Exploring the Folding Mechanism of Dimeric Superoxide Dismutase.

The Cu/Zn Human Superoxide Dismutase (SOD1) is a dimeric metalloenzyme whose genetic mutations are directly related to amyotrophic lateral sclerosis (ALS), so understanding its folding mechanism is of fundamental importance. Currently, the SOD1 dimer formation is studied via molecular dynamics simulations using a simplified structure-based model and an all-atom model. Results from the simplified model reveal a mechanism dependent on distances between monomers, which are limited by constraints to mimic concentration dependence. The stability of intermediates (during the int state) is significantly affected by this distance, as well as by the presence of two folded monomers prior to dimer formation. The kinetics of interface formation are also highly dependent on the separation distance. The folding temperature of the dimer is about 4.2% higher than that of the monomer, a value not too different from experimental data. All-atom simulations on the apo dimer give binding free energy between monomers similar to experimental values. An intermediate state is evident for the apo form at a separation distance between monomers slightly larger than the native distance which has little formed interface between monomers. We have shown that this intermediate is stabilized by non-native intra- and intercontacts.

Exploring Folding Aspects of Monomeric Superoxide Dismutase.

Recent studies have associated the absence of bound metals (Apo protein) and mutations in Cu-Zn Human Superoxide Dismutase (SOD1) with amyotrophic lateral sclerosis (ALS) disease, suggesting mechanisms of SOD1 aggregation. Using a structure-based model and modifying the energy of interaction between amino acids in the metal-binding site, we detected differences between the folding of the apo and holo proteins. The presence of metal ions decreases the free-energy barrier and also suggests that the folding pathway may change to reach the native state. The kinetics of folding of the apo and holo forms also correlates with the amount of free-energy barrier in the folding process. Also, the stability of the native state is significantly affected by the absence of metal ions. Our results, obtained from a very simplified model, correlate with more detailed studies, which also have shown that the transition and the native states are affected by the absence of the metal ions, hindering the folding of SOD1 and decreasing the stability of the native state. Regarding the disulfide bond, the results show that its absence decreases the stability of the native structure but affects the transition state less, suggesting that it is possibly made late in the folding process.

pH and Charged Mutations Modulate Cold Shock Protein Folding and Stability: A Constant pH Monte Carlo Study.

The folding and stability of proteins is a fundamental problem in several research fields. In the present paper, we have used different computational approaches to study the effects caused by changes in pH and for charged mutations in cold shock proteins from Bacillus subtilis (Bs-CspB). First, we have investigated the contribution of each ionizable residue for these proteins to their thermal stability using the TKSA-MC, a Web server for rational mutation via optimizing the protein charge interactions. Based on these results, we have proposed a new mutation in an already optimized Bs-CspB variant. We have evaluated the effects of this new mutation in the folding energy landscape using structure-based models in Monte Carlo simulation at constant pH, SBM-CpHMC. Our results using this approach have indicated that the charge rearrangements already in the unfolded state are critical to the thermal stability of Bs-CspB. Furthermore, the conjunction of these simplified methods was able not only to predict stabilizing mutations in different pHs but also to provide essential information about their effects in each stage of protein folding.