Properties of ligninase from Phanerochaete chrysosporium and their possible applications

Expression of the lignin peroxidase H2 gene from Phanerochaete chrysosporium in Escherichia coli

The DNA sequence for the extracellular lignin peroxidase isozyme H2 from Phanerochaete chrysosporium, obtained from cDNA clone lambda ML-6, was synthesized by PCR and successfully expressed in Escherichia coli under control of the T7 promoter. The portion of the cDNA encoding the signal peptide, not found in the mature native enzyme, was not included. Recombinated lignin peroxidase H2 (rLiPH2) was produced in inclusion bodies in an inactive form. Active enzyme was obtained by refolding with glutathione-mediated oxidation in a medium containing urea, Ca2+, and hemin. The recombinant enzyme had spectral characteristics and kinetic properties identical to that of native enzyme isolated from P. chrysosporium. Surprisingly, rLiPH2, like the native enzyme, also exhibited some manganese peroxidase activity.

In vitro degradation of natural insoluble lignin in aqueous media by the extracellular peroxidases of Phanerochaete chrysosporium

The lignin peroxidases (LIP) and manganese peroxidases (MNP) of Phanerochaete chrysosporium catalyze a wide range of lignin depolymerization reactions with lignin models and synthetic lignins in solution. However, their ability to degrade insoluble natural lignin in aqueous media has not been demonstrated. Insoluble isolated poplar lignin similar to natural lignin was treated in vitro in aqueous media for 12 h with LIP, MNP, and both. Treatment with MNP alone slightly increased the solid mass and produced measurable amounts of lignin-derived 2,6-dimethoxyhydroquinone and 2-methoxyhydroquinone but did not appreciably decrease the total lignin content. Treatment with LIP alone did not decrease the mass but produced measurable amounts of lignin-derived p-hydroxybenzoic acid and slightly decreased the lignin content. Finally, treatment with LIP and MNP together decreased the solid mass by 11%, decreased the lignin content by 5%, and released low-concentration compounds with mass spectra containing the typical lignin-derived electron-impact fragments of mass 107, 137, 151, 167, and 181. These results suggest that MNP increases the effectiveness of LIP-mediated lignin degradation.

2-Chloro-1,4-Dimethoxybenzene as a Novel Catalytic Cofactor for Oxidation of Anisyl Alcohol by Lignin Peroxidase

2-Chloro-1,4-dimethoxybenzene (2Cl-14DMB) is a natural compound produced de novo by several white rot fungi. This chloroaromatic metabolite was identified as a cofactor superior to veratryl alcohol (VA) in the oxidation of anisyl alcohol (AA) by lignin peroxidase (LiP). Our results reveal that good LiP substrates, such as VA and tryptophan, are comparatively poor cofactors in the oxidation of AA. Furthermore, we show that a good cofactor does not necessarily serve a role in protecting LiP against H 2 O 2 inactivation. 2Cl-14DMB was not a direct mediator of AA oxidation, since increasing AA concentrations did not inhibit the oxidation of 2Cl-14DMB at all. However, the high molar ratio of anisaldehyde formed to 2Cl-14DMB consumed, up to 13:1, indicates that a mechanism which recycles the cofactor is present.

Effects of Mn2+ and oxalate on the catalatic activity of manganese peroxidase

Manganese peroxidase from Phanerochaete chrysosporium is an extracellular heme-containing enzyme known to catalyze the oxidation of Mn2+ to Mn3+ in a reaction requiring oxalate or another appropriate manganese chelator. We have found that the enzyme can also catalyze a manganese-dependent disproportionation of hydrogen peroxide when a manganese chelator is not included. The catalatic activity was observed in the pH range from 3.0 to 8.5, and the apparent second-order rate constant for catalatic reaction was about 2 x 10(5) M-1 s-1 at pH 4.5 to 7.0 at 25 degrees C. Oxalate inhibited oxygen production by increasing the apparent K(m) for Mn2+ for catalatic activity from micromolar to millimolar levels and facilitating peroxidase activity. Catalase-type function was recovered by excess of Mn2+ in the presence of oxalate. We propose that catalatic activity may protect the enzyme from inactivation by hydrogen peroxide in an environment where free oxalate may be limited.

Stabilization of lignin peroxidases in white rot fungi by tryptophan

Supplementation of various cultures of white rot fungi with tryptophan was found to have a large stimulatory effect on lignin peroxidase activity levels. This enhancement was greater than that observed in the presence of the lignin peroxidase recycling agent veratryl alcohol. Using reverse transcription-PCR, we found that tryptophan does not act to induce lignin peroxidase expression at the level of gene transcription. Instead, the activity enhancement observed is likely to result from the protective effect of tryptophan against H2O2 inactivation. In experiments using a partially purified lignin peroxidase preparation, tryptophan and its derivative indole were determined to function in the same way as veratryl alcohol in converting compound II, an oxidized form of lignin peroxidase, to ferric enzyme, thereby completing the catalytic cycle. Furthermore, tryptophan was found to be a better substrate for lignin peroxidase than veratryl alcohol. Inclusion of either tryptophan or indole enhanced the oxidation of the azo dyes methyl orange and Eriochrome blue black. Stimulation of azo dye oxidations by veratryl alcohol has previously been shown to be due to its enzyme recycling function. Our data allow us to propose that tryptophan stabilizes lignin peroxidase by acting as a reductant for the enzyme.

Kinetics of calcium release from manganese peroxidase during thermal inactivation

It was previously reported that manganese peroxidase from the white-rot fungus Phanerochaete chrysosporium was susceptible to thermal inactivation because it contains relatively labile Ca2+ ions required for stability and activity [Sutherland and Aust (1996) Arch. Biochem. Biophys. 332, 128-134]. In this work we determined that four Ca2+ ions are present in the enzyme as isolated but this was reduced to 2 mol/mol upon treatment with Ca2+-chelating agents or extensive dialysis of dilute enzyme. One of two relatively tightly bound Ca2+ remaining in the enzyme was released during thermal inactivation at pH 7.2. Inactive enzyme contained one Ca2+ which could be removed in acidic conditions. Inactivation kinetics were biphasic and the rates for the two inactivation steps and the release of Ca2+ during inactivation suggested that the first, faster phase of inactivation was coupled to the removal of Ca2+. The weakly associated Ca2+ normally present in the enzyme did not affect enzyme activity and did not seem to protect the enzyme from thermal inactivation at submicromolar enzyme concentrations. Excess Ca2+ or Mn2+ decreased the rate of the thermal inactivation and Mn2+ stabilized the enzyme more efficiently than Ca2+ at higher temperature. Enzyme stabilization by Mn2+ was proposed to be due to binding of Mn2+ to the Mn2+ substrate binding site. In competition studies, Ca2+ was shown to bind to this site with apparent dissociation constants of 10(-2) and 10(-4) M at pH 4.5 and 7.2, respectively. Moreover, Ca2+ was a poor inhibitor of manganese peroxidase activity at pH 4.5. It is therefore suggested that Ca2+ is absent from the substrate site in physiological conditions but can bind to this site at higher pH and therefore may stabilize the enzyme by binding to both the Mn2+ site and, as previously proposed, to the distal Ca2+ site.

Spectral changes of lignin peroxidase during reversible inactivation

The heme environment of lignin peroxidase (LiP) has been investigated by electronic absorption and electron paramagnetic resonance (EPR) spectroscopy. Native LiP was a pentacoordinate, high-spin ferric iron with a high-spin absorption band at 634 nm and g values at 5.86 and 2.07 in the EPR spectrum. Upon thermal inactivation, calcium ions were released from the enzyme and the Soret absorption decreased and red-shifted about 2 nm, the high-spin absorption band at 634 nm disappeared, and a low-spin absorption band appeared at 532 nm. The EPR spectrum and the temperature dependence of electronic absorption spectra revealed that the heme iron of the thermally inactivated enzyme was a mixture of high- and low-spin states, which was further supported by the changes in the electronic absorption and EPR spectra when cyanide was added to the thermally inactivated enzyme. Addition of various imidazoles or CN- to thermally inactivated enzyme demonstrated that the low-spin heme iron of inactivated enzyme was hexacoordinate with a distal histidine as its sixth ligand, in contrast to the active enzyme, which was pentacoordinate and high-spin. Upon addition of calcium to recover the thermally inactivated LiP, the reactivated enzyme had absorptions at 408, 502, and 634 nm and g values at 5.86 and 2.07 in the EPR spectrum, which demonstrated that the heme iron of the reactivated enzyme was again high-spin and pentacoordinated.

Oxidation of 4-methoxymandelic acid by lignin peroxidase. Mediation by veratryl alcohol

The mechanism of veratryl alcohol-mediated oxidation of 4-methoxymandelic acid by lignin peroxidase was studied by kinetic methods. For monomethoxylated substrates not directly oxidized by lignin peroxidase, veratryl alcohol has been proposed to act as a redox mediator. Our previous study showed that stimulation of anisyl alcohol oxidation by veratryl alcohol was not due to mediation but rather due to the requirement of veratryl alcohol to complete the catalytic cycle. Anisyl alcohol can react with compound I but not with compound II. In contrast, veratryl alcohol readily reduces compound II. We demonstrate in the present report that the oxidation of 4-methoxy mandelic acid is mediated by veratryl alcohol. Increasing veratryl alcohol concentration in the presence of 2 mM 4-methoxymandelic acid resulted in increased oxidation of 4-methoxymandelic acid yielding anisaldehyde. This is in contrast to results obtained with anisyl alcohol where increased concentrations of veratryl alcohol caused a decrease in product formation. ESR spectroscopy demonstrated that 4-methoxymandelic acid caused a decrease in the enzyme-bound veratryl alcohol cation radical signal, which is consistent with its reaction at the active site of the enzyme.

Role of calcium in maintaining the heme environment of manganese peroxidase

We previously demonstrated that manganese peroxidase from the white-rot fungus Phanerochaete chrysosporium was very susceptible to thermal inactivation due to the loss of calcium from the enzyme [Sutherland & Aust (1996) Arch. Biochem. Biophys. 332, 128-134]. The structural changes that occur during thermal inactivation and the release of calcium from manganese peroxidase have now been characterized. Thermal inactivation caused distinct alterations in the heme environment and slight changes in the overall protein structure, both of which were reversed upon reactivation of the enzyme with calcium. The absorption spectrum of inactivated manganese peroxidase was similar to that of low-spin ferric heme proteins, indicating that a sixth ligand had bound to the distal side of the heme iron. Consistent with disruption of the distal heme environment, thermally inactivated manganese peroxidase did not react with hydrogen peroxide to form compound I. The inactive enzyme exhibited a pH-dependent absorption transition with a pKa of 5.7. Studies involving imidazole indicated that the sixth ligand may be a distal histidine. Low-temperature electron paramagnetic resonance spectroscopy confirmed that the heme iron of the inactivated form of manganese peroxidase was predominantly in a low-spin state. The near-ultraviolet/visible circular dichroism spectrum also supported the proposed formation of a highly symmetric bis(imidazole) heme complex upon thermal inactivation of the enzyme. A recombinant manganese peroxidase, in which the distal calcium binding site was altered such that calcium binding would be minimized, was also characterized. This enzyme, D47A, had the same catalytic and spectroscopic properties and calcium content as thermally inactivated manganese peroxidase. Therefore, the inactivation and structural changes that occurred during thermal incubation of manganese peroxidase could be explained by the loss of the distal calcium.

Further studies on the inactivation by sodium azide of lignin peroxidase from Phanerochaete chrysosporium

Azide ion is a mechanism-based inactivator of horseradish peroxidase [Ortiz de Montellano et al. (1988) Biochemistry 27, 5470-5476] and the peroxidase from the coprophilic fungus Coprinus macrorhizus [DePillis and Ortiz de Montellano (1989) Biochemistry 28, 7947-7952]. These peroxidases mediate the one-electron oxidation of azide ion-forming azidyl radical. Inactivation of these enzymes is caused by covalent modification of the heme prosthetic groups by azidyl radical. Lignin peroxidases from the wood-rotting fungus Phanerochaete chrysosporium are also inactivated when they catalyze oxidation of azide ion [Tuisel et al. (1991) Arch. Biochem. Biophys. 288, 456-462; DePillis et al. (1990) Arch. Biochem. Biophys. 280, 217-223]. Following inactivation of horseradish peroxidase and the peroxidase from C. macrorhizus substantial amounts of azidyl-heme adducts have been found. Only trace amounts of such adducts have been found following azide-mediated inactivation of lignin peroxidase. Nevertheless, we have shown that during oxidation of azide by lignin peroxidase H8 destruction of heme occurred and a substantial fraction of the enzyme is irreversibly inactivated. However, the rest of the enzyme forms a relatively stable ferrous-nitric oxide (NO) complex. Although this complex appears to be an inactivated form of the enzyme, we have shown that, when present as the ferrous-NO complex, the enzyme is actually protected from inactivation. The lignin peroxidase ferrous-NO complex reverts slowly (t1/2 = 6.3 x 10(3) s) to the ferric form. Reversion is accelerated if the complex is chromatographed on a PD-10 (Sephadex G-25) column or if veratryl alcohol is added. If azide and hydrogen peroxide (a required cosubstrate) are present (or added), the enzyme undergoes another cycle of catalysis and further inactivation. A detailed reaction mechanism is proposed that is consistent with our experimental observations, the chemistry of azide, and our current understanding of peroxidases.

Metabolic engineering: prospects for crop improvement through the genetic manipulation of phenylpropanoid biosynthesis and defense responses--a review

In leguminous plants such as the forage legume alfalfa, products of the phenylpropanoid pathway of secondary metabolism are involved in interactions with beneficial microorganisms (flavonoid inducers of the Rhizobium symbiosis), and in defense against pathogens (isoflavonoid phytoalexins). In addition, the phenylpropane polymer lignin is a major structural component of secondary vascular tissue and fibers in higher plants. the recent isolation of genes encoding key enzymes of the various phenylpropanoid branch pathways opens up the possibility of engineering important crop plants such as alfalfa for: (a) improved forage digestibility, by modification of lignin composition and/or content; (b) increased or broader-spectrum disease resistance, by introducing novel phytoalexins or structural variants of the naturally occurring phytoalexins, or by modifying expression of transcriptional regulators of phytoalexin pathways; and (c) enhanced nodulation efficiency, by engineering over-production of flavonoid nod gene inducers. The basic biochemistry and molecular biology underlying these strategies is briefly reviewed, and recent progress with transgenic plants summarized. The potential importance of metabolic compartmentation for attempts to engineer phenylpropanoid biosynthetic pathways is also discussed. Over-expression of an alfalfa glucanase-encoding gene confers significant protection against Phytophthora in alfalfa, possibly via indirect effects on phenylpropanoid metabolism.

Biodegradation of lignin

Lignin is an aromatic polymer forming up to 30% of woody plant tissues, providing rigidity and resistance to biological attack. Because it is insoluble, chemically complex, and lacking in hydrolysable linkages, lignin is a difficult substrate for enzymatic depolymerization. Certain fungi, mostly basidiomycetes, are the only organisms able to extensively biodegrade it; white-rot fungi can completely mineralize lignin, whereas brown-rot fungi merely modify lignin while removing the carbohydrates in wood. Several oxidative and reductive extracellular enzymes (lignin peroxidase, manganese peroxidase, laccase, and cellobiose:quinone oxidoreductase) have been isolated from ligninolytic fungi; the role of these enzymes in lignin biodegradation is being intensively studied. Enzymatic combustion, a process wherein enzymes generate reactive intermediates, but do not directly control the reactions leading to lignin breakdown, has been proposed as the mechanism of lignin biodegradation. The economic consequences of lignin biodegradation include wood decay and the biogeochemical cycling of woody biomass. Efforts are being made to harness the delignifying abilities of white-rot fungi to aid wood and straw pulping and pulp bleaching. These fungi can also be used to degrade a variety of pollutants in wastewaters and soils, to increase the digestibility of lignocellulosics, and possibly to bioconvert lignins to higher value products. Key words: delignification, white-rot fungi, biobleaching, lignin peroxidase, manganese peroxidase, laccase.

Evidence for veratryl alcohol as a redox mediator in lignin peroxidase-catalyzed oxidation

We have examined the hypothesis that veratryl alcohol (VA) may act as a redox mediator in lignin peroxidase (LiP)-catalyzed oxidations. The oxidation of chlorpromazine (CPZ) by this system was used to evaluate this hypothesis. Chlorpromazine can be oxidized by one electron to form a stable cation radical (CPZ.+). This cation radical can be oxidized by another electron to the sulfoxide (CPZSO). These oxidation steps are easily monitored, making CPZ a useful chemical to investigate redox mediation by VA. Lignin peroxidase oxidized CPZ to CPZ.+ whether or not VA was present. The inclusion of VA, however, stimulated CPZ oxidation to CPZ.+ and subsequent oxidation of CPZ.+ to CPZSO. In the absence of VA, the initial rates of CPZ oxidation by LiP were CPZ concentration-dependent. However, when saturating concentrations of VA were added, the oxidation of CPZ and CPZ.+ became independent of CPZ concentration. When the oxidation of VA to veratryl aldehyde was examined, increasing concentrations of CPZ produced a lag in veratryl aldehyde appearance proportional to the concentration of CPZ. Conversely, increasing concentrations of VA never inhibited CPZ oxidation. Transient-state kinetic studies indicated that both VA and CPZ reduced the compound I and compound II forms of LiP. However, when saturating concentrations of VA were utilized, LiP turnover was independent of CPZ concentration. We suggest these data demonstrate that VA may act as a redox mediator for the indirect oxidation of compounds by LiP.

Homology models of two isozymes of manganese peroxidase: prediction of a Mn(II) binding site

The three-dimensional structures of two isozymes of manganese peroxidase (MnP) have been predicted from homology modeling using lignin peroxidase as a template. Although highly homologous, MnP differs from LiP by the requirement of Mn(II) as an intermediate in its oxidation of substrates. The Mn(II) site is absent in LiP and unique to the MnP family of peroxidases. The model structures were used to identify the unique Mn(II) binding sites, to determine to what extent they were conserved in the two isozymes, and to provide insight into why this site is absent in LiP. For each isozyme of MnP, three candidate Mn(II) binding sites were identified. Energy optimizations of the three possible Mn(II) enzyme complexes allowed the selection of the most favorable Mn(II) binding site as one with the most anionic oxygen moieties best configured to act as ligands for the Mn(II). At the preferred site, the Mn(II) is coordinated to the carboxyl oxygens of Glu-35, Glu-39, and Asp-179, and a propionate group of the heme. The predicted Mn(II) binding site is conserved in both isozymes. Comparison between the residues at this site in MnP and the corresponding residues in LiP shows that two of the three anionic residues in MnP are replaced by neutral residues in LiP, explaining why LiP does not bind Mn(II).

Quantitative relationship between phenylalanine ammonia-lyase levels and phenylpropanoid accumulation in transgenic tobacco identifies a rate-determining step in natural product synthesis

Phenylalanine ammonia-lyase (PAL) catalyzes the first step in phenylpropanoid synthesis. The role of PAL in pathway regulation was investigated by measurement of product accumulation as a function of enzyme activity in a collection of near-isogenic transgenic tobacco plants exhibiting a range of PAL levels from wild type to 0.2% of wild type. In leaf tissue, PAL level is the dominant factor regulating accumulation of the major product chlorogenic acid and overall flux into the pathway. In stems, PAL at wild-type levels contributes, together with downstream steps, in the regulation of lignin deposition and becomes the dominant, rate-determining step at levels 3- to 4-fold below wild type. The metabolic impact of elevated PAL levels was investigated in transgenic leaf callus that overexpressed PAL. Accumulation of the flavonoid rutin, the major product in wild-type callus, was not increased, but several other products accumulated to similarly high levels. These data indicate that PAL is a key step in the regulation of overall flux into the pathway and, hence, accumulation of major phenylpropanoid products, with the regulatory architecture of the pathway poised so that downstream steps control partitioning into different branch pathways.

The biomimetic oxidation of beta-1, beta-0-4, beta-5, and biphenyl lignin model compounds by synthetic iron porphyrins

The degradation of four dimeric lignin model compounds by meso-tetra(2,6-dichloro-3-sulfonatophenyl)porphyrin iron chloride (TDCSPPFeCl) (2) are reported. 4-Ethoxy-3-methoxyphenylglycerol-beta-guaiacyl ether (3) (a beta-0-4 dimer) was cleaved to give 4-ethoxy-3-methoxybenzaldehyde (4) and guaiacol (5) as major products. The oxidation of 1-(4-ethoxy-3-methoxyphenyl)-2-(4-methoxyphenyl)-1,3-propandiol (6, a beta-1 dimer) gave 4, 4-methoxybenzaldehyde (7), and 4-methoxy-alpha-hydroxyacetophenone (8) as major products. Side chain oxidation and aromatic ring cleavage reactions were found to occur for the phenylcoumaran (alpha-5) model compound, ethyl dehydrodiisoeugenol (12). A biphenyl model compound, 4,4'-diethyldehydrodivanillin (20), was oxidized to give mono- and dicarboxy derivatives, as well as ring-cleaved products of the acid derivatives.

A histone H4 promoter for expression of a phleomycin-resistance gene in Phanerochaete chrysosporium

In this study, two transformation vectors (pMG101 and pMG103) for Phanerochaete chrysosporium were constructed, based on the ble phleomycin-resistance-encoding gene and a homologous histone H4 promoter. Transformation frequencies were 6-10 per micrograms of DNA. Transformed vector DNA could either exist as an unstable replicating plasmid or could be stably integrated. Integrated vector DNA from pMG101, which also contains a histone-encoding H3 gene in the promoter fragment, becomes methylated, resulting in inactivation of ble-dependent resistance. Plasmid pMG103, which lacks the H3, does not show methylation.

Physiology and molecular biology of the lignin peroxidases of Phanerochaete chrysosporium

The white-rot basidiomycete Phanerochaete chrysosporium produces lignin peroxidases (LiPs), a family of extracellular glycosylated heme proteins, as major components of its lignin-degrading system. Up to 15 LiP isozymes, ranging in M(r) values from 38,000 to 43,000, are produced depending on culture conditions and strains employed. Manganese-dependent peroxidases (MnPs) are a second family of extracellular heme proteins produced by P. chrysosporium that are also believed to be important in lignin degradation by this organism. LiP and MnP production is seen during secondary metabolism and is completely suppressed under conditions of excess nitrogen and carbon. Excess Mn(II) in the medium, on the other hand, suppresses LiP production but enhances MnP production. Nitrogen regulation of LiP and MnP production is independent of carbon and Mn(II) regulation. LiP activity is also affected by idiophasic extracellular proteases. Intracellular cAMP levels appear to be important in regulating the production of LiPs and MnPs, although LiP production is affected more than MnP production. Studies on the sequencing and characterization of lip cDNAs and genes of P. chrysosporium have shown that the major LiP isozymes are each encoded by a separate gene. Each lip gene encodes a mature protein that is 343-344 amino acids long, contains 1 putative N-glycosylation site, a number of putative O-glycosylation sites, and is preceded by a 27-28-amino acid leader peptide ending in a Lys-Arg cleavage site. The coding region of each lip gene is interrupted by 8-9 introns (50-63 bp), and the positions of the last two introns appear to be highly conserved. There are substantial differences in the temporal transcription patterns of the major lip genes. The sequence data suggest the presence of three lip gene subfamilies. The genomic DNA of P. chrysosporium strain BKMF-1767 was resolved into 10 chromosomes (genome size of 29 Mb), and that of strain ME-446 into 11 chromosomes (genome size of 32 Mb). The lip genes have been localized to five chromosomes in BKMF-1767 and to four chromosomes in ME-446. DNA transformation studies have reported both integrative and non-integrative transformation in P. chrysosporium.

Pollutant degradation by white rot fungi

The white rot fungi technology is very different from other more well-established methods of bioremediation (e.g., bacterial systems). The differences are primarily due to the mechanisms discussed previously. The unusual mechanisms used by the fungi provide them with several advantages for pollutant degradation, but the complexity of these mechanisms has also made the technology slow to emerge as a viable method of bioremediation. One distinct advantage that white rot fungi have over bacterial systems is that they do not require preconditioning to a particular pollutant. Bacteria must be preexposed to a pollutant to allow the enzymes that degrade the pollutant to be induced. The pollutant must also be present in a significant concentration, otherwise induction of enzyme synthesis will not occur. Therefore, there is a finite level to which pollutants can be degraded by bacteria. In contrast, the degradative enzymes of white rot fungi are induced by nutrient limitation. Thus, cultivate the fungus on a nutrient that is limited in something, and the degradative process will be initiated. Also, because the induction of the lignin-degrading system is not dependent on the chemical, pollutants are degraded to near-nondetectable levels by white rot fungi. Another unique feature of pollutant degradation by white rot fungi involves kinetics. The process of chemical conversion by these fungi occurs via a free-radical process, and thus the degradation of chemicals often follows pseudo-first-order kinetics. In fact, in several studies, it has been found that the rate of mineralization or disappearance of a pollutant is proportional to the concentration of the pollutant. This makes the time required to achieve decontamination more important than the rate of degradation. Because the metabolism of chemicals by bacteria involves mostly enzymatic conversions, pollutant degradation often follows Michaelis-Menton-type kinetics. Therefore, Km values of various degradative enzymes with respect to the pollutant must be considered when using bacteria for bioremediation. Considering this, the solubility of a pollutant or a mixture of pollutants might also present a problem for bacterial degradation. In contrast, using a nonspecific free-radical-based mechanism, the fungi are able to degrade insoluble complex mixtures of pollutants, such as creosote (Aust and Bumpus 1989) and Arochlor (Bumpus and Aust 1987b). Inexpensive nutrient sources, such as sawdust, wood chips, surplus grains, and agricultural wastes, can be used to effectively cultivate white rot fungi.(ABSTRACT TRUNCATED AT 400 WORDS)

Lignin-degrading peroxidases of Phanerochaete chrysosporium

Lignin and manganese peroxidases are secreted by the basidiomycete Phanerochaete chrysosporium during secondary metabolism. These enzymes play major roles in lignin degradation. The active site amino acid sequence of these lignin-degrading peroxidases is similar to that of horseradish peroxidase (HRP) and cytochrome c peroxidase (CcP). The mechanism by which they oxidize substrates also appears to be the similar. pH has a similar effect on lignin peroxidase compound I formation as on HRP or CcP; however, the pKa controlling compound I formation for lignin peroxidase appears to be much lower. Lignin-degrading peroxidases are able to catalyze the oxidation of substrates with high redox potential. This unique ability is consistent with a heme active site of low electron density, which is indicated by high redox potential.

Overproduction of lignin peroxidase by Phanerochaete chrysosporium (BKM-F-1767) under nonlimiting nutrient conditions

The ligninolytic enzymes synthesized by Phanerochaete chrysosporium BKM-F-1767 immobilized on polyurethane foam were characterized under limiting, sufficient, and excess nutrient conditions. The fungus was grown in a nonimmersed liquid culture system under conditions close to those occurring in nature, with nitrogen concentrations ranging from 2.4 to 60 mM. This nonimmersed liquid culture system consisted of fungal mycelium immobilized on porous pieces of polyurethane foam saturated with liquid medium and highly exposed to gaseous oxygen. Lignin peroxidase (LIP) activity decreased to almost undetectable levels as the initial NH4+ levels were increased over the range from 2.4 to 14 mM and then increased with additional increases in initial NH4+ concentration. At 45 mM NH4+, LIP was overproduced, reaching levels of 800 U/liter. In addition, almost simultaneous secretion of LIP and secretion of manganese-dependent lignin peroxidase were observed on the third day of incubation. Manganese-dependent lignin peroxidase activity was maximal under nitrogen limitation conditions (2.4 mM NH4+) and then decreased to 40 to 50% of the maximal level in the presence of sufficient or excess initial NH4+ concentrations. Overproduction of LIP in the presence of a sufficient nitrogen level (24 mM NH4+) and excess nitrogen levels (45 to 60 mM NH4+) seemed to occur as a response to carbon starvation after rapid glucose depletion. The NH4+ in the extracellular fluid reappeared as soon as glucose was depleted, and an almost complete loss of CO2 was observed, suggesting that an alternative energy source was generated by self-proteolysis of cell proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Oligomers of 4-chloroaniline are intermediates formed during its biodegradation by Phanerochaete chrysosporium

Lignin peroxidase H2 (LP-H2) from Phanerochaete chrysosporium oxidized 4-chloroaniline to form several oligomers. Included among the compounds identified were: 4,4'-dichloroazobenzene, 2-(4-chloroanilino)-5-hydroxybenzoquinone-di-4-chloroanil and 2-amino-5-(4-chloroanilino) benzoquinone-di-4-chloroanil. In contrast to results by others, we showed that oligomers of 4-chloroaniline were also formed by the fungus in vivo. It was also demonstrated that, although these potentially toxic intermediates are made, they are also degraded.

Low pH crystal structure of glycosylated lignin peroxidase from Phanerochaete chrysosporium at 2.5 A resolution

The heme-containing glycoprotein lignin peroxidase (pI 4.15) has been crystallized at pH 4.0. The structure of the peroxidase from the orthorhombic crystals has been determined by multiple isomorphous replacement. The model comprises all 343 amino acids, one heme molecule, and three sugar residues. It has been refined to an R-factor of 20.3%. The chain fold of residues 15 to 275 is in general similar to those of cytochrome c peroxidase. Despite binding of the heme to the same region and a similar arrangement of the proximal and distal histidine as in cytochrome c peroxidase a significantly larger distance of the iron ion to the proximal histidine is observed. Distinct electron density extending from Asn-257 and at the distal side of the heme indicates ordered sugar residues in the crystal.

Use of white rot fungi in the degradation of environmental chemicals

White rot fungi have been shown to mineralize a wide variety of environmental pollutants. These fungi secrete a number of enzymes that are involved in its unique ability to degrade lignin, the structural component of woody plants. Lignin is a very complex heteropolymer that can only be degraded by white rot fungi. Degradation is complete without energy value to the fungus. The evolution of this ability has apparently given the organism the ability to degrade structurally diverse and normally very recalcitrant environmental pollutants such as DDT, PCB, benzo(a)pyrene, TNT, etc. Some of the major enzymes that are secreted by the fungi are peroxidases with unique properties. In addition to their ability to catalyze a wide variety of oxidations, they can also catalyze indirect oxidations and reductions. The fungi synthesize and secrete hydrogen peroxide to activate the peroxidases, veratryl alcohol to serve as a free radical intermediate for indirect oxidations, and electron donors, such as oxalate, which with veratryl alcohol catalyze reductions. Reductions are often required for subsequent oxidation of chemicals by the peroxidases. The enzymes can also reduce molecular oxygen.

Oxidation of phenolic arylglycerol beta-aryl ether lignin model compounds by manganese peroxidase from Phanerochaete chrysosporium: oxidative cleavage of an alpha-carbonyl model compound

Manganese peroxidase (MnP) oxidized 1-(3,5-dimethoxy-4-hydroxyphenyl)-2-(4-(hydroxymethyl)-2-methoxyphenoxy) -1,3-dihydroxypropane (I) in the presence of MnII and H2O2 to yield 1-(3,5-dimethoxy-4-hydroxyphenyl)- 2-(4-(hydroxymethyl)-2-methoxyphenoxy)-1-oxo-3-hydroxypropane (II), 2,6-dimethoxy-1,4-benzoquinone (III), 2,6-dimethoxy-1,4-dihydroxybenzene (IV), 2-(4-(hydroxymethyl)-2-methoxyphenoxy)-3-hydroxypropanal (V), syringaldehyde (VI), vanillyl alcohol (VII), and vanillin (VIII). MnP oxidized II to yield 2,6-dimethoxy-1,4-benzoquinone (III), 2,6-dimethoxy-1,4-dihydroxybenzene (IV), vanillyl alcohol (VII), vanillin (VIII), syringic acid (IX), and 2-(4-(hydroxymethyl)-2-methoxyphenoxy)-3-hydroxypropanoic acid (X). A chemically prepared MnIII-malonate complex catalyzed the same reactions. Oxidation of I and II in H2(18)O under argon resulted in incorporation of one atom of 18O into the quinone III and into the hydroquinone IV. Incorporation of one atom of oxygen from H2(18)O into syringic acid (IX) and the phenoxypropanoic acid X was also observed in the oxidation of II. These results are explained by mechanisms involving the initial one-electron oxidation of I or II by enzyme-generated MnIII to produce a phenoxy radical. This intermediate is further oxidized by MnIII to a cyclohexadienyl cation. Loss of a proton, followed by rearrangement of the quinone methide intermediate, yields the C alpha-oxo dimer II as the major product from substrate I. Alternatively, cyclohexadienyl cations are attacked by water. Subsequent alkyl-phenyl cleavage yields the hydroquinone IV and the phenoxypropanal V from I, and IV and the phenoxypropanoic acid X from II, respectively. The initial phenoxy radical also can undergo C alpha-C beta bond cleavage, yielding syringaldehyde (VI) and a C6-C2-ether radical from I and syringic acid (IX) and the same C6-C2-ether radical from II. The C6-C2-ether radical is scavenged by O2 or further oxidized by MnIII, subsequently leading to release of vanillyl alcohol (VII). VII and IV are oxidized to vanillin (VIII) and the quinone III, respectively.

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.

Structure of the active site of lignin peroxidase isozyme H2: native enzyme, compound III, and reduced form

The wood-degrading fungus Phanerochaete chrysosporium secretes a number of extracellular enzymes called lignin peroxidases which are involved in the degradation of both lignin and a number of persistent environmental pollutants. Lignin peroxidase isozyme H2, a glycosylated protein of approximately 40 kDa, contains a single heme. X-ray absorption spectroscopy (XAS) has been used to probe the local environment of the iron in the active site of resting enzyme, reduced enzyme, and compound III. For the native and reduced forms, respectively, the average Fe-pyrrole nitrogen distances are 2.055 and 2.02 A (+/- 0.015 A); the Fe-proximal nitrogen distance is 1.93 and 1.91 A (+/- 0.02 A) while the Fe-distal ligand distance is 2.17 and 2.10 A (+/- 0.03 A). Although the results are not as well-defined, the active-site structure of compound III is largely 2.02 +/- 0.015 A for the average Fe-pyrrole nitrogen distance, 1.90 +/- 0.02 for the Fe-proximal nitrogen, and 1.74 +/- 0.03 A for the Fe-distal ligand distance. The heme iron-pyrrole nitrogen distance is more expanded in ligninase H2 than in other peroxidases. The possible significance of this is discussed in relation to other heme proteins.