Kinetics and thermodynamics of free flavins and the flavin-based redox active site within glucose oxidase dissolved in solution or sequestered within a sol-gel-derived glass.

Abstract

We report on the steady-state and time-resolved fluorescence from the redox active site flavine adenine dinucleotides (FADs) that are bound to glucose oxidase (GOx) when this enzyme is dissolved in aqueous solution or sequestered within a sol-gel-derived glass. To the best of our knowledge, this represents the first report on the actual dynamics of an enzyme active site when the enzyme is part of a sol-gel-derived glass. The results from these experiments show that the "free" FAD intramolecular folding/unfolding kinetics are slowed 3-10-fold within the glass vs solution. The intramolecular exciplex formation event (i.e., excited-state FAD residue folding/unfolding) is completely arrested for the GOx-bound FAD if the enzyme is sequestered within a glass in the absence of glucose. This is significantly different from the behavior of GOx dissolved in solution. However, despite this difference in behavior, the GOx molecules that are sequestered within the glasses continue to function somewhat like GOx dissolved in aqueous solution if they are challenged with glucose. We also found that the GOx molecules do not leach from the glass and they exhibit rotational mobility that is only 2-fold less than GOx dissolved in aqueous solution at 20 degrees C. In aqueous solution or within these glasses, the enzyme pocket that hosts the FAD redox sites opens up by 25-30% when GOx is challenged with glucose. Finally, we present preliminary analytical results for film-based sol-gel-derived biosensors that contain GOx, L-amino acid oxidase or cholesterol oxidase wherein the intrinsic FAD fluorescence produces the analytical signal.