Engineering of Thermal Stability in a Cold-Active Oligo-1,6-Glucosidase from Exiguobacterium sibiricum with Unusual Amino Acid Content

Challenges and prospects of microbial α-amylases for industrial application: a review

α-Amylases are essential biocatalysts representing a billion-dollar market with significant long-term global demand. They have varied applications ranging from detergent, textile, and food sectors such as bakery to, more recently, biofuel industries. Microbial α-amylases have distinct advantages over their plant and animal counterparts owing to generally good activities and better stability at temperature and pH extremes. With the scope of applications expanding, the need for new and improved α-amylases is ever-growing. However, scaling up microbial α-amylase technology from the laboratory to industry for practical applications is impeded by several issues, ranging from mass transfer limitations, low enzyme yields, and energy-intensive product recovery that adds to high production costs. This review highlights the major challenges and prospects for the production of microbial α-amylases, considering the various avenues of industrial bioprocessing such as culture-independent approaches, nutrient optimization, bioreactor operations with design improvements, and product down-streaming approaches towards developing efficient α-amylases with high activity and recyclability. Since the sequence and structure of the enzyme play a crucial role in modulating its functional properties, we have also tried to analyze the structural composition of microbial α-amylase as a guide to its thermodynamic properties to identify the areas that can be targeted for enhancing the catalytic activity and thermostability of the enzyme through varied immobilization or selective enzyme engineering approaches. Also, the utilization of inexpensive and renewable substrates for enzyme production to isolate α-amylases with non-conventional applications has been briefly discussed.

Display of Oligo-α-1,6-Glycosidase from Exiguobacterium sibiricum on the Surface of Escherichia coli Cells

Cell-surface display using anchor motifs of outer membrane proteins allows exposure of target peptides and proteins on the surface of microbial cells. Previously, we obtained and characterized highly catalytically active recombinant oligo-α-1,6-glycosidase from the psychrotrophic bacterium Exiguobacterium sibiricum (EsOgl). It was also shown that the autotransporter AT877 from Psychrobacter cryohalolentis and its deletion variants efficiently displayed type III fibronectin (¹⁰Fn3) domain 10 on the surface of Escherichia coli cells. The aim of the work was to obtain an AT877-based system for displaying EsOgl on the surface of bacterial cells. The genes for the hybrid autotransporter EsOgl877 and its deletion mutants EsOgl877Δ239 and EsOgl877Δ310 were constructed, and the enzymatic activity of EsOgl877 was investigated. Cells expressing this protein retained ~90% of the enzyme maximum activity within a temperature range of 15-35°C. The activity of cells expressing EsOgl877Δ239 and EsOgl877Δ310 was 2.7 and 2.4 times higher, respectively, than of the cells expressing the full-size AT. Treatment of cells expressing EsOgl877 deletion variants with proteinase K showed that the passenger domain localized to the cell surface. These results can be used for further optimization of display systems expressing oligo-α-1,6-glycosidase and other heterologous proteins on the surface of E. coli cells.

Multiple approaches of loop region modification for thermostability improvement of 4,6-α-glucanotransferase from Limosilactobacillus fermentum NCC 3057

4,6-α-glucanotransferase (4,6-α-GT), as a member of the glycoside hydrolase 70 (GH70) family, converts starch/maltooligosaccharides into α,1-6 bond-containing α-glucan and possesses potential applications in food, medical and related industries but does not satisfy the high-temperature requirement due to its poor thermostability. In this study, a 4,6-α-GT (ΔGtfB) from Limosilactobacillus fermentum NCC 3057 was used as a model enzyme to improve its thermostability. The loops of ΔGtfB as the target region were optimized using directed evolution, sequence alignment, and computer-aided design. A total of 11 positive mutants were obtained and iteratively combined to obtain a combined mutant CM9, with high resistance to temperature (50 °C). The activity of mutant CM9 was 2.08-fold the activity of the wild type, accompanied by a 5 °C higher optimal temperature, a 5.76 °C higher melting point (T_m, 59.46 °C), and an 11.95-fold longer half-life time (t_1/2). The results showed that most of the polar residues in the loop region of ΔGtfB are mutated into rigid proline residues. Molecular dynamics simulation demonstrated that the root mean square fluctuation of CM9 significantly decreased by "Breathing" movement reduction of the loop region. This study provides a new strategy for improving the thermostability of 4,6-α-GT through rational loop region modification.

Designing liquefaction and saccharification processes of highly concentrated starch slurry: Challenges and recent advances

Starch-based sugars are an important group of starch derivatives used in food, medicine, chemistry, and other fields. The production of starch sugars involves starch liquefaction and saccharification processes. The production cost of starch sugars can be reduced by increasing the initial concentration of starch slurry. However, the usage of the highly concentrated starch slurry is characterized by challenges such as low reaction efficiency and poor product performance during the liquefaction and saccharification processes. In this study, we endeavored to provide a reference guide for improving high-concentration starch sugar production. Thus, we reviewed the effects of substrate concentration on the starch sugar production process and summarized several potential strategies. These regulation strategies, such as physical field pretreatment, complex enzyme-assisted, and temperature control, can significantly increase the starch concentration and mitigate the challenges of using highly concentrated starch slurry. We believe that highly concentrated starch sugar production will achieve a qualitative leap in the future. This review provides theoretical guidance and highlights the importance of high concentration in starch-based sugar production. Further studies are needed to explore the fine structure and enzyme attack mode during the liquefaction and saccharification processes to regulate the production of more targeted products.

Improved thermostability of D-allulose 3-epimerase from Clostridium bolteae ATCC BAA-613 by proline residue substitution

d-allulose, a rare sugar that is scarce in nature, exerts several beneficial effects and has commercial potential. d-allulose 3-epimerase (DAEase) plays a vital role in catalyzing the isomerization from d-fructose to d-allulose. However, the industrial application of DAEase for d-allulose production is hindered by its poor long-term thermostability. In the present research, we introduced a proline residue (i) to restrict its spatial conformation and (ii) to reduce the entropy of the unfolded state of DAEase. The t_1/2 value of the double-site Clostridium bolteae DAEase mutant Cb-51P/89P was prolonged to 58 min at 55 °C, a 2.32-fold increase compared with wild-type DAEase. The manipulation did not cause obvious changes in the enzymatic properties, including optimum pH, optimal temperature, optimum metal ion, and enzymatic activity. As the accumulation of multiple small effects through proline substitution could dramatically improve the thermostability of the mutant protein, our method to improve the thermostability while roughly retaining the original enzymatic properties is promising.

Thermostability engineering of industrial enzymes through structure modification

Thermostability is an essential requirement of enzymes in the industrial processes to catalyze the reactions at high temperatures; thus, enzyme engineering through directed evolution, semi-rational design and rational design are commonly employed to construct desired thermostable mutants. Several strategies are implemented to fulfill enzymes' thermostability demand including decreasing the entropy of the unfolded state through substitutions Gly → Xxx or Xxx → Pro, hydrogen bond, salt bridge, introducing two different simultaneous interactions through single mutant, hydrophobic interaction, filling the hydrophobic cavity core, decreasing surface hydrophobicity, truncating loop, aromatic-aromatic interaction and introducing positively charged residues to enzyme surface. In the current review, horizons about compatibility between secondary structures and substitutions at preferable structural positions to generate the most desirable thermostability in industrial enzymes are broadened. KEY POINTS: • Protein engineering is a powerful tool for generating thermostable industrial enzymes. • Directed evolution and rational design are practical approaches in enzyme engineering. • Substitutions in preferable structural positions can increase thermostability.