Isolation and Study of Some Properties of Laccase from the Basidiomycetes Cerrena maxima

A new strain producing extracellular laccase (Cerrena maxima 0275) was found by screening of isolates of Basidiomycetes, and the dynamics of laccase biosynthesis by this strain was studied. The enzyme was purified to homogeneity. The molecular weight of the enzyme is 57 kD, and its pI is 3.5. The activity is constant at pH values in the range 3.0-5.0. The temperature optimum for activity is 50°C. The thermal stability of the laccase was studied. The catalytic and Michaelis constants for catechol, hydroquinone, sinapinic acid, and K₄ Fe(CN)₆ were determined. The standard redox potential of type 1 copper in the enzyme is 750 ± 5 mV. Thus, the investigated laccase is a high redox potential laccase.     

Isolation and Characterization of Humin-Like Substances Produced by Wood-Degrading White Rot Fungi

Three samples of high-molecular-weight humin-like substances were obtained by solid-phase cultivation of Coriolus hirsutus and/or Cerrena maxima on oat straw. The yield of humin-like substances amounted to 1.38–2.26% of the weight of the plant substrate consumed. These substances, produced both by individual and mixed cultures of the basidiomycetes, were shown to be similar in their structure and physicochemical properties. According to the data of IR and ¹³C-NMR spectroscopy, the substances contained aromatic fragments and were close to soil humic acids. Studies of the dynamics of laccase production suggested that the humin-like substances were produced via direct degradation of lignin macromolecules with direct involvement of extracellular laccase.     

Fungal Decomposition of Oat Straw during Liquid and Solid-State Fermentation

White rot fungi (Coriolus hirsutus, Coriolus zonatus, and Cerrena maxima from the collection of the Komarov Botanical Institute of the Russian Academy of Sciences) and filamentous fungi (Mycelia sterilia INBI 2-26 and Trichoderma reesei6/16) were grown on oat straw–based liquid and solid media, as well as in a bench-scale reactor, either individually or as cocultures. All fungi grew well on solid agar medium supplemented with powdered oat straw as the sole carbon source. Under these conditions, the mold Trichoderma reesei fully suppressed the growth of all basidiomycetes studied; conversely,Mycelia sterilia neither affected the development of any of the cultures, nor did it show any substantial susceptibility to suppression by their presence. Pure solid cultures of basidiomycetes, as well as the coculture of Coriolus hirsutus andCerrena maxima,caused a notable bleaching of the oat straw during its consumption. When grown on the surface of oat straw–based liquid medium, the basidiomycetes consumed up to 40% of the polysaccharides without measurable lignin degradation (a concomitant process). Under these conditions, Mycelia sterilia decomposed no more than 25% of the lignin in 60 days, but this was observed only after polysaccharide exhaustion and biomass accumulation. In contrast, during solid-state straw fermentation, white rot fungi consumed up to 75% of cellulose and 55% of lignin in 83 days (C. zonarus), whereas the corresponding consumption levels for cocultures ofMycelia sterilia and Trichoderma reesei equaled 70 and 45%, respectively (total loss of dry weight ranged from 55 to 60%). Carbon dioxide–monitored solid-state fermentation of oat straw by the coculture of filamentous fungi was successfully performed in an aerated bench-scale reactor.