Oxidation of phenolic compounds catalyzed by immobilized multi-enzyme systems with integrated hydrogen peroxide production

Abstract

Suicide inactivation of peroxidases by hydrogen peroxide is the major deterrent to using such biocatalysts in oxidative processes. This has been successfully addressed by the in situ generation of H₂O₂. In this study, we have developed a novel multi-enzyme biocatalyst that has been immobilized on agarose-type carriers to oxidize phenols using oxygen and formic acid as indirect oxidants. This original system couples the in situ production of H₂O₂ to the phenol oxidation via an enzymatic cascade that involved three different enzymes (formate dehydrogenase, NADH-oxidase and peroxidase) and two different redox cofactors: nicotinamide and flavin derivatives. The cascade reaction only works when enzymes are immobilized on the solid support since soluble enzymes are dramatically inactivated under the reaction conditions. The highest oxidation efficiency was achieved by combining two different solid biocatalysts: (1) formate dehydrogenase and NADH-oxidase co-immobilized onto agarose beads activated with glyoxyl groups and (2) peroxidase immobilized onto agarose beads as well but activated with boronate groups. Unlike conventional peroxidase-mediated oxidations with exogenous H₂O₂, this novel system enables the quantitative oxidation of phenol without the addition of H₂O₂. Furthermore, this novel system can use a broad range of redox cofactors with similar oxidative effectiveness. Therefore, this novel immobilized tri-enzyme system removes important pollutants such as hydroxylated aromatic derivatives (phenol, 4-aminophenol, 2,4-dichloro-phenol or α-naphthol) using formic acid and molecular oxygen as substrates. In addition, this system generates CO₂ as waste beyond the oxidized phenols that can be easily separated from the aqueous solution. The sustainability of this system is supported by an E-factor of 1.3 and an atom economy of 43%.