Exploring the Effect of Cu2+ on Sludge Hydrolysis and Interaction Mechanism between Cu2+ and Xylanase by Multispectral and Thermodynamic Methods

Abstract

Although hydrolysis of sludge can be improved by enzymes, it not only depends on the nature and concentration of hydrolase but also rests with factors affecting the activity of hydrolase, such as heavy metals. In this research, the impact of Cu2+ on sludge hydrolysis on xylanase is studied with respect to the concentration and components of soluble organic matter in sludge using three-dimensional fluorescence spectra. Results showed that Cu2+ exposure not only inhibited the hydrolysis of sludge due to the denaturation of xylanase but also affected the components of soluble organic matter in sludge. In order to illuminate the interaction mechanism between Cu2+ and xylanase, UV–Vis, steady-state fluorescence, circular dichroism, synchronous fluorescence, light scattering spectra, and isothermal titration calorimetry techniques were applied. The results show that Cu2+ spontaneously interacts with xylanase by a hydrophobic bond, hydrogen bond, or van der Waals force with two binding sites in the activity region of xylanase. This interaction not only causes a looser skeleton structure of xylanase and a more hydrophobic microenvironment of tyrosine and tryptophan residues but also leads to the static quenching of fluorophore due to the formation of complexes (xylanase-Cu2+). This work establishes a new strategy to investigate the interaction between enzymes and heavy metals at a molecular level, which is helpful for clarifying the bioactivities of heavy metals.