatively high bioactivity, suggesting that the surface display system using CotX as the anchoring protein was successful. The stability of the anchored SIase was also significantly better. Thermal stability analysis showed that 80% of relative activity was retained after incubation at 40 °C and 45 °C for 60 min. To develop an economical industrial fermentation medium, untreated beet molasses (30 g/L) and cold-pressed soybean powder (50 g/L) were utilised as the main broth components for SIase pilot-scale production. Under the optimal conditions, the productive spores converted 92% of sucrose after 6 h and the conversion rate was 45% after six cycles. Isomaltulose production with this system using the agricultural residues, untreated beet molasses and soybean powder, as substrates is cost-effective and environmentally friendly and can help to overcome issues due to the genetic background.

A new GH13 α-glucosidase from alkaliphilic Bacillus pseudofirmus 703 with both exo-α-l, 4-glucosidase and oligo-l, 6-glucosidase activities toward amylopectin

Debranching of (1, 6)-α-linkages in starch is of great significance as it is widely used in different industries. In recent years, identifying a single potential enzyme that could debranch (1, 6)-α-linkages and cleave (1, 4)-α-linkages brought great interest as amylopectin possessed both (1, 4)-α- and (1, 6)-α-linkages. In the present study, a 64\,kDa exo-α-l, 4-glucosidase from Bacillus pseudofirmus 703, annotated as Amy112, was cloned and characterized. Biochemical analysis indicated that heterologous Amy112 expressed in Escherichia coli exhibited a high activity against amylopectin, with the optimal temperature and pH of 40°C and pH 7.0, respectively. Addition of K⁺ ions improved the amy112 activity by 12%, but Li⁺, Ca²⁺ and Mg²⁺ ions showed no significant effect. Amy112 sequence homology revealed that it belonged to glycoside hydrolase family 13, showing 65% identity with α-glucosidase GSJ from Geobacillus sp. HTA-462. This is the first report indicating that Amy112 from B. pseudofirmus 703 belongs to GH13 enzyme family, having both exo-α-1, 4-glucosidase and oligo-l, 6-glucosidase activities. However, no transglycosylation activity was detected in LC-MS analysis. Amy112 would be of great significance of being utilized to debranch starch in different industries in a cost effective manner.

Enhancing the Thermostability of Serratia plymuthica Sucrose Isomerase Using B-Factor-Directed Mutagenesis

The sucrose isomerase of Serratia plymuthica AS9 (AS9 PalI) was expressed in Escherichia coli BL21(DE3) and characterized. The half-life of AS9 PalI was 20 min at 45°C, indicating that it was unstable. In order to improve its thermostability, six amino acid residues with higher B-factors were selected as targets for site-directed mutagenesis, and six mutants (E175N, K576D, K174D, G176D, S575D and N577K) were designed using the RosettaDesign server. The E175N and K576D mutants exhibited improved thermostability in preliminary experiments, so the double mutant E175N/K576D was constructed. These three mutants (E175N, K576D, E175N/K576D) were characterized in detail. The results indicate that the three mutants exhibit a slightly increased optimal temperature (35°C), compared with that of the wild-type enzyme (30°C). The mutants also share an identical pH optimum of 6.0, which is similar to that of the wild-type enzyme. The half-lives of the E175N, K576D and E175N/K576D mutants were 2.30, 1.78 and 7.65 times greater than that of the wild-type enzyme at 45°C, respectively. Kinetic studies showed that the K_m values for the E175N, K576D and E175N/K576D mutants decreased by 6.6%, 2.0% and 11.0%, respectively, and their k_cat/K_m values increased by 38.2%, 4.2% and 19.4%, respectively, compared with those of the wild-type enzyme. After optimizing the conditions for isomaltulose production at 45°C, we found that the E175N, K576D and E175N/K576D mutants displayed slightly improved isomaltulose yields, compared with the wild-type enzyme. Therefore, the mutants produced in this study would be more suitable for industrial biosynthesis of isomaltulose.

The structural basis of Erwinia rhapontici isomaltulose synthase

Sucrose isomerase NX-5 from Erwiniarhapontici efficiently catalyzes the isomerization of sucrose to isomaltulose (main product) and trehalulose (by-product). To investigate the molecular mechanism controlling sucrose isomer formation, we determined the crystal structures of native NX-5 and its mutant complexes E295Q/sucrose and D241A/glucose at 1.70 Å, 1.70 Å and 2.00 Å, respectively. The overall structure and active site architecture of NX-5 resemble those of other reported sucrose isomerases. Strikingly, the substrate binding mode of NX-5 is also similar to that of trehalulose synthase from Pseudomonasmesoacidophila MX-45 (MutB). Detailed structural analysis revealed the catalytic RXDRX motif and the adjacent 10-residue loop of NX-5 and isomaltulose synthase PalI from Klebsiella sp. LX3 adopt a distinct orientation from those of trehalulose synthases. Mutations of the loop region of NX-5 resulted in significant changes of the product ratio between isomaltulose and trehalulose. The molecular dynamics simulation data supported the product specificity of NX-5 towards isomaltulose and the role of the loop^330-339 in NX-5 catalysis. This work should prove useful for the engineering of sucrose isomerase for industrial carbohydrate biotransformations.

Simple, fast, and efficient process for producing and purifying trehalulose

A new property of recombinant trehalose synthase (GTase) from Thermus thermophilus HB-8 (ATCC 27634) was found and described in this study. GTase can act on sucrose and catalyze trehalulose formation without isomaltose, isomaltulose, or isomelezitose, releasing small amounts of glucose and fructose as byproducts. Maximum trehalulose yield (approximately 81%) was obtained at an optimum temperature of 65°C and was independent of substrate concentration. A simple, fast, and efficient method of producing and purifying trehalulose is then described. In the first step, GTase catalyzed trehalulose formation using a 20% sucrose substrate. Miscellaneous sugars were then rapidly removed, while trehalulose was completely preserved by Saccharomyces cerevisiae cells. Finally, the cells were separated by centrifugation, and salt ions were removed by an ion-exchange resin, subsequently obtaining a high-purity trehalulose solution. A trehalulose recovery rate of over 95% was achieved using this process. This method has a simple process, fast separation efficiency, and low investment in production equipment, so greatly to improve production efficiency and reduce production costs.

Gene cloning, protein characterization, and alteration of product selectivity for the trehalulose hydrolase and trehalulose synthase from "Pseudomonas mesoacidophila" MX-45

The naturally occurring structural isomer of sucrose, trehalulose, is produced by sucrose isomerase (SI). Screening of chromosomal DNA from "Pseudomonas mesoacidophila" MX-45 with an SI-specific probe facilitated the cloning of two adjacent gene homologs, mutA and mutB. Both genes were expressed separately in Escherichia coli, and their enzyme products were characterized. MutA hydrolyzed the substrates trehalulose, isomaltulose, and sucrose into glucose and fructose. Due to its highest activity on trehalulose, MutA was referred to as trehalulase. mutB encodes the SI (trehalulose synthase) and catalyzes the isomerization of sucrose to mainly trehalulose. From Northern blot analysis it is apparent that the mutB gene is not transcribed as part of an operon and was transcriptionally upregulated when P. mesoacidophila MX-45 cells were grown in sucrose medium, whereas under investigated conditions no transcript for mutA was detected. Mutants of mutB were created by a random mutagenesis approach in order to alter the product specificity of MutB. Two types of mutants have emerged, one type that prefers the hydrolytic reaction on sucrose and another type that still acts as an SI but with a significant shift in the product from trehalulose to isomaltulose. The hydrolytic character of MutB R311C was demonstrated through its higher catalytic efficiency for glucose production over trehalulose production. MutB D442N favored the transfer reaction, with an isomer preference for isomaltulose.

Isomaltose production by modification of the fructose-binding site on the basis of the predicted structure of sucrose isomerase from "Protaminobacter rubrum"

"Protaminobacter rubrum" sucrose isomerase (SI) catalyzes the isomerization of sucrose to isomaltulose and trehalulose. SI catalyzes the hydrolysis of the glycosidic bond with retention of the anomeric configuration via a mechanism that involves a covalent glycosyl enzyme intermediate. It possesses a (325)RLDRD(329) motif, which is highly conserved and plays an important role in fructose binding. The predicted three-dimensional active-site structure of SI was superimposed on and compared with those of other alpha-glucosidases in family 13. We identified two Arg residues that may play important roles in SI-substrate binding with weak ionic strength. Mutations at Arg(325) and Arg(328) in the fructose-binding site reduced isomaltulose production and slightly increased trehalulose production. In addition, the perturbed interactions between the mutated residues and fructose at the fructose-binding site seemed to have altered the binding affinity of the site, where glucose could now bind and be utilized as a second substrate for isomaltose production. From eight mutant enzymes designed based on structural analysis, the R(325)Q mutant enzyme exhibiting high relative activity for isomaltose production was selected. We recorded 40.0% relative activity at 15% (wt/vol) additive glucose with no temperature shift; the maximum isomaltose concentration and production yield were 57.9 g liter(-1) and 0.55 g of isomaltose/g of sucrose, respectively. Furthermore, isomaltose production increased with temperature but decreased at a temperature of >35 degrees C. Maximum isomaltose production (75.7 g liter(-1)) was recorded at 35 degrees C, and its yield for the consumed sucrose was 0.61 g g(-1) with the addition of 15% (wt/vol) glucose. The relative activity for isomaltose production increased progressively with temperature and reached 45.9% under the same conditions.

Trehalulose Synthase Native and Carbohydrate Complexed Structures Provide Insights into Sucrose Isomerization

Various diseases related to the overconsumption of sugar make a growing need for sugar substitutes. Because sucrose is an inexpensive and readily available d-glucose donor, the industrial potential for enzymatic synthesis of the sucrose isomers trehalulose and/or isomaltulose from sucrose is large. The product specificity of sucrose isomerases that catalyze this reaction depends essentially on the possibility for tautomerization of sucrose, which is required for trehalulose formation. For optimal use of the enzyme, targeting controlled synthesis of these functional isomers, it is necessary to minimize the side reactions. This requires an extensive analysis of substrate binding modes and of the specificity-determining sites in the structure. The 1.6-2.2-A resolution three-dimensional structures of native and mutant complexes of a trehalulose synthase from Pseudomonas mesoacidophila MX-45 mimic successive states of the enzyme reaction. Combined with mutagenesis studies they give for the first time thorough insights into substrate recognition and processing and reaction specificities of these enzymes. Among the important outcomes of this study is the revelation of an aromatic clamp defined by Phe(256) and Phe(280) playing an essential role in substrate recognition and in controlling the reaction specificity, which is further supported by mutagenesis studies. Furthermore, this study highlights essential residues for binding the glucosyl and fructosyl moieties. The introduction of subtle changes informed by comparative three-dimensional structural data observed within our study can lead to fundamental modifications in the mode of action of sucrose isomerases and hence provide a template for industrial catalysts.

A new colorimetric method for determining the isomerization activity of sucrose isomerase

A new colorimetric method for determining the isomerization activity of sucrose isomerase was developed. This colorimetric method is based on the enzymatic reactions of invertase and glucose oxidase-peroxidase (GOD-POD). The main scheme for assaying sucrose isomerase activity is to degrade sucrose in the reaction mixture to glucose and fructose by invertase and to detect the concentration of glucose generated using GOD-POD. The concentrations of trehalulose and isomaltulose, reaction products of sucrose isomerase, are calculated from the concentration of glucose. This method allows rapid and accurate determination of the isomerization activity of sucrose isomerase without inhibition by hydrolysis activity.

Conversion of sucrose into isomaltulose by Enterobacter sp. FMB1, an isomaltulose-producing microorganism isolated from traditional Korean food

Over 500 microorganisms isolated from Korean traditional foods, Maeju (source of soybean paste) and Nuruk (Korean koji), were screened to obtain an isomaltulose-producing microorganism. It was identified as Enterobacter sp. FMB-1 by 16S rRNA sequencing and the API 20E system. It had a greater than 90% conversion of sucrose (as 4 g/l) to isomaltulose in 2 days. Small amounts of trehalulose, glucose, and fructose were produced as byproducts, implying that this strain could be possibly employed in the production of isomaltulose in industry.

Overexpression, purification, crystallization and preliminary diffraction studies of the Protaminobacter rubrum sucrose isomerase SmuA

Palatinose (isomaltulose, alpha-D-glucosylpyranosyl-1,6-D-fructofuranose), a nutritional and acariogenic reducing sugar, is industrially obtained from sucrose by using immobilized cells of Protaminobacter rubrum that produce the sucrose isomerase SmuA. The isomerization of sucrose catalyzed by this enzyme also results in the formation of trehalulose (alpha-D-glucosylpyranosyl-1,1-D-fructofuranose) in smaller amounts and glucose, fructose and eventually isomaltose as by-products, which lower the yield of the reaction and complicate the recovery of palatinose. The determination of the three-dimensional structure of SmuA will provide a basis for rational protein-engineering studies in order to optimize the industrial production of palatinose. A recombinant form of the 67.3 kDa SmuA enzyme has been crystallized in the native state by the vapour-diffusion method. Crystals belong to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 61.6, b = 81.4, c = 135.6 A, and diffract to 1.95 A resolution on a synchrotron-radiation source.

Characterization of the highly efficient sucrose isomerase from Pantoea dispersa UQ68J and cloning of the sucrose isomerase gene

Sucrose isomerase (SI) genes from Pantoea dispersa UQ68J, Klebsiella planticola UQ14S, and Erwinia rhapontici WAC2928 were cloned and expressed in Escherichia coli. The predicted products of the UQ14S and WAC2928 genes were similar to known SIs. The UQ68J SI differed substantially, and it showed the highest isomaltulose-producing efficiency in E. coli cells. The purified recombinant WAC2928 SI was unstable, whereas purified UQ68J and UQ14S SIs were very stable. UQ68J SI activity was optimal at pH 5 and 30 to 35 degrees C, and it produced a high ratio of isomaltulose to trehalulose (>22:1) across its pH and temperature ranges for activity (pH 4 to 7 and 20 to 50 degrees C). In contrast, UQ14S SI showed optimal activity at pH 6 and 35 degrees C and produced a lower ratio of isomaltulose to trehalulose (<8:1) across its pH and temperature ranges for activity. UQ68J SI had much higher catalytic efficiency; the Km was 39.9 mM, the Vmax was 638 U mg(-1), and the Kcat/Km was 1.79 x 10(4) M(-1) s(-1), compared to a Km of 76.0 mM, a Vmax of 423 U mg(-1), and a Kcat/Km of 0.62 x 10(4) M(-1) s(-1) for UQ14S SI. UQ68J SI also showed no apparent reverse reaction producing glucose, fructose, or trehalulose from isomaltulose. These properties of the P. dispersa UQ68J enzyme are exceptional among purified SIs, and they indicate likely differences in the mechanism at the enzyme active site. They may favor the production of isomaltulose as an inhibitor of competing microbes in high-sucrose environments, and they are likely to be highly beneficial for industrial production of isomaltulose.

Expression, purification, crystallization and preliminary X-ray crystallographic studies of the trehalulose synthase MutB from Pseudomonas mesoacidophila MX-45

The trehalulose synthase (MutB) from Pseudomonas mesoacidophila MX-45, belonging to glycoside hydrolase family 13, catalyses the isomerization of sucrose to trehalulose (alpha-D-glucosylpyranosyl-1,1-D-fructofuranose) and isomaltulose (alpha-D-glucosylpyranosyl-1,6-D-fructofuranose) as main products and glucose and fructose in residual amounts from the hydrolytic reaction. To date, a three-dimensional structure of a sucrose isomerase that produces mainly trehalulose, as is the case for MutB, has been lacking. Crystallographic studies of this 64 kDa enzyme have therefore been initiated in order to contribute to the understanding of the molecular basis of sucrose decomposition, isomerization and of the selectivity of this enzyme that leads to the formation of different products. The MutB protein has been overexpressed, purified and crystallized using the hanging-drop vapour-diffusion method. Two different crystal forms have been obtained: one diffracts X-rays to 1.6 A resolution using synchrotron radiation and belongs to space group P1, with unit-cell parameters a = 63.8, b = 72.0, c = 82.2 A, alpha = 67.5, beta = 73.1, gamma = 70.8 degrees, while the other form diffracts to 1.8 A resolution using synchrotron radiation and belongs to space group P2(1), with unit-cell parameters a = 63.7, b = 85.9, c = 119.7 A, beta = 97.7 degrees. A molecular-replacement solution has been found using the structure of the isomaltulose synthase (PalI) from Klebsiella sp. LX3 as a search model.

Characterization of Pantoea dispersa UQ68J: producer of a highly efficient sucrose isomerase for isomaltulose biosynthesis

AIMS

Isolation, identification and characterization of a highly efficient isomaltulose producer.

METHODS AND RESULTS

After an enrichment procedure for bacteria likely to metabolize isomaltulose in sucrose-rich environments, 578 isolates were screened for efficient isomaltulose biosynthesis using an aniline/diphenylamine assay and capillary electrophoresis. An isolate designated UQ68J was exceptionally efficient in sucrose isomerase activity. Conversion of sucrose into isomaltulose by UQ68J (enzyme activity of 90-100 U mg(-1) DW) was much faster than the current industrial strain Protaminobacter rubrum CBS574.77 (41-66 U mg(-1) DW) or a reference strain of Erwinia rhapontici (0.3-0.9 U mg(-1) DW). Maximum yield of isomaltulose at 78-80% of supplied sucrose was achieved in less than half the reaction time needed by CBS574.77, and the amount of contaminating trehalulose (4%) was the lowest recorded from an isomaltulose-producing microbe. UQ68J is a Gram negative, facultatively anaerobic, motile, noncapsulate, straight rod-shaped bacterium producing acid but no gas from glucose. Based on 16S rDNA analysis UQ68J is closest to Klebsiella oxytoca, but it differs from Klebsiella in defining characteristics and most closely resembles Pantoea dispersa in phenotype.

SIGNIFICANCE AND IMPACT OF STUDY

This organism is likely to have substantial advantage over previously characterized sucrose isomerase producers for the industrial production of isomaltulose.

Isomaltulose synthase (PalI) of Klebsiella sp. LX3. Crystal structure and implication of mechanism

Isomaltulose synthase from Klebsiella sp. LX3 (PalI, EC 5.4.99.11) catalyzes the isomerization of sucrose to produce isomaltulose (alpha-D-glucosylpyranosyl-1,6-D-fructofuranose) and trehalulose (alpha-D-glucosylpyranosyl-1,1-d-fructofuranose). The PalI structure, solved at 2.2-A resolution with an R-factor of 19.4% and Rfree of 24.2%, consists of three domains: an N-terminal catalytic (beta/alpha)8 domain, a subdomain between N beta 3 and N alpha 3, and a C-terminal domain having seven beta-strands. The active site architecture of PalI is identical to that of other glycoside hydrolase family 13 members, suggesting a similar mechanism in substrate binding and hydrolysis. However, a unique RLDRD motif in the proximity of the active site has been identified and shown biochemically to be responsible for sucrose isomerization. A two-step reaction mechanism for hydrolysis and isomerization, which occurs in the same pocket is proposed based on both the structural and biochemical data. Selected C-terminal truncations have been shown to reduce and even abolish the enzyme activity, consistent with the predicted role of the C-terminal residues in the maintenance of enzyme conformation and active site topology.

Distinct sucrose isomerases catalyze trehalulose synthesis in whiteflies, Bemisia argentifolii, and Erwinia rhapontici

Isomaltulose [alpha-D-glucopyranosyl-(1,6)-D-fructofuranose] and trehalulose [alpha-D-glucopyranosyl-(1,1)-D-fructofuranose] are commercially valuable sucrose-substitutes that are produced in several microorganisms by the palI gene product, a sucrose isomerase. Trehalulose also occurs in the silverleaf whitefly, Bemisia argentifoli, as the major carbohydrate in the insect's honeydew. To determine if the enzyme that synthesizes trehalulose in whiteflies was similar to the well-characterized sucrose isomerase from microbial sources, the properties of the enzymes from whiteflies and the bacterium, Erwinia rhapontici, were compared. Partial purification of both enzymes showed that the enzyme from whiteflies was a 116 kD membrane-associated polypeptide, in contrast to the enzyme from E. rhapontici, which was soluble and 66 kD. The enzyme from E. rhapontici converted sucrose to isomaltulose and trehalulose in a 5:1 ratio, whereas the enzyme from whiteflies produced only trehalulose. Unlike the E. rhapontici enzyme, the whitefly enzyme did not convert isomaltulose to trehalulose, but both enzymes catalyzed the transfer of fructose to trehalulose using sucrose as the glucosyl donor. The results indicate that trehalulose synthase from whiteflies is structurally and functionally distinct from the sucrose isomerases described in bacteria. The whitefly enzyme is the first reported case of an enzyme that converts sucrose to exclusively trehalulose.

A motif rich in charged residues determines product specificity in isomaltulose synthase

Isomaltulose synthase (PalI) catalyzes hydrolysis of sucrose and formation of alpha-1,6 and alpha-1,1 bonds to produce isomaltulose (alpha-D-glucosylpyranosyl-1,6-D-fructofranose) and small amount of trehalulose (alpha-D-glucosylpyranosyl-1,1-D-fructofranose). A potential isomaltulose synthase-specific motif ((325)RLDRD(329)), that contains a 'DxD' motif conserved in many glycosyltransferases, was identified based on sequence comparison with reference to the secondary structural features of PalI and homologs. Site-directed mutagenesis analysis of the motif showed that the four charged amino acid residues (Arg(325), Arg(328), Asp(327) and Asp(329)) influence the enzyme kinetics and determine the product specificity. Mutation of these four residues increased trehalulose formation by 17-61% and decreased isomaltulose by 26-67%. We conclude that the 'RLDRD' motif controls the product specificity of PalI.