The influence of non-phenolic mediators and phenolic co-substrates on the oxidation of 4-bromophenol by lignin peroxidase

Organic Contaminant Biodegradation by Oxidoreductase Enzymes in Wastewater Treatment

Organic contaminants (OCs), such as pharmaceuticals, personal care products, flame retardants, and plasticisers, are societally ubiquitous, environmentally hazardous, and structurally diverse chemical compounds whose recalcitrance to conventional wastewater treatment necessitates the development of more effective remedial alternatives. The engineered application of ligninolytic oxidoreductase fungal enzymes, principally white-rot laccase, lignin peroxidase, and manganese peroxidase, has been identified as a particularly promising approach for OC remediation due to their strong oxidative power, broad substrate specificity, low energy consumption, environmental benignity, and cultivability from lignocellulosic waste. By applying an understanding of the mechanisms by which substrate properties influence enzyme activity, a set of semi-quantitative physicochemical criteria (redox potential, hydrophobicity, steric bulk and pKa) was formulated, against which the oxidoreductase degradation susceptibility of twenty-five representative OCs was assessed. Ionisable, compact, and electron donating group (EDG) rich pharmaceuticals and antibiotics were judged the most susceptible, whilst hydrophilic, bulky, and electron withdrawing group (EWG) rich polyhalogenated compounds were judged the least susceptible. OC susceptibility scores were in general agreement with the removal rates reported for experimental oxidoreductase treatments (R2 = 0.60). Based on this fundamental knowledge, and recent developments in enzyme immobilisation techniques, microbiological enzymic treatment strategies are proposed to formulate a new generation of biological wastewater treatment processes for the biodegradation of environmentally challenging OC compounds.

Oxidative degradation of 2,6-dibromophenol using anion-exchange resin supported supramolecular catalysts of iron(III)-5,10,15,20-tetrakis (p-hydroxyphenyl)porphyrin bound to humic acid prepared via formaldehyde and urea-formaldehyde polycondensation

An iron(III)-porphyrin catalyst, iron(III)-tetrakis(p-hydroxyphenyl)porphyrin (FeTHP), was introduced into a humic acid via a formaldehyde or urea-formaldehyde polycondensation reaction to stabilize the catalyst. The prepared supramolecular catalysts were then attached to Dowex-22, an anion-exchange resin. The oxidation of 2,6-dibromophenol (2,6-DBP) was then used, to evaluate the catalytic activities of the supported catalysts. The supported catalyst prepared via the urea-formaldehyde polycondensation reaction showed the highest catalytic activity of all catalysts tested. However, no debromination was observed under any conditions. To examine the reusability of the supported catalysts, they were evaluated on the basis of the decrease in the percent degradation of 2,6-DBP for the number times that they could be used. To determine why the catalytic activities decreased with increasing use, the surface of the supported catalysts were observed by scanning electron microscopy and energy dispersive X-ray spectrometry (SME-EDS) after each use. The poor reusability of the supported catalysts can be attributed to the fact that 2,6-DBP and/or brominated byproducts are tightly absorbed to the catalyst in the vicinity of the active site, which leads to inactivation of the supported catalysts.

Transformation of 17beta-estradiol mediated by lignin peroxidase: the role of veratryl alcohol

Lignin peroxidases (LiPs) are a group of extracellular enzymes excreted by certain fungi, e.g., Phanerochaete chrysosporium. These fungi also produce veratryl alcohol (VA) as a secondary metabolite to regulate the performance of LiP. 17ss-Estradiol (E2) is a natural female hormone that is strongly endocrine disruptive when released to the natural environment. The widespread occurrence of E2 and related hormonal chemicals in soil and water environments has been identified, representing an emerging contamination of concern. We report in this study that E2 can be effectively transformed and removed through reactions mediated by LiP and such reactions are significantly enhanced in the presence of VA. We systematically investigated LiP activity and enzymatic reaction kinetics in systems having VA absent or present. The results suggest that VA enhanced the transformation and removal of E2 by the combination of two effects: (i) mitigating LiP inactivation and (ii) modifying the enzyme catalytic kinetics. These findings provide insights into an important pathway that may govern the environmental transformation of E2 and other emerging endocrine-disrupting contaminants of similar nature in the environment, and provide a basis for potential development and optimization of enzyme-based processes for remediation and removal of these contaminants.

Biodegradation of brominated aromatics by cultures and laccase of Trametes versicolor

2-Bromophenol (1), 4-bromophenol (2), 2,4-dibromophenol (3), 2,6-dibromophenol (4), 2,4,6-tribromophenol (5) and tetrabromobisphenol A (6) (1 mM each) added to growing submerged cultures of Trametes versicolor CCBAS 612 were eliminated by 65-85% from the culture medium within 4d. Extracellular laccase activity in the culture medium was influenced by the type of brominated compound added. Maximum level of laccase (63 U L(-1)) was found in the culture with 2-bromophenol. Tetrabromobisphenol A was degraded by a commercial laccase from Trametes versicolor in absence of any oxidation mediator, hydroxylated dibrominated compounds being detected as soluble reaction products by LC/MS. A significant degradation of brominated phenols by laccase was achieved only in the presence of ABTS structural characterization of major products suggesting reaction between bromophenol and ABTS radicals.

Mechanistic features of lignin peroxidase-catalyzed oxidation of substituted phenols and 1,2-dimethoxyarenes

The steady state kinetic parameters Km and kcat for the oxidation of phenolic substrates by lignin peroxidase correlated with the presteady state kinetic parameters Kd and k for the reaction of the enzyme intermediate compound II with the substrates, indicating that the latter is the rate-limiting step in the catalytic cycle. ln Km and ln Kd values for phenolic substrates correlated with redox properties, unlike ln kcat and ln k. This finding suggests that in contrast to horseradish peroxidase, electron transfer is not the rate-limiting step during oxidation by lignin peroxidase compound II. A mechanism is proposed for lignin peroxidase compound II reactions consisting of an equilibrium electron transfer step followed by a subsequent rate-limiting step. Analysis of the correlation coefficients for linear relationships between ln Kd and ln Km and different calculated redox parameters supports a mechanism in which the acidic forms of phenols are oxidized by lignin peroxidase and electron transfer is coupled with proton transfer. 1,2-Dimethoxyarenes did not comply with the trend for phenolic substrates, which may be a result of more than one substrate binding site on lignin peroxidase and/or alternative binding modes. This behavior was supported by analogue studies with the 1,2-dimethoxyarenes veratric acid and veratryl aldehyde, both of which are not oxidized by lignin peroxidase. Inclusion of either had little effect on the rate of oxidation of phenolic substrates yet resulted in a decrease in the oxidation rate of 1,2-dimethoxyarene substrates, which was considerable for veratryl alcohol and less pronounced for 3,4-dimethoxyphenethylalcohol and 3,4-dimethoxycinnamic acid, in particular in the presence of veratric acid.

Cyclometalated ruthenium(II) complexes as efficient redox mediators in peroxidase catalysis

Cyclometalated ruthenium(II) complexes, [Ru(II)(C~N)(N~N)(2)]PF(6) [HC~N=2-phenylpyridine (Hphpy) or 2-(4'-tolyl)pyridine; N~N=2,2'-bipyridine, 1,10-phenanthroline, or 4,4'-dimethyl-2,2'-bipyridine], are rapidly oxidized by H(2)O(2) catalyzed by plant peroxidases to the corresponding Ru(III) species. The commercial isoenzyme C of horseradish peroxidase (HRP-C) and two recently purified peroxidases from sweet potato (SPP) and royal palm tree (RPTP) have been used. The most favorable conditions for the oxidation have been evaluated by varying the pH, buffer, and H(2)O(2) concentrations and the apparent second-order rate constants ( k(app)) have been measured. All the complexes studied are oxidized by HRP-C at similar rates and the rate constants k(app) are identical to those known for the best substrates of HRP-C (10(6)-10(7) M(-1) s(-1)). Both cationic (HRP-C) and anionic (SPP and RPTP) peroxidases show similar catalytic efficiency in the oxidation of the Ru(II) complexes. The mediating capacity of the complexes has been evaluated using the SPP-catalyzed co-oxidation of [Ru(II)(phpy)(bpy)(2)]PF(6) and catechol as a poor peroxidase substrate as an example. The rate of enzyme-catalyzed oxidation of catechol increases more than 10000-fold in the presence of the ruthenium complex. A simple routine for calculating the rate constant k(c) for the oxidation of catechol by the Ru(III) complex generated enzymatically from [Ru(II)(phpy)(bpy)(2)](+) is proposed. It is based on the accepted mechanism of peroxidase catalysis and involves spectrophotometric measurements of the limiting Ru(II) concentration at different concentrations of catechol. The calculated k(c) value of 0.75 M(-1) s(-1) shows that the cyclometalated Ru(II) complexes are efficient mediators in peroxidase catalysis.

Oxidation of 4-bromophenol by the recombinant fused protein cellulose-binding domain-horseradish peroxidase immobilized on cellulose

A fused protein consisting of cellulose-binding domain (CBD) and horseradish peroxidase (HRP) was constructed and expressed in Escherichia coli. Refolded recombinant CBD-HRP (95% recovery yield) was bound to microcrystalline cellulose and applied for the oxidation of a model toxic phenol, 4-bromophenol (BP). Oxidation of BP by CBD-HRP resulted in the formation of dimers to pentamers as evidenced by mass spectrometry analysis. When immobilized, the vast majority of the oxidation products adsorbed to the cellulose matrix. CBD-HRP (0.75 pyrogallol units) bound to 0.1 g cellulose was packed in a column, connected to an HPLC pump and monitoring system, and column performance and capacity were studied under various operating conditions. When performance was studied as a function of BP loading rate at a constant H(2)O(2) loading rate of 1500 nmol/min, V(app) (max) and K(m) (app) were calculated to be 5.29 +/- 0.46 micromol mL min and 644.9 +/- 114.3 microM, respectively. Immobilized CBD-HRP exhibited enhanced stability to H(2)O(2) and oxidized considerably more BP than free CBD-HRP. Inclusion of gelatin, which suppresses product-dependent inactivation, further increased the amount of BP oxidation. These findings may have potential impact in terms of enzyme supply in high-rate treatment of wastewater contaminated with toxic phenols, since the susceptibility of peroxidases to both H(2)O(2) - and product-dependent inactivation demands continuous supply of fresh enzyme.