Thermal inactivation of recombinant wild-type and P5′N-1 mutant enzymes. (A) Thermal stability of wild-type and mutant enzymes at 46°C. Aliquots were collected at intervals for measuring residual activity. (B) Plot of the residual activities at 5 minutes versus temperatures. Each enzyme was subjected to heat inactivation in a range of temperature from 26°C to 60°C. After 5 minutes of incubation at a given temperature, the enzyme sample was chilled and the residual activity measured. Residual activity was expressed as percentage of initial activity. • indicates wild type; □, D87V; ▴, L131P; ♦, N179S; ▿, G230R. The solid line differentiates the wild-type from mutants (dashed lines).