Enzymatic synthesis of tyrosol glycosides

Special Issue: "Microbial Enzymes for Biotechnological Applications"

Microbial biocatalysts are at the forefront of modern biotechnology, offering sustainable solutions to some of the world's most pressing environmental and industrial challenges [...].

Enzymatic β-Mannosylation of Phenylethanoid Alcohols

Phenylethanoid glycosides (PhGs) are widely occurring secondary metabolites of medicinal plants with interesting biological activities such as antioxidant, anti-inflammatory, neuroprotective, antiviral, hepatoprotective, immunomodulatory, etc. They are characterized by a structural core formed by a phenethyl alcohol, usually tyrosol or hydroxytyrosol, attached to β-D-glucopyranose via a glycosidic bond. This core is usually further decorated by attached phenolic acids or another saccharide. Several studies suggest an important role of the saccharidic fragment in the biological activities of PhGs, provoking demand for new glycovariants of natural PhGs. This study presents the preparation of β-mannosylated analogs of tyrosol β-D-glucopyranoside (salidroside) and hydroxytyrosol β-D-glucopyranoside (hydroxysalidroside). While the chemical synthesis of β-D-mannopyranosides is rather challenging, they can be prepared by enzymatic catalysis. We found that Novozym 188, an industrial β-glucosidase, also contains β-mannosidase and used this enzyme in the preparation of tyrosol β-D-mannopyranoside and hydroxytyrosol β-D-mannopyranoside in 12 and 16% chemical yields, respectively, by transglycosylation from β-D-mannopyranosyl-(1→4)-D-mannose. The mannosylation was chemoselective and occurred exclusively on the primary hydroxyls of tyrosol and hydroxytyrosol, and the glycosylation of phenolic moieties of the aglycons was observed.

Medium engineering of phenylethanoid transfructosylation catalysed by yeast β-fructofuranosidase

Tyrosol and hydroxytyrosol, by-products of olive oil production, are valuable substrates for enzymatic transglycosylation that can provide products with pharmaceutical potential. Phenylethanoid fructosides are produced from sucrose and phenylethanoids by the catalytic action of β-fructofuranosidases. This work dealt with the potential of the most abundant β-fructofuranosidase, baker's yeast invertase, for this bioconversion. The effects of sucrose and phenylethanoid concentrations were investigated with a focus on the selectivity of phenylethanoid transfructosylation and fructoside yields. For this purpose, initial rate and progress curve experiments were carried out for the initial (hydroxy)tyrosol and sucrose concentrations of 0.072-0.3 M and 1-2 M, respectively. Reaction courses exhibited either a maximum or plateau of fructoside yield in the range of about 10-18%. The addition of deep eutectic solvents was applied in the concentration range from 5 to 70% (v/v) to investigate the possibility of shifting the reaction equilibrium towards fructoside synthesis.

Enzymatic Biosynthesis of Simple Phenolic Glycosides as Potential Anti-Melanogenic Antioxidants

Simple phenolics (SPs) and their glycosides have recently gained much attention as functional skin-care resources for their anti-melanogenic and antioxidant activities. Enzymatic glycosylation of SP aglycone make it feasible to create SP glycosides with updated bioactive potentials. Herein, a glycosyltransferase (GT)-encoding gene was cloned from the fosmid libraries of Streptomyces tenjimariensis ATCC 31603 using GT-specific degenerate PCR followed by in silico analyses. The recombinant StSPGT was able to flexibly catalyze the transfer of two glycosyl moieties towards two SP acceptors, (hydroxyphenyl-2-propanol [HPP2] and hydroxyphenyl-3-propanol [HPP3]), generating stereospecific α-anomeric glycosides as follows: HPP2-O-α-glucoside, HPP2-O-α-2″-deoxyglucoside, HPP3-O-α-glucoside and HPP3-O-α-2″-deoxyglucoside. This enzyme seems not only to prefer UDP-glucose and HPP2 as a favorable glycosyl donor and acceptor, respectively but also differentiates the positional difference of the hydroxyl function as acceptor catalytic sites. Paired in vitro and in vivo antioxidant assays represented SPs and their corresponding glycosides as convincing antioxidants in a time- and concentration-dependent manner by scavenging DPPH radicals and intracellular ROS. Even compared to the conventional agents, HPP2 and glycoside analogs displayed improved tyrosinase inhibitory activity in vitro and still suppressed in vivo melanogenesis. Both HPP2 glycosides are further likely to exert the best inhibitory activity against elastase, eventually highlighting these glycosides with enhanced anti-melanogenic and antioxidant activities as promising anti-wrinkle hits.

Synthesis of Tyrosol and Hydroxytyrosol Glycofuranosides and Their Biochemical and Biological Activities in Cell-Free and Cellular Assays

Tyrosol (T) and hydroxytyrosol (HOT) and their glycosides are promising candidates for applications in functional food products or in complementary therapy. A series of phenylethanoid glycofuranosides (PEGFs) were synthesized to compare some of their biochemical and biological activities with T and HOT. The optimization of glycosylation promoted by environmentally benign basic zinc carbonate was performed to prepare HOT α-L-arabino-, β-D-apio-, and β-D-ribofuranosides. T and HOT β-D-fructofuranosides, prepared by enzymatic transfructosylation of T and HOT, were also included in the comparative study. The antioxidant capacity and DNA-protective potential of T, HOT, and PEGFs on plasmid DNA were determined using cell-free assays. The DNA-damaging potential of the studied compounds for human hepatoma HepG2 cells and their DNA-protective potential on HepG2 cells against hydrogen peroxide were evaluated using the comet assay. Experiments revealed a spectrum of different activities of the studied compounds. HOT and HOT β-D-fructofuranoside appear to be the best-performing scavengers and protectants of plasmid DNA and HepG2 cells. T and T β-D-fructofuranoside display almost zero or low scavenging/antioxidant activity and protective effects on plasmid DNA or HepG2 cells. The results imply that especially HOT β-D-fructofuranoside and β-D-apiofuranoside could be considered as prospective molecules for the subsequent design of supplements with potential in food and health protection.

Design of immobilized biocatalyst and optimal conditions for tyrosol β-galactoside production

Tyrosol β-galactoside (TG) is a phenylethanoid glycoside with proven neuroprotective properties. This work deals with its biocatalytic production from tyrosol and lactose using Aspergillus oryzae β-galactosidase in immobilized form. Six commercial carriers were examined to find the optimal biocatalyst. Besides standard biocatalyst performance characteristics, adsorption of the hydrophobic substrate on immobilization carrier matrices was also investigated. The adsorption of tyrosol was significant, but it did not have adverse effects on TG production. On the contrary, TG yield was improved for some biocatalysts. A biocatalyst prepared by covalent binding of β-galactosidase on an epoxy-activated carrier was used for detailed investigation of the effect of reaction conditions on glycoside production. Temperature had a surprisingly weak effect on the overall process rate. A lactose concentration of 0.83 M was found to be optimal to enhance TG formation. The impact of tyrosol concentration was rather complex. This substrate caused inhibition of all reactions. Its concentration had a strong effect on the hydrolysis of lactose and all products. Higher tyrosol concentrations, 30-40 g/L, were favorable as pseudo-equilibrium concentrations of TG and galactooligosaccharide were reached. Repeated batch results revealed excellent operational stability of the biocatalyst.

Transrutinosylation of tyrosol by flower buds of Sophora japonica

Dried flower buds of Japanese sophora (Sophora japonica) comprising rutinosidase activity were tested in rutinosylation of tyrosol via transglycosylation process from rutin. Optimal conditions for transrutinosylation of tyrosol were 49 mM rutin and 290 mM tyrosol, giving maximum conversion up to 66.4% and 24% yield of isolated and purified rutinoside. The rutinosylation proceeded exclusively on the primary hydroxyl of tyrosol, thus forming rhamnosylated derivative of salidroside. This strict regioselectivity differentiates the sophora biocatalyst from microbial rutinosidases.

Comparative study of relationship between structure of phenylethanoid glycopyranosides and their activities using cell-free assays and human cells cultured in vitro

The study focused on protective potential of phytochemicals applicable in prevention and health protection is of great importance. Various structures of these compounds and a wide range of their biological activities have inspired organic chemists to sythesize their effective analogues in order to further increase their efficacy. The aims of our study were (i) to synthesize phenylethanoid glycopyranosides: salidroside (SALI - tyrosol β-d-glucopyranoside), tyrosol β-d-galactopyranoside (TYBGAL), tyrosol α-d-galactopyranoside (TYAGAL), tyrosol α-d-mannopyranoside (TYAMAN), hydroxytyrosol α-d-mannopyranoside (HOTAMA), homosyringyl β-d-glucopyranoside (HSYGLU), hydroxytyrosol β-d-xylopyranoside (HOTXYL) and hydroxysalidroside (HOSALI); (ii) to determine their antioxidant capacities (cell-free approaches); (iii) to evaluate their cytotoxicity (MTT test), protectivity against hydrogen peroxide (H₂O₂; comet assay) and effect on the intracellular glutathione level (iGSH; flow cytometry) in experimental system utilizing human hepatoma HepG2 cells. HOSALI, HOTAMA, HOTXYL and HSYGLU manifested the highest antioxidant capacity in cell-free assays and they were most active in protection of HepG2 cells against H₂O₂. On the other hand, pre-treatment of HepG2 cells with SALI had protective effects even though SALI displayed almost no activity in cell-free assays. Differences in the efficacy of the analogues revealed that structures of their molecules in terms of aglycone combined with sugar moiety affect their activities.

Optimization of Regioselective α-Glucosylation of Hesperetin Catalyzed by Cyclodextrin Glucanotransferase

The regioselective α-glucosylation of hesperetin was achieved by a transglycosylation reaction catalyzed by cyclodextrin glucanotransferase (CGTase) from Thermoanaerobacter sp. using soluble starch as glucosyl donor. By combining mass spectrometry (ESI-TOF) and 2D-NMR analysis, the main monoglucosylated derivative was fully characterized (hesperetin 7-O-α-d-glucopyranoside). In order to increase the yield of monoglucoside, several reaction parameters were optimized: Nature and percentage of cosolvent, composition of the aqueous phase, glucosyl donor, temperature, and the concentrations of hesperetin and soluble starch. Under the optimal conditions, which included the presence of 30% of bis(2-methoxyethyl) ether as cosolvent, the maximum concentration of monoglucoside was approximately 2 mM, obtained after 24 h of reaction. To our knowledge, this is the first report of direct glucosylation of hesperetin employing free enzymes instead of whole cells.

Salidroside, a Chemopreventive Glycoside, Diminishes Cytotoxic Effect of Cisplatin in Vitro

Natural products represent the source or the inspiration for the majority of the active ingredients of medicines because of their structural diversity and a wide range of biological effects. Our aims in this study were (i) to synthesize enzymatically salidroside (SAL), the most effective phenylethanoid glycoside in Rhodiola species; (ii) to examine its antioxidant capacity using cell-free assays (reducing power, DPPH radicals scavenging and Fe²⁺ -chelating assays); (iii) to assess its DNA-protective potential on plasmid DNA (DNA topology assay) and in HepG2 cells (comet assay) damaged by Fe²⁺ ions and hydrogen peroxide, respectively; and (iv) to investigate the effects of SAL, cisplatin (CDDP) and combined treatments of SAL + CDDP on cell viability (MTT test), level of DNA damage (comet assay), proliferation, cell cycle (flow cytometry) and the expression of signalling molecules associated with cell growth and apoptotic pathways (Western immunoblotting). We found out that SAL manifested low antioxidant and DNA-protective capacity in all assays used. In both parental A2780 and CDDP-resistant A2780/CP human ovarian carcinoma cells, SAL itself exerted in fact no impact on the viability, while in combination with CDDP it showed antagonistic effect supporting the chemopreventive activity on the CDDP-induced cell damage. These results were confirmed by the partial reversal of the cell cycle alterations and the DNA damage level, as well as with partial restoration of cell survival/signalling pathways, when the expression of these molecules partially returned to their proper levels.

Mining of efficient microbial UDP-glycosyltransferases by motif evolution cross plant kingdom for application in biosynthesis of salidroside

The plant kingdom provides a large resource of natural products and various related enzymes are analyzed. The high catalytic activity and easy genetically modification of microbial enzymes would be beneficial for synthesis of natural products. But the identification of functional genes of target enzymes is time consuming and hampered by many contingencies. The potential to mine microbe-derived glycosyltransferases (GTs) cross the plant kingdom was assessed based on alignment and evolution of the full sequences and key motifs of target enzymes, such as Rhodiola-derived UDP-glycosyltransferase (UGT73B6) using in salidroside synthesis. The GTs from Bacillus licheniformis ZSP01 with high PSPG motif similarity were speculated to catalyze the synthesis of salidroside. The UGT_BL1, which had similarity (61.4%) PSPG motif to UGT73B6, displayed efficient activity and similar regioselectivity. Highly efficient glycosylation of tyrosol (1 g/L) was obtained by using engineered E. coli harboring UGT_BL1 gene, which generated 1.04 g/L salidroside and 0.99 g/L icariside D2. All glycosides were secreted into the culture medium and beneficial for downstream purification. It was the first report on the genome mining of UGTs from microorganisms cross the plant kingdom. The mining approach may have broader applications in the selection of efficient candidate for making high-value natural products.

A selective and mild glycosylation method of natural phenolic alcohols

Several bioactive natural p-hydroxyphenylalkyl β-D-glucopyranosides, such as vanillyl β-D-glucopyranoside, salidroside and isoconiferin, and their glycosyl analogues were prepared by a simple reaction sequence. The highly efficient synthetic approach was achieved by utilizing acetylated glycosyl bromides as well as aromatic moieties and mild glycosylation promoters. The aglycones, p-O-acetylated arylalkyl alcohols, were prepared by the reduction of the corresponding acetylated aldehydes or acids. Various stereoselective 1,2-trans-O-glycosylation methods were studied, including the DDQ-iodine or ZnO-ZnCl2 catalyst combination. Among them, ZnO-iodine has been identified as a new glycosylation promoter and successfully applied to the stereoselective glycoside synthesis. The final products were obtained by conventional Zemplén deacetylation.