FIG 6.

Chelator titration of the rate of inactivation of the WT and D818N/D820N mutant at 95°C. The enzymes (2.0 μg/ml) were incubated in buffer A containing different concentrations of EGTA or EDTA at 95°C. Residual activity toward azocasein was measured at appropriate time intervals, as described in Materials and Methods. Half-lives for thermal inactivation of the enzymes are plotted as a function of the negative log EGTA or EDTA concentration using the four-parameter logistic nonlinear regression model. The values are expressed as means ± SDs (bars) of the results of three independent experiments.