Transformation of humic acids by two-domain laccase from Streptomyces anulatus

The properties of two-domain laccase of Streptomyces anulatus (SaSL) and its role in transformation of humic acids (HA) from chernozem, sod-podzolic soil and peat at alkaline pH values were studied. The SaSL was cloned, expressed in E. coil and obtained in an electrophoretically homogeneous state. SaSL is an oligomeric protein with a molecular weight of 235-300 kDa and six or eight subunits in the molecule. The molecular weight of the subunit is 37.7 kDa. Its catalytic properties are similar to those of the previously described two-domain laccase. The enzyme catalyzed oxidation of electron donors (K-4[Fe(CN)(6)], ABTS) at acidic pH and phenolic substrates (2-methoxyphenol, 2,6-dimethixyphenol) at alkaline pH values. The efficiency of catalysis was higher in the case of electron donors than phenolic substrates. The enzyme showed a high thermal stability and was more stable at neutral and alkaline pH values. The enzyme effectively transformed humic acids at alkaline pH values. It was found that polymerization reactions took place during interaction of SaSL with HA, as well as with their high molecular weight (> 80 kDa) and low-molecular weight (< 5 kDa) fractions. Our results suggest a possible involvement of the two-domaim laccases in httmification processes in alkaline soils.