Laccases from Pleurotus ostreatus Applied to the Oxidation of Furfuryl Alcohol for the Synthesis of Key Compounds for Polymer Industry.

Laccases are oxidative enzymes with high synthetic potential. In this work, their value in biocatalysis is shown through the green and selective oxidation of furfuryl alcohol into furfural with the aid of mediators. The influence of different parameters, such as pH, enzyme/mediator composition, buffer type, cosolvent tolerance, and reaction times, is investigated. Under the optimal conditions, 20 mol % of TEMPO as mediator and 5.8 U mL-1 of laccases POXC and POXA1b from Pleurotus ostreatus, quantitative production of furfural is attained after 16 h. POXC laccase stands out for its ability to catalyze the reaction at pH 6.5, whereas POXA1b is notable for its high stability. Furfural conversions reach excellent values (95 %) after 72 h using only 5 mol % of TEMPO at 100 mM. Furthermore, furfuryl alcohol bioamination is achieved by employing the amine transaminase from Chromobacterium violaceum, providing furfuryl amine, a key compound for the polymer industry, through a one-pot sequential approach.

Laccase pretreatment for agrofood wastes valorization.

Apple pomace, potato peels, and coffee silverskin are attractive agrofood wastes for the production of biofuels and chemicals, due to their abundance and carbohydrate content. As lignocellulosic biomasses, their conversion is challenged by the presence of lignin that prevents hydrolysis of polysaccharides, hence demanding a pretreatment step. In this work, the effectiveness of Pleurotus ostreatus laccases (with and without mediator) to remove lignin, improving the subsequent saccharification, was assessed. Optimized conditions for sequential protocol were set up for all agrofood wastes reaching delignification and detoxification yields correlated with high saccharification. Especially noteworthy were results for apple pomace and coffee silverskin for which 83% of and 73% saccharification yields were observed, by using laccase and laccase mediator system, respectively. The herein developed sequential protocol, saving soluble sugars and reducing the amount of wastewater, can improve the overall process for obtaining chemicals or fuels from agrofood wastes.

Oxidoreductases on their way to industrial biotransformations.

Fungi produce heme-containing peroxidases and peroxygenases, flavin-containing oxidases and dehydrogenases, and different copper-containing oxidoreductases involved in the biodegradation of lignin and other recalcitrant compounds. Heme peroxidases comprise the classical ligninolytic peroxidases and the new dye-decolorizing peroxidases, while heme peroxygenases belong to a still largely unexplored superfamily of heme-thiolate proteins. Nevertheless, basidiomycete unspecific peroxygenases have the highest biotechnological interest due to their ability to catalyze a variety of regio- and stereo-selective monooxygenation reactions with H2O2 as the source of oxygen and final electron acceptor. Flavo-oxidases are involved in both lignin and cellulose decay generating H2O2 that activates peroxidases and generates hydroxyl radical. The group of copper oxidoreductases also includes other H2O2 generating enzymes - copper-radical oxidases - together with classical laccases that are the oxidoreductases with the largest number of reported applications to date. However, the recently described lytic polysaccharide monooxygenases have attracted the highest attention among copper oxidoreductases, since they are capable of oxidatively breaking down crystalline cellulose, the disintegration of which is still a major bottleneck in lignocellulose biorefineries, along with lignin degradation. Interestingly, some flavin-containing dehydrogenases also play a key role in cellulose breakdown by directly/indirectly "fueling" electrons for polysaccharide monooxygenase activation. Many of the above oxidoreductases have been engineered, combining rational and computational design with directed evolution, to attain the selectivity, catalytic efficiency and stability properties required for their industrial utilization. Indeed, using ad hoc software and current computational capabilities, it is now possible to predict substrate access to the active site in biophysical simulations, and electron transfer efficiency in biochemical simulations, reducing in orders of magnitude the time of experimental work in oxidoreductase screening and engineering. What has been set out above is illustrated by a series of remarkable oxyfunctionalization and oxidation reactions developed in the frame of an intersectorial and multidisciplinary European RTD project. The optimized reactions include enzymatic synthesis of 1-naphthol, 25-hydroxyvitamin D3, drug metabolites, furandicarboxylic acid, indigo and other dyes, and conductive polyaniline, terminal oxygenation of alkanes, biomass delignification and lignin oxidation, among others. These successful case stories demonstrate the unexploited potential of oxidoreductases in medium and large-scale biotransformations.

Low impact strategies to improve ligninolytic enzyme production in filamentous fungi: the case of laccase in Pleurotus ostreatus.

The ever-increasing demand of laccases for biodelignification, industrial oxidative processes and environmental bioremediation requires the production of large quantities of enzymes at low cost. The present work was carried out to reduce laccase production costs in liquid fermentations of the white-rot fungus Pleurotus ostreatus through two different approaches. In the first, screening of fungal spent media as natural laccase inducer was performed, eliminating the presence of potentially toxic/recalcitrant and expensive exogenous inducers in the culture broth. In the latter, breeding of different strains of P. ostreatus, screened for their laccase productivity, was performed by cross-hybridisation, avoiding genetic transformation and mutagenic treatments that could produce organisms not suitable for "natural or safe processes". A laccase production level close to 80,000U/L by combining the two approaches was achieved. Autoinduction and classical breeding represent promising tools for the improvement of fungal fermentation without affecting the disposable costs that also depend on the eco-compatibility of the whole process.

Fungal laccases: versatile tools for lignocellulose transformation.

Conversion of lignocellulosic materials to useful, high value products normally requires a pre-treatment step to transform or deconstruct the recalcitrant and heterogeneous lignin fraction. The development of "green tools" for the transformation of lignocellulosic feedstocks is in high demand for a sustainable exploitation of such resources. This multi-faceted challenge is being addressed by an ever-increasing suite of ligninolytic enzymes isolated from various sources. Among these, fungal laccases are known to play an important role in lignin degradation/modification processes. The white-rot fungus Pleurotus ostreatus expresses multiple laccase genes encoding isoenzymes with different properties. The availability of established recombinant expression systems for P. ostreatus laccase isoenzymes has allowed to further enrich the panel of P. ostreatus laccases by the construction of mutated, "better performing" enzymes through molecular evolution techniques. New oxidative catalysts with improved activity and stability either at high temperature and at acidic and alkaline pH have been isolated and characterized.