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

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.

Amperometric phenol biosensor based on a thermostable phenol hydroxylase

Phenol hydroxylase (EC 1.14.13.7) was produced using Bacillus stearothermophilus in a 5-1 batch fermentation leading to approximately 17 units after 6 h. The partially purified phenol hydroxylase was entrapped in a sol-gel matrix. The enzyme-loaded silica gel was attached to the sensitive top of a Clark-type oxygen electrode and its application as a phenol biosensor was tested. There was linearity between the maximal rate of oxygen consumption and phenol concentration in the range between 2.5 and 400 microM at 40 degrees C and pH 7.6. The signal could be read off after 10 s at a concentration of 400 microM phenol. The sensor lost 20% of its activity within 7 days. Para-substituted phenols were not detectable.