Katrin Scheibner

Novel Haloperoxidase from the Agaric Basidiomycete Agrocybe aegerita Oxidizes Aryl Alcohols and Aldehydes

ABSTRACT Agrocybe aegerita, a bark mulch- and wood-colonizing basidiomycete, was found to produce a peroxidase (AaP) that oxidizes aryl alcohols, such as veratryl and benzyl alcohols, into the corresponding aldehydes and then into benzoic acids. The enzyme also catalyzed the oxidation of typical peroxidase substrates, such as 2,6-dimethoxyphenol (DMP) or 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonate) (ABTS). A. aegerita peroxidase production depended on the concentration of organic nitrogen in the medium, and highest enzyme levels were detected in the presence of soybean meal. Two fractions of the enzyme, AaP I and AaP II, which had identical molecular masses (46 kDa) and isoelectric points of 4.6 to 5.4 and 4.9 to 5.6, respectively (corresponding to six different isoforms), were identified after several steps of purification, including anion- and cation-exchange chromatography. The optimum pH for the oxidation of aryl alcohols was found to be around 7, and the enzyme required relatively high concentrations of H2O2 (2 mM) for optimum activity. The apparent Km values for ABTS, DMP, benzyl alcohol, veratryl alcohol, and H2O2 were 37, 298, 1,001, 2,367 and 1,313 μM, respectively. The N-terminal amino acid sequences of the main AaP II spots blotted after two-dimensional gel electrophoresis were almost identical and exhibited almost no homology to the sequences of other peroxidases from basidiomycetes, but they shared the first three amino acids, as well as two additional amino acids, with the heme chloroperoxidase (CPO) from the ascomycete Caldariomyces fumago. This finding is consistent with the fact that AaP halogenates monochlorodimedone, the specific substrate of CPO. The existence of haloperoxidases in basidiomycetous fungi may be of general significance for the natural formation of chlorinated organic compounds in forest soils.

Molecular characterization of aromatic peroxygenase from Agrocybe aegerita

Recently, a novel group of fungal peroxidases, known as the aromatic peroxygenases (APO), has been discovered. Members of these extracellular biocatalysts produced by agaric basidiomycetes such as Agrocybe aegerita or Coprinellus radians catalyze reactions—for example, the peroxygenation of naphthalene, toluene, dibenzothiophene, or pyridine—which are actually attributed to cytochrome P450 monooxygenases. Here, for the first time, genetic information is presented on this new group of peroxide-consuming enzymes. The gene of A. aegerita peroxygenase (apo1) was identified on the level of messenger RNA and genomic DNA. The gene sequence was affirmed by peptide sequences obtained through an Edman degradation and de novo peptide sequencing of the purified enzyme. Quantitative real-time reverse transcriptase polymerase chain reaction demonstrated that the course of enzyme activity correlated well with that of mRNA signals for apo1 in A. aegerita. The full-length sequences of A. aegerita peroxygenase as well as a partial sequence of C. radians peroxygenase confirmed the enzymes’ affiliation to the heme-thiolate proteins. The sequences revealed no homology to classic peroxidases, cytochrome P450 enzymes, and only little homology (<30%) to fungal chloroperoxidase produced by the ascomycete Caldariomyces fumago (and this only in the N-terminal part of the protein comprising the heme-binding region and part of the distal heme pocket). This fact reinforces the novelty of APO proteins. On the other hand, homology retrievals in genetic databases resulted in the identification of various APO homologous genes and transcripts, particularly among the agaric fungi, indicating APO’s widespread occurrence in the fungal kingdom.

Transformation and mineralization of 2,4,6-trinitrotoluene (TNT) by manganese peroxidase from the white-rot basidiomycete Phlebia radiata.

The degradation of the nitroaromatic pollutant 2,4,6-trinitrotoluene (TNT) by the manganese-dependent peroxidase (MnP) of the white-rot fungus Phlebia radiata and the main reduction products formed were investigated. In the presence of small amounts of reduced glutathione (10 mM), a concentrated cell-free preparation of MnP from P. radiata exhibiting an activity of 36 nkat/ml (36 nmol Mn(II) oxidized per sec and per ml) transformed 10 mg/l of TNT within three days. The same preparation was capable of completely transforming the reduced derivatives of TNT. When present at 10 mg/l, the aminodinitrotoluenes were transformed in less than two days and the diaminonitrotoluenes in less than three hours. Experiments with 14C-U-ring labeled TNT and 2-amino-4,6-dinitrotoluene showed that these compounds were mineralized by 22% and 76%, respectively, within 5 days. Higher concentrations of reduced glutathione (50 mM) led to a severe inhibition of the degradation process. It is concluded that Phlebia radiata is a good candidate for the biodegradation of TNT as well as its reduction metabolites.

Structural Basis of Substrate Conversion in a New Aromatic Peroxygenase CYTOCHROME P450 FUNCTIONALITY WITH BENEFITS

Background: Aromatic peroxygenases (APOs) are the “missing link” between heme peroxidases and P450-monooxygenases. Results: Based on two crystal structures the substrate conversion of APOs is elucidated. Conclusion: The specific design of the heme cavity and the distal heme access channel govern substrate specificity. Significance: APOs can be utilized in biotechnology and organic synthesis having significant advantages when compared with cytochrome P450 enzymes. Aromatic peroxygenases (APOs) represent a unique oxidoreductase sub-subclass of heme proteins with peroxygenase and peroxidase activity and were thus recently assigned a distinct EC classification (EC 1.11.2.1). They catalyze, inter alia, oxyfunctionalization reactions of aromatic and aliphatic hydrocarbons with remarkable regio- and stereoselectivities. When compared with cytochrome P450, APOs appear to be the choice enzymes for oxyfunctionalizations in organic synthesis due to their independence from a cellular environment and their greater chemical versatility. Here, the first two crystal structures of a heavily glycosylated fungal aromatic peroxygenase (AaeAPO) are described. They reveal different pH-dependent ligand binding modes. We model the fitting of various substrates in AaeAPO, illustrating the way the enzyme oxygenates polycyclic aromatic hydrocarbons. Spatial restrictions by a phenylalanine pentad in the active-site environment govern substrate specificity in AaeAPO.

Stereoselective benzylic hydroxylation of alkylbenzenes and epoxidation of styrene derivatives catalyzed by the peroxygenase of Agrocybe aegerita

Here we report on the stereoselective benzylic hydroxylation and C1–C2 epoxidation of alkylbenzenes and styrene derivatives, respectively, by a heme-thiolate peroxygenase (EC 1.11.2.1) from the fungus Agrocybe aegerita. Benzylic hydroxylation led exclusively to the (R)-1-phenylalkanols. For (R)-1-phenylethanol, (R)-1-phenylpropanol and (R)-1-tetralol, the ee reached >99%. For longer chain lengths, the enantiomeric excesses (ee) and total turnover numbers (TTN) decreased while the number of by-products, e.g. 1-phenylketones, increased. Epoxidation of straight chain and cyclic styrene derivatives gave a heterogeneous picture and resulted in moderate to excellent ee values and TTN: e.g., in the case of (1R,2S)-cis-β-methylstyrene oxide formation, an ee >99% and a TTN of 110 000 was achieved. Hydroxylation and epoxidation were true peroxygenations, which was demonstrated by the incorporation of 18O from H218O2 into the products. The use of fed-batch devices and varying feeding strategies for the substrate and co-substrate turned out to be a suitable approach to optimize peroxygenase catalysis.

Preparation of human drug metabolites using fungal peroxygenases.

The synthesis of hydroxylated and O- or N-dealkylated human drug metabolites (HDMs) via selective monooxygenation remains a challenging task for synthetic organic chemists. Here we report that aromatic peroxygenases (APOs; EC 1.11.2.1) secreted by the agaric fungi Agrocybe aegerita and Coprinellus radians catalyzed the H₂O₂-dependent selective monooxygenation of diverse drugs, including acetanilide, dextrorphan, ibuprofen, naproxen, phenacetin, sildenafil and tolbutamide. Reactions included the hydroxylation of aromatic rings and aliphatic side chains, as well as O- and N-dealkylations and exhibited different regioselectivities depending on the particular APO used. At best, desired HDMs were obtained in yields greater than 80% and with isomeric purities up to 99%. Oxidations of tolbutamide, acetanilide and carbamazepine in the presence of H₂¹⁸O₂ resulted in almost complete incorporation of ¹⁸O into the corresponding products, thus establishing that these reactions are peroxygenations. The deethylation of phenacetin-d₁ showed an observed intramolecular deuterium isotope effect [(k(H)/k(D))(obs)] of 3.1±0.2, which is consistent with the existence of a cytochrome P450-like intermediate in the reaction cycle of APOs. Our results indicate that fungal peroxygenases may be useful biocatalytic tools to prepare pharmacologically relevant drug metabolites.

High-yield production of aromatic peroxygenase by the agaric fungus Marasmius rotula.

An extracellular peroxygenase from Marasmius rotula was produced in liquid culture, chromatographically purified and partially characterized. This is the third aromatic peroxygenase (APO) that has been characterized in detail and the first one that can be produced in high yields. The highest enzyme levels of about 41,000 U l-1 (corresponding to appr. 445 mg l-1 APO protein) exceeded the hitherto reported levels more than 40-fold and were detected in carbon- and nitrogen-rich complex media. The enzyme was purified by FPLC to apparent homogeneity (SDS-PAGE) with a molecular mass of 32 kDa (27 kDa after deglycosylation) and isoelectric points between 4.97 and 5.27. The UV-visible spectrum of the native enzyme showed a characteristic maximum (Soret band) at 418 nm that shifted after reduction with sodium dithionite and flushing with carbon monoxide to 443 nm. The pH optimum of the M. rotula enzyme was found to vary between pH 5 and 6 for most reactions studied. The apparent Km- values for 2,6-dimethoxyphenol, benzyl alcohol, veratryl alcohol, naphthalene and H2O2 were 0.133, 0.118, 0.279, 0.791 and 3.14 mM, respectively. M. rotula APO was found to be highly stable in a pH range from 5 to 10 as well as in the presence of organic solvents (50% vol/vol) such as methanol, acetonitrile and N,N-dimethylformamide. Unlike other APOs, the peroxygenase of M. rotula showed neither brominating nor chlorinating activities.

Depolymerization of Straw Lignin by Manganese Peroxidase from Nematoloma frowardii is Accompanied by Release of Carbon Dioxide

Summary Manganese peroxidase preparations (MnP) from the white-rot fungus Nematoloma frowardii were able to release 14CO2 directly from 14C-labeled milled wheat straw (MWS; total lignin fraction) and milled straw lignin (MSL; dioxane soluble part of MWS). Apart from the formation of 14CO2 (4–10 %) the treatment of insoluble MWS and MSL with MnP resulted in the formation of water-soluble 14C-lignin fragments (lignin solubilization, 14–25%). Analyses with gel permeation chromatography (GPC) demonstrated the formation of lignin fragments with predominant molecular masses around 1 kDa. The extent of MWS mineralization and solubilization was enhanced in the presence of reduced glutathione (GSH) acting as thiol mediator, whereas MSL mineralization was not stimulated by GSH. The principle of direct extracellular mineralization of lignin catalyzed by the MnP system may make a significant contribution to the formation of carbon dioxide in lignincellulose containing habitats.

A spectrophotometric assay for the detection of fungal peroxygenases.

Rapid and simple spectrophotometric methods are required for the unambiguous detection of recently discovered fungal peroxygenases in vivo and in vitro. This paper describes a peroxygenase-specific assay using 5-nitro-1,3-benzodioxole as substrate. The product, 4-nitrocatechol, produces a yellow color at pH 7, which can be followed over time at 425 nm (ε(425)=9,700 M(-1) cm(-1)), and a red color when adjusted to pH >12, which can be measured in form of an end-point determination at 514 nm (ε(514)=11,400 M(-1) cm(-1)). The assay is suitable for detecting peroxygenase activities in complex growth media and environmental samples as well as for high-throughput screenings.

Oxidative cleavage of non-phenolic ß-0-4 lignin model dimers by an extracellular aromatic peroxygenase

Abstract The extracellular aromatic peroxygenase of the agaric fungus Agrocybe aegerita catalyzed the H2O2-dependent cleavage of non-phenolic arylglycerol-β-aryl ethers (β-O-4 ethers). For instance 1-(3,4-dimethoxyphenyl)-2-(2-methoxy-phenoxy)propane-1,3-diol, a recalcitrant dimeric lignin model compound that represents the major non-phenolic substructure in lignin, was selectively O-demethylated at the para-methoxy group to give formaldehyde and 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol. The phenol moiety of the latter compound was then enzymatically oxidized into phenoxy radicals and a quinoid cation, which initiated the autocatalytic cleavage of the dimer and the formation of monomers such as 2-methoxy-1,4-benzoquinone and phenoxyl-substituted propionic acid. The introduction of 18O from H2 18O2 and H2 18O at different positions into the pro-ducts provided information about the routes of ether cleavage. Studies with a 14C-labeled lignin model dimer showed that more than 70% of the intermediates formed were further coupled to form polymers with molecular masses above 10 kDa. The results indicate that fungal aromatic peroxygenases may be involved in the bioconversion of methoxylated plant ingredients originating from lignin or other sources.