Laccases: blue enzymes for green chemistry

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Laccases are oxidoreductases belonging to the multinuclear copper-containing oxidases; they catalyse the monoelectronic oxidation of substrates at the expense of molecular oxygen. Interest in these essentially ‘eco-friendly’ enzymes – they work with air and produce water as the only by-product – has grown significantly in recent years: their uses span from the textile to the pulp and paper industries, and from food applications to bioremediation processes. Laccases also have uses in organic synthesis, where their typical substrates are phenols and amines, and the reaction products are dimers and oligomers derived from the coupling of reactive radical intermediates. Here, we provide a brief discussion of this interesting group of enzymes, increased knowledge of which will promote laccase-based industrial processes in the future.

Introduction

Laccases are a group of oxidative enzymes whose exploitation as biocatalysts in organic synthesis has been neglected in the past, probably because they were not commercially available. The search for new, efficient and environmentally benign processes for the textile and pulp and paper industries has increased interest in these essentially ‘green’ catalysts, which work with air and produce water as the only by-product, making them more generally available to the scientific community. Consequently, a significant number of reports have been published in the past decade that have focused on the biochemical properties of these proteins and/or on their applications in technological and bioremediation processes in addition to their use in chemical reactions. Here, we provide a brief discussion of this interesting group of enzymes and their exploitation as biocatalysts, focusing particularly on their use in organic synthesis.

Section snippets

Laccases: what are they and how do they work?

Laccases (EC 1.10.3.2, p-diphenol:dioxygen oxidoreductase) belong to the so-called blue-copper family of oxidases. They are glycoproteins, which are ubiquitous in nature – they have been reported in higher plants and virtually every fungus that has been examined for them [1]. An enzyme of this group was first described by Yoshida at the end of the 19th century as a component of the resin ducts of the lacquer tree Rhus vernicifera [2]; more recently, proteins with features typical of laccases

Exploitation of laccases in manufacturing industries

Several applications of laccases have been proposed, which use one of the protocols described in Figure 2. Specifically, the direct substrate oxidation of phenol derivatives has been investigated in bioremediation efforts to decontaminate industrial wastewaters. The polymeric polyphenolic derivatives that result from the laccase-catalyzed oxidative couplings are usually insoluble and can be separated, easily, by filtration or sedimentation [10].

The selective removal of phenol derivatives is

Laccases and chemical mediators: mimicking nature

The structure of wood can be represented as an agglomerate of long, slim fibres (mainly celluloses and hemicelluloses) and of lignin, a complex polymer composed of hydroxyphenyl-propane subunits, linked together by various types of bonds. Lignin is present in the middle lamella, where it acts as a natural glue, and in the secondary cell wall of wood fibres, where it acts as a matrix and forms linkages between the cellulose and hemicellulose chains. To produce paper pulp, lignin must be

Exploitation of laccases as biocatalysts for chemical reactions

Laccase-based processes clearly have great potential in manufacturing industries. More recently, the information collected on these enzymes has interested organic chemists, whose efforts in this area of biocatalysis are aimed at developing new synthetic applications and to answer, at the molecular level, some open questions, such as what is the mechanism of action of laccases mediators? and what does the ‘ideal’ mediator structure look like?

What are the best reaction conditions for laccase enzymes?

The poor solubility in water of most potential laccase substrates has raised interest in the behaviour of these oxidases in non-conventional reaction media with restricted water content. In fact, one of the first – if not the first – examples of enzymatic catalysis in water–organic solvent biphasic systems described the oxidation of β-estradiol (Figure 5a, i) dissolved in ethyl acetate, catalyzed by a laccase from Polyporus versicolor dissolved in acetate buffer [59]. Additionally, several of

Conclusions

The goal of this review was to offer a brief presentation of what is known about an interesting group of oxidoreductases, the laccases, focusing specifically on the latest advances in their use as biocatalysts in organic synthesis. The increasing availability of these biocatalysts and the improved biochemical knowledge about these enzymes have been matched by a significant number of new and promising technological applications, whereby three industrial processes were using laccases at the end

Acknowledgements

Poul Poulsen's (NovozymesLtd) and Yves Galante's (Lamberti S.p.A.) useful comments on the industrial applications of laccases are gratefully acknowledged.

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