Simple models with a single bead representation (C R models) have been successful in providing a qualitative understanding of the folding mechanism of small globular proteins. Can we go beyond this qualitative understanding and make more detailed quantitative connections to experiments? To achieve this goal, a tractable framework of protein representations whose complexity falls between C R and all-atom representations is needed to address different energetic competing factors during folding events. Such a model conserves the low computational expense inherent in minimalist models while enhancing the understanding of side-chain packing not existent in simple C R models. In this work, we present a minimalist representation of protein structures that are used to investigate the competition between native side-chain contacts and nonspecific backbone hydrogen bonds. Our results suggest that native tertiary contacts and dihedrals force the nonspecific hydrogen bonds to adopt native configurations and retain a funneled landscape. In addition, the use of an angular component in the hydrogen bond interaction prevents non-native conformations.

The structures of partially-folded states appearing during the folding of a (βα) 8 TIM barrel protein, the indole-3-glycerol phosphate synthase from S. solfataricus (sIGPS), was assessed by hydrogen exchange mass spectrometry (HX-MS) and Gō-model simulations. HX-MS analysis of the peptic peptides derived from the pulse-labeled product of the sub-millisecond folding reaction from the urea-denatured state revealed strong protection in the (βα) 4 region, modest protection in the neighboring (βα) 1-3 and (βα) 5 β 6 segments and no significant protection in the remaining N-and Cterminal segments. These results demonstrate that this species is not a collapsed form of the unfolded state under native-favoring conditions nor is it the native state formed via fast-track folding. However, the striking contrast of these results with the strong protection observed in the (βα) 2-5 β 6 region after 5 s of folding demonstrates that these species represent kinetically-distinct folding intermediates that are not identical as previously thought. A re-examination of the kinetic folding mechanism by chevron analysis of fluorescence data confirmed distinct roles for these two species: the burst-phase intermediate is predicted to be a misfolded, off-pathway intermediate while the subsequent 5 s intermediate corresponds to an on-pathway equilibrium intermediate. Comparison with the predictions using a C α Gō-model simulation of the kinetic folding reaction for sIGPS shows good agreement with the core of structure offering protection against exchange in the on-pathway intermediate (s). Because the native-centric Gō-model simulations do not explicitly include sequencespecific information, the simulation results support the hypothesis that the topology of TIM barrel proteins is a primary determinant of the folding free energy surface for the productive folding reaction. The early misfolding reaction must involve aspects of non-native structure not detected by the Gō-model simulation.

A point mutation, lysine 97 --> isoleucine (K97I), in a surface loop in the beta-sheet protein interleukin 1beta (IL-1beta), exhibits increased levels of inclusion body (IB) formation relative to the wild-type protein (WT) when expressed in Escherichia coli. Despite the common observation that less stable proteins are often found in IBs, K97I is more stable than WT. We examined the folding pathway of the mutant and wild-type proteins at pH 6.5 and 25 degrees C with manual-mixing and stopped-flow optical spectroscopy to determine whether changes in the properties of transiently populated species in vitro correlate with the observation of increased aggregation in vivo. The refolding reactions of the WT and K97I proteins are both described by three exponential processes. Two exponential processes characterize fast events (0.1-1.0 s) in folding while the third exponential process correlates with a slow (70 s) single pathway to and from the native state. The K97I replacement affects the earlier steps in the refolding pathway. Aggregation, absent in the WT refolding reaction, occurs in K97I above a critical protein concentration of 18 microM. This observation is consistent with an initial nucleation step mediating protein aggregation. Stopped-flow kinetic studies of the K97I aggregation process demonstrate that K97I aggregates most rapidly during the earliest refolding times, when unfolded protein conformers remain highly populated and the concentration of folding intermediates is low. Folding and aggregation studies together support a model in which the formation of stable folding intermediates afford protection against further K97I aggregation.