Design and Engineering of an Efficient Peroxidase Using Myoglobin for Dye Decolorization and Lignin Bioconversion

The treatment of environmental pollutants such as synthetic dyes and lignin has received much attention, especially for biotechnological treatments using both native and artificial metalloenzymes. In this study, we designed and engineered an efficient peroxidase using the O₂ carrier myoglobin (Mb) as a protein scaffold by four mutations (F43Y/T67R/P88W/F138W), which combines the key structural features of natural peroxidases such as the presence of a conserved His-Arg pair and Tyr/Trp residues close to the heme active center. Kinetic studies revealed that the quadruple mutant exhibits considerably enhanced peroxidase activity, with the catalytic efficiency (k_cat/K_m) comparable to that of the most efficient natural enzyme, horseradish peroxidase (HRP). Moreover, the designed enzyme can effectively decolorize a variety of synthetic organic dyes and catalyze the bioconversion of lignin, such as Kraft lignin and a model compound, guaiacylglycerol-β-guaiacyl ether (GGE). As analyzed by HPLC and ESI-MS, we identified several bioconversion products of GGE, as produced via bond cleavage followed by dimerization or trimerization, which illustrates the mechanism for lignin bioconversion. This study indicates that the designed enzyme could be exploited for the decolorization of textile wastewater contaminated with various dyes, as well as for the bioconversion of lignin to produce more value-added products.

Vibrational spectra of guaiacylglycerol-β-guaiacyl ether: experiment and theory

As an important inter-unit of lignin, guaiacylglycerol-β-guaiacyl (GG) ether has been synthesized, and characterized using terahertz time-domain spectroscopy in the frequency range of 5-85 cm(-1). Seven absorption peaks have been observed. Among these peaks, the 49.8 cm(-1) and 57.6 cm(-1) vibrations are propose to be characteristic absorption peaks of GG ether. Raman spectra were also measured in the range of 50-3500 cm(-1). The vibrations of the two lowest energy forms, i.e., erythro 1r4s and threo 1s4s, were calculated using density functional theory at the B3LYP/6-311G∗∗ level and assigned according to potential energy distribution. In addition, the contents of erythro and threo forms in GG sample could be estimated by comparing the waveform similarities between theoretical and observed curves in the 33.0-80.0 cm(-1) range. Results showed that the observed curve of GG sample is a combination of erythro 1s4r and threo 1s4s. The four absorption vibrations below 33.0 cm(-1) could be assigned to phonon, inter-molecular modes and/or hydrogen bond vibrations. Terahertz spectra and Raman spectra, together with theoretical calculations, could be powerful methods for predicting contents of different isomers in sample.

Laccase catalyzed covalent coupling of fluorophenols increases lignocellulose surface hydrophobicity

This work presents for the first time the mechanistic evidence of a laccase-catalyzed method of covalently grafting hydrophobicity enhancing fluorophenols onto Fagus sylvatica veneers. Coupling of fluorophenols onto complex lignin model compounds guaiacylglycerol beta-guaiacyl ether and syringylglycerol beta-guaiacyl ether was demonstrated by LC-MS and NMR. Laccase-mediated coupling increased binding of 4-[4-(trifluoromethyl)phenoxy]phenol (4,4-F3MPP) and 4-(trifluoromethoxy)phenol (4-F3MP) to veneers by 77.1% and 39.2%, respectively. XPS studies showed that laccase-catalyzed grafting of fluorophenols resulted in a fluorine content of 6.39% for 4,4-F3MPP, 3.01% for 4-F3MP and 0.26% for 4-fluoro-2-methylphenol (4,2-FMP). Grafting of the fluorophenols 4,2-FMP, 4-F3MP and 4,4-F3MPP led to a 9.6%, 28.6% and 65.5% increase in hydrophobicity, respectively, when compared to treatments with the respective fluorophenols in the absence of laccase, in good agreement with XPS data.

Disassembly of lignin and chemical recovery in supercritical water and p-cresol mixture. Studies on lignin model compounds

The aim of the study was to gain insight into the role of the each unit of lignin in the formation of products. Glycerol, guaiacol, the mixture of glycerol and guaiacol (Gly&Gua), and guaiacylglycerol-beta-guaiacyl ether (GGGE) were used as lignin model compounds to study fragmentation of lignin in an excess of water and p-cresol at 400 degrees C. The products have been analyzed employing gas chromatography (GC)-mass spectrometer (MS) and gas chromatography-frame ionization detector for qualitative and quantitative analysis. GC-MS analysis indicated that phenol, o-cresol, methyl-anisole, di-methyl-phenol, ethyl-methyl-phenol, 2-(hydroxy-benzyl)-4-methyl-phenol (BMP) and 2-(2-hydroxy-5-methyl-benzyl)-4-methyl-phenol were formed. The products were similar to the products by the fragmentation of lignin. The products, except o-cresol, were primarily derived from glycerol and/or guaiacol. We also found that the amount of BMP derived from GGGE, which has glycerol unit and guaiacol unit in its structure, is much more than that derived from Gly&Gua. The increase of the BMP yield by reaction with GGGE compared with (glycerol and/or guaiacol) indicates that guaiacylglycerol unit with linkage of Gly&Gua plays an important role in the formation of BMP and also it is suggested that the BMP formation from GGGE has pathways other than that from Gly&Gua.

Wood stimulates the demethoxylation of [O14CH3]-labeled lignin model compounds by the white-rot fungi Phanerochaete chrysosporium and Phlebia radiata

Mineralization of polymeric wood lignin and its substructures is a result of complex reactions involving oxidizing and reducing enzymes and radicals. The degradation of methoxyl groups is an essential part of this process. The presence of wood greatly stimulates the demethoxylation of a non-phenolic lignin model compound (a [O(14)CH(3)]-labeled beta-O-4 dimer) by the lignin-degrading white-rot fungi Phlebia radiata and Phanerochaete chrysosporium. When grown on wood, both fungi produced up to 47 and 40% (14)CO(2) of the applied (14)C activity, respectively, under air and oxygen in 8 weeks. Without wood, the demethoxylation of the dimer by both fungi was lower, varying between 0.5 and 35%. Addition of nutrient nitrogen together with glucose decreased demethoxylation when the fungi were grown on spruce wood under air. Because the evolution of (14)CO(2) in the absence of wood was poor, the fungi may have preferably used wood as a carbon and nitrogen source. The amount of fungal mycelium, as determined by the ergosterol assay, did not show connection to demethoxylation. P. radiata also showed a high demethoxylation of [O(14)CH(3)]-labeled vanillic acid in the presence of birch wood. The degradation of lignin and lignin-related substances should be studied in the presence of wood, the natural substrate for white-rot fungi.

Non-enzymatic reduction of quinone methides during oxidative coupling of monolignols: implications for the origin of benzyl structures in lignins

Lignin is believed to be synthesized by oxidative coupling of 4-hydroxyphenylpropanoids. In native lignin there are some types of reduced structures that cannot be explained solely by oxidative coupling. In the present work we showed via biomimetic model experiments that nicotinamide adenine dinucleotide (NADH), in an uncatalyzed process, reduced a beta-aryl ether quinone methide to its benzyl derivative. A number of other biologically significant reductants, including the enzyme cellobiose dehydrogenase, failed to produce the reduced structures. Synthetic dehydrogenation polymers of coniferyl alcohol synthesized (under oxidative conditions) in the presence of the reductant NADH produced the same kind of reduced structures as in the model experiment, demonstrating that oxidative and reductive processes can occur in the same environment, and that reduction of the in situ-generated quinone methides was sufficiently competitive with water addition. In situ reduction of beta-beta-quinone methides was not achieved in this study. The origin of racemic benzyl structures in lignins therefore remains unknown, but the potential for simple chemical reduction is demonstrated here.

The cellulose/lignin assembly assessed by molecular modeling. Part 1: adsorption of a threo guaiacyl beta-O-4 dimer onto a Ibeta cellulose whisker

The assembly of the two major cell wall components, cellulose and lignin, were investigated at the atomistic scale using molecular dynamics simulations. To this end, a molecular model of a cellulose crystal corresponding to the allomorph Ibeta and exhibiting different surfaces was considered to mimic the carbohydrate matrix present in native wood cell wall. The lignin model compound considered here is a threo guaiacyl beta-O-4 dimer. The dynamical process of adsorption of the lignin dimer onto the different surfaces of the cellulose crystal was examined. The modes of association between the two constituents were analyzed; energies of adsorption of the dimer are calculated favorable and of the same order of magnitude on all sides of the cellulosic model, suggesting that the deposition of lignin precursors onto cellulose fibers is non-specific from an enthalpic point of view. Interestingly, geometrical characteristics and energetical details of the adsorption are surface-dependent. Computed data have underlined the predominant contribution of van der Waals interactions for adsorption onto the (200) face, as well as the major influence of H-bonding interactions in the dynamical process of adsorption onto (110) and (1-10) faces. A large number of adsorption sites have been identified and a noticeable "flat" geometry of adsorption of the lignin dimer has been observed, as a consequence of the stacking interactions between lignin aromatic rings and C-H groups of cellulose. Importantly, these dispersive interactions lead to a preferential parallel orientation of lignin aromatic rings relative to the cellulose surface, notably on the (200) face. Such a parallel orientation is consistent with previously reported experimental observations.

Conformational study of a guaiacyl beta-O-4 lignin model compound by NMR. Examination of intramolecular hydrogen bonding interactions and conformational flexibility in solution

Intramolecular H-bonding interactions were investigated in solution for the threo and erythro diastereomeric forms of a guaiacyl beta-O-4 lignin model compound by using the NMR data obtained from hydroxyl protons. Temperature coefficients of the chemical shifts (ddelta/dT) and coupling constants (3J(HCOH)) were measured in aprotic and protic solutions: DMSO-d6, acetone-d6 and acetone-d6-water. The NMR parameters do not support the existence of strong and persistent intramolecular H-bonds that could participate in the stabilization of the guaiacyl beta-O-4 structure in solution, but instead indicate that intermolecular H-bonds to solvent predominate. 1D NOE experiments nevertheless revealed the presence of a direct chemical exchange between the hydroxyl protons, suggesting the possible existence of weak and transient intramolecular H-bonding interactions. The conformational flexibility of the threo structure was also investigated in acetone solution from the measurement of long-range 1H, 1H and 1H, 13C coupling constants and from NOESY experiments. The NMR data are not consistent with any single conformation, indicating that different conformers co-exist in solution. The experimental results support the conformational flexibility predicted by molecular dynamics simulations performed in a previous study. Finally, both experimental and theoretical approaches indicate that weak intramolecular H-bonds can exist transiently in solution, breaking and reforming as the beta-O-4 molecule undergoes conformational interconversion, but cannot be invoked as possible means of conferring rigidity to the beta-O-4 structure.

Polymerization of Guaiacol and a Phenolic β-O-4-Substructure by Trametes hirsuta Laccase in the Presence of ABTS

The reaction of the monomeric lignin model compound guaiacol and the beta-O-4-type dimer erol (1-(4-hydroxy-3-methoxyphenyl)-2(2-methoxyphenoxy)-propane-1,3-diol with laccase from Trametes hirsuta was studied in the presence of the mediator ABTS (2,2'-azino-di[3ethylbenzothiazoline-6-sulfonic acid]). The product mixtures were analyzed by means of aqueous-phase size exclusion chromatography (SEC) with 50 mM NaOH as eluent. Interestingly, in the laccase-catalyzed reaction with both substrates, the mediator not only functioned as an electron carrier but underwent coupling reactions with the substrate to give polymeric coupling products. The molecular weight of these copolymeric products was significantly higher than the molecular weight of products obtained without ABTS. After ultrafiltration, 33% and 21% of the initially applied ABTS could be found in the polymeric product fraction for the substrates guaiacol and erol, respectively, on the basis of nitrogen analysis. When ABTS was added to substrates after full laccase-catalyzed polymerization, the reaction proceeded toward higher molecular weights.

Detection and characterization of a novel extracellular fungal enzyme that catalyzes the specific and hydrolytic cleavage of lignin guaiacylglycerol beta-aryl ether linkages

Cleavage of the arylglycerol beta-aryl ether linkage is the most important process in the biological degradation of lignin. The bacterial beta-etherase was described previously and shown to be tightly associated with the cellular membrane. In this study, we aimed to detect and isolate a new extracellular function that catalyses the beta-aryl ether linkage cleavage of high-molecular lignin in the soil fungi. We screened and isolated 2BW-1 cells by using a highly sensitive fluorescence assay system. The beta-aryl ether cleavage enzyme was produced by a newly isolated fungus, 2BW-1, and is secreted into the extracellular fraction. The beta-aryl ether cleavage enzyme converts the guaiacylglycerol beta-O-guaiacyl ether (GOG) to guaiacylglycerol and guaiacol. It requires the C alpha alcohol structure and p-hydroxyl group and specifically attacks the beta-aryl ether linkage of high-molecular mass lignins with addition of two water molecules at the C alpha and C beta positions.

Guaiacylglycerol-7'-O-methyl 8'-vanillic acid ether and related compounds from Boreava orientalis

The threo and erythro forms of guaiacylglycerol-7'-O-methyl 8'-vanillic acid ethers, threo and erythro guaiacylglycerol 8'-vanillin ethers, and threo guaiacylglycerol 8'-(4-hydroxymethyl-2-methoxyphenyl) ether have been isolated from fruits of Boreava orientalis. Structural determinations were made on the basis of UV, MS, 1H- and 13C-NMR spectral data, including two-dimensional shift correlation. The relative configurations were assigned on the basis of 1H-NMR chemical shifts.

Abuse of guaifenesin-containing medications generates an excess of a carboxylate salt of beta-(2-methoxyphenoxy)-lactic acid, a guaifenesin metabolite, and results in urolithiasis

OBJECTIVES

Several urinary calculi were submitted to our institution for compositional analysis. The typical techniques of analysis, polarized light microscopy, electron microprobe analysis, and infrared spectroscopy proved inadequate for a definitive identification. As a result, a more detailed organic analysis was conducted to determine the exact chemical structure of the material.

METHODS

Infrared spectroscopy and mass spectrometric analysis were carried out on the solid material, providing information concerning the functional groups and the molecular mass of the organic constituent and its components. The stone was solubilized in deuterated solvents and analyzed by nuclear magnetic resonance spectroscopy, which resulted in a definitive chemical structure.

RESULTS

The spectroscopic analysis indicated that the stones were composed of a calcium salt of beta-(2-methoxyphenoxy)-lactic acid, a metabolite of the pharmaceutical guaifenesin, which is used as an expectorant.

CONCLUSIONS

Guaifenesin, an expectorant common in over-the-counter cold and allergy remedies, can cause urolithiasis if taken in excess. Discussions with physicians and their patients confirmed that most patients admitted to taking large doses of guaifenesin-containing medications.

Lignin peroxidase L3 from Phlebia radiata. Pre-steady-state and steady-state studies with veratryl alcohol and a non-phenolic lignin model compound 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol

The catalytic cycle of lignin peroxidase (LiP, ligninase) isozyme L3 from the white-rot fungus Phlebia radiata was investigated using stopped-flow techniques. Veratryl (3,4-dimethoxybenzyl) alcohol and a lignin model compound, non-phenolic beta-O-4 dimer 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol, were used as electron donors. This is the first report on the detailed kinetic analysis of a LiP-catalysed C alpha-C beta bond cleavage of the dimer, representing the major depolymerisation reaction in the lignin polymer. The native enzyme showed a typical heme peroxidase absorbance spectrum with a Soret maximum at 407 nm. Following the reaction with H2O2, the Soret band decreased in absorbance, shifted to 403 nm and then to 421 nm, demonstrating the formation of compound I followed by the formation of compound II, respectively. Similar results have been reported for the LiP from Phanerochaete chrysosporium upon reaction with H2O2. However, compound I of L3 was more stable in the absence of additional electron donors. The second-order rate constant of compound I formation by H2O2 was determined to be 6 x 10(5) M-1 s-1 and was the same at pH 3.0 and 6.0. Compound I was rapidly reduced to compound II and further to native enzyme when either veratryl alcohol or the beta-O-4 dimer was supplied as electron donor and in both cases veratraldehyde appeared as the major product. At pH 6.0, the second-order rate constant for compound II formation was similar with either veratryl alcohol or the beta-O-4 dimer (6.7 x 10(3) and 6.5 x 10(3) M-1 s-1, respectively). At pH 3.0 formation of compound II with either reductant proceeded so rapidly that determination of the respective rate constants was not possible. The results point to identical catalytic cycles of L3 with veratryl alcohol or the beta-O-4 dimer involving both compounds I and II as intermediates and participation of the same veratryl alcohol radical as the most appropriate reductant for compound II. Chemical evidence of such a radical, formed after the initial LiP-catalysed one-electron oxidation of beta-O-4 dimeric lignin models, is presented in a separate article [Lundell, T., Schoemaker, H., Hatakka, A. & Brunow, G. (1993) Holzforschung, in the press]. The catalytic redox-cycle and the oxidation mechanism presented here reconcile seemingly contradictory results obtained in previous studies on LiP kinetics during the last decade.

Degradation of labelled lignins and veratrylglycerol-beta-guaiacyl ether by Acinetobacter sp

Acinetobacter sp. evolved 14CO2 from 14C-(ring)DHP lignin and 14C-teakwood lignin. Veratrylglycerol-beta-guaiacyl ether, a lignin model compound with beta-o-4 linkage was cleaved by Acinetobacter sp. Veratrylglycerol-beta-guaiacyl ether into 2(o-methoxyphenoxy) ethanol and veratrylalcohol 2(o-methoxyphenoxy) ethanol was degraded to guaiacol and then to catechol whereas veratrylalcohol was converted to veratraldehyde, veratric acid, vanillic acid, protocatechuic acid and catechol. Both catechol 1,2-dioxygenase and protocatechuate 3,4-dioxygenase were detected in veratrylglycerol-beta-guaiacyl ether grown cultures.

Biomimetic oxidation of lignin model compounds by simple inorganic complexes

Copper peroxydisulfate has been shown to mimic "ligninases" in the oxidative degradation of Dihydroanisoin, Veratrylglycerol-beta-guaiacyl ether and veratryl alcohol. A unified mechanism leads to predictable degradative pathways. These are initiated by single-electron oxidation of aromatic substrates to aryl cation radicals as common intermediates to both the enzymic and biomimetic reactions. Our preliminary results show that simple complexes can facilitate the oxidative degradation of lignin model compounds.

Anaerobic degradation of veratrylglycerol-beta-guaiacyl ether and guaiacoxyacetic acid by mixed rumen bacteria

Veratrylglycerol-beta-guaiacyl ether (0.2 g/liter), a lignin model compound, was found to be degraded by mixed rumen bacteria in a yeast extract medium under strictly anaerobic conditions to the extent of 19% within 24 h. Guaiacoxyacetic acid, 2-(o-methoxyphenoxy)ethanol, vanillic acid, and vanillin were detected as degradation products of veratrylglycerol-beta-guaiacyl ether by thin-layer chromatography, gas chromatography, and gas chromatography-mass spectrometry. Guaiacoxyacetic acid (0.25 g/liter), when added into the medium as a substrate, was entirely degraded within 36 h, resulting in the formation of phenoxyacetic acid, guaiacol, and phenol. These results suggest that the beta-arylether bond, an important intermonomer linkage in lignin, can be cleaved completely by these rumen anaerobes.

A new bacterial dehydrogenase oxidizing the lignin model compound guaiacylglycerol beta-O-4-guaiacyl ether

A lignin model compound, named in short guaiagylglycerol beta-guaiacyl ether (GGE), contains the beta-0-4 ether linkage that is common in the chemical structure of lignin. A Pseudomonas sp. (GU5) had been isolated as an organism able to grow with GGE as the sole source of carbon and energy. When grown on vanillate, the bacteria contained a NAD+ -dependent dehydrogenase converting GGE to a 355 nm absorbing product. The enzyme, named GGE-dehydrogenase, was purified about 160-fold using gel permeation, ion exchange on DEAE-Sephadex, and dye-ligand affinity chromatography. The new protein was about 52 kDa in apparent size with but one polypeptide chain after denaturation and reduction. According to several criteria, the product of GGE oxidation (Km = 12 microM) was identified as the corresponding conjugated ketone at the alpha-carbon of the C3 side-chain. The secondary alcohol function in GGE was apparently the sole target of the enzyme action. However the conversion of GGE into ketone catalyzed by the enzyme was only partial, and did not exceed 50%, probably because only one of the alpha-enantiomers was susceptible to enzyme attack. In contrast the ketone, either made by organic synthesis or by enzymic oxidation of GGE, could be totally reduced back to GGE (Km = 13 microM at pH 8.4, 8 microM at neutral pH), with NADH as the reductant, as confirmed by UV absorption and NMR spectra. Other model compounds with no primary alcoholic function, ether linkage or phenolic group were also substrates for the enzyme, confirming the specificity of GGE-dehydrogenase for the alpha-carbon position. Conjugation of the alpha-ketone with an adjacent phenolic nucleus interfered strongly with equilibrium constants and redox potentials of the system according to pH, and the enzyme displayed widely different optima with pH over 9 when oxidizing GGE, below 7 when reducing the ketone. Equilibrium studies showed that the ketone/GGE potential was -0.37 volt at pH 8.7, -0.23 volt at pH 7 (30 degrees C). The significance of this new dehydrogenase and its properties are discussed, especially in the general concern of lignin biodegradation.

Catabolism of arylglycerol-beta-aryl ethers lignin model compounds by Pseudomonas cepacia 122

Pseudomonas cepacia 122 can grow on several lignin model compounds including the arylglycerol-beta-aryl ethers guaiacylglycerol-beta-coniferyl ether and guaiacylglycerol-beta-guaiacyl ether. Non-phenolic lignin model compounds are not degraded by this bacterium. The enzyme system catalyzing guaiacylglycerol-beta-guaiacyl ether dissimilation in Pseudomonas cepacia 122 is inducible and repressed by glucose. Guaiacylglycerol and guaiacylglycerol-beta-guaiacyl ether were identified as intermediates in guaiacylglycerol-beta-coniferyl ether catabolism. Guaiacol, guaiacoxyethanol, vanillin and vanillic acid were identified as intermediates of guaiacylglycerol-beta-guaiacyl ether breakdown indicating that a C alpha-C beta splitting mechanism is involved in the degradation of aryl-alkyl ethers by this bacterium.