Purification and characterization of a thermally stable yellow laccase from Daedalea flavida MTCC-145 with higher catalytic performance towards selective synthesis of substituted benzaldehydes

Abstract

A laccase from the culture filtrate of white rot fungus Daedalea flavida MTCC-145 has been purified and characterized. The method involved concentration of the culture filtrate by ultrafiltration and an anion exchange chromatography on diethylaminoethyl (DEAE) cellulose. The sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and native polyacrylamide gel electrophoresis (native PAGE) both gave single protein bands indicating that the enzyme preparation was pure. The molecular mass of the enzyme determined from SDS-PAGE analysis was 75.0 kDa. Purification fold was 21.5 while recovery of the enzyme activity was 11.52%. Using 2,6-dimethoxyphenol, diammonium salt of 2,2'-[azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid)] and 3,5-dimethoxy-4-hydroxybenzaldehyde azine as substrates, the Km, kcat, and k_cat/K_m values of the laccase were found to be 440 µM, 6.45 s^–1, 1.47 × 10⁴ M^–1 s^–1; 366 µM, 6.45 s^–1, 1.76 × 10⁴ M^–1 s^–1; and 226 µM, 6.45 s^–1, 2.85 × 10⁴ M^–1 s^–1, respectively. The pH and temperature optima were 4.5 and 50°C, respectively. The enzyme was most stable at pH 5.0 when exposed for 1 h. The purified laccase has yellow color and shows no absorption band around 610 nm characteristic of blue laccases. The enzyme transforms toluene and substituted toluenes to corresponding benzaldehyde and substituted benzaldehydes in the absence of mediator molecules with higher catalytic efficiency as compared to other known laccases.