Stability and Catalytic Kinetics of Horseradish Peroxidase Confined in Nanoporous SBA-15

We have synthesized nanoporous silica, SBA-15 in the 1 m size range with the pore diameter of 7.6 nm. The redox enzyme horseradish peroxidase (HRP) was entrapped in the pores to form nanostructured hybrid materials. The catalytic activity of free and immobilized enzyme was first compared at room temperature. Details of the enzyme kinetics including the apparent Michaelis constant (KM) and maximum rate (Vmax) were determined. Both thermal stability and stability, towards the denaturing agents guanidinium chloride and urea, of free and immobilized enzymes were compared next. The thermal stability of the immobilized enzyme is significantly improved in comparison with free HRP. The catalytic kinetics is slowed down notably, but Vmax is much more robust to heat than the free enzyme. The stability resistance of the enzyme towards the denaturing agents depends on the chemical nature of the denaturing agents and interactions between enzyme and silica nanopore walls. Guanidinium chloride showed similar attenuation of the catalytic activity of immobilized and free enzyme. In contrast, the immobilized HRP was much more resistant to urea than the free enzyme. The different behaviour of free and immobilized enzyme is most likely due to different hydrogen bonding of water and increased hydration strength of the protein inside the nanopores.