Immobilization of Recombinant Glucose Isomerase for Efficient Production of High Fructose Corn Syrup

Xylitol binding to the M1 site of glucose isomerase induces a conformational change in the substrate binding channel

Glucose isomerase (GI) is extensively used in the food industry for production of high-fructose corn syrup and for the production of biofuels and other renewable chemicals. Structure-based studies on GI inhibitors are important for improving its efficiency in industrial applications. Here, we report the subatomic crystal structure of Streptomyces rubiginosus GI (SruGI) complexed with its inhibitor, xylitol, at 0.99 Å resolution. Electron density map and temperature factor analysis showed partial binding of xylitol to the M1 metal binding site of SruGI, providing two different conformations of the metal binding site and the substrate binding channel. The xylitol molecule induced a conformational change in the M2 metal ion-interacting Asp255 residue, which subsequently led to a conformational change in the side chain of Asp181 residue. This led to the positional shift of Pro25 by 1.71 Å and side chain rotation of Phe26 by 21°, where located on the neighboring protomer in tetrameric SruGI. The conformation change of these two residues affect the size of the substrate-binding channel of GI. Therefore, xylitol binding to M1 site of SruGI induces not only a conformational changes of the metal-binding site, but also conformational change of substrate-binding channel of the tetrameric SruGI. These results expand our knowledge about the mechanism underlying the inhibitory effect of xylitol on GI.

Microbial glucoamylases: structural and functional properties and biotechnological uses

Glucoamylases (GAs) are one of the principal groups of enzymes involved in starch hydrolysis and belong to the glycosylhydrolase family. They are classified as exo-amylases due to their ability to hydrolyze α-1,4 glycosidic bonds from the non-reducing end of starch, maltooligosaccharides, and related substrates, releasing β-D-glucose. Structurally, GAs possess a characteristic catalytic domain (CD) with an (α/α)6 fold and exhibit five conserved regions within this domain. The CD may or may not be linked to a non-catalytic domain with variable functions depending on its origin. GAs are versatile enzymes with diverse applications in food, biofuel, bioplastic and other chemical industries. Although fungal GAs are commonly employed for these purposes, they have limitations such as their low thermostability and an acidic pH requirement. Alternatively, GAs derived from prokaryotic organisms are a good option to save costs as they exhibit greater thermostability compared to fungal GAs. Moreover, a group of cold-adapted GAs from psychrophilic organisms demonstrates intriguing properties that make them suitable for application in various industries. This review provides a comprehensive overview of the structural and sequential properties as well as biotechnological applications of GAs in different industrial processes.

Novel Recyclable UCST-Type Immobilized Glucose Isomerase Biocatalyst with Excellent Performance for Isomerization of Glucose to Fructose

The development of a suitable immobilization strategy to improve the performance of immobilized glucose isomerase for the isomerization of glucose to fructose is crucial to promoting the industrial production of high-fructose syrup. In this work, a novel recyclable upper critical solution temperature (UCST)-type mVBA-b-P(AAm-co-AN)@glucose isomerase biocatalyst (PVAA@GI) was prepared, and the immobilized glucose isomerase could capture the glucose substrate through the affinity of 4-vinylbenzeneboronic acid (4-VBA) and the glucose substrate, which led to the enhanced substrate affinity and catalytic efficiency of the PVAA@GI. The biocatalyst exhibited excellent stability in pH, thermal, storage, and recycling compared to the free enzyme. The mVBA-b-P(AAm-co-AN)@glucose isomerase biocatalyst displayed reversibly soluble-insoluble characteristics with temperature change, which was in the soluble state during the enzyme reaction process but could be recovered in an insoluble form by lowering the temperature after the reaction. The highest fructose production rate reached 62.79%, which would have potential application in the industrial production of high-fructose syrup.

Crystal structure of a novel putative sugar isomerase from the psychrophilic bacterium Paenibacillus sp. R4

Sugar isomerases (SIs) catalyze the reversible conversion of aldoses to ketoses. A novel putative SI gene has been identified from the genome sequence information on the psychrophilic bacterium Paenibacillus sp. R4. Here, we report the crystal structure of the putative SI from Paenibacillus sp. R4 (PbSI) at 2.98 Å resolution. It was found that the overall structure of PbSI adopts the triose-phosphate isomerase (TIM) barrel fold. PbSI was also identified to have two heterogeneous metal ions as its cofactors at the active site in the TIM barrel, one of which was confirmed as a Zn ion through X-ray anomalous scattering and inductively coupled plasma mass spectrometry analysis. Structural comparison with homologous SI proteins from mesophiles, hyperthermophiles, and a psychrophile revealed that key residues in the active site are well conserved and that dimeric PbSI is devoid of the extended C-terminal region, which tetrameric SIs commonly have. Our results provide novel structural information on the cold-adaptable SI, including information on the metal composition in the active site.

Room-Temperature Structure of Xylitol-Bound Glucose Isomerase by Serial Crystallography: Xylitol Binding in the M1 Site Induces Release of Metal Bound in the M2 Site

Glucose isomerase (GI) is an important enzyme that is widely used in industrial applications, such as in the production of high-fructose corn syrup or bioethanol. Studying inhibitor effects on GI is important to deciphering GI-specific molecular functions, as well as potential industrial applications. Analysis of the existing xylitol-bound GI structure revealed low metal occupancy at the M2 site; however, it remains unknown why this phenomenon occurs. This study reports the room-temperature structures of native and xylitol-bound GI from Streptomyces rubiginosus (SruGI) determined by serial millisecond crystallography. The M1 site of native SruGI exhibits distorted octahedral coordination; however, xylitol binding results in the M1 site exhibit geometrically stable octahedral coordination. This change results in the rearrangement of metal-binding residues for the M1 and M2 sites, the latter of which previously displayed distorted metal coordination, resulting in unstable coordination of Mg²⁺ at the M2 site and possibly explaining the inducement of low metal-binding affinity. These results enhance the understanding of the configuration of the xylitol-bound state of SruGI and provide insights into its future industrial application.

Immobilization of recombinant Escherichia coli cells expressing glucose isomerase using modified diatomite as a carrier for effective production of high fructose corn syrup in packed bed reactor

To improve the operational stability of glucose isomerase in E. coli TEGI-W139F/V186T, the immobilized cells were prepared with modified diatomite as a carrier and 74.1% activity of free cells was recovered after immobilization. Results showed that the immobilized cells still retained 86.2% of the initial transformational activity after intermittent reused 40 cycles and the yield of D-fructose reached above 42% yield at 60 °C. Moreover, the immobilized cells were employed in the continuous production of High Fructose Corn Syrup (HFCS) in a recirculating packed bed reactor for 603 h at a constant flow rate. It showed that the immobilized cells exhibited good operational stability and the yield of D-fructose retained above 42% within 603 h. The space-time yield of high fructose corn syrup reached 3.84 kg L^-1 day^-1. The investigation provided an efficient immobilization method for recombinant cells expressing glucose isomerase with higher stability, and the immobilized cells are a promising biocatalyst for HFCS production.

Activity, stability, and binding capacity of β-galactosidase immobilized on electrospun nylon-6 fiber membrane

In this research, we explored various immobilized enzyme support materials, including the novel nylon-6 fiber membrane (NFM), and evaluated the increase in surface area and its effect on enzyme binding potential. We also manipulated incubation and reaction conditions and assessed the subsequent effects on activity and stability of β-galactosidase, with comparisons between various solid support materials and free (dissolved) enzyme. Nylon-6 fiber membranes were created by electrospinning and were compared with other materials as solid supports for enzyme binding. The other materials included polyvinylidene fluoride 5-kDa nanofiltration dairy membranes, nylon-6 pellets, and silica glass beads. Scanning electron microscopy revealed the large surface area of NFM, which correlated with greater enzyme activity compared with the relatively flatter surfaces of the other solid support materials. Enzyme activity was measured spectrophotometrically with the color-changing substrate o-nitrophenyl-β-d-galactopyranoside. Compared with the other solid supports, NFM had greater maximum enzyme binding potential. Across pH conditions ranging from 3.5 to 6.0 (including the optimal pH of 4.0-5.0), enzyme activity was maintained on the membrane-immobilized samples, whereas free enzyme did not maintain activity. Altering the storage temperature (4, 22, and 50°C) affected enzyme stability (i.e., the ability of the enzyme to maintain activity over time) of free and polyvinylidene fluoride membrane samples. However, NFM samples maintained stability across the varying storage temperatures. Increasing the immobilization solution enzyme concentration above the maximum enzyme binding capacity had no significant effect on enzyme stability for membrane-immobilized samples; however, both had lower mean stability than free enzyme by approximately 74%. With further development, β-galactosidase immobilized on NFM or other membranes could be used in continuous processing in the dairy industry for a combination of filtration and lactose hydrolysis-creating products that are reduced in lactose and increased in sweetness, with no requirement for "added sugars" on the nutrition label and no enzyme listed as final product ingredient.

A Novel Glucose Isomerase from Caldicellulosiruptor bescii with Great Potentials in the Production of High-Fructose Corn Syrup

Glucose isomerase (GI) that catalyzes the conversion of D-glucose to D-fructose is one of the most important industrial enzymes for the production of high-fructose corn syrup (HFCS). In this study, a novel GI (CbGI) was cloned from Caldicellulosiruptor bescii and expressed in Escherichia coli. The purified recombinant CbGI (rCbGI) showed neutral and thermophilic properties. It had optimal activities at pH 7.0 and 80°C and retained stability at 85°C. In comparison with other reported GIs, rCbGI exhibited higher substrate affinity (Km = 42.61 mM) and greater conversion efficiency (up to 57.3% with 3M D-glucose as the substrate). The high catalytic efficiency and affinity of this CbGI is much valuable for the cost-effective production of HFCS.

Purification and kinetic behavior of glucose isomerase from Streptomyces lividans RSU26

Glucose isomerase (GI), an enzyme with deserved high potential in the world market. GI plays a major role in high Fructose Corn Syrup Production (HFCS). HFCS is used as a sweetener in food and pharmaceutical industries. Streptomyces are well-known producers of various industrially valuable enzymes, including Glucose isomerase. Currently, recombinant strains have been available for the production of various enzymes, but it has limitation in the large scale production. Therefore, identifying effective streptomyces strains have emerged. The current study, the novel S. lividans RSU26 was isolated from a marine source and optimized its potential to produce glucose isomerase at different physical and chemical conditions. The optimum pH and temperature for GI and biomass production were 7.5 and 35 °C, respectively at 96 h. Characterization study revealed that the approximate molar mass of GI was 43 kDa for monomeric and 170 kDa for tetrameric forms. Kinetic behavior exhibits Km, and Vmax values for the conversion of fructose to glucose conversion were 48.8 mM and 2.54 U mg⁻¹ at 50 °C and glucose to fructose were 29.4 mM and 2.38 U mg⁻¹ at 65 °C protein, respectively. Therefore, the present study suggested that the wild–type S. lividans RSU26 has strong potential to produce glucose isomerase for various industrial applications.

Immobilization of recombinant Escherichia coli whole cells harboring xylose reductase and glucose dehydrogenase for xylitol production from xylose mother liquor

In this study, recombinant E. coli BL21(DE3)/pCDFDuet-1-XR-GDH harboring xylose reductase (XR) and glucose dehydrogenase (GDH) were immobilized and applied for the production of xylitol from xylose mother liquor (XML). Various immobilization methods were screened and the cross-linking approach with diatomite and polyetherimide as the raw materials and glutaraldehyde as the cross-linking agent was the optimal one, and the recovery activity reached of 80.3% after immobilization. The half-life of immobilized cells was 1.52 times to that of free cells. Batch experiments showed that the enzyme activity of immobilized cells remained 70.5% of the initial activity after 10 batches and the space-time yield of xylitol reached of 11.5 g/(L h). The production of xylitol from xylose mother liquor by immobilized E. coli cells containing xylose reductase and glucose dehydrogenase was reported for the first time, which paved a foundation for industrial production of xylitol from waste xylose mother liquor.

Structural analysis of substrate recognition by glucose isomerase in Mn2+ binding mode at M2 site in S. rubiginosus

Glucose isomerase (GI) catalyzes the reversible enzymatic isomerization of d-glucose and d-xylose to d-fructose and d-xylulose, respectively. This is one of the most important enzymes in the production of high-fructose corn syrup (HFCS) and biofuel. We recently determined the crystal structure of GI from S. rubiginosus (SruGI) complexed with a xylitol inhibitor in one metal binding mode. Although we assessed inhibitor binding at the M1 site, the metal binding at the M2 site and the substrate recognition mechanism for SruGI remains the unclear. Here, we report the crystal structure of the two metal binding modes of SruGI and its complex with glucose. This study provides a snapshot of metal binding at the SruGI M2 site in the presence of Mn²⁺, but not in the presence of Mg²⁺. Metal binding at the M2 site elicits a configuration change at the M1 site. Glucose molecule can only bind to the M1 site in presence of Mn²⁺ at the M2 site. Glucose and Mn²⁺ at the M2 site were bridged by water molecules using a hydrogen bonding network. The metal binding geometry of the M2 site indicates a distorted octahedral coordination with an angle of 55-110°, whereas the M1 site has a relatively stable octahedral coordination with an angle of 85-95°. We suggest a two-step sequential process for SruGI substrate recognition, in Mn²⁺ binding mode, at the M2 site. Our results provide a better understanding of the molecular role of the M2 site in GI substrate recognition.

Crystal structure of glucose isomerase in complex with xylitol inhibitor in one metal binding mode

Glucose isomerase (GI) is an intramolecular oxidoreductase that interconverts aldoses and ketoses. These characteristics are widely used in the food, detergent, and pharmaceutical industries. In order to obtain an efficient GI, identification of novel GI genes and substrate binding/inhibition have been studied. Xylitol is a well-known inhibitor of GI. In Streptomyces rubiginosus, two crystal structures have been reported for GI in complex with xylitol inhibitor. However, a structural comparison showed that xylitol can have variable conformation at the substrate binding site, e.g., a nonspecific binding mode. In this study, we report the crystal structure of S. rubiginosus GI in a complex with xylitol and glycerol. Our crystal structure showed one metal binding mode in GI, which we presumed to represent the inactive form of the GI. The metal ion was found only at the M1 site, which was involved in substrate binding, and was not present at the M2 site, which was involved in catalytic function. The O₂ and O₄ atoms of xylitol molecules contributed to the stable octahedral coordination of the metal in M1. Although there was no metal at the M2 site, no large conformational change was observed for the conserved residues coordinating M2. Our structural analysis showed that the metal at the M2 site was not important when a xylitol inhibitor was bound to the M1 site in GI. Thus, these findings provided important information for elucidation or engineering of GI functions.