IRREVERSIBLE THERMAL-DENATURATION OF TORPEDO-CALIFORNICA ACETYLCHOLINESTERASE

Thermal denaturation of Torpedo californica acetylcholinesterase, a di sulfide-linked homodimer with 537 amino acids in each subunit, was stu died by differential scanning calorimetry. It displays a single calori metric peak that is completely irreversible, the shape and temperature maximum depending on the scan rate. Thus, thermal denaturation of ace tylcholinesterase is an irreversible process, under kinetic control, w hich is described well by the two-state kinetic scheme N -->(k) D, wit h activation energy 131 +/- 8 kcal/mol. Analysis of the kinetics of de naturation in the thermal transition temperature range, by monitoring loss of enzymic activity, yields activation energy of 121 +/- 20 kcal/ mol, similar to the value obtained by differential scanning calorimetr y. Thermally denatured acetylcholinesterase displays spectroscopic cha racteristics typical of a molten globule state, similar to those of pa rtially unfolded enzyme obtained by modification with thiol-specific r eagents. Evidence is presented that the partially unfolded states prod uced by the two different treatments are thermodynamically favored rel ative to the native state.