Enhanced Production of β-D-glycosidase and α-L-arabinofuranosidase in Recombinant Escherichia coli in Fed-batch Culture for the Biotransformation of Ginseng Leaf Extract to Ginsenoside Compound K

Biotransformation of High Concentrations of Ginsenoside Substrate into Compound K by β-glycosidase from Sulfolobus solfataricus

The rare ginsenoside Compound K (CK) is an attractive ingredient in traditional medicines, cosmetics, and the food industry because of its various biological activities. However, it does not exist in nature. The commonly used method for the production of CK is enzymatic conversion. In order to further improve the catalytic efficiency and increase the CK content, a thermostable β-glycosidase from Sulfolobus solfataricus was successfully expressed in Pichia pastoris and secreted into fermentation broth. The recombinant SS-bgly in the supernatant showed enzyme activity of 93.96 U/mg at 120 h when using pNPG as substrate. The biotransformation conditions were optimized at pH 6.0 and 80 °C, and its activity was significantly enhanced in the presence of 3 mM Li⁺. When the substrate concentration was 10 mg/mL, the recombinant SS-bgly completely converted the ginsenoside substrate to CK with a productivity of 507.06 μM/h. Moreover, the recombinant SS-bgly exhibited extraordinary tolerance against high substrate concentrations. When the ginsenoside substrate concentration was increased to 30 mg/mL, the conversion could still reach 82.5% with a productivity of 314.07 μM/h. Thus, the high temperature tolerance, resistance to a variety of metals, and strong substrate tolerance make the recombinant SS-bgly expressed in P. pastoris a potential candidate for the industrial production of the rare ginsenoside CK.

Increased Production of Ginsenoside Compound K by Optimizing the Feeding of American Ginseng Extract during Fermentation by Aspergillus tubingensis

The ginsenoside compound K (C-K) is widely used in traditional medicines, nutritional supplements, and cosmetics owing to its diverse pharmacological activities. Although many studies on C-K production have been conducted, fermentation is reported to produce C-K with low concentration and productivity. In the present study, addition of an inducer and optimization of the carbon and nitrogen sources in the medium were performed using response surface methodology to increase the C-K production via fermentation by Aspergillus tubingensis, a generally recognized as safe fungus. The optimized inducer and carbon and nitrogen sources were 2 g/l rice straw, 10 g/l sucrose, and 10 g/l soy protein concentrate, respectively, and they resulted in a 3.1-fold increase in the concentration and productivity of C-K (0.22 g/l and 1.52 mg/l/h, respectively) compared to those used before optimization without inducer (0.071 g/l and 0.49 mg/l/h, respectively). The feeding methods of American ginseng extract (AGE), including feeding timing, feeding concentration, and feeding frequency, were also optimized. Under the optimized conditions, A. tubingensis produced 3.96 mM (2.47 g/l) C-K at 144 h by feeding two times with 8 g/l AGE at 48 and 60 h, with a productivity of 17.1 mg/l/h. The concentration and productivity of C-K after optimization of feeding methods were 11-fold higher than those before the optimization (0.22 g/l and 1.52 mg/l/h, respectively). Thus, the optimization for the feeding methods of ginseng extract is an efficient strategy to increase C-K production. To our knowledge, this is the highest reported C-K concentration and productivity via fermentation reported so far.

Spectroscopic and in silico investigation of the interaction between GH1 β-glucosidase and ginsenoside Rb1

The function and application of β-glucosidase attract attention nowadays. β-glucosidase was confirmed of transforming ginsenoside Rb1 to rare ginsenoside, but the interaction mechanism remains not clear. In this work, β-glucosidase from GH1 family of Paenibacillus polymyxa was selected, and its gene sequence bglB was synthesized by codon. Then, recombinant plasmid was transferred into Escherichia coli BL21 (DE3) and expressed. The UV-visible spectrum showed that ginsenoside Rb1 decreased the polarity of the corresponding structure of hydrophobic aromatic amino acids (Trp) in β-glucosidase and increased new π-π* transition. The fluorescence quenching spectrum showed that ginsenoside Rb1 inhibited intrinsic fluorescence, formed static quenching, reduced the surface hydrophobicity of β-glucosidase, and KSV was 8.37 × 103 L/M (298K). Circular dichroism (CD) showed that secondary structure of β-glucosidase was changed by the binding action. Localized surface plasmon resonance (LSPR) showed that β-glucosidase and Rb1 had strong binding power which KD value was 5.24 × 10-4 (±2.35 × 10-5) M. Molecular docking simulation evaluated the binding site, hydrophobic force, hydrogen bond, and key amino acids of β-glucosidase with ginsenoside Rb1 in the process. Thus, this work could provide basic mechanisms of the binding and interaction between β-glucosidase and ginsenoside Rb1.

Two Key Amino Acids Variant of α-l-arabinofuranosidase from Bacillus subtilis Str. 168 with Altered Activity for Selective Conversion Ginsenoside Rc to Rd

α-l-arabinofuranosidase is a subfamily of glycosidases involved in the hydrolysis of l-arabinofuranosidic bonds, especially in those of the terminal non-reducing arabinofuranosyl residues of glycosides, from which efficient glycoside hydrolases can be screened for the transformation of ginsenosides. In this study, the ginsenoside Rc-hydrolyzing α-l-arabinofuranosidase gene, BsAbfA, was cloned from Bacilus subtilis, and its codons were optimized for efficient expression in E. coli BL21 (DE3). The recombinant protein BsAbfA fused with an N-terminal His-tag was overexpressed and purified, and then subjected to enzymatic characterization. Site-directed mutagenesis of BsAbfA was performed to verify the catalytic site, and the molecular mechanism of BsAbfA catalyzing ginsenoside Rc was analyzed by molecular docking, using the homology model of sequence alignment with other β-glycosidases. The results show that the purified BsAbfA had a specific activity of 32.6 U/mg. Under optimal conditions (pH 5, 40 °C), the kinetic parameters K_m of BsAbfA for pNP-α-Araf and ginsenoside Rc were 0.6 mM and 0.4 mM, while the K_cat/K_m were 181.5 s^-1 mM^-1 and 197.8 s^-1 mM^-1, respectively. More than 90% of ginsenoside Rc could be transformed by 12 U/mL purified BsAbfA at 40 °C and pH 5 in 24 h. The results of molecular docking and site-directed mutagenesis suggested that the E173 and E292 variants for BsAbfA are important in recognizing ginsenoside Rc effectively, and to make it enter the active pocket to hydrolyze the outer arabinofuranosyl moieties at C₂₀ position. These remarkable properties and the catalytic mechanism of BsAbfA provide a good alternative for the effective biotransformation of the major ginsenoside Rc into Rd.

Effect of surface charge conditions of carriers on the immobilization of β-d-glucosidase

In this study, a series of acidic or alkaline polypeptide chains were designed and grafted onto DEG-AM resin using Fmoc solid-phase synthesis to study the relationship between enzyme conformation and carrier surface charge. β-d-glucosidase (βGase) was then immobilized onto these modified carriers by adsorption. Each form of immobilized βGase showed decreasing specific activity compared to that of the free. It could be attributed to both the changes in the enzyme conformation and the decrease in mass transfer efficiency. The optimum temperature of free βGase, DEG@B3-βGase is 55 °C, which of DEG@A3-βGase is 65 °C and they all have the highest activity at pH 5. The E_a values ​​of free βGase, DEG@A3-βGase, and DEG@B3-βGase are 0.546 kJ/mol, 0.224 kJ/mol, and 0.446 kJ/mol, and the K_m values were 1.30 mmol/L, 1.44 mmol/L and 2.63 mmol/L, respectively. It shows that free βGase and DEG@A3-βGase are more similar. Meanwhile, the free βGase (1.0 g/L, pH 5.0) stored at 4 °C has a shorter half-life (t_1/2), which is only 9 days. However, the half-life of DEG@B3-βGase and DEG@A3-βGase is 20 days and over 60 days, indicating that the negative charged surface was conducive to maintenance of the structure and catalytic property of βGase.

Endophytes, biotransforming microorganisms, and engineering microbial factories for triterpenoid saponins production

Triterpenoid saponins are structurally diverse secondary metabolites. They are the main active ingredient of many medicinal plants and have a wide range of pharmacological effects. Traditional production of triterpenoid saponins, directly extracted from cultivated plants, cannot meet the rapidly growing demand of pharmaceutical industry. Microorganisms with triterpenoid saponins production ability (especially Agrobacterium genus) and biotransformation ability, such as fungal species in Armillaria and Aspergillus genera and bacterial species in Bacillus and Intestinal microflora, represent a valuable source of active metabolites. With the development of synthetic biology, engineering microorganisms acquired more potential in terms of triterpenoid saponins production. This review focusses on potential mechanisms and the high yield strategies of microorganisms with inherent production or biotransformation ability of triterpenoid saponins. Advances in the engineering of microorganisms, such as Saccharomyces cerevisiae, Yarrowia lipolytica, and Escherichia coli, for the biosynthesis triterpenoid saponins de novo have also been reported. Strategies to increase the yield of triterpenoid saponins in engineering microorganisms are summarized following four aspects, that is, introduction of high efficient gene, optimization of enzyme activity, enhancement of metabolic flux to target compounds, and optimization of fermentation conditions. Furthermore, the challenges and future directions for improving the yield of triterpenoid saponins biosynthesis in engineering microorganisms are discussed.