The structure and function of fungal laccases

Laccase is a copper-containing polyphenol oxidase enzyme discovered in both plants and fungi, with one bacterial example known. This review examines fungal laccases, which are extracellular glycoproteins typically 60-80 kDa in size containing four copper atoms. The enzyme catalyzes one-electron oxidation of phenolic substrates coupled to four-electron reduction of oxygen to water, with substrate specificity varying among different laccases. Proposed functions include roles in lignin degradation by wood-rotting basidiomycetes, though evidence remains inconclusive since some effective lignin degraders lack laccase. Laccase functions in morphogenesis and pigment formation in several fungi, including involvement in fruiting body development in Agaricus bisporus and Schizophyllum commune. In plant pathogens like Botrytis cinerea and Cryphonectria parasitica, laccase contributes to pathogenesis. Structural studies reveal laccase similarity to plant ascorbate oxidases and mammalian ceruloplasmin, with conservation of copper-binding residues arranged in a trinuclear cluster. Gene sequences from five fungi show similar overall architecture but limited sequence identity, particularly regarding substrate specificity differences among organisms.

Key findings

• Laccase is a copper-containing oxidase found in fungi and some bacteria that catalyzes one-electron substrate oxidation coupled to oxygen reduction, with highly variable substrate specificity among different fungal sources
• The enzyme's role in lignin degradation remains unresolved despite evidence it can cleave lignin structures; some effective lignin-degrading fungi do not produce laccase, while laccase-deficient mutants show reduced ligninolytic ability
• Laccase functions in fungal morphogenesis and pigment synthesis, with regulated expression during fruiting body development in cultivated mushrooms and other fungi
• Purified fungal laccases are extracellular glycoproteins of 60-80 kDa containing four copper atoms arranged in a trinuclear cluster, showing structural homology to plant ascorbate oxidases and mammalian ceruloplasmin
• Laccase sequences from different fungi show conservation of copper-binding residues but limited overall sequence identity, possibly reflecting different substrate specificities and biological functions