Production, characterization and Congo red dye decolourizing efficiency of a laccase from Pleurotus ostreatus MTCC 142 cultivated on co-substrates of paddy straw and corn husk

Abstract

A laccase produced by <i>Pleurotus ostreatus</i> MTCC 142 under solid-state fermentation using co-substrates of paddy straw and corn husk (1.5:1.5, g w/w) showed an activity of 2.54 U gds^-1. Laccase activity was determined spectrophotometrically using 0.5 mM 2,2'- azino-bis (3-ethylbenzthiazoline-6-sulphonic acid) (ABTS). Supplementation with fructose and potassium nitrate resulted in maximum enzyme production at initial pH 5.8 ± 0.2 and initial moisture content of 70%. A carbon: nitrogen ratio of 0.5:0.1 yielded highest laccase activity in the presence of surfactant Tween 20 (0.05%, w/v). Incorporation of vanillin (5 mM) and copper sulphate (10 mM) facilitated enhanced synthesis of laccase. A 4.8-fold increase in enzyme activity was recorded after optimization of nutritional parameters. The apparent molecular mass of this enzyme was revealed as 43 kDa by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The laccase showed optimal activity at pH 3 and 35 °C with 82.8% residual activity after 1 h of incubation. The <i>K</i> _m and <i>V</i> _max values on ABTS were found to be 0.52 mM and 9.33 U gds^-1, respectively. The enzyme activity was enhanced by Cu²⁺ and remained unaffected with Ba²⁺, Mn²⁺, Pb²⁺, Mg²⁺, Ca²⁺ and Fe³⁺. However, pre-incubation of the enzyme with reagents like sodium azide, sodium lauryl sulphate and 2-mercaptoethanol demonstrated an inhibition of its activity. Addition of crude laccase to Congo red dye solution resulted in 36.84% decolourization after 20 h of incubation at 35 ± 2 °C. This study discusses the production and characterization of a laccase from <i>P. ostreatus</i> strain with potential for azo dye decolourization.