Chapter 7 - Laccases—properties and applications

Abstract

The cost-effective transformation of nonfood lignocellulose could revolutionize agriculture and reshape the bioeconomy, while maintaining biodiversity, minimizing agriculture’s environmental footprint, and conserving fresh water. Lignin is the second most abundant naturally produced organic polymer on Earth. It is a highly complex, heterogeneous, three-dimensional aromatic polymer that is recalcitrant to biodegradation. In lignin, links with cellulose and hemicellulose create a barrier to lignocellulosic enzymes. Lignin-modifying enzyme (LME)-types of laccases are produced by the white-rot fungi as products of secondary metabolism, since lignin degradation does not provide any energy to the fungus. LME-types of laccases have also been reported for some species of bacteria. Laccases belong to the so-called blue-copper family of oxidases. The enzyme laccase catalyzes the oxidation of paradiphenols, aminophenols, polyphenols, polyamines, lignin, and aryl diamines, as well as some inorganic ions. During this reaction, one molecule of atmospheric oxygen is reduced to two molecules of water, so that the catalysis is taking place via transfer of four electrons per round of catalysis. Laccase is also able to catalyze the oxidation of nonphenolic lignin units (C4-esterified) to radicals, and during this reaction laccase acts via a mediator. The need for a capacity to produce large volumes of highly active enzyme can be met with the use of recombinant organisms or screening for natural hypersecretory strains. This chapter concludes with the properties and applications of laccases and implications for lignin degradation.