Purification and characterization of a novel β-glucuronidase precisely converts glycyrrhizin to glycyrrhetinic acid 3-O-mono-β-D-glucuronide from plant endophytic Chaetomium globosum DX-THS3

Production of bilirubin via whole-cell transformation utilizing recombinant Corynebacterium glutamicum expressing a β-glucuronidase from Staphylococcus sp. RLH1

Bilirubin, a key active ingredient of bezoars with extensive clinical applications in China, is produced through a chemical process. However, this method suffers from inefficiency and adverse environmental impacts. To address this challenge, we present a novel and efficient approach for bilirubin production via whole-cell transformation. In this study, we employed Corynebacterium glutamicum ATCC13032 to express a β-glucuronidase (StGUS), an enzyme from Staphylococcus sp. RLH1 that effectively hydrolyzes conjugated bilirubin to bilirubin. Following the optimization of the biotransformation conditions, a remarkable conversion rate of 79.7% in the generation of bilirubin was obtained at temperate 40 °C, pH 7.0, 1 mM Mg2+ and 6 mM antioxidant NaHSO3 after 12 h. These findings hold significant potential for establishing an industrially viable platform for large-scale bilirubin production.

Site-specific crosslinking and assembly of tetrameric β-glucuronidase improve glycyrrhizin hydrolysis

In this study, eight nonconserved residues with exposed surfaces and flexible conformations of the homotetrameric PGUS (β-glucuronidase from Aspergillus oryzae Li-3) were identified. Single-point mutation into cysteine enabled the thiol-maleimide reaction and site-specific protein assembly using a two-arm polyethylene glycol (PEG)-maleimide crosslinker (Mal2 ). The Mal2 (1k) (with 1 kDa PEG spacer)-crosslinked PGUS assemblies showed low crosslinking efficiency and unimproved thermostability except for G194C-Mal2 (1k). To improve the crosslinking efficiency, a lengthened crosslinker Mal2 (2k) (with 2 kDa PEG spacer) was used to produce PGUS assembly and a highly improved thermostability was achieved with a half-life of 47.2-169.2 min at 70°C, which is 1.04-3.74 times that of wild type PGUS. It is found that the thermostability of PGUS assembly was closely associated with the formation of inter-tetramer assembly and intratetramer crosslinking, rather than the PEGylation of the enzyme. Therefore, the four-arm PEG-maleimide crosslinker Mal4 (2k) (with 2 kDa PEG spacer) was employed to simultaneously increase the inter-tetramer assembly and intratetramer crosslinking, and the resulting PGUS assemblies showed further improved thermostabilities compared with Mal2 (2k)-crosslinked assemblies. Finally, the application of PGUS assemblies with significantly improved thermostability to the bioconversion of GL proved that the PGUS assembly is a strong catalyst for glycyrrhizin (GL) hydrolysis in industrial applications.

Tetramerization of GH2 β-Glucuronidases is Essential for Catalyzing the Hydrolysis of the Large Substrate Glycyrrhizin

In this study, structural analysis was employed to identify three hotspot residues that contribute most to the tetramer formation of a glycoside hydrolase family 2 (GH2) β-glucuronidase (GUS) from Aspergillus oryzae Li-3. Single-point mutation at these sites completely disrupted the tetramer structure and abolished the glycyrrhizin (GL)-hydrolyzing activity. Then, the W522A dimer was refactored into a tetramer by disulfide bonding, and partial GL activity was restored. Further saturated mutation showed a strong correlation between the GL activity of the mutants and their tetramer ratios. Molecular simulations were employed to illustrate the critical role of the tetramer interface in maintaining a functional active-site structure. The three highly conserved tetramer-forming residues were finally applied to two other GH2 GUSs for tetramer dissociation and demonstrated the significance of the homotetramerization for GL-hydrolyzing activity of GH2 GUSs. This study lays foundation for engineering GL-hydrolyzing GUSs at the quaternary structure level for function regulations.

18β-Glycyrrhetinic acid monoglucuronide (GAMG) alleviates single-walled carbon nanotubes (SWCNT)-induced lung inflammation and fibrosis in mice through PI3K/AKT/NF-κB signaling pathway

Carbon nanotubes (CNTs) have become far and wide used in a number of technical and merchant applications as a result of substantial advances in nanotechnology, therein single-walled carbon nanotubes (SWCNT) are one of the most promising nanoparticles. Inhaling CNTs has been linked to a variety of health problems, including lung fibrosis. Glycyrrhetinic acid 3-O-mono-β-D-glucuronide (GAMG), a natural sweetener, has anti-inflammatory and antioxidant capacities. The purpose of this study was to evaluate the potential for GAMG to alleviate SWCNT-induced lung inflammation and fibrosis. During days 3-28 after SWCNT intratracheal administration, we observed a remarkable increase of IL-1β, IL-6 and TNF-α in bronchoalveolar lavage fluid (BALF) on day 3 and collagen deposition on day 28. GAMG treatment remarkably ameliorated SWCNT-induced pulmonary fibrosis and attenuated SWCNT-induced inflammation and collagen deposition, and suppressed the activation of PI3K/AKT/NF-κB signaling pathway in the lungs. Therefore, GAMG has a therapeutic potential for the treatment of SWCNT-induced pulmonary fibrosis. Targeting PI3K/AKT/NF-κB signaling pathway may be a potential therapeutic approach to treat pulmonary fibrosis in mice with SWCNT.

Physiology-Based Pharmacokinetic Study on 18β-Glycyrrhetic Acid Mono-Glucuronide (GAMG) Prior to Glycyrrhizin in Rats

To understand that 18β-Glycyrrhetic acid 3-O-mono-β-D-glucuronide (GAMG) showed better pharmacological activity and drug-like properties than 18β-Glycyrrhizin (GL); a rapid and sensitive HPLC-MS/MS method was established for the simultaneous determination of GAMG and its metabolite 18β-Glycyrrhetinic acid (GA) in rat plasma and tissues after oral administration of GAMG or GL. This analytical method was validated by linearity, LLOQ, specificity, recovery rate, matrix effect, etc. After oral administration, GAMG exhibited excellent C_max (2377.57 ng/mL), T_max (5 min) and AUC_0-T (6625.54 mg/L*h), which was much higher than the C_max (346.03 ng/mL), T_max (2.00 h) and AUC_0-T (459.32 mg/L*h) of GL. Moreover, GAMG had wider and higher tissue distribution in the kidney, spleen, live, lung, brain, etc. These results indicated that oral GAMG can be rapidly and efficiently absorbed and be widely distributed in tissues to exert stronger and multiple pharmacological activities. This provided a physiological basis for guiding the pharmacodynamic study and clinical applications of GAMG.

Endophytic fungi: a potential source of industrial enzyme producers

Microbial enzymes have gained interest for their widespread use in various industries and medicine due to their stability, ease of production, and optimization. Endophytic fungi in plant tissues produce a wide range of secondary metabolites and enzymes, which exhibit a variety of biological activities. The present review illustrates promising applications of enzymes produced by endophytic fungi and discusses the characteristic features of the enzymes, application of the endophytic fungal enzymes in therapeutics, agriculture, food, and biofuel industries. Endophytic fungi producing ligninolytic enzymes have possible biotechnological applications in lignocellulosic biorefineries. The global market of industrially important enzymes, challenges, and future prospects are illustrated. However, the commercialization of endophytic fungal enzymes for industrial purposes is yet to be explored. The present review suggests that endophytic fungi can produce various enzymes and may become a novel source for upscaling the production of enzymes of industrial use.

Endophytic fungi from Dongxiang wild rice (Oryza rufipogon Griff.) show diverse catalytic potential for converting glycyrrhizin

Endophytic fungi inhabiting niche environments are novel biocatalyst resources that need to be exploited urgently. In this study, 63 endophytic fungi isolated from Dongxiang wild rice (Oryza rufipogon Griff.) were tested to assess their potentials to transform glycyrrhizin (GL) into glycyrrhetinic acid monoglucuronide (GAMG) or glycyrrhetinic acid (GA), of which 12 strains were shown to have β-d-glucuronidase activity. Based on morphological characteristics and rDNA ITS sequence analysis, the strains S59, L138, L55 and R57 with high GL molar conversion rates (55%, 45%, 65% and 89%) were further identified as Microsphaeropsis arundinis S59, Penicillium rubens L138, Aspergillus flavus L55 and Eupenicillium javanicum R57, respectively. These four strains with four different types of GL conversion processes were identified, i.e., (1) GL → GAMG in M. arundinis S59, (2) GL → GAMG and GA in A. flavus L55, (3) GL → GA in P. rubens L138, and (4) GL → GAMG → GA in E. javanicum R57, in which the bioconversion type (4) is reported for the first time. The study not only provided abundant and diverse β-d-glucuronidase resources that can be used for GL bioconversion, especially for GAMG biosynthesis from endophytic fungi, but also expanded our knowledge of potential roles of endophytes as new biocatalysts in biotransformation.

Concurrent production of glycyrrhetic acid 3-O-mono-β-d-glucuronide and lignocellulolytic enzymes by solid-state fermentation of a plant endophytic Chaetomium globosum

Glycyrrhetic acid 3-O-mono-β-d-glucuronide (GAMG) as an important derivative of glycyrrhizin (GL) shows stronger biological activities and higher sweetness than GL. The biotransformation process is considered as an efficient strategy for GAMG production, due to its mild reaction, high production efficiency and environmentally friendly status. In this study, licorice straw was used for the first time as a medium for GAMG and lignocellulosic enzyme production via solid-state fermentation (SSF) of endophytic fungus Chaetomium globosum DX-THS3. The fermentation conditions including particle size, temperature, seed age, inoculum size, and moisture of substrate were optimized. Furthermore, additional nitrogen sources and carbon sources were screened for GAMG production by C. globosum DX-THS3 of SSF. Under optimal fermentation conditions, the percent conversion of glycyrrhizin reached 90% in 15 days, whereas the control needed 35 days to achieve the same result. The productivity of optimization (P = 2.1 mg/g/day) was 2.33-fold that of non-optimization (P = 0.9 mg/g/day). Meanwhile, high activities of filter paper enzyme (FPase) (245.80 U/g), carboxymethyl cellulase (CMCase) (33.67 U/g), xylanase (83.44 U/g), and β-glucuronidase activity (271.42 U/g) were obtained faster than those in the control during SSF. Our study provides a novel and efficient strategy for GAMG production and indicates C. globosum DX-THS3 as a potential producer of lignocellulolytic enzymes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40643-021-00441-y.

Endophytic Fungi-Mediated Biocatalysis and Biotransformations Paving the Way Toward Green Chemistry

Catalysis is a process carried out in the presence of a heterogenous catalyst for accelerating the rate of a chemical reaction. It plays a pivotal role in transition from take, make, and dispose technology to sustainable technology via chemo- and biocatalytic processes. However, chemocatalyzed reactions are usually associated with copious amounts of perilous/hazardous environmental footprints. Therefore, whole-cell biotransformations or enzyme cocktails serve as cleaner biocatalytic alternatives in replacing the classical chemical procedures. These benchmark bioconversion reactions serve as important key technology in achieving the goals of green chemistry by eliminating waste generation at source. For this, nature has always been a driving force in fuelling natural product discovery and related applications. The fungal endophytic community, in particular, has undergone co-evolution with their host plant and has emerged as a powerful tool of genetic diversity. They can serve as a treasure trove of biocatalysts, catalyzing organic transformations of a wide range of substances into enantiopure compounds with biotechnological relevance. Additionally, the biocatalytic potential of endophytic fungi as whole-intact organisms/isolated enzyme systems has been greatly expanded beyond the existing boundaries with the advancement in high-throughput screening, molecular biology techniques, metabolic engineering, and protein engineering. Therefore, the present review illustrates the promising applications of endophytic fungi as biocatalysts for the synthesis of new structural analogs and pharmaceutical intermediates and refinement of existing proteins for novel chemistries.

Amentoflavone from Selaginella tamariscina as a potent inhibitor of gut bacterial β-glucuronidase: Inhibition kinetics and molecular dynamics stimulation

Gut bacterial β-glucuronidase (GUS) plays a pivotal role in the metabolism and reactivation of a vast of glucuronide conjugates of both endogenous and xenobiotic compounds in the gastrointestinal tract of human, which has been implicated in certain drug-induced gastrointestinal tract (GI) toxicity in clinic. Inhibitors of gut microbial GUS exhibited great potentials in relieving the drug-induced GI toxicity. In this study, Selaginella tamariscina and its major biflavonoid amentoflavone (AMF) were evaluated for their inhibitory activity against Escherichia coli GUS. Two selective probe substrates for GUS (a specific fluorescent probe substrate for GUS, DDAOG and a classical drug substrate for GUS, SN38G) were used in parallel for charactering the inhibition behaviors. Both the extract of S. tamariscina and its major biflavonoid AMF displayed evident inhibitory effects on GUS, and the IC₅₀ values of AMF against GUS mediated DDAOG and SN-38G hydrolysis were 0.62 and 0.49 μM, respectively. Inhibition kinetics studies indicated that AMF showed mixed type inhibition for GUS-mediated DDAOG hydrolysis, while displayed competitive type inhibition against GUS-mediated SN-38G hydrolysis, with the K_i values of 0.24 and 1.25 μM, respectively. Molecular docking studies and molecular dynamics stimulation results clarified the role of amino acid residues Leu361, Ile363, and Glu413 in the inhibition of AMF on GUS. These results provided some foundations for the potential clinical utility of S. tamariscina and its major biflavonoid AMF for treating drug-induced enteropathy.

Efficient and clean preparation of rare prosaikogenin D by enzymatic hydrolysis of saikosaponin B2 and response surface methodology optimization

Prosaikogenin D, a rare secondary saponin in Radix Bupleuri, has much higher in vivo bioactivities than its original glycoside saikosaponin B₂. Its preparation methods, such as conventional acid hydrolysis and column chromatograph, are unfriendly to environment with serious pollution and undesired products. The aim of this study was to establish an efficient and clean approach for convenient preparation of this rare steroid saponin based on the enzymatic hydrolysis. Cellulase was selected from four commercial enzymes due to its higher hydrolysis performance. Then the hydrolysis conditions were optimized by response surface methodology after preliminary investigation on affecting factors by single-factor experiments. The reaction system was constructed by 100 μg/mL of saikosaponin B₂ and 8.00 mg/mL of cellulase, which was incubated in HAc-NaAc buffer (pH 4.7) at 60 °C for 33 h. Consequently, a high conversion ratio of the substrate has been achieved at 95.04 %. The newly developed strategy is an efficient and clean approach for the preparation of prosaikogenin D and it is a promising technology in industrial application.