Protein Folding, Stability, and Denaturation (1A) Practice Test

Researchers monitored thermal unfolding of a monomeric enzyme by circular dichroism at 222 nm while heating from 20°C to 90°C in buffers of varying pH. The apparent melting temperature ($T_m$) was defined as the midpoint of the unfolding transition. Measured $T_m$ values were: pH 5.0: 64°C; pH 7.0: 72°C; pH 9.0: 62°C. Based on the data, which inference about protein stability is most supported?

A soluble enzyme shows maximal activity at 37°C. When incubated at 65°C for 5 minutes, activity drops to 5%. If the 65°C-treated sample is cooled and supplemented with a molecular chaperone system plus ATP, activity returns to 40% over 30 minutes. No proteolysis is detected. Which statement is most consistent with the experiment?