Hydrated proteins exhibit a broad dielectric loss extending over the frequency range from 1 MHz to 10 GHz which can be decomposed into a number of constituent dispersions. One of these dispersions with a relaxation time of approximately 18 ns has been attributed to the relaxation of protein backbone peptide groups in the protein interior. In the work reported here, this dielectric dispersion was investigated as a function of temperature for the enzyme glucose oxidase. In the low temperature region, the temperature-dependence of the dispersion magnitude showed a marked increase in gradient at a critical temperature indicating a transition from a relatively rigid to a more mobile protein structure. At higher temperatures, the response increased rapidly, reaching a maximum value at a second critical temperature. Glucose oxidase samples raised above this temperature showed significantly reduced enzyme activity. Both critical temperatures decreased with increasing protein water content. This is consistent with a scheme in which the hydrated glassy protein undergoes a change in structural mobility at the glass transition temperature and experiences an irreversible change in conformation at a higher denaturation temperature. Both glass transition and denaturation temperatures are key indicators of protein stability and are important in the production and storage of protein based pharmaceuticals.