Cloning and expression of pyranose oxidase cDNA from Coriolus versicolor in Escherichia coli

Localization of Pyranose 2-Oxidase from Kitasatospora aureofaciens: A Step Closer to Elucidate a Biological Role

Lignin degradation in fungal systems is well characterized. Recently, a potential for lignin depolymerization and modification employing similar enzymatic activities by bacteria is increasingly recognized. The presence of genes annotated as peroxidases in Actinobacteria genomes suggests that these bacteria should contain auxiliary enzymes such as flavin-dependent carbohydrate oxidoreductases. The only auxiliary activity subfamily with significantly similar representatives in bacteria is pyranose oxidase (POx). A biological role of providing H₂O₂ for peroxidase activation and reduction of radical degradation products suggests an extracellular localization, which has not been established. Analysis of the genomic locus of POX from Kitasatospora aureofaciens (KaPOx), which is similar to fungal POx, revealed a start codon upstream of the originally annotated one, and the additional sequence was considered a putative Tat-signal peptide by computational analysis. We expressed KaPOx including this additional upstream sequence as well as fusion constructs consisting of the additional sequence, the KaPOx mature domain and the fluorescent protein mRFP1 in Streptomyces lividans. The putative signal peptide facilitated secretion of KaPOx and the fusion protein, suggesting a natural extracellular localization and supporting a potential role in providing H₂O₂ and reducing radical compounds derived from lignin degradation.

Phylogeny and Structure of Fatty Acid Photodecarboxylases and Glucose-Methanol-Choline Oxidoreductases

Glucose-methanol-choline (GMC) oxidoreductases are a large and diverse family of flavin-binding enzymes found in all kingdoms of life. Recently, a new related family of proteins has been discovered in algae named fatty acid photodecarboxylases (FAPs). These enzymes use the energy of light to convert fatty acids to the corresponding Cn-1 alkanes or alkenes, and hold great potential for biotechnological application. In this work, we aimed at uncovering the natural diversity of FAPs and their relations with other GMC oxidoreductases. We reviewed the available GMC structures, assembled a large dataset of GMC sequences, and found that one active site amino acid, a histidine, is extremely well conserved among the GMC proteins but not among FAPs, where it is replaced with alanine. Using this criterion, we found several new potential FAP genes, both in genomic and metagenomic databases, and showed that related bacterial, archaeal and fungal genes are unlikely to be FAPs. We also identified several uncharacterized clusters of GMC-like proteins as well as subfamilies of proteins that lack the conserved histidine but are not FAPs. Finally, the analysis of the collected dataset of potential photodecarboxylase sequences revealed the key active site residues that are strictly conserved, whereas other residues in the vicinity of the flavin adenine dinucleotide (FAD) cofactor and in the fatty acid-binding pocket are more variable. The identified variants may have different FAP activity and selectivity and consequently may prove useful for new biotechnological applications, thereby fostering the transition from a fossil carbon-based economy to a bio-economy by enabling the sustainable production of hydrocarbon fuels.

Pyranose oxidase: A versatile sugar oxidoreductase for bioelectrochemical applications

Pyranose oxidase (POx) is an FAD-dependent oxidoreductase, and like glucose oxidase (GOx) it is a member of the glucose-methanol-choline (GMC) superfamily of oxidoreductases. POx oxidizes several monosaccharides including D-glucose, D-galactose, and D-xylose, while concurrently oxygen is reduced to hydrogen peroxide. In addition to this oxidase activity, POx shows pronounced activity with alternative electron acceptors that include various quinones or (complexed) metal ions. Even though POx in general shows properties that are more favourable than those of GOx (e.g., a considerably higher catalytic efficiency (k_cat/K_m) for D-glucose, significantly lower Michaelis constants K_m for D-glucose, reactivity with both anomeric forms of D-glucose) it is much less frequently used for both biosensor and biofuel cell applications than GOx. POx has been applied in biosensing of D-glucose, D-galactose, and D-xylose, and in combination with α-glucosidase also maltose. An attractive application is in biosensors constructed for the measurement of 1,5-anhydro-D-glucitol, a recognised biomarker in diabetes. Bioelectrochemical applications of POx had been restricted to enzymes of fungal origin. The recent discovery and characterisation of POx from bacterial sources, which show properties that are very distinct from the fungal enzymes, might open new possibilities for further applications in bioelectrochemistry.

Biochemical and mutational analyses of a Trametes pyranose oxidase and comparison of its mutants in breadmaking

Pyranose oxidase (POx) is a homotetrameric flavoprotein that catalyzes the oxidation of pyranose-configured sugars at position C-2 to corresponding 2-ketoaldoses. The wide substrate specificity makes POx potential for application in various biotechnological industries. In the present study we reported the gene cloning and heterologous expression of a POx from the basidiomycete Trametes sp. and functionally expressed the gene in Escherichia coli BL21(DE3). Based on sequence alignment, three residues were chosen for site-directed mutagenesis to obtain two single mutants (K312E and E539K) and two double mutants (T166A/E539K and K312E/E539K). In comparison to the wild-type, K312E shifted its optimal pH to 5.5 while the optimal temperature of E539K and K312E/E539K increased by 10 °C. The mutants retained more activities over broader pH ranges and higher temperatures and catalyzed d-glucose at higher efficiency (5800‒12,667 M⁻¹ s⁻¹ for the mutants versus 5083 M⁻¹ s⁻¹ for the wild-type). The recombinant POx and its mutants were all useful in gluten agglomeration and enlarging the loaf volume, which depends on the amounts of enzymes added. Interestingly, adding the same amount (0.5 nkat/g of flour) of wild-type and mutant enzymes differed in the change of loaf volumes, pinpointing that the catalytic activity is not the sole determinant in applying POx in breadmaking.

Membrane-associated glucose-methanol-choline oxidoreductase family enzymes PhcC and PhcD are essential for enantioselective catabolism of dehydrodiconiferyl alcohol

Sphingobium sp. strain SYK-6 is able to degrade various lignin-derived biaryls, including a phenylcoumaran-type compound, dehydrodiconiferyl alcohol (DCA). In SYK-6 cells, the alcohol group of the B-ring side chain of DCA is initially oxidized to the carboxyl group to generate 3-(2-(4-hydroxy-3-methoxyphenyl)-3-(hydroxymethyl)-7-methoxy-2,3-dihydrobenzofuran-5-yl) acrylic acid (DCA-C). Next, the alcohol group of the A-ring side chain of DCA-C is oxidized to the carboxyl group, and then the resulting metabolite is catabolized through vanillin and 5-formylferulate. In this study, the genes involved in the conversion of DCA-C were identified and characterized. The DCA-C oxidation activities in SYK-6 were enhanced in the presence of flavin adenine dinucleotide and an artificial electron acceptor and were induced ca. 1.6-fold when the cells were grown with DCA. Based on these observations, SLG_09480 (phcC) and SLG_09500 (phcD), encoding glucose-methanol-choline oxidoreductase family proteins, were presumed to encode DCA-C oxidases. Analyses of phcC and phcD mutants indicated that PhcC and PhcD are essential for the conversion of (+)-DCA-C and (-)-DCA-C, respectively. When phcC and phcD were expressed in SYK-6 and Escherichia coli, the gene products were mainly observed in their membrane fractions. The membrane fractions of E. coli that expressed phcC and phcD catalyzed the specific conversion of DCA-C into the corresponding carboxyl derivatives. In the oxidation of DCA-C, PhcC and PhcD effectively utilized ubiquinone derivatives as electron acceptors. Furthermore, the transcription of a putative cytochrome c gene was significantly induced in SYK-6 grown with DCA. The DCA-C oxidation catalyzed by membrane-associated PhcC and PhcD appears to be coupled to the respiratory chain.

Calixarene modified montmorillonite: a novel design for biosensing applications

Here we report the synthesis, characterization and application of calixarene (Calix) modified montmorillonite (Mt) as a platform for bio-applications such as biomolecule immobilization and biosensing technologies.

Crystal structures of Phanerochaete chrysosporium pyranose 2-oxidase suggest that the N-terminus acts as a propeptide that assists in homotetramer assembly

The flavin-dependent homotetrameric enzyme pyranose 2-oxidase (P2O) is found mostly, but not exclusively, in lignocellulose-degrading fungi where it catalyzes the oxidation of β-d-glucose to the corresponding 2-keto sugar concomitantly with hydrogen peroxide formation during lignin solubilization. Here, we present crystal structures of P2O from the efficient lignocellulolytic basidiomycete Phanerochaete chrysosporium. Structures were determined of wild-type PcP2O from the natural fungal source, and two variants of recombinant full-length PcP2O, both in complex with the slow substrate 3-deoxy-3-fluoro-β-d-glucose. The active sites in PcP2O and P2O from Trametes multicolor (TmP2O) are highly conserved with identical substrate binding. Our structural analysis suggests that the 17 °C higher melting temperature of PcP2O compared to TmP2O is due to an increased number of intersubunit salt bridges. The structure of recombinant PcP2O expressed with its natural N-terminal sequence, including a proposed propeptide segment, reveals that the first five residues of the propeptide intercalate at the interface between A and B subunits to form stabilizing, mainly hydrophobic, interactions. In the structure of mature PcP2O purified from the natural source, the propeptide segment in subunit A has been replaced by a nearby loop in the B subunit. We propose that the propeptide in subunit A stabilizes the A/B interface of essential dimers in the homotetramer and that, upon maturation, it is replaced by the loop in the B subunit to form the mature subunit interface. This would imply that the propeptide segment of PcP2O acts as an intramolecular chaperone for oligomerization at the A/B interface of the essential dimer.

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Structures of pyranose 2-oxidase from Phanerochaete chrysosporium were determined.

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The N-terminus may act as a propeptide with a role in homotetramer assembly.

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A large number of salt bridges between subunits provides thermostability.

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The substrate is bound in the productive binding mode for oxidation at C2.

Heterologous expression and biochemical characterization of novel pyranose 2-oxidases from the ascomycetes Aspergillus nidulans and Aspergillus oryzae

A gene encoding a pyranose 2-oxidase (POx; pyranose/oxygen 2-oxidoreductase; glucose 2-oxidase; EC 1.1.3.10) was identified in the genome of the ascomycete Aspergillus nidulans. Attempts to isolate POx directly from A. nidulans cultures or to homologously overexpress the native POx (under control of the constitutive gpdA promoter) in A. nidulans were unsuccessful. cDNA encoding POx was synthesized from mRNA and expressed in Escherichia coli, and the enzyme was subsequently purified and characterized. A putative pyranose 2-oxidase-encoding gene was also identified in the genome of Aspergillus oryzae. The coding sequence was synthetically produced and was also expressed in E. coli. Both purified enzymes were shown to be flavoproteins consisting of subunits of 65 kDa. The A. nidulans enzyme was biochemically similar to POx reported in literature. From all substrates, the highest catalytic efficiency was found with D-glucose. In addition, the enzyme catalyzes the two-electron reduction of 1,4-benzoquinone, several substituted benzoquinones and 2,6-dichloroindophenol. As judged by the catalytic efficiencies (k (cat)/k(m)), some of these quinone electron acceptors are better substrates for pyranose oxidase than oxygen. The enzyme from A. oryzae was physically similar but showed lower kinetic constants compared to the enzyme from A. nidulans. Distinct differences in the stability of the two enzymes may be attributed to a deletion and an insertion in the sequence, respectively.

Bioconversion of D-glucose into D-glucosone by glucose 2-oxidase from Coriolus versicolor at moderate pressures

Glucose 2-oxidase (pyranose oxidase, pyranose:oxygen-2-oxidoreductase, EC 1.1.3.10) from Coriolus versicolor catalyses the oxidation of D-glucose at carbon 2 in the presence of molecular O₂ producing D-glucosone (2-keto-glucose and D-arabino-2-hexosulose) and H₂O₂. It was used to convert D-glucose into D-glucosone at moderate pressures (i.e. up to 150 bar) with compressed air in a modified commercial batch reactor. Several parameters affecting biocatalysis at moderate pressures were investigated as follows: pressure, [enzyme], [glucose], pH, temperature, nature of fluid and the presence of catalase. Glucose 2-oxidase was purified by immobilized metal affinity chromatography on epoxy-activated Sepharose 6B-IDA-Cu(II) column at pH 6.0. The rate of bioconversion of D-glucose increased with the pressure since an increase in the pressure with compressed air resulted in higher rates of conversion. On the other hand, the presence of catalase increased the rate of reaction which strongly suggests that H₂O₂ acted as inhibitor for this reaction. The rate of bioconversion of D-glucose by glucose 2-oxidase in the presence of either nitrogen or supercritical CO₂ at 110 bar was very low compared with the use of compressed air at the same pressure. The optimum temperature (55 °C) and pH (5.0) of D-glucose bioconversion as well as kinetic parameters for this enzyme were determined under moderate pressure. The activation energy (E (a)) was 32.08 kJ mol⁻¹ and kinetic parameters (V(max), K(m), K(cat) and K(cat)/K(m)) for this bioconversion were 8.8 U mg⁻¹ protein, 2.95 mM, 30.81 s⁻¹ and 10,444.06 s⁻¹ M⁻¹, respectively. The biomass of C. versicolor as well as the cell-free extract containing glucose 2-oxidase activity were also useful for bioconversion of D-glucose at moderate pressures. The enzyme was apparently stable at moderate pressures since such pressures did not affect significantly the enzyme activity.

Pyranose 2-oxidase from Phanerochaete chrysosporium--expression in E. coli and biochemical characterization

The presented work reports the isolation and heterologous expression of the p2ox gene encoding the flavoprotein pyranose 2-oxidase (P2Ox) from the basidiomycete Phanerochaete chrysosporium. The p2ox cDNA was inserted into the bacterial expression vector pET21a(+) and successfully expressed in Escherichia coli. We obtained active, fully flavinylated recombinant P2Ox in yields of approximately 270 mg/l medium. The recombinant enzyme was provided with an N-terminal T7-tag and a C-terminal His(6)-tag to facilitate simple one-step purification. We obtained an apparently homogenous enzyme preparation with a specific activity of 16.5 U/mg. Recombinant P2Ox from P. chrysosporium was characterized in some detail with respect to its physical and catalytic properties, both for electron donor (sugar substrates) and - for the first time - alternative electron acceptors (1,4-benzoquinone, substituted quinones, 2,6-dichloroindophenol and ferricenium ion). As judged from the catalytic efficiencies k(cat)/K(m), some of these alternative electron acceptors are better substrates than oxygen, which might have implications for the proposed in vivo function of pyranose 2-oxidase.

Gene cloning and heterologous expression of pyranose 2-oxidase from the brown-rot fungus, Gloeophyllum trabeum

A pyranose 2-oxidase gene from the brown-rot basidiomycete Gloeophyllum trabeum was isolated using homology-based degenerate PCR. The gene structure was determined and compared to that of several pyranose 2-oxidases cloned from white-rot fungi. The G. trabeum pyranose 2-oxidase gene consists of 16 coding exons with canonical promoter CAAT and TATA elements in the 5'UTR. The corresponding G. trabeum cDNA was cloned and contains an ORF of 1,962 base pairs encoding a 653 amino acid polypeptide with a predicted molecular weight of 72 kDa. A Hisx6 tagged recombinant G. trabeum pyranose 2-oxidase was generated and expressed heterologously in Escherichia coli yielding 15 U enzyme activity per ml of induced culture. Structural alignment and phylogenetic analysis were performed and are discussed.

The C-terminal region controls correct folding of genus Trametes pyranose 2-oxidases

The pyranose 2-oxidases from Trametes ochracea and Trametes pubescens share markedly similar amino acid sequences with identity of 93.4%. When expressed from the recombinant plasmids based on the same vector in the Escherichia coli host strain BL21(DE3) at higher growth temperatures, they differ strikingly in the formation of the inclusion bodies. Upon overexpression in the cultures performed at 28 degrees C, the specific activity of pyranose 2-oxidase from T. pubescens was eight times higher than that from T. ochracea: 93% of pyranose 2-oxidase from T. ochracea and only 15% of that from T. pubescens was present in the form of inclusion bodies. To ascertain the cause of this difference, both cloned genes were shuffled. Site-directed recombination of p2o cDNAs revealed that DNA constructs ending with 3' end of p2o cDNA from T. pubescens code for proteins that are folded into an active form to the greater extent, regardless of the gene expression level. "In silicio" analysis of physico-chemical properties of the protein sequences of pyranose 2-oxidases revealed that the sequence of amino acid residues 368-430, constituting the small, head domain of pyranose 2-oxidase from T. pubescens, affects positively the enzyme folding at higher cultivation temperatures. The domain differs in six amino acid residues from that of T. ochracea.

N-ethylmaleimide-resistant acyl-coenzyme A oxidase from Arthrobacter ureafaciens NBRC 12140: molecular cloning, gene expression and characterization of the recombinant enzyme

N-ethylmaleimide (NEM)-resistant acyl-coenzyme A oxidase (ACO) has been desired for the determination of free fatty acids (FFAs). In order to meet this demand, we prepared recombinant ACO from Arthrobacter ureafaciens NBRC 12140. The coding region of the gene was 2109, encoding a protein of 703 amino acids with a predicted molecular mass of 76.5 kDa. The heterologous expression level in Escherichia coli was 520-fold higher than that in the native strain. The purified enzyme retained more than 60% activity after incubation in the presence of 10 mM NEM at 37 degrees C for 4 h, while other commercially available ACOs showed only less than 10% activities after the same NEM treatment. We presume that this is due to the presence of only three cysteines in ACO from A. ureafaciens. Site-directed mutagenesis studies and close scrutiny of the three-dimensional structures of other related ACOs suggested that these cysteines were buried in the protein and unreactive to NEM. The recombinant enzyme was used for the colorimetric determination of free fatty acid, which gave a linear calibration.

Improvement of the fungal enzyme pyranose 2-oxidase using protein engineering

Native pyranose 2-oxidase (P2Ox) was purified from Peniophora sp. and characterized. To improve its catalytic efficiencies and stabilities by protein engineering, we cloned and expressed the P2Ox gene in Escherichia coli and received active, fully flavinylated recombinant P2OxA. Selenomethionine-labeled P2OxA was used for X-ray analysis and the resulting crystal structure enabled the rational design using variant P2OxA1 with the substitution E542K as template. Besides increased thermal and pH stabilities this variant showed improved catalytic efficiencies (k(cat)/K(m)) for the main substrates. A new variant, P2OxA2H, with an additional substitution T158A and a C-terminal His(6)-tag exhibited significantly decreased apparent K(m) values for D-glucose (0.47 mM), l-sorbose (1.79 mM), and D-xylose (1.35 mM). Compared to native P2Ox, the catalytic efficiencies were substantially improved for D-glucose (230-fold), L-sorbose (874-fold), and D-xylose (1751-fold). This P2Ox variant was used for the bioconversion of L-sorbose under O(2)-saturation in a molar scale. The structure-activity relationships of the amino acid substitutions were analyzed by modelling of the mutated P2Ox structures. Molecular docking calculations of various carbohydrates into the crystal structure of P2OxA and the analysis of the protein-ligand interactions in the docked complexes enabled us to explain the substrate specificity of the enzyme by a conserved hydrogen bond pattern which is formed between the protein and all substrates.

Histamine dehydrogenase from Rhizobium sp.: gene cloning, expression in Escherichia coli, characterization and application to histamine determination

The gene encoding histamine dehydrogenase in Rhizobium sp. 4--9 has been cloned and overexpressed in Escherichia coli. The coding region of the gene was 2,079 bp and encoded a protein of 693 amino acids with a calculated molecular mass of 76,732 Da. This histamine dehydrogenase was related to histamine dehydrogenase from Nocardioides simplex (54.5% identical), trimethylamine dehydrogenase from Methylophilus methylotrophus (39.3% identical) and dimethylamine dehydrogenase from Hyphomicrobium X (38.1% identical), which have a covalent 6-S-cysteinyl flavin mononucleotide and a [4Fe--4S] cluster as redox cofactors. Sequence alignment and a UV-visible absorption spectrum supported the presence of these cofactors in this histamine dehydrogenase. The investigation of the enzymatic properties suggested that this enzyme exhibited the most excellent substrate specificity toward histamine among all amine oxidases or dehydrogenases found to date. The recombinant enzyme was able to be used for the colorimetric determination of histamine, which gave a linear calibration curve and identical data with conventional methods.

Expression of the pyranose 2-oxidase from Trametes pubescens in Escherichia coli and characterization of the recombinant enzyme

cDNA-encoding pyranose 2-oxidase (P2O) from Trametes pubescens was sequenced and cloned into Escherichia coli strain BL21/DE3 on a multicopy plasmid under the control of trc promoter. The synthesis of P2O was studied in a batch culture in M9-based mineral medium: the enzyme was synthesized constitutively at 28 degrees C in amount corresponding to 8% of the cell soluble protein (0.6 Umg(-1)). Only small portion of P2O (11%) was in the form of non-active inclusion bodies. Purified recombinant enzyme has similar physico-chemical and kinetic parameters with other P2Os. When compared to the expression of p2o of Trametes ochracea, a ratio of the mature enzyme to inclusion bodies found in the same E. coli host at 28 degrees C is as much as nine times higher. The finding makes the enzyme from T. pubescens preferable for the large-scale production by recombinant bacteria. The difference in amino acid sequences of the P2O from T. ochracea and T. pubescens may explain the favourable trait of the latter enzyme regarding protein folding.

Engineering of pyranose 2-oxidase from Peniophora gigantea towards improved thermostability and catalytic efficiency

To improve the stability and catalytic efficiency of pyranose 2-oxidase (P2Ox) by molecular enzyme evolution, we cloned P2Ox cDNA by RACE-PCR from a cDNA library derived from the basidiomycete Peniophora gigantea. The P2Ox gene was expressed in Escherichia coli BL21(DE3), yielding an intracellular and enzymatically active P2OxB with a volumetric yield of 500 units/l. Site-directed mutagenesis was employed to construct the P2Ox variant E540K (termed P2OxB1), which exhibited increased thermo- and pH-stability compared with the wild type, concomitantly with increased catalytic efficiencies (k(cat)/K(m)) for D-xylose and L-sorbose. P2OxB1 was provided with a C-terminal His(6)-tag (termed P2OxB1H) and subjected to directed evolution using error-prone PCR. Screening based on a chromogenic assay yielded the new P2Ox variant K312E (termed P2OxB2H) that showed significant improvements with respect to k(cat)/K(m) for D-glucose (5.3-fold), methyl-beta-D-glucoside (2.0-fold), D-galactose (4.8-fold), D-xylose (59.9-fold), and L-sorbose (69.0-fold), compared with wild-type P2Ox. The improved catalytic performance of P2OxB2H was demonstrated by bioconversions of L-sorbose that initially was a poor substrate for wild-type P2Ox. This is the first report on the improvement of a pyranose 2-oxidase by a dual approach of site-directed mutagenesis and directed evolution, and the application of the engineered P2Ox in bioconversions.

Isolation and purification of pyranose 2-oxidase from Phanerochaete chrysosporium and characterization of gene structure and regulation

Pyranose 2-oxidase (POX) was recovered from Phanerochaete chrysosporium BKM-F-1767 solid substrate culture using mild extraction conditions and was purified. (13)C-nuclear magnetic resonance confirmed production of d-arabino-hexos-2-ulose (glucosone) from d-glucose with the oxidase. Peptide fingerprints generated by liquid chromatography-tandem mass spectrometry of tryptic digests and analysis of the corresponding cDNA revealed a structurally unusual sequence for the P. chrysosporium POX. Relatively high levels of pox transcript were detected under carbon-starved culture conditions but not under nutrient sufficiency. This regulation pattern is similar to that observed for lignin peroxidases, manganese peroxidases, and glyoxal oxidase of P. chrysosporium, supporting evidence that POX has a role in lignocellulose degradation.

Crystal Structure of Pyranose 2-Oxidase from the White-Rot Fungus Peniophora sp.,

Pyranose 2-oxidase catalyzes the oxidation of a number of carbohydrates using dioxygen. The enzyme forms a D(2) symmetric homotetramer and contains one covalently bound FAD per subunit. The structure of the enzyme from Peniophora sp. was determined by multiwavelength anomalous diffraction (MAD) based on 96 selenium sites per crystallographic asymmetric unit and subsequently refined to good-quality indices. According to its chain fold, the enzyme belongs to the large glutathione reductase family and, in a more narrow sense, to the glucose-methanol-choline oxidoreductase (GMC) family. The tetramer contains a spacious central cavity from which the substrate enters one of the four active centers by penetrating a mobile barrier. Since this cavity can only be accessed by glucose-sized molecules, the enzyme does not convert sugars that are part of a larger molecule. The geometry of the active center and a comparison with an inhibitor complex of the homologous enzyme cellobiose dehydrogenase allow the modeling of the reaction at a high confidence level.

Crystal Structure of the 270 kDa Homotetrameric Lignin-degrading Enzyme Pyranose 2-Oxidase

Pyranose 2-oxidase (P2Ox) is a 270 kDa homotetramer localized preferentially in the hyphal periplasmic space of lignocellulolytic fungi and has a proposed role in lignocellulose degradation to produce the essential co-substrate, hydrogen peroxide, for lignin peroxidases. P2Ox oxidizes D-glucose and other aldopyranoses regioselectively at C2 to the corresponding 2-keto sugars; however, for some substrates, the enzyme also displays specificity for oxidation at C3. The crystal structure of P2Ox from Trametes multicolor has been determined using single anomalous dispersion with mercury as anomalous scatterer. The model was refined at 1.8A resolution to R and Rfree values of 0.134 and 0.171, respectively. The overall fold of the P2Ox subunit resembles that of members of the glucose-methanol-choline family of long-chain oxidoreductases, featuring a flavin-binding Rossmann domain of class alpha/beta and a substrate-binding subdomain with a six-stranded central beta sheet and three alpha helices. The homotetramer buries a large internal cavity of roughly 15,000 A3, from which the four active sites are accessible. Four solvent channels lead from the surface into the cavity through which substrate must enter before accessing the active site. The present structure shows an acetate molecule bound in the active site with the carboxylate group positioned immediately below the flavin N5 atom, and with one carboxylate oxygen atom interacting with the catalytic residues His548 and Asn593. The entrance to the active site is blocked by a loop (residues 452 to 461) with excellent electron density but elevated temperature factors. We predict that this loop is dynamic and opens to allow substrate entry and exit. In silico docking of D-glucose in the P2Ox active site shows that with the active-site loop in the closed conformation, monosaccharides cannot be accommodated; however, after removing the loop from the model, a tentative set of protein-substrate interactions for beta-D-glucose have been outlined.

High-level expression of a fungal pyranose oxidase in high cell-density fed-batch cultivations of Escherichia coli using lactose as inducer

Expression of a recombinant pyranose oxidase (P2O) from the basidiomycete Trametes ochracea has been increased 10-fold in shaking flask cultures of Escherichia coli BL21(DE3) harboring plasmid pSE33 by optimizing the composition of the culture medium using an experimental design approach. Inexpensive lactose was used as a medium component and inducer of expression of the P2O gene, which is under the control of a trc promoter. The expression system was studied in detail in batch and fed-batch cultivations with the aim to improve the expression level of active recombinant protein and to minimize the formation of inclusion bodies. In batch cultivations, the highest specific P2O activity of 0.9 U (mg of soluble protein)(-1) was measured in oxygen-limited cultures grown at 25 degrees C. The highest overall volumetric productivity of 33 mg of active P2O per liter and hour (corresponding to 345U (L h)(-1)) has been determined in a high-density fed-batch process with a feed-forward exponential feeding strategy. During the fed-batch process, lactose was added intermittently to the culture. A final biomass concentration of 33 g L(-1) (based on cell dry weight) was obtained. Compared to shaking flask cultures in not optimized culture media, the overall volumetric P2O productivity has been improved by a factor of 110 using the fed-batch strategy and the optimized culture medium. Recombinant P2O was expressed in the cytoplasm with 9% of the total soluble protein being active P2O. In terms of physical and enzyme kinetic properties, the purified recombinant P2O was found to be similar to the previously published data of P2O isolated from its original host.

Molecular cloning and expression of the pyranose 2-oxidase cDNA from Trametes ochracea MB49 in Escherichia coli

A cDNA of a structural gene encoding pyranose 2-oxidase (P2O) from Trametes ochracea strain MB49 was cloned into Escherichia coli strain BL21(DE3) on a multicopy plasmid under the control of the trc promoter. Synthesis of P2O was studied in batch cultures in LB or M9-based mineral medium at 28 degrees C. While there was a low specific activity of P2O in LB medium, the enzyme was synthesised constitutively in mineral medium and represented 3% of the cell soluble protein (0.3 U mg(-1)). The effect of isopropyl beta-D-thiogalactoside on the expression of P2O was studied in mineral medium at 25 and 28 degrees C. The synthesis of P2O at 28 degrees C corresponded to 39% of the cell soluble protein but the major portion of P2O (93%) was in the form of non-active inclusion bodies (activity of P2O equalled 0.19 U mg(-1)). At 25 degrees C, the amount of P2O represented 14% of the cell soluble protein and the activity of P2O was 1.1 U mg(-1). The soluble enzyme represented 70% of the total amount of P2O.

Identification of the covalent flavin adenine dinucleotide-binding region in pyranose 2-oxidase from Trametes multicolor

We present the first report on characterization of the covalent flavinylation site in flavoprotein pyranose 2-oxidase. Pyranose 2-oxidase from the basidiomycete fungus Trametes multicolor, catalyzing C-2/C-3 oxidation of several monosaccharides, shows typical absorption maxima of flavoproteins at 456, 345, and 275 nm. No release of flavin was observed after protein denaturation, indicating covalent attachment of the cofactor. The flavopeptide fragment resulting from tryptic/chymotryptic digestion of the purified enzyme was isolated by anion-exchange and reversed-phase high-performance liquid chromatography. The flavin type, attachment site, and mode of its linkage were determined by mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy of the intact flavopeptide, without its prior enzymatic degradation to the central aminoacyl moiety. Mass spectrometry identified the attached flavin as flavin adenine dinucleotide (FAD). Post-source decay analysis revealed that the flavin is covalently bound to histidine residue in the peptide STHW, consistent with the results of N-terminal amino acid sequencing by Edman degradation. The type of the aminoacyl flavin covalent link was determined by NMR spectroscopy, resulting in the structure 8alpha-(N(3)-histidyl)-FAD.

Thermostabilization of bacterial fructosyl-amino acid oxidase by directed evolution

We succeeded in isolating several thermostable mutant fructosyl-amino acid oxidase (FAOX; EC 1.5.3) without reduction of productivity by directed evolution that combined an in vivo mutagenesis and membrane assay screening system. Five amino acid substitutions (T60A, A188G, M244L, N257S, and L261M) occurred in the most thermostable mutant obtained by a fourth round of directed evolution. This altered enzyme, FAOX-TE, was stable at 45 degrees C, whereas the wild-type enzyme was not stable above 37 degrees C. The K(m) values of FAOX-TE for D-fructosyl-L-valine and D-fructosyl-glycine were 1.50 and 0.58 mM, respectively, in contrast with corresponding values of 1.61 and 0.74 mM for the wild-type enzyme. This altered FAOX-TE will be useful in the diagnosis of diabetes.

Purification and characterization of pyranose oxidase from the white rot fungus Trametes multicolor

We purified an intracellular pyranose oxidase from mycelial extracts of the white rot fungus Trametes multicolor by using ammonium sulfate fractionation, hydrophobic interaction, ion-exchange chromatography, and gel filtration. The native enzyme has a molecular mass of 270 kDa as determined by equilibrium ultracentrifugation and is composed of four identical 68-kDa subunits as determined by matrix-assisted laser desorption ionization mass spectrometry. Each subunit contains one covalently bound flavin adenine dinucleotide as its prosthetic group. The enzyme oxidizes several aldopyranoses specifically at position C-2, and its preferred electron donor substrates are D-glucose, D-xylose, and L-sorbose. During this oxidation reaction electrons are transferred to oxygen, yielding hydrogen peroxide. In addition, the enzyme catalyzes the two-electron reduction of 1,4-benzoquinone, several substituted benzoquinones, and 2,6-dichloroindophenol, as well as the one-electron reduction of the ABTS [2,2'-azinobis(3-ethylbenzthiazolinesulfonic acid)] cation radical. As judged by the catalytic efficiencies (k(cat)/K(m)), some of these quinone electron acceptors are much better substrates for pyranose oxidase than oxygen. The optimum pH of the pyranose oxidase-catalyzed reaction depends strongly on the electron acceptor employed and varies from 4 to 8. It has been proposed that the main metabolic function of pyranose oxidase is as a constituent of the ligninolytic system of white rot fungi that provides peroxidases with H(2)O(2). An additional function could be reduction of quinones, key intermediates that are formed during mineralization of lignin.

Kinetic model discrimination via step-by-step experimental and computational procedure in the enzymatic oxidation of D-glucose

The enzymatic oxidation of D-glucose to 2-keto-D-glucose (D-arabino-hexos-2-ulose, D-glucosone) is of prospective industrial interest. Pyranose oxidase (POx) from Peniphora gigantea is deactivated during the reaction. To develop a kinetic model including the main reaction and the enzyme inactivation, possible side-reactions of the non-stabilised enzyme with D-glucosone, hydrogen peroxide, and peroxide radicals were considered. A developed step-by-step combined experimental and computational procedure allowed to discriminate among alternative inactivation mechanisms and provides an increased model reliability. The most probable scheme is the enzyme inactivation by hydroxyl radicals formed from produced H2O2 in the presence of Fe2+ ions. This .OH reaction is supported by matrix assisted laser desorption ionisation-mass spectrometry (MALDI-MS) measurement. The estimated kinetic parameter values for the main reaction are of the same order of magnitude as those reported in the literature. The identified model allows a satisfactory process simulation and highlights measures to prevent the enzyme activity loss.

A novel carbohydrate:acceptor oxidoreductase from Microdochium nivale

A Microdochium nivale carbohydrate:acceptor oxidoreductase was purified, cloned, heterologously expressed, and characterized. The gene encoding the protein showed one intron, and the ORF showed a sequence with low homology (< or = 25% identity or 65% similarity) to other known flavin-containing carbohydrate oxidases. The maturation of the protein required the cleavage of a tetrameric propeptide in addition to an 18 amino-acid signal peptide. The enzyme was found to have a relative molecular mass of 55 000 Da, an isoelectric point of 9, and one FAD per protein. It could oxidize mono-, oligo-, or polymeric saccharides, and transfer their electrons to O2 or other acceptors. When D-glucose served as electron-donating substrate, an activity of 2 s(-1) was observed at pH 5.5 and 23 degrees C. Among various oligosaccharides, the enzyme preferred tetrameric dextrins, indicating a favorable interaction of four linked glucose units with the substrate pocket. The unique structure and ability of oxidizing oligo/polymeric saccharides suggest a promising prospect of this enzyme for various industrial/medicinal applications.

Peptide Sequencing by Matrix-Assisted Laser Desorption/Ionisation and Post-Source Decay Mass Spectrometry: A Rapid Method to Design Oligonucleotide Hybridisation Probes for Cloning cDNA Encoding Pyranose 2-Oxidase from Trametes multicolor

A rapid and elegant method for designing oligonucleotide hybridisation probes for cloning cDNA encoding a biologically and/or biotechnologically important protein is presented. The approach is based on proteolytic digestion of a protein of interest and subsequent matrix/assisted laser desorption/ionisation mass spectrometric (MALDI-MS) analysis of the resulting peptide mixture. The method is demonstrated on the analysis of pyranose 2-oxidase (P2O), a homotetrameric flavoprotein used as a catalyst component in several biotechnologically important carbohydrate conversions. P2O from the fungusTrametes multicolorwas cleaved directly in the gel by two different proteases and the peptides formed were subjected to MALDI-MS. A comparison of the obtained peptide maps to those theoretically derived from the known sequence of homologous P2O (Trametes versicolor) allowed us to select peptide candidates for designing hybridisation probes. The suitable peptides were sequenced by post-source decay (PSD) analysis. The acquired sequence data are aimed at cloning and sequencing ofT. multicolor p2ocDNA and at production of the recombinant enzyme.

Pyruvate phosphate dikinase from a thermophilic actinomyces Microbispora rosea subsp. aerata: purification, characterization and molecular cloning of the gene

Various thermophilic bacteria were analyzed by Southern hybridization analysis using oligonucleotide probes coding for the pyruvate phosphate dikinase (PPDK) gene from Clostridium symbiosum, and positive hybridization signals were observed in the chromosomal DNAs from Microbispora rosea subsp. aerata (IFO 14047). PPDK activity was detected in lactose induced cells and the enzyme was purified to homogeneity. The molecular mass of PPDK was estimated to be 230000 by gel filtration chromatography and 91000 by SDS-PAGE, suggesting that PPDK is a dimeric enzyme. This enzyme was specific for adenine nucleotide and the apparent Km values for AMP, PPi, and phosphoenolpyruvate were 5, 38, and 280 microM, respectively. It was stable in the pH range 6 to 11, and retained 80% activity after 60 min heat treatment at 60 degrees C. We cloned the PPDK gene from M. rosea. It consists of 878 amino acids with a molecular mass of 95514. Sequence comparison indicates around 50% similarity with other PPDKs and it has all the highly conserved regions of the related enzymes. We expressed the PPDK gene in Escherichia coli and obtained enzymatically active protein.

Glucose 1- and 2-oxidases from fungal strains: isolation and production of monoclonal antibodies

Monoclonal antibodies (Mabs) against purified glucose 2-oxidase (EC 1.1.3.10) from Coriolus versicolor were raised by hybridoma technology using Sp2/0 myeloma cells as a fusion partner. Hybrid growth was observed in 42% of culture wells and 30% of these (i.e. 30 culture wells) contained anti-glucose 2-oxidase activity. Three positive wells containing hybrid cell lines were selected and cloned twice by the limiting dilution method and two hybridoma clones (E1A5 and E1A6) secreting Mabs were selected at random for purification and characterisation purposes. Both cell lines secreted Mabs of IgM class which were purified by gel filtration chromatography on a Sephacryl S-200 column with a final recovery of 80% and a purification factor of 16. The purified preparations were apparently homogeneous on native PAGE running with a M(r) of 950 kDa. Mabs were highly specific for glucose 2-oxidase as determined by Western blotting. These Mabs also crossreacted with glucose 1- and 2-oxidases from other fungal sources (Phanerochaeta chrysosporium, Penicillium amagasakiense and Aspergillus niger) as determined by Western blotting and by ELISA. Both glucose 1- and 2-oxidases from C. versicolor, P. chrysosporium, P. amagasakiense and A. niger were purified by hydrophobic interaction chromatography on Sepharose 4B-triazine dye with a recovery of enzyme activity in the range 85-92%. Purified preparations of glucose oxidases from fungal strains were apparently homogeneous on native PAGE. Glucose 2-oxidases were more hydrophobic than glucose 1-oxidases as determined by their chomatographic behaviour on Sepharose 4B-Cibacron Red G-E which could be used to study their roles in lignin biodegradation.

Sequencing and characterization of the xyl operon of a gram-positive bacterium, Tetragenococcus halophila

The xyl operon of a gram-positive bacterium, Tetragenococcus halophila (previously called Pediococcus halophilus), was cloned and sequenced. The DNA was about 7.7 kb long and contained genes for a ribose binding protein and part of a ribose transporter, xylR (a putative regulatory gene), and the xyl operon, along with its regulatory region and transcription termination signal, in this order. The DNA was AT rich, the GC content being 35.8%, consistent with the GC content of this gram-positive bacterium. The xyl operon consisted of three genes, xylA, encoding a xylose isomerase, xylB, encoding a xylulose kinase, and xylE, encoding a xylose transporter, with predicted molecular weights of 49,400, 56,400, and 51,600, respectively. The deduced amino acid sequences of the XylR, XylA, XylB, and XylE proteins were similar to those of the corresponding proteins in other gram-positive and -negative bacteria, the similarities being 37 to 64%. Each polypeptide of XylB and XylE was expressed functionally in Escherichia coli. XylE transported D-xylose in a sodium ion-dependent manner, suggesting that it is the first described xylose/Na+ symporter. The XylR protein contained a consensus sequence for binding catabolites of glucose, such as glucose-6-phosphate, which has been discovered in glucose and fructose kinases in bacteria. Correspondingly, the regulatory region of this operon contained a putative binding site of XylR with a palindromic structure. Furthermore, it contained a consensus sequence, CRE (catabolite-responsive element), for binding CcpA (catabolite control protein A). We speculate that the transcriptional regulation of this operon resembles the regulation of catabolite-repressible operons such as the amy, lev, xyl, and gnt operons in various gram-positive bacteria. We discuss the significance of the regulation of gene expression of this operon in T. halophila.