Use of whole cells of Pseudomonas aeruginosa for synthesis of the antioxidant hydroxytyrosol via conversion of tyrosol

4-Hydroxyphenylacetate 3-Hydroxylase (4HPA3H): A Vigorous Monooxygenase for Versatile O-Hydroxylation Applications in the Biosynthesis of Phenolic Derivatives

4-Hydroxyphenylacetate 3-hydroxylase (4HPA3H) is a long-known class of two-component flavin-dependent monooxygenases from bacteria, including an oxygenase component (EC 1.14.14.9) and a reductase component (EC 1.5.1.36), with the latter being accountable for delivering the cofactor (reduced flavin) essential for o-hydroxylation. 4HPA3H has a broad substrate spectrum involved in key biological processes, including cellular catabolism, detoxification, and the biosynthesis of bioactive molecules. Additionally, it specifically hydroxylates the o-position of the C4 position of the benzene ring in phenolic compounds, generating high-value polyhydroxyphenols. As a non-P450 o-hydroxylase, 4HPA3H offers a viable alternative for the de novo synthesis of valuable natural products. The enzyme holds the potential to replace plant-derived P450s in the o-hydroxylation of plant polyphenols, addressing the current significant challenge in engineering specific microbial strains with P450s. This review summarizes the source distribution, structural properties, and mechanism of 4HPA3Hs and their application in the biosynthesis of natural products in recent years. The potential industrial applications and prospects of 4HPA3H biocatalysts are also presented.

Cold adapted Pseudomonas: ecology to biotechnology

The cold adapted microorganisms, psychrophiles/psychrotolerants, go through several modifications at cellular and biochemical levels to alleviate the influence of low temperature stress conditions. The low temperature environments depend on these cold adapted microorganisms for various ecological processes. The ability of the microorganisms to function in cold environments depends on the strategies directly associated with cell metabolism, physicochemical constrains, and stress factors. Pseudomonas is one among such group of microorganisms which is predominant in cold environments with a wide range of ecological and biotechnological applications. Bioformulations of Pseudomonas spp., possessing plant growth promotion and biocontrol abilities for application under low temperature environments, are well documented. Further, recent advances in high throughput sequencing provide essential information regarding the prevalence of Pseudomonas in rhizospheres and their role in plant health. Cold adapted species of Pseudomonas are also getting recognition for their potential in biodegradation and bioremediation of environmental contaminants. Production of enzymes and bioactive compounds (primarily as an adaptation mechanism) gives way to their applications in various industries. Exopolysaccharides and various biotechnologically important enzymes, produced by cold adapted species of Pseudomonas, are making their way in food, textiles, and pharmaceuticals. The present review, therefore, aims to summarize the functional versatility of Pseudomonas with particular reference to its peculiarities along with the ecological and biotechnological applications.

Effect of Tween Type Non-Ionic Detergent on the Activity of Lipase of Pseudomonas aeruginosa

Pseudomonas aeruginosa is a Gram-negative and rod-shaped bacterium. It can use a variety of carbon sources and grow in different culture media. Its versatile extracellular enzymes give it the ability to grow on complex carbon sources. One of the most important enzymes of this bacterium is lipase, which is an extracellular enzyme. Lipases are one of the most useful enzymes in medicine and industry, especially in the detergent industry. In recent years, lipases have become an important component of detergent powders, so it is important to evaluate the performance of lipases in the presence of detergents. The aim of this study was to investigate the effect of non-ionic detergents Tween 20 and 80 on the activity of the Pseudomonas lipase. These detergents reduced Km and increased Vmax of the enzyme. The enzyme activity increased in the presence of these detergents at optimal pH and temperature. Conformational studies with the purified enzyme by fluorescence spectrophotometry showed that in the presence of Tween 20 and 80, there was a hypochromicity in emission peak of the enzyme, which indicated that the enzyme became less compact in vicinity of these detergents.

Half-Preparative Scale Synthesis of (S)-1-Phenylethane-1,2-Diol as a Result of 2-Phenylethanol Hydroxylation with Aspergillus niger (IAFB 2301) Assistance

Aspergillus niger (IAFB 2301) was employed for bioconversions of 2-phenylethanol as an immobilized or free mycelium and also as a spore suspension. Experiments were conducted on laboratory and half-preparative scale (bioreactor New Brunswick Scientific, BioFlo Model C32). Thus, A. niger applied as free mycelium, depending on the outcome, supported formation of the mixture of 4-hydroxyphenylacetic acid and hydroxytyrosol (final concentration of 13.8 mg/L and 3.7% efficiency) or 4-hydroxyphenylacetic acid, as single product (final concentration of 140 mg/L and 18% efficiency). In case of scaling experiments conducted with flow and batch reactors, accordingly, the following results were achieved: 1. mixture of antioxidants 4-hydroxyphenylacetic acid and hydroxytyrosol formed with final concentration of 76 mg/L and 10% efficiency (simplified flow system and immobilized mycelium); 2. (S)-1-phenylethane-1,2-diol synthesized with a final concentration of 447 mg/L and 65% (1.3 L batch reactor).

A coupled enzymatic reaction of tyrosinase and glucose dehydrogenase for the production of hydroxytyrosol

Hydroxytyrosol (HT) is a diphenolic compound prevalent mainly in olives with pronounced antioxidant activity and proven benefits for human health. Current production limitations have motivated studies concerning the hydroxylation of tyrosol to HT with tyrosinase; however, accumulation of the diphenol is restricted due to its rapid subsequent oxidation to 3,4-quinone-phenylethanol. In this study, a continuous two-enzyme reaction system of sol-gel-immobilized tyrosinase and glucose dehydrogenase (GDH) was developed for the synthesis of HT. Purified tyrosinase from Bacillus megaterium (TyrBm) and E. coli cell extract expressing GDH from B. megaterium were encapsulated in a sol-gel matrix based on triethoxysilane precursors. While tyrosinase oxidized tyrosol to 3,4-quinone-phenylethanol, GDH catalyzed the simultaneous reduction of the cofactor NAD⁺ to NADH, which was the reducing agent enabling the accumulation of HT. Using 50 mM tyrosol, the immobilized system under optimized conditions, enabled a final HT yield of 7.68 g/L with productivity of 2.30 mg HT/mg TyrBm beads. Furthermore, the immobilized bi-enzyme system showed the feasibility for HT production from 1 mM tyrosol using a 0.5-L bioreactor as well as stable activity over 8 repeated cycles. The production of other diphenols with commercial importance such as L-dopa (3,4-dihydroxyphenylalanine) or piceatannol may be synthesized with this efficient approach.

Dual pathway for metabolic engineering of Escherichia coli to produce the highly valuable hydroxytyrosol

One of the most abundant phenolic compounds traced in olive tissues is hydroxytyrosol (HT), a molecule that has been attributed with a pile of beneficial effects, well documented by many epidemiological studies and thus adding value to products containing it. Strong antioxidant capacity and protection from cancer are only some of its exceptional features making it ideal as a potential supplement or preservative to be employed in the nutraceutical, agrochemical, cosmeceutical, and food industry. The HT biosynthetic pathway in plants (e.g. olive fruit tissues) is not well apprehended yet. In this contribution we employed a metabolic engineering strategy by constructing a dual pathway introduced in Escherichia coli and proofing its significant functionality leading it to produce HT. Our primary target was to investigate whether such a metabolic engineering approach could benefit the metabolic flow of tyrosine introduced to the conceived dual pathway, leading to the maximalization of the HT productivity. Various gene combinations derived from plants or bacteria were used to form a newly inspired, artificial biosynthetic dual pathway managing to redirect the carbon flow towards the production of HT directly from glucose. Various biosynthetic bottlenecks faced due to feaB gene function, resolved through the overexpression of a functional aldehyde reductase. Currently, we have achieved equimolar concentration of HT to tyrosine as precursor when overproduced straight from glucose, reaching the level of 1.76 mM (270.8 mg/L) analyzed by LC-HRMS. This work realizes the existing bottlenecks of the metabolic engineering process that was dependent on the utilized host strain, growth medium as well as to other factors studied in this work.

Efficient Synthesis of Hydroxytyrosol from l-3,4-Dihydroxyphenylalanine Using Engineered Escherichia coli Whole Cells

Hydroxytyrosol is a high-value-added compound with a variety of biological and pharmacological activities. In this study, a whole-cell catalytic method for the synthesis of hydroxytyrosol was developed: aromatic amino acid aminotransferase (TyrB), l-glutamate dehydrogenase (GDH), α-keto acid decarboxylase (PmKDC), and aldehyde reductase (YahK) were co-expressed in Escherichia coli to catalyze the synthesis of hydroxytyrosol from l-3,4-dihydroxyphenylalanine (l-DOPA). The plasmids with different copy numbers were used to balance the expression of the four enzymes, and the most appropriate strain (pRSF- yahK- tyrB and pCDF- gdh- Pmkdc) was identified. After determination of the optimum temperature (35 °C) and pH (7.5) for whole-cell catalysis, the yield of hydroxytyrosol reached 36.33 mM (5.59 g/L) and the space-time yield reached 0.70 g L^-1 h^-1.

Chemical, physical and biotechnological approaches to the production of the potent antioxidant hydroxytyrosol

Hydroxytyrosol (HT) is a polyphenol of interest to the food, feed, supplements and pharmaceutical sectors. It is one of the strongest known natural antioxidants and has been shown to confer other benefits such as anti-inflammatory and anti-carcinogenic properties, and it has the potential to act as a cardio- and neuroprotectant. It is known to be one of the compounds responsible for the health benefits of the Mediterranean diet. In nature, HT is found in the olive plant (Olea europaea) as part of the secoiridoid compound oleuropein, in its leaves, fruit, oil and oil production waste products. HT can be extracted from these olive sources, but it can also be produced by chemical synthesis or through the use of microorganisms. This review looks at the production of HT using plant extraction, chemical synthesis and biotechnological approaches.

Bioconversion of p-Tyrosol into Hydroxytyrosol under Bench-Scale Fermentation

Tyrosol hydroxylating Pseudomonas strain was previously isolated from olive mill wastewaters-irrigated soil. In the present work, experimental design was used to study the bioconversion of tyrosol in laboratory fermenters aiming at the recovery of the highest yields of hydroxytyrosol. The effects of biocatalyst loading and tyrosol concentration were studied. The bioconversion yield reached 86.9% (37.3 mM hydroxytyrosol) starting from a tyrosol concentration of 43 mM. Under these conditions, the specific productivity relative to the biocatalyst was 4.78 μM/min/g. The established model to predict bioconversion yield was validated in two bench-scale fermenters. At the downstream stage, the reaction product was recovered as a hydroxytyrosol rich solution after microfiltration and concentration under vacuum. Subsequent to this operation, hydroxytyrosol composition yielded 73.8% of the total dry matter.

Isolation, characterization and antibacterial effect of biosurfactant from Candida parapsilosis

In the present study, a biosurfactant producing Candida parapsilosis strain was isolated and identified by our laboratory. Different biosurfactant screening tests such as drop collapse, oil spreading, emulsification index and hemolytic activity confirmed the production of biosurfactant by the isolated Candida parapsilosis strain. The biosurfactant showed significant emulsifying index, drop collapse and oil-spread activity. The partially purified biosurfactant was characterized by Fourier Transform Infrared Spectroscopy (FT-IR) and Gas Chromatography-Mass Spectroscopy (GC-MS). The FT-IR results indicated phenol (O-H), amide (N-H) and carbon functional group peaks like C[bond, double bond]O and C[bond, double bond]C at their identified places. GC-MS analysis revealed the presence of 13-docosenamide type of compound with a molecular weight of 337.5 g mol^-1. The isolated biosurfactant showed significant antibacterial activity against pathogenic Escherichia coli and Staphylococcus aureus strains at the concentrations of 10 and 5 mg ml^-1 respectively. Growth inhibition of both Gram positive and Gram negative pathogenic strains designated the future prospect of exploring the isolated biosurfactant as broad spectrum antibacterial agent.

Establishing an Artificial Pathway for Efficient Biosynthesis of Hydroxytyrosol

Hydroxytyrosol (HT) is a valuable natural phenolic compound with strong antioxidant activity and various physiological and pharmaceutical functions. In this study, we established an artificial pathway for HT biosynthesis. First, efficient enzymes were selected to construct a tyrosol biosynthetic pathway. Aro10 from Saccharomyces cerevisiae was shown to be a better ketoacid decarboxylase than Kivd from Lactococcus lactis for tyrosol production. While knockout of feaB significantly decreased accumulation of the byproduct 4-hydroxyphenylacetic acid, overexpression of alcohol dehydrogenase ADH6 further improved tyrosol production. The titers of tyrosol reached 1469 ± 56 mg/L from tyrosine and 620 ± 23 mg/L from simple carbon sources, respectively. The pathway was further extended for HT production by overexpressing Escherichia coli native hydroxylase HpaBC. To enhance transamination of tyrosine to 4-hydroxyphenylpyruvate, NH₄Cl was removed from the culture media. To decrease oxidation of HT, ascorbic acid was added to the cell culture. To reduce the toxicity of HT, 1-dodecanol was selected as the extractant for in situ removal of HT. These efforts led to an additive increase in HT titer to 1243 ± 165 mg/L in the feeding experiment. Assembly of the full pathway resulted in 647 ± 35 mg/L of HT from simple carbon sources. This work provides a promising alternative for sustainable production of HT, which shows scale-up potential.

Biotransformation: a green and efficient way of antioxidant synthesis

Antioxidant compounds play a vital role in human physiology. They prevent the oxidation of biomolecules by scavenging free radicals produced during physiochemical processes and/or as a result of several pathological states. A balance between the reactive oxygen species (free radicals) and antioxidants is essential for proper physiological conditions. Excessive free radicals cause oxidative stress which can lead to several human diseases. Therefore, synthesis of the effective antioxidants is crucial in managing the oxidative stress. Biotransformation has evolved as an effective technique for the production of structurally diverse molecules with a wide range of biological activities. This methodology surpasses the conventional chemical synthesis due to the fact that enzymes, being specific in nature, catalyze reactions affording products with excellent regio- and stereoselectivities. Structural transformation of various classes of compounds such as alkaloids, steroids, flavonoids, and terpenes has been carried out through this technique. Several bioactive molecules, especially those having antioxidant potential have also been synthesized by using different biotransformation techniques and enzymes. Hydroxylated, glycosylated, and acylated derivatives of phenols, flavonoids, cinnamates, and other molecules have proven abilities as potential antioxidants. A critical review of the biotransformation of these compounds into potent antioxidant metabolites is presented here.

Involvement of antioxidant activity of Lactobacillus plantarum on functional properties of olive phenolic compounds

Eight lactic acid bacteria strains isolated from traditional fermented foods were investigated for their antioxidant activity against DPPH free radicals, β-carotene bleaching assay and linoleic acid test. L. plantarum LAB 1 at a dose of 8.2 10(9) CFU/ml showed the highest DPPH scavenging activity, with inhibition rate of 57.07 ± 0.57 % and an antioxidant activity (TAA = 43.47 ± 0.663 % and AAC = 172.65 ± 5.57), which increase with cell concentrations. When L. plantarum LAB 1 was administered to oxidative enzymes, residual activities decreased significantly with cell concentrations. The use of L. plantarum LAB 1 on olives process, favours the increase of the antioxidant activity (24 %). HPLC results showed a significant increase of orthodiphenols (74 %). Viable cells of strain were implicated directly on minimum media growth with 500 mg/l of olive phenolic compounds. Results showed an increase in their antioxidant activity. CG-SM analysis, identify the presence of compounds with higher antioxidant activity as vinyl phenol and hydroxytyrosol.

Enzymatic hydrolysis of oleuropein from Olea europea (olive) leaf extract and antioxidant activities

Oleuropein (OE), the main polyphenol in olive leaf extract, is likely to decompose into hydroxytyrosol (HT) and elenolic acid under the action of light, acid, base, high temperature. In the enzymatic process, the content of OE in olive leaf extract and enzyme are key factors that affect the yield of HT. A selective enzyme was screened from among 10 enzymes with a high OE degradation rate. A single factor (pH, temperature, time, enzyme quantity) optimization process and a Box-Behnken design were studied for the enzymatic hydrolysis of 81.04% OE olive leaf extract. Additionally, enzymatic hydrolysis results with different substrates (38.6% and 81.04% OE) were compared and the DPPH antioxidant properties were also evaluated. The result showed that the performance of hydrolysis treatments was best using hemicellulase as a bio-catalyst, and the high purity of OE in olive extract was beneficial to biotransform OE into HT. The optimal enzymatic conditions for achieving a maximal yield of HT content obtained by the regression were as follows: pH 5, temperature 55 °C and enzyme quantity 55 mg. The experimental result was 11.31% ± 0.15%, and the degradation rate of OE was 98.54%. From the present investigation of the antioxidant activity determined by the DPPH method, the phenol content and radical scavenging effect were both decreased after enzymatic hydrolysis by hemicellulase. However, a high antioxidant activity of the ethyl acetate extract enzymatic hydrolysate (IC₅₀ = 41.82 μg/mL) was demonstated. The results presented in this work suggested that hemicellulase has promising and attractive properties for industrial production of HT, and indicated that HT might be a valuable biological component for use in pharmaceutical products and functional foods.

The antioxidant hydroxytyrosol: biotechnological production challenges and opportunities

Hydroxytyrosol (HT) is a highly potent antioxidant originating in nature as a second metabolite of plants, most abundantly in olives (Olea europaea). In the last decade, numerous research studies showed the health benefits of antioxidants in general and those of HT in particular. As olive oil is a prime constituent of the health-promoting Mediterranean diet, HT has obtained recognition for its attributes, supported by a recent health claim of the European Food Safety Authority. HT is already used as a food supplement and in cosmetic products, but it has the potential to be used as a food additive and drug, based on its anticarcinogenic, anti-inflammatory, antiapoptotic and neuroprotective activity. Nevertheless, there is a large gap between the potential of HT and its current availability in the market due to its high price tag. In this review, the challenges of producing HT using biotechnological methods are described with an emphasis on the substrate source, the biocatalyst and the process parameters, in order to narrow the gap towards an efficient bio-based industrial process.

Biotransformation of polyphenols for improved bioavailability and processing stability

Research on the functions and effects of polyphenols has gained considerable momentum in recent times. This is attributed to their bioactivities, ranging from antioxidant to anticancer activities. But their potential is seldom fully realized since their solubility and stability is quite low and their bioavailability is hampered due to extensive metabolism in the body. Biotransformation of polyphenols using enzymes, whole cell microbes, or plant cell cultures may provide an effective solution by modifying their structure while maintaining their original bioactivity. Lipase, protease, cellulase, and transferases are commonly used enzymes, with lipase being the most popular for carrying out acylation reactions. Among the whole cell microbes, Aspergillus, Bacillus, and Streptomyces sp. are the most widely used, while Eucalyptus perriniana and Capsicum frutescens are the plant cell cultures used for the production of secondary metabolites. This chapter emphasizes the development of green solvents and identification of different sources/approaches to maximize polyphenol transformation for varied applications.

Engineering of a tyrosol-producing pathway, utilizing simple sugar and the central metabolic tyrosine, in Escherichia coli

Metabolic engineering was applied to the development of Escherichia coli capable of synthesizing tyrosol (2-(4-hydroxyphenyl)ethanol), an attractive phenolic compound with great industrial value, from glucose, a renewable carbon source. In this strain, tyrosine, which was supplied not only from the culture medium but also from the central metabolism, was converted into tyrosol via three steps: decarboxylation, amine oxidation, and reduction. The engineered strain synthesized both tyrosol and 4-hydroxyphenylacetate (4HPA), but disruption of the endogenous phenylacetaldehyde dehydrogenase gene shut off 4HPA production and improved the production of tyrosol as a sole product. The engineered mutant strain was capable of producing 0.5 mM tyrosol from 1% (w/v) glucose during a 48 h shake flask cultivation.

Changes in the content of bioactive polyphenolic compounds of olive mill wastewater by the action of exogenous enzymes

The aim behind the present research is to develop an enzymatic treatment for olive mill wastewater (OMW) to release high amounts of simple phenolics having high antioxidant value. OMW was hydrolyzed by a mixed enzyme preparation rich in β-glucosidase produced by Aspergillus niger . This research shows that A. niger β-glucosidase played a major role in the release of simple phenolic compounds from OMW. These compounds were recovered by ethyl acetate extraction and identified by HPLC and LC-MS. The main identified phenolic compound is hydroxytyrosol. The results of enzymatic hydrolysis of OMW under optimum conditions indicated a maximum hydroxytyrosol concentration of 2.9 g L(-1) compared to 0.015 g L(-1) contained in the control (test without added enzyme). The above results prove that OMW is a potential substrate for producing hydroxytyrosol through enzymatic hydrolysis of its glycosides.

Conversion of isoeugenol to vanillin by Psychrobacter sp. strain CSW4

To screen strains of halotolerant or halophile bacteria which are able to convert isoeugenol to vanillin, 36 different strains of bacteria isolated from the salty environments in Iran were investigated. During growth on isoeugenol, a moderately halotolerant Gram-negative coccobacil showed capability of converting isoeugenol to vanillin. Based on morphological, physiological, and phylogenetic studies, strain CSW4 was classified as a bacterium belonging to the genus Psychrobacter. The bioconversion products were confirmed by thin-layer chromatography, high-performance liquid chromatography, and spectral data obtained from UV/Vis spectroscopy, FTIR, and mass-spectroscopy. Using growing cells, vanillin reached its maximum level of 88.18 mg L(-1) after 24 h of reaction time in the presence of 1 g L(-1) isoeugenol, resulting in a molar yield of 10.2%. The use of resting cells led to the optimal yield of vanillin (16.4%) which was obtained after 18-h reaction using 1 g L(-1) isoeugenol and 3.1 g of dry weight of cells per liter harvested at the end of the exponential growth phase. To improve vanillin yield, the effect of substrate concentration on vanillin production under resting cells conditions was also investigated. Using 10 g L(-1) isoeugenol, the maximal vanillin concentration (1.28 g L(-1)) was achieved after a 48-h reaction, without further optimization. The present study brings the first evidence for biotransformation of isoeugenol to vanillin in the genus Psychrobacter.

Tuning the specificity of the recombinant multicomponent toluene o-xylene monooxygenase from Pseudomonas sp. strain OX1 for the biosynthesis of tyrosol from 2-phenylethanol

Biocatalysis is today a standard technology for the industrial production of several chemicals, and the number of biotransformation processes running on a commercial scale is constantly increasing. Among biocatalysts, bacterial multicomponent monooxygenases (BMMs), a diverse group of nonheme diiron enzymes that activate dioxygen, are of primary interest due to their ability to catalyze a variety of complex oxidations, including reactions of mono- and dihydroxylation of phenolic compounds. In recent years, both directed evolution and rational design have been successfully used to identify the molecular determinants responsible for BMM regioselectivity and to improve their activity toward natural and nonnatural substrates. Toluene o-xylene monooxygenase (ToMO) is a BMM isolated from Pseudomonas sp. strain OX1 which hydroxylates a wide spectrum of aromatic compounds. In this work we investigate the use of recombinant ToMO for the biosynthesis in recombinant cells of Escherichia coli strain JM109 of 4-hydroxyphenylethanol (tyrosol), an antioxidant present in olive oil, from 2-phenylethanol, a cheap and commercially available substrate. We initially found that wild-type ToMO is unable to convert 2-phenylethanol to tyrosol. This was explained by using a computational model which analyzed the interactions between ToMO active-site residues and the substrate. We found that residue F176 is the major steric hindrance for the correct positioning of the reaction intermediate leading to tyrosol production into the active site of the enzyme. Several mutants were designed and prepared, and we found that the combination of different mutations at position F176 with mutation E103G allows ToMO to convert up to 50% of 2-phenylethanol into tyrosol in 2 h.

Isolation, identification and characterization of a new lipolytic pseudomonas sp., strain AHD-1, from Tunisian soil

A novel, lipid-degrading bacterium (strain AHD-1) was isolated from soil regularly contaminated with washing-machine wastewater in Sfax, Tunisia. When this strain was grown in a medium containing 2% triacylglycerol, the hydrolysis products were found to be diacylglycerols, monoacylglycerols and free fatty acids. This strain was an aerobic, mesophilic, Gram-negative, motile, non-sporulating bacterium, capable of growing optimally at pH 7 and 27 degrees C. The predominant fatty acids were found to be C16:1omega7c (31%), C16:0 (28.1%), C18:1 omega7c (16.3%) and C17:0 (5.8%). Phylogenetic analysis of the 16S rRNA gene showed that this isolate is a new strain belonging to the genus Pseudomonas. Strain AHD-1 was found to be closely related to Pseudomonas azotoformans IAM 1603T, Pseudomonas gessardii CIP 105469T and Pseudomonas libanensis CIP 105460T with 99.7%, 99.56% and 99.54% of similarity, respectively.

Hydroxytyrosol from tyrosol using hydroxyphenylacetic acid-induced bacterial cultures and evidence of the role of 4-HPA 3-hydroxylase

Hydroxytyrosol (HTyr) is a potent natural antioxidant found in olive mill wastewaters. Bacterial conversion of 4-tyrosol (2-(4-hydroxyphenyl)-ethanol) to HTyr was reported in a limited number of bacterial species including Pseudomonas aeruginosa. In this work, we studied this conversion, taking as a model the newly isolated Halomonas sp. strain HTB24. It was first hypothesized that the enzyme responsible for 4-tyrosol hydroxylation in HTyr was a 4-hydroxyphenylacetic acid 3-hydroxylase (HPAH, EC 1.14.13.3), previously known to convert 4-hydroxyphenylacetic acid (4-HPA) into 3,4-dihydroxyphenylacetic acid (3,4-DHPA) in P. aeruginosa. Cloning and expression of hpaB (oxygenase component) and hpaC (reductase component) genes from P. aeruginosa confirmed this hypothesis. Furthermore, using cultures of HTB24 containing 4-tyrosol, it was shown that 4-HPA accumulation preceded 4-tyrosol hydroxylation. We further demonstrated that the synthesis of HPAH activity was induced by 4-HPA, with the latter compound being formed from 4-tyrosol oxidation by aryl-dehydrogenases. Interestingly, similar results were obtained with other 4-HPA-induced bacteria, including P. aeruginosa, Serratia marcescens, Escherichia coli, Micrococcus luteus and other Halomonas, thus demonstrating general hydroxylating activity of 4-tyrosol by the HPAH enzyme. E. coli W did not have aryl-dehydrogenase activity and hence were unable to oxidize 4-tyrosol to 4-HPA and HTyr to 3,4-DHPA, making this bacterium a good candidate for achieving better HTyr production.

Tyrosol degradation via the homogentisic acid pathway in a newly isolated Halomonas strain from olive processing effluents

AIMS

To isolate a new Halomonas sp. strain capable of degrading tyrosol, a toxic compound present in olive mill wastewater, through the homogentisic acid (HGA) pathway.

METHODS AND RESULTS

A moderately halophilic Gram-negative bacterium belonging to the Halomonas genus and designated strain TYRC17 was isolated from olive processing effluents. This strain was able to completely degrade tyrosol (2-(p-hydroxyphenyl)-ethanol), a toxic compound found in such effluent. Tyrosol degradation begins by an oxidation to 4-hydroxyphenylacetic acid (HPA), which is then converted into HGA by an HPA 1-monooxygenase, while closest Halomonas species degrade tyrosol through 3,4-dihydroxyphenylacetic acid (DHPA). In the presence of transition metals, HGA underwent a pH-dependent abiotic conversion into benzoquinone acetic acid, then into 2,5-dihydroxybenzaldehyde (gentisaldehyde) and pyomelanin, by oxidative decarboxylation and polymerization, respectively.

CONCLUSIONS

Tyrosol degradation via HGA by the new Halomonas sp. strain TYRC17 was complete in the absence of trace elements. In their presence, HGA was abiotically converted into gentisaldehyde and pyomelanin.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report on tyrosol degradation via the HGA pathway under hypersaline conditions and on the oxidative decarboxylation of HGA into gentisaldehyde. It underlines the importance of the Halomonas genus in the bioremediation of toxic-contaminated sites.

Bioconversion of tyrosol into hydroxytyrosol and 3,4-dihydroxyphenylacetic acid under hypersaline conditions by the new Halomonas sp. strain HTB24

This paper reports the characterization of a Halomonas sp. strain (named HTB24) isolated from olive-mill wastewater and capable of transforming tyrosol into hydroxytyrosol (HT) and 3,4-dihydroxyphenylacetic acid (DHPA) in hypersaline conditions. This is the first time that a halophile has been shown to perform such reactions. The potent natural antioxidant HT was obtained through a C3 hydroxylation on the ring cycle, whereas DHPA was synthesized via the 4-hydroxyphenylacetic acid (HPA) pathway, which has been well described from other bacterial sources. HT was produced first, and then DHPA was detected in the medium accompanied by traces of HPA. HPA involved another pathway resulting from the activity of an aryl-dehydrogenase, which is suggested to be responsible for both tyrosol and hydroxytyrosol oxidation. Maximal HT content (2.30 mM) and maximal DHPA (5.15+/-0.42 mM) were obtained from a culture inoculated in the presence of 20 mM tyrosol and 0.5 g L(-1) yeast extract. Following this, DHPA was quickly degraded into 5-carboxymethyl-2-hydroxymuconic semialdehyde by a 2,3-dioxygenase, finally resulting in succinate and pyruvate. Phylogenetic analysis of the 16S rRNA gene revealed that this isolate was a member of the genus Halomonas. Strain HTB24, with a G+C content of 55.3 mol%, is closely related to Halomonas neptunia DSM 15720(T), 'Halomonas alkaliantarctica' DSM 15686(T) and Halomonas boliviensis DSM 15516(T).

Mild photochemical synthesis of the antioxidant hydroxytyrosol via conversion of tyrosol

Hydroxytyrosol, a naturally occurred orthodiphenolic antioxidant molecule found in olive oil and olive mill wastewaters, was obtained from the wet hydrogen peroxide photocatalytic oxidation of its monophenolic precursor tyrosol. The liquid-phase oxidation of tyrosol to hydroxytyrosol was performed by use of an iron-containing heterogeneous catalyst (Al-Fe)PILC with the assistance of UV irradiation at 254 nm and at room temperature. The spectroscopic and HPLC data of the synthesized compound proved to coincide fully with those of a pure sample obtained by continuous countercurrent extraction. This reaction was found to be light-induced. The hydroxytyrosol synthesis reaction reached its maximum yield of 64.36% under the optimized operating conditions of 3.6 mM tyrosol, 0.5 g L(-1) catalyst, and 10(-2) M H2O2 with the assistance of UV light. Increasing the initial hydrogen peroxide concentration more than 10(-2) M has a diminishing return on the reaction efficiency. Catalyst can be recuperated by means of filtration and then reused in a next run after regeneration since its activity did not significantly decrease (<10%). The reaction synthesis is operationally simple and could find application for industrial purposes.

Use of Pseudomonas mendocina, or recombinant Escherichia coli cells expressing toluene-4-monooxygenase, and a cell-free tyrosinase for the synthesis of 4-fluorocatechol from fluorobenzene

The transformation of fluorobenzene (FB) by whole cell expressing toluene-4-monooxygenase (T4MO) resulted in the formation of various hydroxylated products. The predominant product was either 4-fluorophenol (4FP) or 4-fluorocatechol (4Fcat) depending on the ratio of biocatalyst to substrate concentration. The transformation of 1 mM FB by whole cells (1.5 mg CDW/ml) gave a 52% yield of 4Fcat as a single product. The yield of 4Fcat was improved 1.6-fold (80%) by adding 10 mM ascorbic acid to the biotransformations. A combination of two biocatalysts (whole cells expressing T4MO and cell free mushroom tyrosinase) also resulted in the transformation of FB (5 mM) to higher concentrations of 4Fcat (1.8 mM) compared to a whole cell biotransformation alone. However, mixed products were formed and the yield of 4Fcat from FB was lower using the two-step (tandem) method (27%) compared to the use of whole cells of P. mendocina KR1 alone (80%).

Production of high hydroxytyrosol yields via tyrosol conversion by Pseudomonas aeruginosa immobilized resting cells

An immobilized whole cell system was successfully performed to produce the most powerful antioxidant, hydroxytyrosol. Bioconversion of tyrosol into hydroxytyrosol was achieved via the immobilization of Pseudomonas aeruginosa resting cells in calcium alginate beads. Immobilization was advantageous as it allows immobilized cells to tolerate a greater tyrosol concentration than free cells. The bioconversion yield reached 86% in the presence of 5 g L-1 of tyrosol when cells immobilized in alginate beads were carried out in single batches. Evaluation of kinetic parameters showed the maintenance of the same catalytic efficiency expressed as Kcat/Km for both free and immobilized cells. The use of immobilized cells in repeated batches demonstrated a notable activity stabilization since the biocatalyst reusability was extended for at least four batches with a molar yield greater than 85%.

Production of triacetylhydroxytyrosol from olive mill waste waters for use as stabilized bioantioxidant

A hydroxytyrosol triacetyl derivative was very efficiently produced as a highly pure stabilized antioxidant compound by a short treatment of olive mill waste water (OMWW) organic extracts, rich in hydroxytyrosol, with an acetylating mixture composed of HClO4-SiO2 and Ac2O (Chakborti and Gulhane reaction), in mild and safe conditions. A successive single step of middle pressure liquid chromatography (MPLC) purification of the reaction product was performed, with an overall yield of 35.6%. (This process, including both the Chakborti and Gulhane reaction and the MPLC purification, is protected by an international patent under PCT/IT2005/000781.) The o-diphenol triacetyl derivative was also produced by direct reaction of hydroxytyrosol, previously purified by MPLC, with HClO4-SiO2 and Ac2O, with an overall yield of 29.5%. A further procedure for the production of the hydroxytyrosol triacetyl derivative was consistent with the direct treatment of raw OMWW with the acetylating agent and a single step of MPLC purification, with an overall yield of 27.6%. The purified natural triacetylhydroxytyrosol confirmed the same strong protective effects against the oxidative stress in human cells as the corresponding synthetic compound, likely because of the biochemical activation of the acetyl derivative into the active parent hydroxytyrosol by esterases. We therefore propose the utilization of OMWW for recovering hydroxytyrosol as a natural antioxidant in a chemically stabilized form, with a good yield, which can be potentially used as a nontoxic functional component in nutritional, pharmaceutical, and cosmetic preparations.