Oxidative polymerization of ribonuclease A by lignin peroxidase from Phanerochaete chrysosporium. Role of veratryl alcohol in polymer oxidation.
The mechanism of lignin peroxidase (LiP) was examined using bovine pancreatic ribonuclease A (RNase) as a polymeric lignin model substrate. SDS/PAGE analysis demonstrates that an RNase dimer is the major product of the LiP-catalyzed oxidation of this protein. Fluorescence spectroscopy and amino acid analyses indicate that RNase dimer formation is due to the LiP-catalyzed oxidation of Tyr residues to Tyr radicals, followed by intermolecular radical coupling. The LiP-catalyzed polymerization of RNase in strictly dependent on the presence of veratryl alcohol (VA). In the presence of 100 microM H2O2, relatively low concentrations of RNase and VA, together but not individually, can protect LiP from H2O2 inactivation. The presence of RNase strongly inhibits VA oxidation to veratraldehyde by LiP; whereas the presence of VA does not inhibit RNase oxidation by LiP. Stopped-flow and rapid-scan spectroscopy demonstrate that the reduction of LiP compound I (LiPI) to the native enzyme by RNase occurs via two single-electron steps. At pH 3.0, the reduction of LiPI by RNase obeys second-order kinetics with a rate constant of 4.7 x 10(4) M-1.s-1, compared to the second-order VA oxidation rate constant of 3.7 x 10(5) M-1.s-1. The reduction of LiP compound II (LiPII) by RNase also follows second-order kinetics with a rate constant of 1.1 x 10(4) M-1.s-1, compared to the first-order rate constant for LiPII reduction by VA. When the reductions of LiPI and LiPIi are conducted in the presence of both VA and RNase, the rate constants are essentially identical to those obtained with VA alone. These results suggest that VA is oxidized by LiP to its cation radical which, while still in its binding site, oxidizes RNase. (+info)
Homologous expression of recombinant lignin peroxidase in Phanerochaete chrysosporium.
The glyceraldehyde-3-phosphate dehydrogenase (gpd) promoter was used to drive expression of lip2, the gene encoding lignin peroxidase (LiP) isozyme H8, in primary metabolic cultures of Phanerochaete chrysosporium. The expression vector, pUGL, also contained the Schizophyllum commune ura1 gene as a selectable marker. pUGL was used to transform a P. chrysosporium Ura11 auxotroph to prototrophy. Ura+ transformants were screened for peroxidase activity in liquid cultures containing high-carbon and high-nitrogen medium. Recombinant LiP (rLiP) was secreted in active form by the transformants after 4 days of growth, whereas endogenous lip genes were not expressed under these conditions. Approximately 2 mg of homogeneous rLiP/liter was obtained after purification. The molecular mass, pI, and optical absorption spectrum of rLiPH8 were essentially identical to those of the wild-type LiPh8 (wt LiPH8), indicating that heme insertion, folding, and secretion functioned normally in the transformant. Steady-state and transient-state kinetic properties for the oxidation of veratryl alcohol between wtLiPH8 and rLiPH8 were also identical. (+info)
Organization and differential regulation of a cluster of lignin peroxidase genes of Phanerochaete chrysosporium.
The lignin peroxidases of Phanerochaete chrysosporium are encoded by a minimum of 10 closely related genes. Physical and genetic mapping of a cluster of eight lip genes revealed six genes occurring in pairs and transcriptionally convergent, suggesting that portions of the lip family arose by gene duplication events. The completed sequence of lipG and lipJ, together with previously published sequences, allowed phylogenetic and intron/exon classifications, indicating two main branches within the lip family. Competitive reverse transcription-PCR was used to assess lip transcript levels in both carbon- and nitrogen-limited media. Transcript patterns showed differential regulation of lip genes in response to medium composition. No apparent correlation was observed between genomic organization and transcript levels. Both constitutive and upregulated transcripts, structurally unrelated to peroxidases, were identified within the lip cluster. (+info)
Biotransformation of 2,4,6-trinitrotoluene with Phanerochaete chrysosporium in agitated cultures at pH 4.5.
The biotransformation of 2,4,6-trinitrotoluene (TNT) (175 microM) by Phanerochaete chrysosporium with molasses and citric acid at pH 4.5 was studied. In less than 2 weeks, TNT disappeared completely, but mineralization (liberated 14CO2) did not exceed 1%. A time study revealed the presence of several intermediates, marked by the initial formation of two monohydroxylaminodinitrotoluenes (2- and 4-HADNT) followed by their successive transformation to several other products, including monoaminodinitrotoluenes (ADNT). A group of nine acylated intermediates were also detected. They included 2-N-acetylamido-4,6-dinitrotoluene and its p isomer, 2-formylamido-4, 6-dinitrotoluene and its p isomer (as acylated ADNT), 4-N-acetylamino-2-amino-6-nitrotoluene and 4-N-formylamido-2-amino-6-nitrotoluene (as acetylated DANT), 4-N-acetylhydroxy-2,6-dinitrotoluene and 4-N-acetoxy-2, 6-dinitrotoluene (as acetylated HADNT), and finally 4-N-acetylamido-2-hydroxylamino-6-nitrotoluene. Furthermore, a fraction of HADNTs were found to rearrange to their corresponding phenolamines (Bamberger rearrangement), while another group dimerized to azoxytoluenes which in turn transformed to azo compounds and eventually to the corresponding hydrazo derivatives. After 30 days, all of these metabolites, except traces of 4-ADNT and the hydrazo derivatives, disappeared, but mineralization did not exceed 10% even after the incubation period was increased to 120 days. The biotransformation of TNT was accompanied by the appearance of manganese peroxidase (MnP) and lignin-dependent peroxidase (LiP) activities. MnP activity was observed almost immediately after TNT disappearance, which was the period marked by the appearance of the initial metabolites (HADNT and ADNT), whereas the LiP activity was observed after 8 days of incubation, corresponding to the appearance of the acyl derivatives. Both MnP and LiP activities reached their maximum levels (100 and 10 U/liter, respectively) within 10 to 15 days after inoculation. (+info)
Direct probing of the surface ultrastructure and molecular interactions of dormant and germinating spores of Phanerochaete chrysosporium.
Atomic force microscopy (AFM) has been used to probe, under physiological conditions, the surface ultrastructure and molecular interactions of spores of the filamentous fungus Phanerochaete chrysosporium. High-resolution images revealed that the surface of dormant spores was uniformly covered with rodlets having a periodicity of 10 +/- 1 nm, which is in agreement with earlier freeze-etching measurements. In contrast, germinating spores had a very smooth surface partially covered with rough granular structures. Force-distance curve measurements demonstrated that the changes in spore surface ultrastructure during germination are correlated with profound modifications of molecular interactions: while dormant spores showed no adhesion with the AFM probe, germinating spores exhibited strong adhesion forces, of 9 +/- 2 nN magnitude. These forces are attributed to polysaccharide binding and suggested to be responsible for spore aggregation. This study represents the first direct characterization of the surface ultrastructure and molecular interactions of living fungal spores at the nanometer scale and offers new prospects for mapping microbial cell surface properties under native conditions. (+info)
Reversible alkaline inactivation of lignin peroxidase involves the release of both the distal and proximal site calcium ions and bishistidine co-ordination of the haem.
Phanerochaete chrysosporium lignin peroxidase isoenzyme H2 (LiP H2) exhibits a transition to a stable, inactive form at pH 9.0 with concomitant spectroscopic changes. The Soret peak intensity decreases some 55% with a red shift from 408 to 412 nm; the bands at 502 nm and 638 nm disappear and the peak at 536 nm increases. The EPR spectrum changes from a signal typical of high spin ferric haem to an exclusively low spin spectrum with g=2.92, 2.27, 1.50. These data indicate that the active pentaco-ordinated haem is converted into a hexaco-ordinated species at alkaline pH. Room temperature near-IR MCD data coupled with the EPR spectrum allow us to assign the haem co-ordination of alkali-inactivated enzyme as bishistidine. Re-acidification of the alkali-inactivated enzyme to pH 6 induces further spectroscopic changes and generates an irreversibly inactivated species. By contrast, a pH shift from 9.0 to 6.0 with simultaneous addition of 50 mM CaCl(2) results in the recovery of the initial activity together with the spectroscopic characteristics of the native ferric enzyme. Incubating with 50 mM CaCl(2) at a pH between 6.0 and 9.0 can also re-activate the enzyme. Divalent metals other than Ca(2+) do not result in restoration of activity. Experiments with (45)Ca indicate that two tightly bound calcium ions per enzyme monomer are lost during inactivation and reincorporated during subsequent re-activation, consistent with the presence of two structural Ca(2+) ions in LiP H2. It is concluded that both the structural Ca(2+) ions play key roles in the reversible alkaline inactivation of LiP H2. (+info)
Peroxyl radicals are potential agents of lignin biodegradation.
Past work has shown that the extracellular manganese-dependent peroxidases (MnPs) of ligninolytic fungi degrade the principal non-phenolic structures of lignin when they peroxidize unsaturated fatty acids. This reaction is likely to be relevant to ligninolysis in sound wood, where enzymes cannot penetrate, only if it employs a small, diffusible lipid radical as the proximal oxidant of lignin. Here we show that a non-phenolic beta-O-4-linked lignin model dimer was oxidized to products indicative of hydrogen abstraction and electron transfer by three different peroxyl radical-generating systems: (a) MnP/Mn(II)/linoleic acid, (b) arachidonic acid in which peroxidation was initiated by a small amount of H(2)O(2)/Fe(II), and (c) the thermolysis in air of either 4,4'-azobis(4-cyanovaleric acid) or 2,2'-azobis(2-methylpropionamidine) dihydrochloride. Some quantitative differences in the product distributions were found, but these were attributable to the presence of electron-withdrawing substituents on the peroxyl radicals derived from azo precursors. Our results introduce a new hypothesis: that biogenic peroxyl radicals may be agents of lignin biodegradation. (+info)
Identification of catalytic residues in glyoxal oxidase by targeted mutagenesis.
Glyoxal oxidase is a copper metalloenzyme produced by the wood-rot fungus Phanerochaete chrysosporium as an essential component of its extracellular lignin degradation pathways. Previous spectroscopic studies on glyoxal oxidase have demonstrated that it contains a free radical-coupled copper active site remarkably similar to that found in another fungal metalloenzyme, galactose oxidase. Alignment of primary structures has allowed four catalytic residues of glyoxal oxidase to be targeted for site-directed mutagenesis in the recombinant protein. Three glyoxal oxidase mutants have been heterologously expressed in both a filamentous fungus (Aspergillus nidulans) and in a methylotrophic yeast (Pichia pastoris), the latter expression system producing as much as 2 g of protein per liter of culture medium under conditions of high density methanol-induced fermentation. Biochemical and spectroscopic characterization of the mutant enzymes supports structural correlations between galactose oxidase and glyoxal oxidase, clearly identifying the catalytically important residues in glyoxal oxidase and demonstrating the functions of each of these residues. (+info)