Purification and characterization of a secreted laccase of Gaeumannomyces graminis var. tritici

Abstract

We purified a secreted fungal laccase from filtrates of Gaeumannomyces graminis var. tritici cultures induced with copper and xylidine. The active protein had an apparent molecular mass of 190 kDa and yielded subunits with molecular masses of 60 kDa when denatured and deglycosylated. This laccase had a pI of 5.6 and an optimal pH of 4.5 with 2,6-dimethoxyphenol as its substrate. Like other, previously purified laccases, this one contained several copper atoms in each subunit, as determined by inductively coupled plasma spectroscopy. The active enzyme catalyzed the oxidation of 2,6- dimethoxyphenol (K(m) = 2.6 x 10-5 ± 7 x 10-6 M), catechol (K(m) = 2.5 x 10-4 ± 1 x 10-5 M), pyrogallol (K(m) = 3.1 x 10-4 ± 4 x 10-5 M), and guaiacol (K(m) = 5.1 x 10-4 ± 2 x 10-5 M). In addition, the laccase catalyzed the polymerization of 1,8-dihydroxynaphthalene, a natural fungal melanin precursor, into a high-molecular-weight melanin and catalyzed the oxidation, or decolorization, of the dye poly B-411, a lignin-like polymer. These findings indicate that this laccase may be involved in melanin polymerization in this phytopathogen's hyphae and/or in lignin depolymerization in its infected plant host.