Stability of wild-type and mutant TyrH activity at 50°C. The proteins were incubated at 50°C in 100 mM KCl, 10% glycerol, 50 mM HEPES, pH 7.0; at the times indicated in the figure an aliquot was taken and the residual TyrH activity was determined as described in the Materials and Methods. Wild-type (closed squares), T245P (triangles), T283M (closed circles), R306H (open squares), and T463M (open circles) TyrH. The activity is expressed as percentage of the initial activity. The lines are from fits of the data to Equation (2).