Production of hydroxyl radical by the synergistic action of fungal laccase and aryl alcohol oxidase.
ABSTRACT A mechanism for the production of hydroxyl radical (*OH) during the oxidation of hydroquinones by laccase, the ligninolytic enzyme most widely distributed among white-rot fungi, has been demonstrated. Production of Fenton reagent (H2O2 and ferrous ion), leading to *OH formation, was found in reaction mixtures containing Pleurotus eryngii laccase, lignin-derived hydroquinones, and chelated ferric ion. The semiquinones produced by laccase reduced both ferric to ferrous ion and oxygen to superoxide anion radical (O2*-). Dismutation of the latter provided the H2O2 for *OH generation. Although O2*- could also contribute to ferric ion reduction, semiquinone radicals were the main agents accomplishing the reaction. Due to the low extent of semiquinone autoxidation, H2O2 was the limiting reagent in Fenton reaction. The addition of aryl alcohol oxidase and 4-methoxybenzyl alcohol (the natural H2O2-producing system of P. eryngii) to the laccase reaction greatly increased *OH generation, demonstrating the synergistic action of both enzymes in the process.
Article: Biodegradation of lignocellulosics: microbial, chemical, and enzymatic aspects of the fungal attack of lignin.[show abstract] [hide abstract]
ABSTRACT: Wood is the main renewable material on Earth and is largely used as building material and in paper-pulp manufacturing. This review describes the composition of lignocellulosic materials, the different processes by which fungi are able to alter wood, including decay patterns caused by white, brown, and soft-rot fungi, and fungal staining of wood. The chemical, enzymatic, and molecular aspects of the fungal attack of lignin, which represents the key step in wood decay, are also discussed. Modern analytical techniques to investigate fungal degradation and modification of the lignin polymer are reviewed, as are the different oxidative enzymes (oxidoreductases) involved in lignin degradation. These include laccases, high redox potential ligninolytic peroxidases (lignin peroxidase, manganese peroxidase, and versatile peroxidase), and oxidases. Special emphasis is given to the reactions catalyzed, their synergistic action on lignin, and the structural bases for their unique catalytic properties. Broadening our knowledge of lignocellulose biodegradation processes should contribute to better control of wood-decaying fungi, as well as to the development of new biocatalysts of industrial interest based on these organisms and their enzymes.International Microbiology 10/2005; 8(3):195-204. · 1.80 Impact Factor
Article: Author's personal copy[show abstract] [hide abstract]
ABSTRACT: a b s t r a c t Preparation of sol–gel derived materials with tailored porosity is a goal in fields such as catalysis, adsor-bents, separation media and sensors. The aim of this work was to study the simultaneous effects of pH (8–10), temperature (295–314 K), and ethanol:TEOS (6:1–12:1) and water:TEOS (2:1–6:1) molar ratios on the textural and structural properties of xerogels synthesized in alkaline media. We applied a 2 4 fac-torial design with five replicates of the central point to evaluate the effect of each independent variable and their interactions. Samples were characterized by XRD, SEM, TEM, FTIR, mercury porosimetry and gas adsorption (N 2 at 77 K and CO 2 at 273 K). The structural bonds in xerogels were similar; the siloxane microstructure did not change as a function of concentration of reagents and temperature. All xerogels were amorphous and mesoporous. Higher ethanol:TEOS molar ratios yielded xerogels with a higher spe-cific surface area, lower pore volume and average pore size, and narrower pore size distribution. On the other hand, increasing the water:TEOS molar ratio produced xerogels with a lower specific surface area and higher meso, macro and total pore volumes.Microporous and Mesoporous Materials 10/2008; · 3.29 Impact Factor
Article: Effect of pH and oxalate on hydroquinone-derived hydroxyl radical formation during brown rot wood degradation.[show abstract] [hide abstract]
ABSTRACT: The redox cycle of 2,5-dimethoxybenzoquinone (2,5-DMBQ) is proposed as a source of reducing equivalent for the regeneration of Fe2+ and H2O2 in brown rot fungal decay of wood. Oxalate has also been proposed to be the physiological iron reductant. We characterized the effect of pH and oxalate on the 2,5-DMBQ-driven Fenton chemistry and on Fe3+ reduction and oxidation. Hydroxyl radical formation was assessed by lipid peroxidation. We found that hydroquinone (2,5-DMHQ) is very stable in the absence of iron at pH 2 to 4, the pH of degraded wood. 2,5-DMHQ readily reduces Fe3+ at a rate constant of 4.5 x 10(3) M(-1)s(-1) at pH 4.0. Fe2+ is also very stable at a low pH. H2O2 generation results from the autoxidation of the semiquinone radical and was observed only when 2,5-DMHQ was incubated with Fe3+. Consistent with this conclusion, lipid peroxidation occurred only in incubation mixtures containing both 2,5-DMHQ and Fe3+. Catalase and hydroxyl radical scavengers were effective inhibitors of lipid peroxidation, whereas superoxide dismutase caused no inhibition. At a low concentration of oxalate (50 micro M), ferric ion reduction and lipid peroxidation are enhanced. Thus, the enhancement of both ferric ion reduction and lipid peroxidation may be due to oxalate increasing the solubility of the ferric ion. Increasing the oxalate concentration such that the oxalate/ferric ion ratio favored formation of the 2:1 and 3:1 complexes resulted in inhibition of iron reduction and lipid peroxidation. Our results confirm that hydroxyl radical formation occurs via the 2,5-DMBQ redox cycle.Applied and Environmental Microbiology 11/2003; 69(10):6025-31. · 3.83 Impact Factor