Thermal stability of glucose oxidase from Penicillium adametzii

The thermal stability of glucose oxidase was studied at temperatures between 50 and 70 degreesC by kinetic and spectroscopic (circular dichroism) methods. The stability of glucose oxidase was shown to depend on the medium pH, protein concentration, and the presence of protectors in the solution. At low protein concentrations (< 15 mug/ml) and pH > 5.5, the rate constants k(in), s(-1), for thermal inactivation of glucose oxidase were high. Circular dichroic spectra suggested an essential role of beta structures in stabilizing the protein globule. At a concentration of 15 mug protein/ml, the activation energy E-A of thermal inactivation of glucose oxidase in aqueous solution was estimated at 79.1 kcal/mol. Other thermodynamic activation parameters estimated at 60 degreesC had the following values: DeltaH = 78.4 kcal/mol, DeltaG = 25.5 kcal/mol, and DeltaS = 161.9 entropy units. The thermal inactivation of glucose oxidase was inhibited by KCl, polyethylene glycols, and polyols. Among polyols, the best was sorbitol, which stabilized glucose oxidase without affecting its activity. Ethanol, phenol, and citrate exerted destabilizing effects.