Heterophase Synthesis of Humic Substances at Low Substrate Concentrations and Flow-Through Conditions

Eurasian Soil Science - Humic substances (HS) are heterogeneous and polydisperse compounds formed in soils, sediments and waters during the decay and transformation of organic residues (the process...     

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.     

Biotransformation of soil humic acids by blue laccase of Panus tigrinus 8/18: an in vitro study

We describe here the role of the polyphenoloxidases in the oxidation of recalcitrant soil organic compounds and consider what changes occur in their structure during experiments on the biotransformation of soil and peat-derived humic acids (HA). These transformations were carried out by laccase (EC 1.10.3.2.) of the white-rot basidiomycete Panus tigrinus 8/18. It was shown that purified laccase alone is capable both of polymerizing and depolymerizing HA in vitro. The direction of transformations depends on the nature and properties of HA. Those fractions of HA are affected by laccase, which cause the lowest inhibitory effect on the enzyme. Contrary to previous studies depolymerization of HA was not necessarily accompanied by decolorization: chernozem-derived HA showed increase in absorbance of its aqueous solution in the region of 240-500 nm during depolymerization, while peat-derived HA showed decrease in absorbance throughout the entire spectrum during polymerization. All studied HA were competitive inhibitors of laccase towards oxidation of synthetic substrate 2,2′-azino-bis-(3-ethylthiazoline-6-sulfonate) (ABTS). When studying the nature of the inhibitory effect, it was shown that more ‘hydrophobic’ HA as well as more ‘hydrophobic’ HA fragments were stronger inhibitors of blue laccase.