Inhibition of metal ions on Cerrena sp. laccase: Kinetic, decolorization and fluorescence studies

Abstract

Laccases (EC 1.10.3.2) have important industrial values in areas such as bioremediation, but they are often inactivated by heavy metal ions in real applications. In this report, laccase from a high laccase-producing Cerrena sp. HYB07 presented resistance to many metal ions except for Ag+, Hg2+, Li+ and Pb2+, the inhibition of which on the laccase were all reversible. The presence of these four cations decreased Remazol Brilliant Blue R decolorization efficiencies catalyzed by the laccase, and the mediator 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) could restore the decolorization efficiency in spite of Hg2+ inhibition. Hg2+ had the lowest IC50 value (0.17 mM) on the laccase activity, followed by Ag+, Li+ and Pb2+. The inhibition type on laccase activity was competitive for Pb2+, noncompetitive for Hg2+, and mixed type for Ag+ and Li+. The inhibition kinetic model of Hg2+ on laccase activity was established by using the kinetic method of substrate reaction. The microscopic forward inhibition rate constant (k+0) of Hg2+was 3.26 × 10−2/s and the microscopic reverse inhibition rate constant (k-0) was 1.36 × 10−3/s, indicating the laccase would be completely inhibited when Hg2+ concentration was sufficiently high because k+0 was much larger than k-0. Furthermore, Hg2+ reduced thermal and pH stability of the laccase. Fluorescence emission spectra demonstrated that Hg2+ had one binding site per laccase protein regardless of pH, and the binding was stronger at pH 6.0 than that at pH 5.0 or 9.0, in accordance with the lowest stability of the enzyme at pH 6.0 with Hg2+. Dynamic simulation was performed to identify the binding sites of each ion on the Lac7 protein.