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Laccase-mediated oxidation of small organics: bifunctional roles for versatile applications

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Highlights

  • The diverse laccase-catalyzed oxidation reactions with small organics are classified into two types: catabolic and anabolic.

  • These bifunctional actions are readily found in laccase-driven in vivo metabolic pathways.

  • Biological functions of the in vivo metabolisms can inspire discovery of new application areas for in vitro bifunctional reactions of laccases and small organics.

Laccases have been widely used in several biotechnological areas, including organic synthesis, bioremediation, and pulp/textile bleaching. In most applications, the enzymatic actions start with single-electron oxidation of small organics followed by formation of the corresponding radicals. These radicals are subsequently involved in either oxidative coupling (i.e., bond formation) or bond cleavage of target organics. These bifunctional actions – catabolic versus anabolic – are readily identifiable in in vivo metabolic processes involving laccases. Here, we characterize the bifunctionality of laccase-mediated oxidation of small organics and present the view that knowledge of the biological functions of these metabolic processes in vivo can illuminate potential biotechnological applications of this bifunctionality.

Section snippets

Laccases and their substrates

Laccases are copper-containing oxidases that perform the single-electron oxidation of substrates, such as phenols and aliphatic or aromatic amines, to the corresponding radicals at the expense of molecular oxygen. Redox actions of these enzymes are readily found in several biological groups, including prokaryotes, fungi, insects, and plants 1, 2. Depending on the species, laccases are known to be naturally involved in either synthetic or degradation processes. For instance, fungal laccases play

Dual roles of low-molecular-weight natural phenolics in metabolic processes involving laccases

Small phenolics are known to be key substrates for in vivo metabolic processes involving laccases. Depending on the biological species, phenoxyl radicals produced from laccase-catalyzed oxidations contribute to either morphogenesis via polymerization 5, 6, 16, 20, 21 or carbon recycling via depolymerization (Figure 2) 3, 4, 19, 22.

In vivo, laccase-catalyzed anabolic processes generally use low-molecular-weight phenolics as building blocks. Single-electron oxidation by laccases allows the

Lessons from nature offer insights into the bifunctionality of laccase-mediated oxidation of small organics

Laccases perform both anabolic and catabolic functions using natural phenolics as substrates. These bifunctional actions can be readily reproduced in vitro for biotechnological applications. Substrates for in vitro reactions can be extended to include non-natural phenolics and nitroxyl compounds in addition to natural phenolics 4, 9, 10, 11, 12. Laccase-catalyzed oxidation of small organics gives rise to the corresponding radicals, which are subsequently linked to synthetic or degradative

Biotechnological applications of laccase-catalyzed oxidation of small organics

The bifunctionality of laccase-catalyzed in vitro oxidation of small organics manifests as catabolic and anabolic pathways. It has been demonstrated that the bifunctionality of laccase-catalyzed small organics reactions allows for diverse applications (Figure 3). In this section, we provide specific examples showing how catabolic or anabolic reactions pave the way for laccase applications (Table 1).

Oxidative coupling through laccase-catalyzed reactions of small organics is mainly applicable to

Concluding remarks

The diverse laccase-catalyzed oxidation reactions with small organics are classified into two types: catabolic and anabolic. In catabolic pathways, the small organics act as laccase mediators, facilitating redox cycling between the organics and target compounds. Such laccase-mediator systems can be efficiently applied to fiber bleaching and recalcitrant pollutant removal. By contrast, anabolic pathways are based on oxidative coupling of small organics and give rise to adduct and polymeric

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012-0008787), and ‘The GAIA Project’ by Korea Ministry of Environment.

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