Nucleation, Growth, and Activation Energies for Seeded and Unseeded Aggregation of α-Chymotrypsinogen A

The intrinsic time scales for nonnative aggregate nucleation () and chain growth () were determined for α-chymotrypsinogen A as a function of temperature under acidic conditions where the resulting aggregates do not appreciably condense. Previous results (Andrews and Roberts (2007) Biochemistry 46, 7558) indicated that the productincreases with increasing temperature but could not distinguishand. Separate experimental values ofandare reported here from two approaches based on either (i) combining unseeded monomer loss kinetics with static light scattering of the resulting aggregates or (ii) seeded monomer loss kinetics as a function of number concentration of seed. Values ofandfrom (i) and (ii) agree quantitatively, and indicate that nucleation has a large, negative effective activation energy (ca. −76 kcal/mol) while growth has at most a weak dependence on temperature. The results are consistent with a model in which nucleation requires significant conformational changes within a nonnative oligomer, beyond those for monomer unfolding. The results more generally illustrate the potential utility of approaches (i) and (ii) for quantitatively determining in vitro andvalues, as well as how the effects of seeding can be predicted purely from unseeded kinetics and static light scattering measurements prior to significant aggregate condensation.