Factors affecting the oxidative activity of laccase towards biphenyl derivatives in homogeneous aqueous-organic systems

Computational explorations of the interaction between laccase and bisphenol A: influence of surfactant and different organic solvents

Bisphenol A (BPA), as an environmental endocrine disruptor can cause damage to the reproductive, nervous and immune systems. Laccase can be used to degrade BPA. However, laccase is easily deactivated, especially in organic solvents, but the specific details are not clear. Molecular dynamics simulations were used to investigate the reasons for changes in laccase activity in acetonitrile (ACN) and dimethyl formamide (DMF) solutions. In addition, the effects of ACN and DMF on the activity of laccase and surfactant rhamnolipid (RL) on the degradation of BPA by laccase were investigated. Results showed that addition of ACN changed the structure of the laccase, not only decreasing the van der Waals interaction that promoted the binding of laccase with BPA, but also increasing the polar solvation free energy that hindered the binding of laccase with BPA, so it weakened the laccase activity. DMF greatly enhanced the van der Waals interaction between laccase and BPA, and played a positive role in their binding. The addition of surfactant RL alleviated the effect of organic solvent on the activity of laccase by changing the polar solvation energy. The mechanism of surfactant RL affecting laccase activity in ACN and DMF is described, providing support for understanding the effect of organic solvents on laccase.

Enhancement of laccase activity by pre-incubation with organic solvents

Laccases that are tolerant to organic solvents are powerful bio-catalysts with broad applications in biotechnology. Most of these uses must be accomplished at high concentration of organic solvents, during which proteins undergo unfolding, thereby losing enzyme activity. Here we show that organic-solvent pre-incubation provides effective and reversible 1.5- to 4.0-fold enhancement of enzyme activity of fungal laccases. Several organic solvents, including acetone, methanol, ethanol, DMSO, and DMF had an enhancement effect among all laccases studied. The enhancement was not substrate-specific and could be observed by using both phenolic and non-phenolic substrates. Laccase preincubated with organic solvents was sensitive to high temperature but remained stable at 25 °C, for an advantage for long-term storage. The acetone-pre-incubated 3-D structure of DLac, a high-efficiency fungal laccase, was determined and confirmed that the DLac protein structure remains intact and stable at a high concentration of organic solvent. Moreover, the turnover rates of fungal laccases were improved after organic-solvent pre-incubation, with DLac showing the highest enhancement among the fungal laccases examined. Our investigation sheds light on improving fungal laccase usage under extreme conditions and extends opportunities for bioremediation, decolorization, and organic synthesis.

The catalytic activity enhancement and biodegradation potential of free laccase and novel sol-gel laccase in non-conventional solvents

The catalytic activity of free laccase and a novel sol-gel laccase (SOLAC) in ionic liquids and organic solvents was demonstrated by using 2,6-dimethoxyphenol (2,6-DMP) as a substrate. The enhancement of the catalytic activity of the SOLAC was observed and compared to the free laccase in both media. The oxidative biodegradation of o-chlorophenol as a model of phenolic environmental pollutants in organic media shows that the degradation was observed only when using water pre-saturated organic solvents or reverse micelle system. The SOLAC gave higher biodegradation rate in either aqueous or organic solvents, in which the optimum temperature was observed at 40 °C for the reverse micelle system as a reaction medium. All results demonstrated the potential use of the SOLAC for biodegradation of phenolic environmental pollutants in non-conventional media.

Study of enzymatic properties of phenol oxidase from nitrogen-fixing Azotobacter chroococcum

Azotobacter chroococcum is a widespread free-living soil bacterium within the genus of Azotobacter known for assimilation of atmospheric nitrogen and subsequent conversion into nitrogenous compounds, which henceforth enrich the nitrogen content of soils. A. chroococcum SBUG 1484, isolated from composted earth, exhibits phenol oxidase (PO) activity when growing under nitrogen-fixing conditions. In the present study we provide incipient analysis of the crude PO activity expressed by A. chroococcum SBUG 1484 within comparative analysis to fungal crude PO from the white-rot fungus Pycnoporus cinnabarinus SBUG-M 1044 and tyrosinase (PPO) from the mushroom Agaricus bisporus in an attempt to reveal desirable properties for exploitation with future recombinant expression of this enzyme. Catalytic activity increased with pre-incubation at 35°C; however 70% of activity remained after pre-treatment at 50°C. Native A. chroococcum crude PO exhibited not only strong preference for 2,6-dimethoxyphenol, but also towards related methoxy-activated substrates as well as substituted ortho-benzenediols from over 40 substrates tested. Presence of CuSO₄ enhanced crude phenol oxidase activity up to 30%, whereas NaN₃ (0.1 mM) was identified as the most inhibiting substance of all inhibitors tested. Lowest inhibition of crude PO activity occurred after 60 minutes of incubation in presence of 15% methanol and ethanol with 63% and 77% remaining activities respectively, and presence of DMSO even led to increasing oxidizing activities. Substrate scope and inhibitor spectrum strongly differentiated A. chroococcum PO activity comprised in crude extracts from those of PPO and confirmed distinct similarities to fungal PO.

Bacillus pumilus laccase: a heat stable enzyme with a wide substrate spectrum

Background

Laccases are multi-copper oxidases that catalyze the one electron oxidation of a broad range of compounds. Laccase substrates include substituted phenols, arylamines and aromatic thiols. Such compounds are activated by the enzyme to the corresponding radicals. Owing to their broad substrate range laccases are considered to be versatile biocatalysts which are capable of oxidizing natural and non-natural industrial compounds, with water as sole by-product.

Results

A novel CotA-type laccase from Bacillus pumilus was cloned, expressed and purified and its biochemical characteristics are presented here. The molecular weight of the purified laccase was estimated to be 58 kDa and the enzyme was found to be associated with four copper atoms. Its catalytic activity towards 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS), 2,6-dimethoxyphenol (2,6-DMP) and syringaldazine (SGZ) was investigated. The kinetic parameters K_M and k_cat for ABTS were 80 ± 4 μM and 291 ± 2.7 s^-1, for 2,6-DMP 680 ± 27 μM and 11 ± 0.1 s^-1 and for SGZ only k_cat could be estimated to be 66 ± 1.5 s^-1. The pH optimum for ABTS was 4, for 2,6-DMP 7 and for SGZ 6.5 and temperature optima for ABTS and 2,6-DMP were found to be around 70°C. The screening of 37 natural and non-natural compounds as substrates for B. pumilus laccase revealed 18 suitable compounds. Three of them served as redox mediators in the laccase-catalyzed decolorization of the dye indigocarmine (IC), thus assessing the new enzyme's biotechnological potential.

Conclusions

The fully copper loaded, thermostable CotA laccase from Bacillus pumilus is a versatile laccase with potential applications as an industrial biocatalyst.

Carbon-oxygen bond formation by fungal laccases: cross-coupling of 2,5-dihydroxy-N-(2-hydroxyethyl)-benzamide with the solvents water, methanol, and other alcohols

Laccase-catalyzed reactions lead to oxidation of the substrate via a cation radical, which has been described to undergo proton addition to form a quinonoid derivative or nucleophilic attack by itself producing homomolecular dimers. In this study, for the substrate 2,5-dihydroxy-N-(2-hydroxyethyl)-benzamide, we show that, besides the quinonoid form of substrate, all products formed are nonhomomolecular ones. Indeed, without addition of a reaction partner, heteromolecular products are formed from the quinonoid form of the laccase-substrate and the solvents water or methanol present in the incubation assay. Consequently, in laccase catalyzed syntheses performed in aqueous solutions or in the presence of methanol or other alcohols, undesirable heteromolecular coupling reactions between the laccase substrate and solvents must be taken into account. Additionally, it could be shown at the example of methanol and other alcohols that C-O-bound cross-coupling of dihydroxylated aromatic substances with the hydroxyl group of aliphatic alcohols can be catalyzed by fungal laccases.

Heterologous expression of lcc1 gene from Trametes trogii in Pichia pastoris and characterization of the recombinant enzyme

Background

Fungal laccases are useful enzymes for industrial applications; they exhibit broad substrate specificity and thus are able to oxidize a variety of xenobiotic compounds including chlorinated phenolics, synthetic dyes, pesticides and polycyclic aromatic hydrocarbons. Unfortunately, the biotechnological exploitation of laccases can be hampered by the difficulties concerning the enzyme production by the native hosts.

Results

In order to obtain a simple and efficient source of laccase, the lcc1 cDNA isolated from the white-rot fungus Trametes trogii has been successfully expressed in the methylotrophic yeast Pichia pastoris under the control of the methanol induced alcohol oxidase promoter P_AOX1. The recombinant Lcc1 was produced as a secreted protein with the native N-terminal prepropeptide for signal trafficking, and thus easily recovered from the culture medium. At the 1-liter scale, as calculated on the basis of the specific activity, the recombinant protein was produced at a yield of 17 mg/l. The highest production level obtained in fed-batch culture was 2520 U/l, corresponding to a specific productivity of 31.5 U/g biomass.

The purified recombinant laccase exhibited a behaviour similar to the main laccase produced by T. trogii. Lcc1 showed high activity in the presence of organic solvents and a high decolourization capacity towards azo, triarylmethane, indigo carmine and anthraquinonic dyes, that could be significantly enhanced in the presence of the redox mediators 1-hydroxybenzotriazole and violuric acid.

Conclusion

Heterologous expression of T. trogii laccase lcc1 in the methylotrophic yeast P. pastoris was successfully achieved. The biochemical and kinetic characterization of the recombinant protein suggests potential technological applications for this enzyme.

Mediator-assisted laccase-catalyzed oxidation of 4-hydroxybiphenyl

The kinetics of oxidation of 4-hydroxybiphenyl (4-HBP) catalyzed by laccase from Polyporus pinsitus was studied in the presence of methyl syringate (MS), which acts as an electron-transfer mediator. Measurements were performed in 0.05 M acetate buffer, pH 5.5, in the presence of 4-HBP, MS, and laccase. It is shown that the oxidation rate of the lowly reactive substrate 4-HBP significantly increases during synergistic action of the highly reactive substrate MS. Bimolecular kinetic constants of interaction between the oxidized form of laccase and MS, the former and 4-HBP, and the oxidized form of MS and 4-HBP were calculated. A kinetic scheme of the synergistic substrate action is suggested; based on this scheme, the dependence of the initial rate on reagent concentration is derived. Analyzing experimental data, we obtained kinetic constants close to those obtained by modeling the processes.

Applicability of Coriolopsis rigida for Biodegradation of Polycyclic Aromatic Hydrocarbons

Laccase was produced by Coriolopsis rigida using barley bran as substrate in solid-state fermentation (SSF) and also by submerged fermentation (SmF). The best results were obtained in SSF with twice the amount of laccase production. Laccase could be produced from repeated batch cultures of SSF over 30 days. The laccase degraded several polycyclic aromatic hydrocarbons (PAHs) in vivo and in vitro. The addition of an effective mediator, 1-hydroxybenzotriazol (50 microM), during in vitro treatment increased the degradation rate.

Laccase-induced derivatization of unprotected amino acid L-tryptophan by coupling with p-hydroquinone 2,5-dihydroxy-N-(2-hydroxyethyl)-benzamide

We have studied the enzymatic derivatization of amino acids by use of the polyphenol oxidase laccase. Derivatization of L-tryptophan was achieved by enzymatic crosslinking with the laccase substrate 2,5-dihydroxy-N-(2-hydroxyethyl)-benzamide. The main product (yield up to 70%) was identified as the quinoid compound 2-[2-(2-hydroxy-ethylcarbamoyl)-3,6-dioxo-cyclohexa-1,4-dienylamino]-3-(1H-indol-3-yl)- propionic acid and demonstrates that laccase-catalyzed C-N-coupling occurred on the amino group of the aliphatic side chain. These enzyme based reactions provide a simple and fast method for the derivatization of unprotected amino acids.

Biodegradation of phenolic environmental pollutants by a surfactant–laccase complex in organic media

Oxidative degradation of phenolic environmental pollutants in organic media was investigated using a laccase complexed with surfactants. The catalytic activity of the surfactant-laccase complex in isooctane was markedly enhanced by appropriately adjusting the water content of the reaction medium using reverse micelles. The surfactant-laccase complex showed little activity towards the oxidative reaction of bisphenol A in water-saturated isooctane (i.e., 0.0055% [v/v] water) while effectively catalyzed the same reaction in isooctane containing 4% (v/v) water, which is over the maximum water solubility. The latter system was homogeneous and was only achieved by the aid of reverse micelles. With respect to the oxidation of bisphenol A, two products, 4-isopropylphenol and 4-isopropenylphenol, were identified by gas chromatography-mass spectrometry (GC-MS) analyses, indicating the oxidative degradation of the bis-phenolic structure of bisphenol A. We also found that the surfactant-laccase complex turned out to handle other environmental pollutants, chlorophenols, by the simultaneous addition of water and a redox mediator into the reaction medium using reverse micelles.