Evaluation of a silica-coated magnetic nanoparticle for the immobilization of a His-tagged lipase

Magnetic particles of size 10 nm have been coated with silica to a mean diameter of 40 nm and charged with Cu²⁺ ions via a multidentate ligand, iminodiacetic acid (IDA), for the immobilization of His-tagged Bacillus stearothermopilus L1 lipase. Microporous (average pore diameter of 60 Å) silica gel with a mean particle diameter of 115 μm has been used as a comparative support material. The molar ratio of Cu²⁺ to IDA was found to be 1:1.14 and 1:1.99 in the silica gel and the silica-coated magnetic nanoparticles (SiMNs), respectively. The specific activity of the immobilized enzyme was found to conform to the following order: Cu²⁺-charged SiMN>SiMN<Cu²⁺-charged silica gel>silica gel. When it was immobilized on the Cu²⁺-charged SiMNs, over 70% of the initial activity of the lipase remained after it had been reused five times. However, only 20% of the initial activity remained after the enzyme immobilized on the Cu²⁺-charged silica gel had been reused five times. For the enzyme immobilized on supports without Cu²⁺ cations, all activity was lost after threefold reuse. The differences in the specific activities and the efficiencies of reuse of the enzymes immobilized on the various support materials are discussed in terms of immobilization mechanisms (physical adsorption vs. coordination bonding), mass transfer of a substrate and a product of the enzyme reaction, and the status of the Cu (Cu bound to the IDA on the silica layer vs. Cu directly adsorbed on the silica layer).