Insight into the glycosylation and hydrolysis kinetics of alpha-glucosidase in the synthesis of glycosides

Acremonium sp. diglycosidase-aid chemical diversification: valorization of industry by-products

The fungal diglycosidase α-rhamnosyl-β-glucosidase I (αRβG I) from Acremonium sp. DSM 24697 catalyzes the glycosylation of various OH-acceptors using the citrus flavanone hesperidin. We successfully applied a one-pot biocatalysis process to synthesize 4-methylumbellipheryl rutinoside (4-MUR) and glyceryl rutinoside using a citrus peel residue as sugar donor. This residue, which contained 3.5 % [w/w] hesperidin, is the remaining of citrus processing after producing orange juice, essential oil, and peel-juice. The low-cost compound glycerol was utilized in the synthesis of glyceryl rutinoside. We implemented a simple method for the obtention of glyceryl rutinoside with 99 % yield, and its purification involving activated charcoal, which also facilitated the recovery of the by-product hesperetin through liquid-liquid extraction. This process presents a promising alternative for biorefinery operations, highlighting the valuable role of αRβG I in valorizing glycerol and agricultural by-products. KEYPOINTS: • αRβG I catalyzed the synthesis of rutinosides using a suspension of OPW as sugar donor. • The glycosylation of aliphatic polyalcohols by the αRβG I resulted in products bearing a single rutinose moiety. • αRβG I catalyzed the synthesis of glyceryl rutinoside with high glycosylation/hydrolysis selectivity (99 % yield).

Structures, Biochemical Characteristics, and Functions of β-Xylosidases

The complete degradation of abundant xylan derived from plants requires the participation of β-xylosidases to produce the xylose which can be converted to xylitol, ethanol, and other valuable chemicals. Some phytochemicals can also be hydrolyzed by β-xylosidases into bioactive substances, such as ginsenosides, 10-deacetyltaxol, cycloastragenol, and anthocyanidins. On the contrary, some hydroxyl-containing substances such as alcohols, sugars, and phenols can be xylosylated by β-xylosidases into new chemicals such as alkyl xylosides, oligosaccharides, and xylosylated phenols. Thus, β-xylosidases shows great application prospects in food, brewing, and pharmaceutical industries. This review focuses on the molecular structures, biochemical properties, and bioactive substance transformation function of β-xylosidases derived from bacteria, fungi, actinomycetes, and metagenomes. The molecular mechanisms of β-xylosidases related to the properties and functions are also discussed. This review will serve as a reference for the engineering and application of β-xylosidases in food, brewing, and pharmaceutical industries.

Engineering of Cyclodextrin Glycosyltransferase through a Size/Polarity Guided Triple-Code Strategy with Enhanced α-Glycosyl Hesperidin Synthesis Ability

Hesperidin, a flavonoid enriched in citrus peel, can be enzymatically glycosylated using CGTase with significantly improved water solubility. However, the reaction catalyzed by wild-type CGTase is rather inefficient, reflected in the poor production rate and yield. By focusing on the aglycon attacking step, seven residues were selected for mutagenesis in order to improve the transglycosylation efficiency. Due to the lack of high-throughput screening technology regarding to the studied reaction, we developed a size/polarity guided triple-code strategy in order to reduce the library size. The selected residues were replaced by three rationally chosen amino acids with either changed size or polarity, leading to an extremely condensed library with only 32 mutants to be screened. Twenty-five percent of the constructed mutants were proved to be positive, suggesting the high quality of the constructed library. Specific transglycosylation activity of the best mutant Y217F was assayed to be 935.7 U/g, and its k_cat/K_mA is 6.43 times greater than that of the wild type. Homology modeling and docking computation suggest the source of notably enhanced catalytic efficiency is resulted from the combination of ligand transfer and binding effect. IMPORTANCE Size/polarity guided triple-code strategy, a novel semirational mutagenesis strategy, was developed in this study and employed to engineer the aglycon attacking site of CGTase. Screening pressure was set as improved hesperidin glucoside synthesis ability, and eight positive mutants were obtained by screening only 32 mutants. The high quality of the designed library confirms the effectiveness of the developed strategy is potentially valuable to future mutagenesis studies. Mechanisms of positive effect were explained. The best mutant exhibits 6.43 times enhanced k_cat/K_mA value and confirmed to be a superior whole-cell catalyst with potential application value in synthesizing hesperidin glucosides.

Biocatalytic synthesis of 2-O-α-D-glucopyranosyl-L-ascorbic acid using an extracellular expressed α-glucosidase from Oryza sativa

BACKGROUND

2-O-α-D-Glucopyranosyl-L-ascorbic acid (AA-2G) is an important derivative of L-ascorbic acid (L-AA), which has the distinct advantages of non-reducibility, antioxidation, and reproducible decomposition into L-AA and glucose. Enzymatic synthesis is a preferred method for AA-2G production over alternative chemical synthesis owing to the regioselective glycosylation reaction. α-Glucosidase, an enzyme classed into O-glycoside hydrolases, might be used in glycosylation reactions to synthesize AA-2G.

MAIN METHODS AND MAJOR RESULTS

Here, an α-glucosidase from Oryza sativa was heterologously produced in Pichia pastoris GS115 and used for biosynthesis of AA-2G with few intermediates and byproducts. The extracellular recombinant α-glucosidase (rAGL) reached 9.11 U mL^-1 after fed-batch cultivation for 102 h in a 5 L fermenter. The specific activity of purified rAGL is 49.83 U mg^-1 at 37°C and pH 4.0. The optimal temperature of rAGL was 65°C, and it was stable below 55°C. rAGL was active over the range of pH 3.0-7.0, with the maximal activity at pH 4.0. Under the condition of 37°C, pH 4.0, equimolar maltose and ascorbic acid sodium salt, 8.7 ± 0.4 g L^-1  of AA-2G was synthesized by rAGL.

CONCLUSIONS AND IMPLICATIONS

The production of rAGL in P. pastoris was proved to be beneficial in providing enough enzyme and promoting biocatalytic synthesis of AA-2G. These studies lay the basis for the industrial application of α-glucosidase.

GRAPHICAL ABSTRACT LAY SUMMARY

2-O-α-D-Glucopyranosyl-L-ascorbic acid (AA-2G) is an important industrial derivative of L-ascorbic acid (L-AA), which has the distinct advantages of non-reducibility, antioxidation, and reproducible decomposition into L-AA and glucose. In this study, the authors characterized an α-glucosidase from Oryza sativa, which was recombinantly produced in Pichia pastoris GS115, and its potential for AA-2G production via transglycosylation of L-AA was investigated. These studies lay the basis for the industrial application of recombinant α-glucosidase.

Inhibitory potential of 4-hexylresorcinol against α-glucosidase and non-enzymatic glycation: Activity and mechanism

Inhibition of α-glucosidase as well as non-enzymatic glycation is thought as an effective method for treating type-2 diabetes mellitus. In this study, we investigated the inhibitory potential and mechanism of 4-hexylresorcinol against α-glucosidase and non-enzymatic glycation by using multispectroscopic analyses and molecular docking. The results of enzyme kinetics showed that 4-hexylresorcinol reversibly inhibited α-glucosidase activity in a noncompetitive way. Fluorescence quenching then revealed that it increased the hydrophobicity of α-glucosidase and changed the conformation of the enzyme by forming the α-glucosidase-hexylresorcinol complex. Thermodynamic analysis and molecular docking further demonstrated that the inhibition of 4-hexylresorcinol on the α-glucosidase was mainly dependent on hydrogen bond and hydrophobic interaction. Moreover, the 4-hexylresorcinol moderately inhibited the formation of fructosamine, and strongly suppressed the generation of α-dicarbonyl compounds and advanced glycation end products (AGEs). The interaction between 4-hexylresorcinol and bovine serum albumin was mainly driven by hydrophobic interaction. This study showed a novel inhibitor of α-glucosidase as well as non-enzymatic glycation, and provided a drug candidate for the prevention and treatment of type-2 diabetes.

Glycosyl hydrolase catalyzed glycosylation in unconventional media

The reversible hydrolytic property of glycosyl hydrolases (GHs) as well as their acceptance of aglycones other than water has provided the abilities of GHs in synthesizing glycosides. Together with desirable physiochemical properties of glycosides and their high commercial values, research interests have been aroused to investigate the synthetic other than the hydrolytic properties of GHs. On the other hand, just like the esterification processes catalyzed by lipases, GH synthetic effectiveness is strongly obstructed by water both thermodynamically and kinetically. Medium engineering by involving organic solvents can be a viable approach to alleviate the obstacles caused by water. However, as native hydrolyases function in water-enriched environments, most GHs display poor catalytic performance in the presence of organic solvents. Some GHs from thermophiles are more tolerant to organic solvents due to their robust folded structures with strong residue interactions. Other than native sources, immobilization, protein engineering, employment of surfactant, and lyophilization have been proved to enhance the GH stability from the native state, which opens up the possibilities for GHs to be employed in unconventional media as synthases. KEY POINTS: • Unconventional media enhance the synthetic ability but destabilize GHs. • Viable approaches are discussed to improve GH stability from the native state. • GHs robust in unconventional media can be valuable industrial synthases.

A novel strategy for efficient disaccharides synthesis from glucose by β-glucosidase

Oligosaccharides have important therapeutic applications. A useful route for oligosaccharides synthesis is reverse hydrolysis by β-glucosidase. However, the low conversion efficiency of disaccharides from monosaccharides limits its large-scale production because the equilibrium is biased in the direction of hydrolysis. Based on the analysis of the docking results, we hypothesized that the hydropathy index of key amino acid residues in the catalytic site is closely related with disaccharide synthesis and more hydrophilic residues located in the catalytic site would enhance reverse hydrolysis activity. In this study, positive variants TrCel1bI177S, TrCel1bI177S/I174S, and TrCel1bI177S/I174S/W173H, and one negative variant TrCel1bN240I were designed according to the Hydropathy Index For Enzyme Activity (HIFEA) strategy. The reverse hydrolysis with TrCel1bI177S/I174S/W173H was accelerated and then the maximum total production (195.8 mg/mL/mg enzyme) of the synthesized disaccharides was increased by 3.5-fold compared to that of wild type. On the contrary, TrCel1bN240I lost reverse hydrolysis activity. The results demonstrate that the average hydropathy index of the key amino acid residues in the catalytic site of TrCel1b is an important factor for the synthesis of laminaribiose, sophorose, and cellobiose. The HIFEA strategy provides a new perspective for the rational design of β-glucosidases used for the synthesis of oligosaccharides.