Thermal inactivation and product inhibition of Aspergillus terreus CECT 2663 alpha-L-rhamnosidase and their role on hydrolysis of naringin solutions

Computational design of Lactobacillus Acidophilus α-L-rhamnosidase to increase its structural stability

α-L-rhamnosidase catalyzes hydrolysis of the terminal α-L-rhamnose from various natural rhamnoglycosides, including naringin and hesperidin, and has various applications such as debittering of citrus juices in the food industry and flavonoid derhamnosylation in the pharmaceutical industry. However, its activity is lost at high temperatures, limiting its usage. To improve Lactobacillus acidophilus α-L-rhamnosidase stability, we employed molecular dynamics (MD) to identify a highly flexible region, as evaluated by its root mean square fluctuation (RMSF) value, and computational protein design (Rosetta) to increase rigidity and favorable interactions of residues in highly flexible regions. MD results show that five regions have the highest flexibilities and were selected for design by Rosetta. Twenty-one designed mutants with the best ΔΔG at each position and ΔΔG < 0 REU were simulated at high temperature. Eight designed mutants with ΔRMSF of highly flexible regions lower than -10.0% were further simulated at the optimum temperature of the wild type. N88Q, N202V, G207D, Q209M, N211T and Y213K mutants were predicted to be more stable and could maintain their native structures better than the wild type due to increased hydrogen bond interactions of designed residues and their neighboring residues. These designed mutants are promising enzymes with high potential for stability improvement.

Development and characterization of an α-l-rhamnosidase mutant with improved thermostability and a higher efficiency for debittering orange juice

The glycoside hydrolase, α-l-rhamnosidase, could remove the bitter taste of naringin from citrus juices. However, most α-l-rhamnosidases are easily deactivated at high temperatures, limiting the practice in debittering citrus juices. The V529A mutant of the α-l-rhamnosidase r-Rha1 from Aspergillus niger JMU-TS528 was developed with improved thermostability using directed evolution technology and site-directed mutagenesis. The enzyme mutant had a half-live of thermal inactivation T_(1/2) of 1.92 h, 25.00 min, and 2 min at 60, 65, and 70 °C, respectively. In addition, it had improved substrate affinity and better resistance to the inhibition of glucose. The improved substrate affinity was related to its lowered binding energy. Most significantly, the naringin content was reduced to below the bitter taste threshold by treatment with 75 U/mL of the mutant during the preheating process of orange juice production. The comprehensive results indicate that thermostability improvement could promote the practical value of α-l-rhamnosidase in citrus juice processing.

Rhamnosidase activity of selected probiotics and their ability to hydrolyse flavonoid rhamnoglucosides

Bioavailability of flavonoids is low, especially when occurring as rhamnoglucosides. Thus, the hydrolysis of rutin, hesperidin, naringin and a mixture of narcissin and rutin (from Cyrtosperma johnstonii) by 14 selected probiotics was tested. All strains showed rhamnosidase activity as shown using 4-nitrophenyl α-l-rhamnopyranoside as a substrate. Hesperidin was hydrolysed by 8–27% after 4 and up to 80% after 10 days and narcissin to 14–56% after 4 and 25–97% after 10 days. Rutin was hardly hydrolysed with a conversion rate ranging from 0 to 5% after 10 days. In the presence of narcissin, the hydrolysis of rutin was increased indicating that narcissin acts as an inducer. The rhamnosidase activity as well as the ability to hydrolyse flavonoid rhamnoglucosides was highly strain specific. Naringin was not hydrolysed by rhamnosidase from probiotics, not even by the purified recombinant enzyme, only by fungal rhamnosidase. In conclusion, rhamnosidases from the tested probiotics are substrate specific cleaving hesperidin, narcissin and to a small extent rutin, but not naringin.

The possible presence of natural β-D-glucosidase inhibitors in jujube leaf extract

Isoquercitrin is a phenolic compound well-known for having greater health benefits than quercitin, its aglycone derivative, and other related glycosides. However, isoquercitrin is rarely found in nature. Here, we optimized the conditions for the enzymatic transformation of isoquercitrin from rutin that was extracted from jujube leaf using the hesperidinase, enzyme complex containing β-D-glucosidase and α-L-rhamnosidase. The maximum productivity (2.57±0.16mg/mL) was experimentally found under the following conditions: 47.3°C, 52.16h, and pH 5.31, which agreed well with the predicted value (2.65mg/mL). However, the achievement of this maximum yield was due to the absence of β-D-glucosidase activity. Further investigations using a β-D-glucosidase assay and reaction measurements under various conditions revealed that the β-D-glucosidase activity was not blocked by denaturation or known inhibitory factors. Currently, there are no recognized β-D-glucosidase inhibitors present in the jujube leaf; however, our observations strongly suggest that an unidentified β-D-glucosidase inhibitor exists in jujube leaf extract.

Preparatory production of quercetin-3-β-D-glucopyranoside using alkali-tolerant thermostable α-L-rhamnosidase from Aspergillus terreus

Extensive screening for a robust producer of α-L-rhamnosidase activity from well-defined strains of filamentous fungi, including multifactorial optimization (inducers, cultivation conditions) was accomplished. Enzyme production of the optimal producer Aspergillus terreus (non-toxigenic) was scaled up to 50L. α-L-Rhamnosidase, which was fully characterized, proved to be thermo- and alkali-tolerant, thus enabling effective operation at 70°C and pH 8.0. These conditions allow for a very high substrate (rutin) load up to 100-300 g/L, thus enabling very high volumetric productivity of the reaction product quercetin-3-β-D-glucopyranoside (isoquercitrin). Here, a novel concept of "immobilised substrate" is used. Isoquercitrin is a highly effective and biocompatible antioxidant with strong anti-inflammatory activities. Rutin biotransformation was optimized and scaled up to ca 10 kg production and thus the robustness of the large-scale production was demonstrated. Isoquercitrin can be produced to a very high purity (98%) in multikilogram amounts, without any quercetin and directly applicable in nutraceuticals.

Enzymatic Synthesis of the Flavone Glucosides, Prunin and Isoquercetin, and the Aglycones, Naringenin and Quercetin, with Selective α-L-Rhamnosidase and β-D-Glucosidase Activities of Naringinase

The production of flavonoid glycosides by removing rhamnose from rutinosides can be accomplished through enzymatic catalysis. Naringinase is an enzyme complex, expressing both α-L-rhamnosidase and β-D-glucosidase activities, with application in glycosides hydrolysis. To produce monoglycosylated flavonoids with naringinase, the expression of β-D-glucosidase activity is not desirable leading to the need of expensive methods for α-L-rhamnosidase purification. Therefore, the main purpose of this study was the inactivation of β-D-glucosidase activity expressed by naringinase keeping α-L-rhamnosidase with a high retention activity. Response surface methodology (RSM) was used to evaluate the effects of temperature and pH on β-D-glucosidase inactivation. A selective inactivation of β-D-glucosidase activity of naringinase was achieved at 81.5°C and pH 3.9, keeping a very high residual activity of α-L-rhamnosidase (78%). This was a crucial achievement towards an easy and cheap production method of very expensive flavonoids, like prunin and isoquercetin starting from naringin and rutin, respectively.

Antimicrobial properties of prunin, a citric flavanone glucoside, and its prunin 6″-O-lauroyl ester

AIMS

To determine the antimicrobial potential of prunin (P), a flavanone glucoside resulting from the hydrolysis of naringin present in grapefruit, and of its prunin 6″-O-lauroyl ester (PL), synthesized by enzymatic catalysis.

METHODS AND RESULTS

P and its lauroyl ester were tested against Gram-negative and Gram-positive bacteria, yeasts and moulds. P showed no inhibitory effect against the micro-organisms assayed, but stimulated growth of Pseudomonas aeruginosa and different Bacilllus sp. However, 150 μg ml(-1) of PL inhibited Escherichia coli, Salmonella enterica serovar Enteritidis, Salmonella enterica serovar Typhimurium, many Bacillus sp., Staphylococcus aureus ATCC29213, Enterococcus avium DSMZ17511, and different Listeria monocytogenes strains. In the last case, L. monocytogenes, sensitive or bacteriocin-resistant cells, lost nearly 4-log reductions after 30 min of contact. A bactericidal mode of action was determined using both scanning and transmission electronic microscopies.

CONCLUSIONS

PL could be used as a food additive, because at low concentration (150 μg ml(-1)) it exhibited antimicrobial activity against important food-borne pathogens. A bactericidal effect was also determined on L. monocytogenes sensitive and bacteriocin-resistant mutant strains. P did not show any antimicrobial property at all.

SIGNIFICANCE AND IMPACT OF THE STUDY

PL is a potential antimicrobial compound with a high anti-Listeria property.

Potential and utilization of thermophiles and thermostable enzymes in biorefining

In today's world, there is an increasing trend towards the use of renewable, cheap and readily available biomass in the production of a wide variety of fine and bulk chemicals in different biorefineries. Biorefineries utilize the activities of microbial cells and their enzymes to convert biomass into target products. Many of these processes require enzymes which are operationally stable at high temperature thus allowing e.g. easy mixing, better substrate solubility, high mass transfer rate, and lowered risk of contamination. Thermophiles have often been proposed as sources of industrially relevant thermostable enzymes. Here we discuss existing and potential applications of thermophiles and thermostable enzymes with focus on conversion of carbohydrate containing raw materials. Their importance in biorefineries is explained using examples of lignocellulose and starch conversions to desired products. Strategies that enhance thermostablity of enzymes both in vivo and in vitro are also assessed. Moreover, this review deals with efforts made on developing vectors for expressing recombinant enzymes in thermophilic hosts.

Cloning, sequence analysis, and expression of the gene encoding Sphingomonas paucimobilis FP2001 alpha-L -rhamnosidase

The gene (rhaM) encoding the alpha-L-rhamnosidase of Sphingomonas paucimobilis FP2001 was cloned, sequenced, and expressed in Escherichia coli. The rhaM consisted of 3,354 nucleotides and had a promoter and Shine-Dalgarno sequences typical in bacteria. The rhaM encoding a protein (Rham) deducted from the sequence consisted of 1,117 amino acids and had a putative signal peptide of 25 amino acids. Rham has no similarity to other known rhamnosidases. Rham has a sugar-binding domain of glycoside hydrolase family 2, which has been well conserved in beta-glucuronidase, beta-mannosidase, and beta-galactosidase, in its C-terminal region. Rham is possibly a member of a new bacterial subfamily in glycoside hydrolase family 78 (alpha-L-rhamnosidase). RT-PCR analysis of rhaM mRNA indicated that the induction of alpha-L-rhamnosidase by the addition of L-rhamnose occurred on the transcriptional level.

Generation of an ?-L-rhamnosidase library and its application for the selective derhamnosylation of natural products

A screening of 16 different fungal strains was performed under different cultivation conditions, using L-rhamnose or L-rhamnose-containing flavonoid glycosides (rutin, hesperidin, and naringin) as specific inducers. No significant constitutive production of alpha-L-rhamnosidases was detected in noninduced cultures, while high levels of these glycosidase activities were obtained using different inducers. New species, so far unknown for the production of alpha-L-rhamnosidases, were identified. More than 30 different alpha-L-rhamnosidase samples were prepared by ammonium sulfate precipitation. Substrate specificity of this alpha-L-rhamnosidase library was tested with various L-rhamnose-containing natural compounds (flavonoids, terpenoids, and saponins). Most of the enzymatic preparations showed broad substrate specificity, and some of them were also acting on sterically hindered substrates (e.g., quercitrin). The screening of the library under different reaction conditions showed the coexistence, in the same preparation, of more than one alpha-L-rhamnosidase activities with different substrate specificity and different stability towards organic cosolvents. To exploit this enzymatic library for synthetic applications, the presence of contaminating alpha-L-arabinosidases and beta-D-glucosidases was investigated. The latter enzymes were observed in several preparations, while alpha-L-arabinosidase content was generally quite low. The selective derhamnosylation of the saponin desglucoruscin was performed on a preparative scale. The enzyme obtained by rhamnose induction of the Aspergillus niger K2 CCIM strain showed high activity towards this substrate and negligible alpha-L-arabinosidase contamination. Therefore, it was chosen as a catalyst for the selective derhamnosylation reaction, which provided the desired product in 70% yield.