Synthesis of rutinosides and rutinose by reverse hydrolysis catalyzed by fungal α-l-rhamnosidases

Efficiency enhancement in Aspergillus niger α-L-rhamnosidase reverse hydrolysis by using a tunnel site rational design strategy

There has been ongoing interest in improving the efficiency of glycoside hydrolase for synthesizing glycoside compounds through protein engineering, given the potential applications of glycoside compounds. In this study, a strategy of modifying the substrate access tunnel was proposed to enhance the efficiency of reverse hydrolysis catalyzed by Aspergillus niger α-L-rhamnosidase. Analysis of the tunnel dynamics identified Tyr299 as a key modifiable residue in the substrate access tunnel. The location of Tyr299 was near the enzyme surface and at the outermost end of the substrate access tunnel, suggested its role in substrate recognition and throughput. Based on the properties of side chains, six mutants were designed and expressed by Pichia pastoris. Compared to WT, the reverse hydrolysis efficiencies of mutants Y299P and Y299W were increased by 21.3 % and 11.1 %, respectively. The calculation results of binding free energy showed that the binding free energy was inversely proportional to the reverse hydrolysis efficiency. Further, when binding free energy levels were comparable, the mutants with shorter side chains displayed a higher reverse hydrolysis efficiency. These results proved that substrate access tunnel modification was an effective method to improve the reverse hydrolysis efficacy of α-L-rhamnosidase and also provided new insights for modifying other glycoside hydrolases.

Enzymatic rhamnosylation of anticancer drugs by an α-L-rhamnosidase from Alternaria sp. L1 for cancer-targeting and enzyme-activated prodrug therapy

The synthesis of rhamnosylated compounds has gained great importance since these compounds have potential therapeutic applications. The enzymatic approaches for glycosylation of bioactive molecules have been well developed; however, the enzymatic rhamnosylation has been largely hindered by lacking of the glycosyl donor for rhamnosyltransferases. Here, we employed an α-L-rhamnosidase from Alternaria sp. L1 (RhaL1) to perform one-step rhamnosylation of anticancer drugs, including 2'-deoxy-5-fluorouridine (FUDR), cytosine arabinoside (Ara C), and hydroxyurea (Hydrea). The key synthesis conditions including substrate concentrations and reaction time were carefully optimized, and the maximum yields of each rhamnosylated drugs were 57.7 mmol for rhamnosylated Ara C, 68.6 mmol for rhamnosylated Hydrea, and 42.2 mmol for rhamnosylated FUDR. It is worth pointing out that these rhamnosylated drugs exhibit little cytotoxic effects on cancer cells, but could efficiently restore cytotoxic activity when incubated with exogenous α-L-rhamnosidase, suggesting their potential applications in the enzyme-activated prodrug system. To evaluate the cancer-targeting ability of rhamnose moiety, the rhamnose-conjugated fluorescence dye rhodamine B (Rha-RhB) was constructed. The fluorescence probe Rha-RhB displayed much higher cell affinity and cellular internalization rate of oral cancer cell KB and breast cancer cell MDA-MB-231 than that of the normal epithelial cells MCF 10A, suggesting that the rhamnose moiety could mediate the specific internalization of rhamnosylated compounds into cancer cells, which greatly facilitated their applications for cancer-targeting drug delivery.

Enhancement of the thermostability of Aspergillus niger α-l-rhamnosidase based on PoPMuSiC algorithm

α-l-Rhamnosidase is a biotechnologically important enzyme in food industry and in the preparation of drugs and drug precursors. To expand the functionality of our previously cloned α-l-rhamnosidase from Aspergillus niger JMU-TS528, 14 mutants were constructed based on the changes of the folding free energy (ΔΔG), predicted by the PoPMuSiC algorithm. Among them, six single-site mutants displayed higher thermal stability than wild type (WT). The combinational mutant K573V-E631F displayed even higher thermostability than six single-site mutants. The spectra analyses displayed that the WT and K573V-E631F had almost similar secondary and tertiary structure profiles. The simulated protein structure-based interaction analysis and molecular dynamics calculation were further implemented to assess the conformational preferences of the K573V-E631F. The improved thermostability of mutant K573V-E631F may be attributed to the introduction of new cation-π and hydrophobic interactions, and the overall improvement of the enzyme conformation. PRACTICAL APPLICATIONS: The stability of enzymes, particularly with regards to thermal stability remains a critical issue in industrial biotechnology and industrial processing generally tends to higher ambient temperature to inhibit microbial growth. Most of the α-l-rhamnosidases are usually active at temperature from 30 to 60°C, which are apt to denature at temperatures over 60°C. To expand the functionality of our previously cloned α-l-rhamnosidase from Aspergillus niger JMU-TS528, we used protein engineering methods to increase the thermal stability of the α-l-rhamnosidase. Practically, conducting reactions at high temperatures enhances the solubility of substrates and products, increases the reaction rate thus reducing the reaction time, and inhibits the growth of contaminating microorganisms. Thus, the improvement on the thermostability of α-l-rhamnosidase on the one hand can increase enzyme efficacy; on the other hand, the high ambient temperature would enhance the solubility of natural substrates of α-l-rhamnosidase, such as naringin, rutin, and hesperidin, which are poorly dissolved in water at room temperature. Protein thermal resistance is an important issue beyond its obvious industrial importance. The current study also helps in the structure-function relationship study of α-l-rhamnosidase.

Enzymatic Synthesis of Rhamnose Containing Chemicals by Reverse Hydrolysis

Rhamnose containing chemicals (RCCs) are widely occurred in plants and bacteria and are known to possess important bioactivities. However, few of them were available using the enzymatic synthesis method because of the scarcity of the α-L-rhamnosidases with wide acceptor specificity. In this work, an α-L-rhamnosidase from Alternaria sp. L1 was expressed in Pichia pastroris strain GS115. The recombinant enzyme was purified and used to synthesize novel RCCs through reverse hydrolysis in the presence of rhamnose as donor and mannitol, fructose or esculin as acceptors. The effects of initial substrate concentrations, reaction time, and temperature on RCC yields were investigated in detail when using mannitol as the acceptor. The mannitol derivative achieved a maximal yield of 36.1% by incubation of the enzyme with 0.4 M L-rhamnose and 0.2 M mannitol in pH 6.5 buffers at 55°C for 48 h. In identical conditions except for the initial acceptor concentrations, the maximal yields of fructose and esculin derivatives reached 11.9% and 17.9% respectively. The structures of the three derivatives were identified to be α-L-rhamnopyranosyl-(1→6')-D-mannitol, α-L-rhamnopyranosyl-(1→1')-β-D-fructopyranose, and 6,7-dihydroxycoumarin α-L-rhamnopyranosyl-(1→6')-β-D-glucopyranoside by ESI-MS and NMR spectroscopy. The high glycosylation efficiency as well as the broad acceptor specificity of this enzyme makes it a powerful tool for the synthesis of novel rhamnosyl glycosides.

Versatile redox chemistry complicates antioxidant capacity assessment: flavonoids as milieu-dependent anti- and pro-oxidants

Some antioxidants have been shown to possess additional pro-oxidant effects. Diverse methodologies exist for studying redox properties of synthetic and natural chemicals. The latter are substantial components of our diet. Exploration of their contribution to life-extending or -compromising effects is mandatory. Among reactive oxygen species (ROS), hydroxyl radical (^•OH) is the most damaging species. Due to its short half-life, the assay has to contain a specific generation system. Plants synthesize flavonoids, phenolic compounds recognized as counter-agents to coronary heart disease. Their antioxidant activities are affected by their hydroxylation patterns. Moreover, in the plant, they mainly occur as glycosides. We chose three derivatives, quercetin, luteolin, and rutin, in attempts to explore their redox chemistry in contrasting hydrogen peroxide environments. Initial addition of hydrogen peroxide in high concentration or gradual development constituted a main factor affecting their redox chemical properties, especially in case of quercetin. Our study exemplifies that a combination of a chemical assay (deoxyribose degradation) with an electrochemical method (square-wave voltammetry) provides insightful data. The ambiguity of the tested flavonoids to act either as anti- or pro-oxidant may complicate categorization, but probably contributed to their evolution as components of a successful metabolic system that benefits both producer and consumer.

Transglycosylation specificity of Acremonium sp. α-rhamnosyl-β-glucosidase and its application to the synthesis of the new fluorogenic substrate 4-methylumbelliferyl-rutinoside

Transglycosylation potential of the fungal diglycosidase α-rhamnosyl-β-glucosidase was explored. The biocatalyst was shown to have broad acceptor specificity toward aliphatic and aromatic alcohols. This feature allowed the synthesis of the diglycoconjugated fluorogenic substrate 4-methylumbelliferyl-rutinoside. The synthesis was performed in one step from the corresponding aglycone, 4-methylumbelliferone, and hesperidin as rutinose donor. 4-Methylumbelliferyl-rutinoside was produced in an agitated reactor using the immobilized biocatalyst with a 16% yield regarding the sugar acceptor. The compound was purified by solvent extraction and silica gel chromatography. MALDI-TOF/TOF data recorded for the [M+Na](+) ions correlated with the theoretical monoisotopic mass (calcd [M+Na](+): 507.44 m/z; obs. [M+Na](+): 507.465 m/z). 4-Methylumbelliferyl-rutinoside differs from 4-methylumbelliferyl-glucoside in the rhamnosyl substitution at the C-6 of glucose, and this property brings about the possibility to explore in nature the occurrence of endo-β-glucosidases by zymographic analysis.

Enzyme-assisted extraction of flavonoids from Ginkgo biloba leaves: improvement effect of flavonol transglycosylation catalyzed by Penicillium decumbens cellulase

We report a novel enzyme-involved approach to improve the extraction of flavonoids from Ginkgo biloba, in which the enzyme is employed not only for cell wall degradation, but also for increasing the solubility of target compounds in the ethanol-water extractant. Penicillium decumbens cellulase, a commercial cell wall-degrading enzyme with high transglycosylation activity, was found to offer far better performance in the extraction than Trichoderma reesei cellulase and Aspergillus niger pectinase under the presence of maltose as the glycosyl donor. TLC, HPLC and MS analysis indicated that P. decumbens cellulase could transglycosylate flavonol aglycones into more polar glucosides, the higher solubility of which led to improved extraction. The influence of glycosyl donor, pH, solvent and temperature on the enzymatic transglycosylation was investigated. For three predominant flavonoids in G. biloba, the transglycosylation showed similar optimal conditions, which were therefore used for the enzyme-assisted extraction. The extraction yield turned to be 28.3mg/g of dw, 31% higher than that under the pre-optimized conditions, and 102% higher than that under the conditions without enzymes. The utilization of enzymatic bifunctionality described here, naming enzymatic modification of target compounds and facilitation of cell wall degradation, provides a novel approach for the extraction of natural compounds from plants.

Extracellular monoenzyme deglycosylation system of 7-O-linked flavonoid beta-rutinosides and its disaccharide transglycosylation activity from Stilbella fimetaria

We screened for microorganisms able to use flavonoids as a carbon source; and one isolate, nominated Stilbella fimetaria SES201, was found to possess a disaccharide-specific hydrolase. It was a cell-bound ectoenzyme that was released to the medium during conidiogenesis. The enzyme was shown to cleave the flavonoid hesperidin (hesperetin 7-O-alpha-rhamnopyranosyl-beta-glucopyranoside) into rutinose (alpha-rhamnopyranosyl-beta-glucopyranose) and hesperetin. Since only intracellular traces of monoglycosidase activities (beta-glucosidase, alpha-rhamnosidase) were produced, the disaccharidase alpha-rhamnosyl-beta-glucosidase was the main system utilized by the microorganism for hesperidin hydrolysis. The enzyme was a glycoprotein with a molecular weight of 42224 Da and isoelectric point of 5.7. Even when maximum activity was found at 70 degrees C, it was active at temperatures as low as 5 degrees C, consistent with the psychrotolerant character of S. fimetaria. Substrate preference studies indicated that the enzyme exhibits high specificity toward 7-O-linked flavonoid beta-rutinosides. It did not act on flavonoid 3-O-beta-rutinoside and 7-O-beta-neohesperidosides, neither monoglycosylated substrates. In an aqueous medium, the alpha-rhamnosyl-beta-glucosidase was also able to transfer rutinose to other acceptors besides water, indicating its potential as biocatalyst for organic synthesis. The monoenzyme strategy of Acremonium sp. SES201 = DSM 24697, [corrected] as well as the enzyme substrate preference for 7-O-beta-flavonoid rutinosides, is unique characteristics among the microbial flavonoid deglycosylation systems reported.