Degradation of phenolic compounds by the yeast Candida tropicalis HP 15. II. Some properties of the first two enzymes of the degradation pathway

Multisubstrate specific flavin containing monooxygenase from Chlorella pyrenoidosa with potential application for phenolic wastewater remediation and biosensor application

Microbial degradation of phenolic pollutants in industrial wastewater is dependent on enzymatic pathway comprising a cascade of phenol metabolizing enzymes. Phenol hydroxylase is the first enzyme of the pathway catalysing the initial attack on phenol in green algae Chlorella pyrenoidosa. The present work reports cost-effective production of partially purified microalgal phenol hydroylase by single-step purification and characterization of its kinetic properties with the view of application for enzyme-based remediation of phenolic wastewater or in phenolic biosensor. The enzyme with a molecular weight of 25 kDa shows all characteristics of phenol hydroxylases, that is, hydroxylation of phenol to catechol (confirmed by HPLC), substrate-dependent NADPH oxidation, absorption spectrum typical of flavoproteins and peptide mass fingerprint corresponding to flavoprotein hydroxylase. The enzyme utilizes phenol with apparent Michealis constant (K_m) of 1.71 µM, maximal velocity (V_max) of 0.4 µM/min with optimal activity at pH 7 and 35°C. Fe²⁺chelators (Phenanthroline and sodium arsenate), heavy metals, denaturants and oxidizing agents showed inhibitory effect on phenol hydroxylation activity of the enzyme. The enzyme has broad substrate specificity against isomeric diphenols, isomeric methylphenols, halogen-substituted phenols, amino-substituted phenols, nitrophenols, hydroxybenzaldehyde and hydroxylbenzoic acid. The enzyme shows remarkable storage stability at room temperature and at 4°C. The multisubstrate specificity coupled to remarkable storage stability of the microalgal phenol hydroxylase opens up avenues for its application in remediation of a wide range of phenolics released in industrial wastewater or phenolic biosensor application.

Identification and characterization of phenol hydroxylase from phenol-degrading Candida tropicalis strain JH8

The gene phhY encoding phenol hydroxylase from Candida tropicalis JH8 was cloned, sequenced, and expressed in Escherichia coli. The gene phhY contained an open reading frame of 2130 bp encoding a polypeptide of 709 amino acid residues. From its sequence analysis, it is a member of a family of flavin-containing aromatic hydroxylases and shares 41% amino acid identity with phenol hydroxylase from Trichosporon cutaneum. The recombinant phenol hydroxylase exists as a homotetramer structure with a native molecular mass of 320 kDa. Recombinant phenol hydroxylase was insensitive to pH treatment; its optimum pH was at 7.6. The optimum temperature for the enzyme was 30 °C, and its activity was rapidly lost at temperatures above 60 °C. Under the optimal conditions with phenol as substrate, the Km and Vmax of recombinant phenol hydroxylase were 0.21 mmol·L–1 and 0.077 μmol·L–1·min−1, respectively. This is the first paper presenting the cloning and expression in E. coli of the phenol hydroxylase gene from C. tropicalis and the characterization of the recombinant phenol hydroxylase.

Phenol biodegradation by a microbial consortium: application of artificial neural network (ANN) modelling

In this study, an effective microbial consortium for the biodegradation of phenol was grown under different operational conditions, and the effects of phosphate concentration (1.4 g L(-1), 2.8 g L(-1), 4.2 g L(-1)), temperature (25 degrees C, 30 degrees C, 35 degrees C), agitation (150 rpm, 200 rpm, 250 rpm) and pH (6, 7, 8) on phenol degradation were investigated, whereupon an artificial neural network (ANN) model was developed in order to predict degradation. The learning, recall and generalization characteristics of neural networks were studied using data from the phenol degradation system. The efficiency of the model generated by the ANN was then tested and compared with the experimental results obtained. In both cases, the results corroborate the idea that aeration and temperature are crucial to increasing the efficiency ofbiodegradation.

Isolation of cytoplasmic NADPH-dependent phenol hydroxylase and catechol-1,2-dioxygenase from Candida tropicalis yeast

The efficiencies of NADPH-dependent phenol hydroxylase (EC 1.14.13.7) and catechol 1,2-dioxygenase (EC.1.13.11.1) in biodegradation of phenol in the cytosolic fraction isolated from yeast Candida tropicalis were investigated. Enzymatic activities of both NADPH-dependent phenol hydroxylase and catechol 1,2-dioxygenase were detected in the cytosolic fraction of C. tropicalis grown on medium containing phenol. Using the procedure consisting of chromatography on DEAE-Sepharose, fractionation by polyethylene glycol 6000 and gel permeation chromatography on Sepharose 4B the enzyme responsible for phenol hydroxylation in cytosol, NADPH-dependent phenol hydroxylase, was isolated from the cytosolic fraction of C. tropicalis close to homogeneity. However, fractionation with polyethylene glycol 6000 lead to a decrease in catechol 1,2-dioxygenase activity. Therefore, another procedure was tested to purify this enzyme. Gel permeation chromatography of proteins of the eluate obtained by chromatography on a DEAE-Sepharose column was utilized to separate phenol hydroxylase and catechol 1,2-dioxygenase. Among gel permeation chromatography on columns of Sephadex G-100, Sephacryl S-300 and Sepharose 4B tested for their efficiencies to isolate phenol hydroxylase and catechol 1,2-dioxygenase, that on Sephacryl S-300 was found to be suitable for such a procedure. Nevertheless, even this chromatographic method did not lead to obtain catechol 1,2-dioxygenase in sufficient amounts and purity for its further characterization. The data demonstrate the progress in resolving the enzymes responsible for the first two steps of phenol degradation by the C. tropicalis strain.

Optimization of phenol degradation by Candida tropicalis Z-04 using Plackett-Burman design and response surface methodology

Statistical experimental designs were used to optimize the process of phenol degradation by Candida tropicalis Z-04, isolated from phenol-degrading aerobic granules. The most important factors influencing phenol degradation (p < 0.05), as identified by a two-level Plackett-Burman design with 11 variables, were yeast extract, phenol, inoculum size, and temperature. Steepest ascent method was undertaken to determine the optimal regions of these four significant factors. Central composite design (CCD) and response surface analysis were adopted to further investigate the mutual interactions between these variables and to identify their optimal values that would generate maximum phenol degradation. The analysis results indicated that interactions between yeast extract and temperature, phenol and temperature, inoculum size and temperature affected the response variable (phenol degradation) significantly. The predicted results showed that the maximum removal efficiency of phenol (99.10%) could be obtained under the optimum conditions of yeast extract 0.41 g/L, phenol 1.03 g/L, inoculum size 1.43% (V/V) and temperature 30.04 degrees C. These predicted values were further verified by validation experiments. The excellent correlation between predicted and experimental values confirmed the validity and practicability of this statistical optimum strategy. This study indicated the excellent ability of C. tropicalis Z-04 in degrading high-strength phenol. Optimal conditions obtained in this experiment laid a solid foundation for further use of this microorganism in the treatment of high-strength phenol effluents.

19F nuclear magnetic resonance as a tool to investigate microbial degradation of fluorophenols to fluorocatechols and fluoromuconates

A method was developed to study the biodegradation and oxidative biodehalogenation of fluorinated phenols by 19F nuclear magnetic resonance (NMR). Characterization of the 19F NMR spectra of metabolite profiles of a series of fluorophenols, converted by purified phenol hydroxylase, catechol 1,2-dioxygenase, and/or by the yeast-like fungus Exophiala jeanselmei, provided possibilities for identification of the 19F NMR chemical shift values of fluorinated catechol and muconate metabolites. As an example, the 19F NMR method thus defined was used to characterize the time-dependent metabolite profiles of various halophenols in either cell extracts or in incubations with whole cells of E. jeanselmei. The results obtained for these two systems are similar, except for the level of muconates observed. Altogether, the results of the present study describe a 19F NMR method which provides an efficient tool for elucidating the metabolic pathways for conversion of fluorine-containing phenols by microorganisms, with special emphasis on possibilities for biodehalogenation and detection of the type of fluorocatechols and fluoromuconates involved. In addition, the method provides possibilities for studying metabolic pathways in vivo in whole cells.

Evidence of two pathways for the metabolism of phenol by Aspergillus fumigatus

Aspergillus fumigatus (ATCC 28282), a thermotolerant fungus, has been shown to be capable of growth on phenol as the sole carbon and energy source. During growth of the organism on phenol, catechol and hydroquinone accumulated transiently in the medium; cells grown on phenol oxidised these compounds without a lag period. Two different routes operating simultaneously, leading to different ring-fission substrates, are proposed for the metabolism of phenol. In one route, phenol undergoes ortho-hydroxylation to give catechol, which is then cleaved by an intradiol mechanism leading to 3-oxoadipate. In the other route, phenol is hydroxylated in the para-position to produce hydroquinone, which is then converted into 1,2,4-trihydroxybenzene for ring fission by ortho-cleavage to give maleylacetate. Cell-free extracts of phenol-grown mycelia were found to contain enzymic activities for the proposed steps. Two ring-fission dioxygenases, one active towards 1,2,4-trihydroxybenzene, but not catechol, and one active towards both ring-fission substrates, were separated by FPLC. Succinate-grown mycelia did not oxidise any of the intermediates until a clear lag period had elapsed and did not contain any of the enzymic activities for phenol metabolism.

Phenol degradation by Acinetobacter calcoaceticus NCIB 8250

Acinetobacter calcoaceticus NCIB 8250 utilizes phenol as sole source of carbon and energy via an ortho-cleavage pathway. The presence of ethanol in mixed substrate cultivations repressed the utilization of phenol. In fed batch cultivation the phenol tolerance was increased at least 2-fold. Maximum degradation rates of 150 mg phenol/(1 h) and 280 mg phenol/(g h), respectively were observed. Phenol hydroxylase is induced by its substrate and in parallel the catechol-1,2-dioxygenase is detectable. The presence of active phenol hydroxylase is strongly connected with the phenol degradation. Using a spectrophotometric enzyme assay the partially purified phenol hydroxylase was characterized with respect to kinetic parameters. The apparent Km values for phenol, FAD and NADPH were estimated to be 147 microM, 35 microM and 416 microM, respectively. Both FAD and NADPH were essential for maximum activity of the cytoplasmically localized enzyme. No substrate inhibition of phenol hydroxylase by phenol was observed up to 0.8 mM. The pH and temperature optima were pH 7.8 and 33 degrees C, respectively. The partially purified enzyme showed a broad substrate specificity. It hydroxylated the three isomeric cresols, chlorophenols and methylated chlorophenols. Pyrogallol, 3,4-dihydroxy-L-phenylalanine and resorcinol were oxygenated with higher rates than phenol. With the exception of phenol all other enzyme substrates tested did not serve as growth substrates.

Catabolism of benzene compounds by ascomycetous and basidiomycetous yeasts and yeastlike fungi. A literature review and an experimental approach

A literature review is given on growth of yeasts on benzene compounds and on the catabolic pathways involved. Additionally, a yeast collection was screened for assimilation of phenol and 3-hydroxybenzoic acid. Fifteen ascomycetous and thirteen basidiomycetous yeast species were selected and were tested for growth on 84 benzene compounds. It appeared that 63 of these compounds supported growth of one or more yeast species. The black yeast Exophiala jeanselmei assimilated 54 of these compounds. The catechol branch of the 3-oxoadipate pathway and its hydroxyhydroquinone variant were involved in phenol and resorcinol catabolism of ascomycetes as well as of basidiomycetes. However, these two groups of yeasts showed characteristic differences in hydroxybenzoate catabolism. In the yeastlike fungus E. jeanselmei and in basidiomycetes of the genera Cryptococcus, Leucosporidium and Rhodotorula, the protocatechuate branch of the 3-oxoadipate pathway was induced by growth on 3- and 4-hydroxybenzoic acids. In three Trichosporon species and in all ascomycetous yeasts tested, 4-hydroxybenzoic acid was catabolyzed via protocatechuate and hydroxyhydroquinone. These yeasts were unable to cleave protocatechuate. 3-Hydroxybenzoic and 3-hydroxycinnamic acids were catabolized in ascomycetous yeasts via the gentisate pathway, but in basidiomycetes via protocatechuate. Incomplete oxidation of phenol, some chlorophenols, cresols and xylenols was observed in cultures of Candida parapsilosis growing on hydroquinone. Most compounds transformed by the growing culture were also converted by the phenol monooxygenase present in cell-free extracts of this yeast. They did not support growth. The relationship between the ability of ascomycetous yeasts to assimilate n-alkanes, amines and benzene compounds, and the presence of Coenzyme Q9 is discussed.

Effect of pH and inoculum size on phenol degradation by bacteria isolated from landfill waste

Four aerobic species of mesophilic bacteria which catabolise phenol as the sole carbon source were isolated from landfill waste, and identified as Acinetobacter, Arthrobacter, Micrococcus and Nocardia spp. In vitro studies in defined medium revealed variations in abilities of the isolates to catabolise phenol, and in pH growth optima ranging from 6.8 to 7.6. The pH of culture media decreased with growth, and phenol catabolism was impeded below pH 5.4. Phenol catabolism in landfill leachate occurred after longer lag phases than in defined medium, and relative efficiencies of phenol catabolism did not correspond with the patterns in defined medium. Inoculum size affected phenol removal rates. The results describe factors which may be manipulated to prevent pollution problems in the codisposal of phenolic industrial waste.