Thermodynamic Characterization of Stability

The activities of two enzymes as a function of temperature are shown in Table 12.1 and Fig. 12.10. In the lower temperature range, increases in temperature lead to increases in the activity of the enzymes, since the rate of a reaction increases with temperature. However, since enzymes are proteins, higher temperatures also lead to protein denaturation. A consequence of these two competing processes is the existence of a temperature optimum. At temperatures below the optimum, an activation of the reaction 

TABLE 12.1 Relative Activity of Two Enzymes as a Function of Temperature 

The fractional activity of the native enzymes (/n) can be calculated from activity data using Eq. (12.27) (Fig. 12.11). The denaturation midpoint temperature (T ) corresponds to the temperature at which half of the enzyme has lost activity. As can be appreciated in Fig. 12.11, the of enzyme A is lower than that of enzyme B. This could be interpreted as enzyme B being more thermostable than enzyme A. 

The equilibrium constant of denaturation Ko) can easily be calculated from fractional activity data using Eq. (12.25). Changes in as a function of temperature are shown in Fig. 12.12 and the corresponding 

TABLE 12.2 Changes in the Relative Activity of an Enzyme as a Function of Time at Various Temperatures 

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