Dimeric and monomeric laccases of soil-stabilizing lichen Solorina crocea: Purification, properties and reactions with humic acids

Lichens form the dominant plant cover in extreme environments and participate in mineral weathering, fine-earth stabilization and primary accumulation of soil organic matter. However, biochemical role of lichens in soil processes has never been investigated. Recently, laccases and tyrosinases have been discovered in representatives of the order Peltigerales (Laufer et al., 2006a, b; Zavarzina and Zavarzin, 2006). Laccases from most species had unusually large molecular weights (Laufer et al., 2009). Together with oligomeric laccases, we have found monomeric enzymes in Solorina crocea and Peltigera aphthosa (Lisov et al., 2007). In the present work we have purified homodimeric (large) and monomeric (small) laccases of the soil-stabilizing lichen S. crocea, determined their physico-chemical and catalytic properties and studied their reactions with soil humic acids. Our results suggest that oligomeric nature of lichen laccases can be artifactual, because homodimeric laccase was transformed into the monomeric form following hydrophobic interaction chromatography. We hypothesize that large laccase consists of two monomeric enzymes, each of which is bound with additional hydrophobic component(s). Small laccase is similar in its properties to the laccases of basidiomycetes. It is more resistant to elevated temperature and storage than the large form, showed a higher oxidation potential, had different pH-optima in oxidizing substrates and was less inhibited by humic acids. Despite these differences, both laccases depolymerized and decolorized humic acids from soils at comparable rates, with small laccase being slightly more effective. This finding suggests that lichens have a potential to participate in transformation of soil organic matter.