Lipid Peroxidation by the Manganese Peroxidase of Phanerochaete chrysosporium Is the Basis for Phenanthrene Oxidation by the Intact Fungus

The manganese peroxidase (MnP) of Phanerochaete chrysosporium supported Mn(II)-dependent, H(2)O(2)-independent lipid peroxidation, as shown by two findings: linolenic acid was peroxidized to give products that reacted with thiobarbituric acid, and linoleic acid was peroxidized to give hexanal. MnP also supported the slow oxidation of phenanthrene to 2,2'-diphenic acid in a reaction that required Mn(II), oxygen, and unsaturated lipids. Phenanthrene oxidation to diphenic acid by intact cultures of P. chrysosporium occurred to the same extent that oxidation in vitro did and was stimulated by Mn. These results support a role for MnP-mediated lipid peroxidation in phenanthrene oxidation by P. chrysosporium.

Initial Steps in the Degradation of Methoxychlor by the White Rot Fungus Phanerochaete chrysosporium

The white rot fungus Phanerochaete chrysosporium mineralized [ring-(sup14)C]methoxychlor [1,1,1-trichloro-2,2-bis(4-methoxyphenyl)ethane] and metabolized it to a variety of products. The three most prominent of these were identified as the 1-dechloro derivative 1,1-dichloro-2,2-bis(4-methoxyphenyl)ethane, the 2-hydroxy derivative 2,2,2-trichloro-1,1-bis(4-methoxyphenyl)ethanol, and the 1-dechloro-2-hydroxy derivative 2,2-dichloro-1,1-bis(4-methoxyphenyl)ethanol by comparison of the derivatives with authentic standards in chromatographic and mass spectrometric experiments. In addition, the 1-dechloro-2-hydroxy derivative was identified from its (sup1)H nuclear magnetic resonance spectrum. The 1-dechloro and 2-hydroxy derivatives were both converted to the 1-dechloro-2-hydroxy derivative by the fungus; i.e., there was no requirement that dechlorination precede hydroxylation or vice versa. All three metabolites were mineralized and are therefore likely intermediates in the degradation of methoxychlor by P. chrysosporium.

Biosynthetic Pathway for Veratryl Alcohol in the Ligninolytic Fungus Phanerochaete chrysosporium

Veratryl alcohol (VA) is a secondary metabolite of white-rot fungi that produce the ligninolytic enzyme lignin peroxidase. VA stabilizes lignin peroxidase, promotes the ability of this enzyme to oxidize a variety of physiological substrates, and is accordingly thought to play a significant role in fungal ligninolysis. Pulse-labeling and isotope-trapping experiments have now clarified the pathway for VA biosynthesis in the white-rot basidiomycete Phanerochaete chrysosporium. The pulse-labeling data, obtained with C-labeled phenylalanine, cinnamic acid, benzoic acid, and benzaldehyde, showed that radiocarbon labeling followed a reproducible sequence: it peaked first in cinnamate, then in benzoate and benzaldehyde, and finally in VA. Phenylalanine, cinnamate, benzoate, and benzaldehyde were all efficient precursors of VA in vivo. The isotope-trapping experiments showed that exogenous, unlabeled benzoate and benzaldehyde were effective traps of phenylalanine-derived C. These results support a pathway in which VA biosynthesis proceeds as follows: phenylalanine --> cinnamate --> benzoate and/or benzaldehyde --> VA.

Pathways for extracellular Fenton chemistry in the brown rot basidiomycete Gloeophyllum trabeum

The brown rot fungus Gloeophyllum trabeum uses an extracellular hydroquinone-quinone redox cycle to reduce Fe(3+) and produce H(2)O(2). These reactions generate extracellular Fenton reagent, which enables G. trabeum to degrade a wide variety of organic compounds. We found that G. trabeum secreted two quinones, 2,5-dimethoxy-1,4-benzoquinone (2,5-DMBQ) and 4,5-dimethoxy-1,2-benzoquinone (4,5-DMBQ), that underwent iron-dependent redox cycling. Experiments that monitored the iron- and quinone-dependent cleavage of polyethylene glycol by G. trabeum showed that 2,5-DMBQ was more effective than 4,5-DMBQ in supporting extracellular Fenton chemistry. Two factors contributed to this result. First, G. trabeum reduced 2,5-DMBQ to 2,5-dimethoxyhydroquinone (2,5-DMHQ) much more rapidly than it reduced 4,5-DMBQ to 4,5-dimethoxycatechol (4,5-DMC). Second, although both hydroquinones reduced ferric oxalate complexes, the predominant form of Fe(3+) in G. trabeum cultures, the 2,5-DMHQ-dependent reaction reduced O(2) more rapidly than the 4,5-DMC-dependent reaction. Nevertheless, both hydroquinones probably contribute to the extracellular Fenton chemistry of G. trabeum, because 2,5-DMHQ by itself is an efficient reductant of 4,5-DMBQ.

Degradation of nonphenolic lignin by the laccase/1-hydroxybenzotriazole system

Phenolic and nonphenolic (permethylated) synthetic [14C]lignins were depolymerized by Trametes villosa laccase in the presence of a radical mediator, 1-hydroxybenzotriazole (HOBT). Gel permeation chromatography of the treated lignins showed that approximately 10% of their substructures were cleaved. The system also cleaved a beta-O-4-linked model compound, 1-(4-ethoxy-3-methoxy-ring-[14C]phenyl)-2-(2-methoxyphenoxy)-propane- 1,3-diol, and a beta-1-linked model, 1, 2-bis-(3-methoxy-4-[14C]methoxyphenyl)-propane-1,3-diol, that represent nonphenolic substructures in lignin. High performance liquid chromatography of products from the oxidized models showed that they were produced in sufficient yields to account for the ability of laccase/HOBT to depolymerize nonphenolic lignin.

Peroxyl radicals are potential agents of lignin biodegradation

Past work has shown that the extracellular manganese-dependent peroxidases (MnPs) of ligninolytic fungi degrade the principal non-phenolic structures of lignin when they peroxidize unsaturated fatty acids. This reaction is likely to be relevant to ligninolysis in sound wood, where enzymes cannot penetrate, only if it employs a small, diffusible lipid radical as the proximal oxidant of lignin. Here we show that a non-phenolic beta-O-4-linked lignin model dimer was oxidized to products indicative of hydrogen abstraction and electron transfer by three different peroxyl radical-generating systems: (a) MnP/Mn(II)/linoleic acid, (b) arachidonic acid in which peroxidation was initiated by a small amount of H(2)O(2)/Fe(II), and (c) the thermolysis in air of either 4,4'-azobis(4-cyanovaleric acid) or 2,2'-azobis(2-methylpropionamidine) dihydrochloride. Some quantitative differences in the product distributions were found, but these were attributable to the presence of electron-withdrawing substituents on the peroxyl radicals derived from azo precursors. Our results introduce a new hypothesis: that biogenic peroxyl radicals may be agents of lignin biodegradation.

Biodegradative mechanism of the brown rot basidiomycete Gloeophyllum trabeum: evidence for an extracellular hydroquinone-driven fenton reaction

We have identified key components of the extracellular oxidative system that the brown rot fungus Gloeophyllum trabeum uses to degrade a recalcitrant polymer, polyethylene glycol, via hydrogen abstraction reactions. G. trabeum produced an extracellular metabolite, 2,5-dimethoxy-1,4-benzoquinone, and reduced it to 2,5-dimethoxyhydroquinone. In the presence of 2,5-dimethoxy-1,4-benzoquinone, the fungus also reduced extracellular Fe3+ to Fe2+ and produced extracellular H2O2. Fe3+ reduction and H2O2 formation both resulted from a direct, non-enzymatic reaction between 2,5-dimethoxyhydroquinone and Fe3+. Polyethylene glycol depolymerization by G. trabeum required both 2,5-dimethoxy-1,4-benzoquinone and Fe3+ and was completely inhibited by catalase. These results provide evidence that G. trabeum uses a hydroquinone-driven Fenton reaction to cleave polyethylene glycol. We propose that similar reactions account for the ability of G. trabeum to attack lignocellulose.

Rapid polyether cleavage via extracellular one-electron oxidation by a brown-rot basidiomycete

Fungi that cause brown rot of wood are essential biomass recyclers and also the principal agents of decay in wooden structures, but the extracellular mechanisms by which they degrade lignocellulose remain unknown. To test the hypothesis that brown-rot fungi use extracellular free radical oxidants as biodegradative tools, Gloeophyllum trabeum was examined for its ability to depolymerize an environmentally recalcitrant polyether, poly(ethylene oxide) (PEO), that cannot penetrate cell membranes. Analyses of degraded PEOs by gel permeation chromatography showed that the fungus cleaved PEO rapidly by an endo route. 13C NMR analyses of unlabeled and perdeuterated PEOs recovered from G. trabeum cultures showed that a major route for depolymerization was oxidative C---C bond cleavage, a reaction diagnostic for hydrogen abstraction from a PEO methylene group by a radical oxidant. Fenton reagent (Fe(II)/H2O2) oxidized PEO by the same route in vitro and therefore might account for PEO biodegradation if it is produced by the fungus, but the data do not rule out involvement of less reactive radicals. The reactivity and extrahyphal location of this PEO-degrading system suggest that its natural function is to participate in the brown rot of wood and that it may enable brown-rot fungi to degrade recalcitrant organopollutants.

Evidence That Ceriporiopsis subvermispora Degrades Nonphenolic Lignin Structures by a One-Electron-Oxidation Mechanism

The white-rot fungus Ceriporiopsis subvermispora is able to degrade nonphenolic lignin structures but appears to lack lignin peroxidase (LiP), which is generally thought to be responsible for these reactions. It is well established that LiP-producing fungi such as Phanerochaete chrysosporium degrade nonphenolic lignin via one-electron oxidation of its aromatic moieties, but little is known about ligninolytic mechanisms in apparent nonproducers of LiP such as C. subvermispora. To address this question, C. subvermispora and P. chrysosporium were grown on cellulose blocks and given two high-molecular-weight, polyethylene glycol-linked model compounds that represent the major nonphenolic arylglycerol-(beta)-aryl ether structure of lignin. The model compounds were designed so that their cleavage via one-electron oxidation would leave diagnostic fragments attached to the polyethylene glycol. One model compound was labeled with (sup13)C at C(inf(alpha)) of its propyl side chain and carried ring alkoxyl substituents that favor C(inf(alpha))-C(inf(beta)) cleavage after one-electron oxidation. The other model compound was labeled with (sup13)C at C(inf(beta)) of its propyl side chain and carried ring alkoxyl substituents that favor C(inf(beta))-O-aryl cleavage after one-electron oxidation. To assess fungal degradation of the models, the high-molecular-weight metabolites derived from them were recovered from the cultures and analyzed by (sup13)C nuclear magnetic resonance spectrometry. The results showed that both C. subvermispora and P. chrysosporium degraded the models by routes indicative of one-electron oxidation. Therefore, the ligninolytic mechanisms of these two fungi are similar. C. subvermispora might use a cryptic LiP to catalyze these C(inf(alpha))-C(inf(beta)) and C(inf(beta))-O-aryl cleavage reactions, but the data are also consistent with the involvement of some other one-electron oxidant.

Manganese-Dependent Cleavage of Nonphenolic Lignin Structures by Ceriporiopsis subvermispora in the Absence of Lignin Peroxidase

Vol. 62, no. 10, p. 3684, column 2, line 16: "Tri(methylsilyl)" should read "Tri(trimethylsilyl)." Line 17: "Di(methylsilyl)" should read "Di(trimethylsilyl)." Line 20: "Tri(methylsilyl)" should read "Trimethylsilyl." Page 3686, column 1, reference 12: The journal should be Dokl. Akad. Nauk Belarusi. [This corrects the article on p. 3679 in vol. 62.].

Manganese-Dependent Cleavage of Nonphenolic Lignin Structures by Ceriporiopsis subvermispora in the Absence of Lignin Peroxidase

Many ligninolytic fungi appear to lack lignin peroxidase (LiP), the enzyme generally thought to cleave the major, recalcitrant, nonphenolic structures in lignin. At least one such fungus, Ceriporiopsis subvermispora, is nevertheless able to degrade these nonphenolic structures. Experiments showed that wood block cultures and defined liquid medium cultures of C. subvermispora rapidly depolymerized and mineralized a (sup14)C-labeled, polyethylene glycol-linked, high-molecular-weight (beta)-O-4 lignin model compound (model I) that represents the major nonphenolic structure of lignin. The fungus cleaved model I between C(inf(alpha)) and C(inf(beta)) to release benzylic fragments, which were shown in isotope trapping experiments to be major products of model I metabolism. The C(inf(alpha))-C(inf(beta)) cleavage of (beta)-O-4 lignin structures to release benzylic fragments is characteristic of LiP catalysis, but assays of C. subvermispora liquid cultures that were metabolizing model I confirmed that the fungus produced no detectable LiP activity. Three results pointed, instead, to the participation of a different enzyme, manganese peroxidase (MnP), in the degradation of nonphenolic lignin structures by C. subvermispora. (i) The degradation of model I and of exhaustively methylated (nonphenolic), (sup14)C-labeled, synthetic lignin by the fungus in liquid cultures was almost completely inhibited when the Mn concentration of the medium was decreased from 35 (mu)M to approximately 5 (mu)M. (ii) The fungus degraded model I and methylated lignin significantly faster in the presence of Tween 80, a source of unsaturated fatty acids, than it did in the presence of Tween 20, which contains only saturated fatty acids. Previous work has shown that nonphenolic lignin structures are degraded during the MnP-mediated peroxidation of unsaturated lipids. (iii) In experiments with MnP, Mn(II), and unsaturated lipid in vitro, this system mimicked intact C. subvermispora cultures in that it cleaved nonphenolic (beta)-O-4 lignin model compounds between C(inf(alpha)) and C(inf(beta)) to release a benzylic fragment.

Fluorene Oxidation In Vivo by Phanerochaete chrysosporium and In Vitro during Manganese Peroxidase-Dependent Lipid Peroxidation

The oxidation of fluorene, a polycyclic hydrocarbon which is not a substrate for fungal lignin peroxidase, was studied in liquid cultures of Phanerochaete chrysosporium and in vitro with P. chrysosporium extracellular enzymes. Intact fungal cultures metabolized fluorene to 9-hydroxyfluorene via 9-fluorenone. Some conversion to more-polar products was also observed. Oxidation of fluorene to 9-fluorenone was also obtained in vitro in a system that contained manganese(II), unsaturated fatty acid, and either crude P. chrysosporium peroxidases or purified recombinant manganese peroxidase. The oxidation of fluorene in vitro was inhibited by the free-radical scavenger butylated hydroxytoluene but not by the lignin peroxidase inhibitor NaVO(inf3). Manganese(III)-malonic acid complexes could not oxidize fluorene. These results indicate that fluorene oxidation in vitro was a consequence of lipid peroxidation mediated by P. chrysosporium manganese peroxidase. The rates of fluorene and diphenylmethane disappearance in vitro were significantly faster than those of true polycyclic aromatic hydrocarbons or fluoranthenes, whose rates of disappearance were ionization potential dependent. This result indicates that the initial oxidation of fluorene proceeds by mechanisms other than electron abstraction and that benzylic hydrogen abstraction is probably the route for oxidation.

New polymeric model substrates for the study of microbial ligninolysis

Lignin model dimers are valuable tools for the elucidation of microbial ligninolytic mechanisms, but their low molecular weight (MW) makes them susceptible to nonligninolytic intracellular metabolism. To address this problem, we prepared lignin models in which unlabeled and alpha-14C-labeled beta-O-4-linked dimers were covalently attached to 8,000-MW polyethylene glycol (PEG) or to 45,000-MW polystyrene (PS). The water-soluble PEG-linked model was mineralized extensively in liquid medium and in solid wood cultures by the white rot fungus Phanerochaete chrysosporium, whereas the water-insoluble PS-linked model was not. Gel permeation chromatography showed that P. chrysosporium degraded the PEG-linked model by cleaving its lignin dimer substructure rather than its PEG moiety. C alpha-C beta cleavage was the major fate of the PEG-linked model after incubation with P. chrysosporium in vivo and also after oxidation with P. chrysosporium lignin peroxidase in vitro. The brown rot fungus Gloeophyllum trabeum, which unlike P. chrysosporium lacks a vigorous extracellular ligninolytic system, was unable to degrade the PEG-linked model efficiently. These results show that PEG-linked lignin models are a marked improvement over the low-MW models that have been used in the past.

Mechanisms for polycyclic aromatic hydrocarbon degradation by ligninolytic fungi

Ligninolytic fungi accomplish the partial degradation of numerous aromatic organopollutants. Their ability to degrade polycyclic aromatic hydrocarbons (PAHs) is particularly interesting because eukaryotes were previously considered to be unable to cleave fused-ring aromatics. Recent results indicate that extracellular peroxidases of these fungi are responsible for the initial oxidation of PAHs. Fungal lignin peroxidases oxidize certain PAHs directly, whereas fungal manganese peroxidases co-oxidize them indirectly during enzyme-mediated lipid peroxidation.

Fungal degradation of recalcitrant nonphenolic lignin structures without lignin peroxidase

Lignin peroxidases (LiPs) are likely catalysts of ligninolysis in many white-rot fungi, because they have the unusual ability to depolymerize the major, recalcitrant, non-phenolic structures of lignin. Some white-rot fungi have been reported to lack LiP when grown on defined medium, but it is not clear whether they exhibit full ligninolytic competence under these conditions. To address this problem, we compared the abilities of a known LiP producer, Phanerochaete chrysosporium, with those of a reported nonproducer, Ceriporiopsis subvermispora, to degrade a synthetic lignin with normal phenolic content, a lignin with all phenolic units blocked, and a dimer, 1-(4-ethoxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol, that represents the major nonphenolic structure in lignin. P. chrysosporium mineralized all three models rapidly in defined medium, but C. subvermispora showed appreciable activity only toward the more labile phenolic compound under these conditions. However, in wood, its natural environment, C. subvermispora mineralized all of the models as rapidly as P. chrysosporium did. Defined media therefore fail to elicit a key component of the ligninolytic system in C. subvermispora. A double-labeling experiment with the dimeric model showed that a LiP-dependent pathway was responsible for at least half of dimer mineralization in wood by P. chrysosporium but was responsible for no more than 6-7% of mineralization by C. subvermispora in wood. Therefore, C. subvermispora has mechanisms for degradation of nonphenolic lignin that are as efficient as those in P. chrysosporium but that do not depend on LiP.

H2O2 recycling during oxidation of the arylglycerol beta-aryl ether lignin structure by lignin peroxidase and glyoxal oxidase

Oxidative C alpha-C beta cleavage of the arylglycerol beta-aryl ether lignin model 1-(3,4-dimethoxy-phenyl)-2-phenoxypropane-1,3-diol (I) by Phanerochaete chrysosporium lignin peroxidase in the presence of limiting H2O2 was enhanced 4-5-fold by glyoxal oxidase from the same fungus. Further investigation showed that each C alpha-C beta cleavage reaction released 0.8-0.9 equiv of glycolaldehyde, a glyoxal oxidase substrate. The identification of glycolaldehyde was based on 13C NMR spectrometry of reaction product obtained from beta-, gamma-, and beta,gamma-13C-substituted I, and quantitation was based on an enzymatic NADH-linked assay. The oxidation of glycolaldehyde by glyoxal oxidase yielded 0.9 oxalate and 2.8 H2O2 per reaction, as shown by quantitation of oxalate as 2,3-dihydroxyquinoxaline after derivatization with 1,2-diaminobenzene and by quantitation of H2O2 in coupled spectrophotometric assays with veratryl alcohol and lignin peroxidase. These results suggest that the C alpha-C beta cleavage of I by lignin peroxidase in the presence of glyoxal oxidase should regenerate as many as 3 H2O2. Calculations based on the observed enhancement of LiP-catalyzed C alpha-C beta cleavage by glyoxal oxidase showed that approximately 2 H2O2 were actually regenerated per cleavage of I when both enzymes were present. The cleavage of arylglycerol beta-aryl ether structures by ligninolytic enzymes thus recycles H2O2 to support subsequent cleavage reactions.

Oxidative degradation of non-phenolic lignin during lipid peroxidation by fungal manganese peroxidase

A non-phenolic lignin model dimer, 1-(4-ethoxy-3-methoxyphenyl)-2-phenoxypropane-1,3-diol, was oxidized by a lipid peroxidation system that consisted of a fungal manganese peroxidase, Mn(II), and unsaturated fatty acid esters. The reaction products included 1-(4-ethoxy-3-methoxyphenyl)-1-oxo-2-phenoxy-3-hydroxypropane and 1-(4-ethoxy-3-methoxyphenyl)-1-oxo-3-hydroxypropane, indicating that substrate oxidation occurred via benzylic hydrogen abstraction. The peroxidation system depolymerized both exhaustively methylated (non-phenolic) and unmethylated (phenolic) synthetic lignins efficiently. It may therefore enable white-rot fungi to accomplish the initial delignification of wood.

Ligninolysis by a purified lignin peroxidase

The lignin peroxidases (LiPs) of white-rot basidiomycetes are generally thought to catalyze the oxidative cleavage of polymeric lignin in vivo. However, direct evidence for such a role has been lacking. In this investigation, 14C- and 13C-labeled synthetic lignins were oxidized with a purified isozyme of Phanerochaete chrysosporium LiP. Gel permeation chromatography of the radiolabeled polymers showed that LiP catalyzed their cleavage to give soluble lower-M(r) products. To a lesser extent, the enzyme also polymerized the lignins to give soluble higher-M(r) products. This result is attributable to the fact that purified LiP, unlike the intact fungus, provides no mechanism for the removal of lignin fragments that are susceptible to repolymerization. LiP catalysis also gave small quantities of insoluble, perhaps polymerized, lignin, but in lower yield than intact P. chrysosporium does. 13C NMR experiments with 13C-labeled polymer showed that LiP cleaved it between C alpha and C beta of the propyl side chain to give benzylic aldehydes at C alpha, in agreement with the cleavage mechanism hypothesized earlier. The data show that LiP catalysis accounts adequately for the initial steps of ligninolysis by P. chrysosporium in vivo.

Oxidative degradation of phenanthrene by the ligninolytic fungus Phanerochaete chrysosporium

The ligninolytic fungus Phanerochaete chrysosporium oxidized phenanthrene and phenanthrene-9,10-quinone (PQ) at their C-9 and C-10 positions to give a ring-fission product, 2,2'-diphenic acid (DPA), which was identified in chromatographic and isotope dilution experiments. DPA formation from phenanthrene was somewhat greater in low-nitrogen (ligninolytic) cultures than in high-nitrogen (nonligninolytic) cultures and did not occur in uninoculated cultures. The oxidation of PQ to DPA involved both fungal and abiotic mechanisms, was unaffected by the level of nitrogen added, and was significantly faster than the cleavage of phenanthrene to DPA. Phenanthrene-trans-9,10-dihydrodiol, which was previously shown to be the principal phenanthrene metabolite in nonligninolytic P. chrysosporium cultures, was not formed in the ligninolytic cultures employed here. These results suggest that phenanthrene degradation by ligninolytic P. chrysosporium proceeds in order from phenanthrene----PQ----DPA, involves both ligninolytic and nonligninolytic enzymes, and is not initiated by a classical microsomal cytochrome P-450. The extracellular lignin peroxidases of P. chrysosporium were not able to oxidize phenanthrene in vitro and therefore are also unlikely to catalyze the first step of phenanthrene degradation in vivo. Both phenanthrene and PQ were mineralized to similar extents by the fungus, which supports the intermediacy of PQ in phenanthrene degradation, but both compounds were mineralized significantly less than the structurally related lignin peroxidase substrate pyrene was.

Ring fission of anthracene by a eukaryote

Ligninolytic fungi are unique among eukaryotes in their ability to degrade polycyclic aromatic hydrocarbons (PAHs), but the mechanism for this process is unknown. Although certain PAHs are oxidized in vitro by the fungal lignin peroxidases (LiPs) that catalyze ligninolysis, it has never been shown that LiPs initiate PAH degradation in vivo. To address these problems, the metabolism of anthracene (AC) and its in vitro oxidation product, 9,10-anthraquinone (AQ), was examined by chromatographic and isotope dilution techniques in Phanerochaete chrysosporium. The fungal oxidation of AC to AQ was rapid, and both AC and AQ were significantly mineralized. Both compounds were cleaved by the fungus to give the same ring-fission metabolite, phthalic acid, and phthalate production from AQ was shown to occur only under ligninolytic culture conditions. These results show that the major pathway for AC degradation in Phanerochaete proceeds AC----AQ----phthalate + CO2 and that it is probably mediated by LiPs and other enzymes of ligninolytic metabolism.

Comparison of lignin peroxidase, horseradish peroxidase and laccase in the oxidation of methoxybenzenes

Lignin peroxidase oxidizes non-phenolic substrates by one electron to give aryl-cation-radical intermediates, which react further to give a variety of products. The present study investigated the possibility that other peroxidative and oxidative enzymes known to catalyse one-electron oxidations may also oxidize non-phenolics to cation-radical intermediates and that this ability is related to the redox potential of the substrate. Lignin peroxidase from the fungus Phanerochaete chrysosporium, horseradish peroxidase (HRP) and laccase from the fungus Trametes versicolor were chosen for investigation with methoxybenzenes as a homologous series of substrates. The twelve methoxybenzene congeners have known half-wave potentials that differ by as much as approximately 1 V. Lignin peroxidase oxidized the ten with the lowest half-wave potentials, whereas HRP oxidized the four lowest and laccase oxidized only 1,2,4,5-tetramethoxybenzene, the lowest. E.s.r. spectroscopy showed that this congener is oxidized to its cation radical by all three enzymes. Oxidation in each case gave the same products: 2,5-dimethoxy-p-benzoquinone and 4,5-dimethoxy-o-benzoquinone, in a 4:1 ratio, plus 2 mol of methanol for each 1 mol of substrate. Using HRP-catalysed oxidation, we showed that the quinone oxygen atoms are derived from water. We conclude that the three enzymes affect their substrates similarly, and that whether an aromatic compound is a substrate depends in large part on its redox potential. Furthermore, oxidized lignin peroxidase is clearly a stronger oxidant than oxidized HRP or laccase. Determination of the enzyme kinetic parameters for the methoxybenzene oxidations demonstrated further differences among the enzymes.

Biomimetic oxidation of nonphenolic lignin models by Mn(III): new observations on the oxidizability of guaiacyl and syringyl substructures

Mn(III) is a one-electron oxidant, produced in vivo by the Mn peroxidases of white-rot fungi, and thought to be involved in lignin degradation by these organisms. However, Mn(III) has not been shown to oxidize the major nonphenolic substructures of lignin under mild conditions. We have used Mn(III) acetate as a biomimetic model for enzymatically generated Mn(III), and report that low concentrations of this oxidant suffice to oxidize nonphenolic lignin models at physiological temperatures and pH values. Under these conditions, the monomeric lignin model veratryl alcohol was oxidized to veratraldehyde, and the diarylpropane model 1-(3,4-dimethoxyphenyl)-2-phenylpropanol was oxidatively cleaved to veratraldehyde, 1-phenylethanol, and acetophenone. In an attempt to identify other lignin models that might be oxidized by Mn(III) more rapidly, we compared the rates at which Mn(III) was reduced by two guaiacyl models, veratryl alcohol and 1-(3-methoxy-4-isopropoxyphenyl)ethanol, vs two syringyl models, 3,4,5-trimethoxybenzyl alcohol and 1-(3,5-dimethoxy-4-isopropoxyphenyl)ethanol. The results were the opposite of those predicted: the syringyl models were oxidized slower than the guaiacyl models by Mn(III). To investigate the basis for this unexpected result, we recorded the visible absorption spectra of charge-transfer complexes prepared between each of the lignin models and an electron acceptor, tetracyanoethylene or p-chloranil. The results, in general agreement with the kinetic findings, showed that the nonphenolic syringyl lignin models had higher ionization potentials than the guaiacyl models.

Oxidation of polycyclic aromatic hydrocarbons and dibenzo[p]-dioxins by Phanerochaete chrysosporium ligninase

The lignin peroxidase (ligninase) of Phanerochaete chrysosporium catalyzes the oxidation of a variety of lignin-related compounds. Here we report that this enzyme also catalyzes the oxidation of certain aromatic pollutants and compounds related to them, including polycyclic aromatic hydrocarbons with ionization potentials less than or equal to approximately 7.55 eV. This result demonstrates that the H2O2-oxidized states of lignin peroxidase are more oxidizing than the analogous states of classical peroxidases. Experiments with pyrene as the substrate showed that pyrene-1,6-dione and pyrene-1,8-dione are the major oxidation products (84% of total as determined by high performance liquid chromatography), and gas chromatography/mass spectrometry analysis of ligninase-catalyzed pyrene oxidations done in the presence of H2(18)O showed that the quinone oxygens come from water. We found that whole cultures of P. chrysosporium also transiently oxidize pyrene to these quinones. Experiments with dibenzo[p]dioxin and 2-chlorodibenzo[p]dioxin showed that they are also substrates for ligninase. The immediate product of dibenzo[p]dioxin oxidation is the dibenzo[p]dioxin cation radical, which was observed in enzymatic reactions by its electron spin resonance and visible absorption spectra. The cation radical mechanism of ligninase thus applies not only to lignin, but also to other environmentally significant aromatics.

Substrate free radicals are intermediates in ligninase catalysis

The H2O2-requiring ligninase of the basidiomycete Phanerochaete chrysosporium oxidatively cleaves both lignin and lignin model compounds between C alpha and C beta (C-1 and C-2) of their aliphatic side chains. Previous work has demonstrated a reaction mechanism by which ligninase oxidizes aromatic substrates to their cation radicals, which then undergo side chain cleavage to yield carbon-centered free radicals. These carbon-centered radicals add O2 to give substrate peroxyl radicals that react further to yield the hydroxylated and carbonylated end products usually seen in experiments with ligninase. To investigate this radical mechanism, we have now designed three dimeric lignin models: 1-(3,4-dimethoxyphenyl)-2-phenylethanol (I), 1-(3,4-dimethoxyphenyl)-2-phenylpropanol (II), and 1-(3,4-dimethoxyphenyl)-2-methyl-2-phenylpropanol (III). The following results were obtained when these models were oxidized by ligninase: methyl groups at C beta of the substrate favored C alpha-C beta cleavage versus C alpha oxidation to the ketone. GC/MS and HPLC analysis showed that II gave a radical coupling dimer, 2,3-diphenylbutane, as a major (26% yield) reaction product under anaerobic conditions. The anaerobic oxidation of III yielded 2,3-dimethyl-2,3-diphenylbutane. Spin-trapping experiments with nitrosobenzene showed that model II oxidation produced alpha-methylbenzyl radicals, whereas model III oxidation gave alpha, alpha-dimethylbenzyl radicals. TLC and iodometric determinations showed that III gave cumene hydroperoxide as a major (21% yield) reaction product in air. These findings demonstrate that carbon-centered and peroxyl radicals at C beta are major products of C alpha-C beta cleavage by ligninase.

Mechanism of oxidative C alpha-C beta cleavage of a lignin model dimer by Phanerochaete chrysosporium ligninase. Stoichiometry and involvement of free radicals

The hemoprotein ligninase of Phanerochaete chrysosporium Burds. catalyzes the oxidative cleavage of lignin model dimers between C alpha and C beta of their propyl side chains. The model dimers hitherto used give multiple products and complex stoichiometries upon enzymatic oxidation. Here we present experiments with a new model dimer, 1-(3,4-dimethoxyphenyl)-2-phenylethanediol (dimethoxyhydrobenzoin, DMHB) which is quantitatively cleaved by ligninase in air to give benzaldehyde and veratraldehyde according to the stoichiometry: 2DMHB + O2----2PhCHO + 2Ph(OMe)2CHO. Catalytic amounts of H2O2 are required for this aerobic reaction. Under anaerobic conditions, ligninase uses H2O2 as the oxidant for cleavage: DMHB + H2O2----PhCHO + Ph(OMe)2CHO. Electron spin resonance experiments done in the presence of spin traps, 2-methyl-2-nitrosopropane or 5,5-dimethyl-1-pyrroline-N-oxide, show that C alpha-C beta cleavage yields alpha-hydroxybenzyl radicals as intermediate products. Under anaerobic conditions, these radicals react further to give the final aldehyde products. In air, O2 adds to the carbon-centered radicals, probably giving alpha-hydroxybenzylperoxyl radicals which fragment to yield superoxide, benzaldehyde, and veratraldehyde. These results lead us to propose a mechanism for C alpha-C beta cleavage in which attack by ligninase and H2O2 on the methoxylated ring of DMHB yields a cation radical, which then cleaves to give either benzaldehyde and an alpha-hydroxy(dimethoxybenzyl) radical or veratraldehyde and an alpha-hydroxybenzyl radical (cf. Kersten, P. J., Tien, M., Kalyanaraman, B., and Kirk, T.K. (1985) J. Biol. Chem. 260, 2609-2612; Snook, M. E., and Hamilton, G. A. (1974) J. Am. Chem. Soc. 96, 860-869). Similar mechanisms probably apply to the enzymatic C alpha-C beta cleavage of natural lignin.

Evidence for a nickel-containing carbon monoxide dehydrogenase in Methanobrevibacter arboriphilicus

In growing cultures of Methanobrevibacter arboriphilicus (Methanobrevibacter arboriphilus), the synthesis of active carbon monoxide dehydrogenase required nickel. The 21-fold-purified enzyme from 63Ni-labeled cells of M. arboriphilicus comigrated with 63Ni during gel filtration. These results provide evidence that the carbon monoxide dehydrogenase of methanogens is a nickel protein.

Ferredoxin/flavoprotein-linked pathway for the reduction of thioredoxin

Thioredoxins are small redox proteins, alternating between the S-S (oxidized) and SH (reduced) states, that function in a number of important biochemical processes, including DNA synthesis, DNA replication, and enzyme regulation. Reduced ferredoxin is known to serve as the source of reducing power for the reduction of thioredoxins only in photosynthetic cells that evolve oxygen. In all other organisms, the source of hydrogen (electrons) for thioredoxin reduction is considered to be NADPH. We now report evidence that Clostridium pasteurianum, an anaerobic bacterium normally living in the soil unexposed to light, resembles photosynthetic cells in that it uses reduced ferredoxin as the reductant for thioredoxin. Moreover, the transfer of electrons from reduced ferredoxin to thioredoxin is catalyzed by a flavoprotein enzyme that has not been detected in other organisms. Our results reveal the existence of a pathway for the reduction of thioredoxin in which ferredoxin, reduced fermentatively either by molecular hydrogen or by a carbon substrate, provides the reducing power for the flavoprotein enzyme ferredoxin-thioredoxin reductase, which in turn reduces thioredoxin.