Catalytic activity and stability of encapsulated oxidoreductases for biodegradation of organic pollutants

Abstract

The oxidoreductases are widely recognized for their capability to degrade organic pollutants. However, in order to enhance their biochemical stability and reusability to degrade organic pollutants especially in non-conventional solvents, the immobilization is absolutely necessary. The immobilization through encapsulation in sol-gel silica matrices was adopted based on several reports which indicated that enzymes such as lipase encapsulated in sol-gel silica matrices resulted enhanced their catalytic activity and stability. The oxidoreductases namely laccase was used exclusively in this study, while peroxidase and tyrosinase were used as comparisons. The catalytic activity of laccase and peroxidase was assayed using 2,6- dimethoxyphenol as a substrate, while the catalytic activity of the tyrosinase was assayed using L-tyrosine as a substrate. The resulted sol-gel laccase gave larger pore size (96.79 A) and strong intensity of the Fourier Transform Infrared (FTIR) spectra. The catalytic activity and stability of the sol-gel laccase and sol-gel peroxidase were proven to be significantly enhanced to 100* and 20 respectively; however the sol-gel tyrosinase was showed no measurable catalytic activity. No significant leaching of the encapsulated laccase was observed over time, suggesting that the laccase molecule were sterically confined in small pores. It was also found that the catalytic activity and stability of the sol-gel laccase could be enhanced in the presence of additives and the use of different precursors did not show any significant difference on the laccase catalytic activity. The sol-gel laccase yielded lower Km (0.72) and increased in Vmax (1779.36) compared to the free laccase. The present study also showed that the sol-gel laccase was catalytically active in non-conventional solvents (e.g. organic solvents and ionic liquids). The degradation of o-chorophenol as a model organic pollutant was also conducted in aqueous and non-conventional solvents. The High Performance Liquid Chromatography (HPLC) was used to determine the concentration of o-chlorophenol. The degree of degradation was measured by the decrease in the concentration of o-chlorophenol against its initial concentration. The results showed that the sol-gel laccase gave higher degradation than free laccase either in aqueous or non-conventional solvents. It was found that the degradation of o-chlorophenol in organic solvents either by the free or the sol-gel laccase required water and the reaction temperature significantly affected the biodegradation capability of the free and sol-gel laccase. It was also found that the sol-gel laccase could be used 10 times and fairly stable.