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.

This work has been funded by the INDOX European project (KBBE-2013-7-613549), together with the BIO2014-56388-R and AGL2014-53730-R projects of the Spanish Ministry of Economy and Competitiveness (MINECO) co-financed by FEDER funds, and the BBI JU project EnzOx2 (H2020-BBI-PPP-2015-2-720297). The work conducted by the US DOE JGI was supported by the Office of Science of the US DOE under contract number DE-AC02-05CH11231. The authors thank other members of their groups at CIB-CSIC, Novozymes, Technical University of Dresden, JenaBios, University of Naples Federico II, Setas Kimya Sanayy, Wageningen University & Research, Anaxomics, Chiracon, BOKU, Delft University of Technology, INRABBF, Biopolis, Cheminova, CLEA, Solvay, IRNAS-CSIC and ICP-CSIC for their significant contributions to the results presented.

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