Purification, molecular characterization and reactivity with aromatic compounds of a laccase from basidiomycete Trametes sp. strain AH28-2

Abstract.

A recently isolated basidiomycete, Trametes sp. strain AH28-2, can be induced to produce a high level of laccases when grown on a cellobiose-asparagine liquid medium. After induction by kraft lignin, two major isozymes were detected in the fermentation supernatant of the fungus. The principal component laccase A, which accounts for about 85% of the total activity, can be purified to electrophoretic homogeneity by three chromatographic steps: DEAE-Sepharose FF, Superdex-200 and Mono-Q. The solution containing purified laccase is blue in color, and the ratio of absorbance at 280 nm to that at 600 nm is 22. The molecular mass of laccase A is estimated to be 62 kDa by SDS-PAGE, 57 kDa by FPLC, and measured as 58,522 Da by MALDI mass spectrum. Laccase A is a monomeric glycoprotein with a carbohydrate content of 11–12% and an isoelectric point of 4.2. The optimum pH and temperature for oxidizing guaiacol are 4.5 and 50°C, respectively. The half-life of the enzyme at 75°C is 27 min. The enzyme shows a good stability from pH 4.2 to pH 8.0. The K _m values of the enzyme toward substrates 2,2′-azino-bis (3-ethylbenzothazoline-6-sulfonate) (ABTS), guaiacol and 2,6-dimethoxyphenol are 25, 420 and 25.5 µM, respectively, and the corresponding V _max values are 670, 66.8, and 79 µM min^–1 mg^–1, respectively. Laccase A activity is strongly inhibited by 0.1 mM NaN₃ or 0.1 mM cyanide. Two units of laccase A alone is able to completely oxidize 100 µmol 2,6-chlorophenol in 6 h. In the presence of 1 mM ABTS and 1-hydroxybenzotriazole, 15.0 U laccase A is able to oxidize 45% and 70% of 50 µmol fluorene in 12 and 18 h, respectively. The laccase A gene was cloned by a PCR method, and preliminary analysis of its sequence indicates 87.0% similarity to the corresponding segment in the phenoloxidase gene from Coriolus hirsutus.