ABSTRACT
Formaldehyde dehydrogenase (FDH, molecular size of 8.6 nm × 8.6 nm × 19.0 nm) was immobilized on seven types of mesoporous silica (MPS), whose pores were from 2.4 to 31.2 nm sized, and their catalytic activities were evaluated by oxidation of formaldehyde. Among MPSs, FDH immobilized on MPS-4 (pore size of 12.3 nm) showed the best catalytic activity. Enhancement of catalytic activity was obtained by immobilizing onto MPS, whose pore (mesopore) size was similar to the molecular size of FDH. In addition, FDH was immobilized on five types of organo-functionalized MPS-4. Results from assays of enzyme activity showed that FDH immobilized on phenyl-functionalized MPS-4 (MPS-4-Ph) had higher activity than FDH immobilized on non-functionalized one. Immobilized FDH on MPS-4-Ph was active for low formaldehyde concentration form 6.0 µM and more sensitive than conventional formaldehyde detectors. Stability of FDH activity was also evaluated under the various conditions, in which protein denaturation could occur by solvent treatment, such as methanol or sodium dodecyl sulfate. As a result, FDH stability was strongly improved by the immobilization on MPS materials. Further investigation using tryptophan fluorescence and circular dichroism (CD) indicated that the high-order structure of the FDH did not alter upon binding to the non-functionalized MPS surface. On the other hand, FDH immobilized on functionalized-MPS was changed by hydrophobic interaction or covalent binding. Consequently, substrate affinity was improved by the change in the structure of FDH and then the orientation of the active site.