Early events in protein folding: Is there something more than hydrophobic burst?

Computational analysis of the effect of Gly100Ala mutation on the thermostability of SazCA

Maintaining the protein stability upon mutation is a challenging task in protein engineering. In the present computational study, we induced a single point Gly100Ala mutation in SazCA and examined the factors governing the stability and flexibility of the mutated form, and compared it to that of the wildtype using molecular dynamics simulations. We observed higher structural stability and lesser residual mobility in the mutated SazCA. Improved H-bonding due to Gly100Ala was observed. Ala100 was responsible for the increased helical contents in the mutated SazCA while Gly100 compromised the secondary structure contents in the wildtype. A strong network of salt bridges and high local ordering of the solvent molecules at the protein surface contributed to the enhanced stability of the mutated protein. Our simulations conclusively highlight Gly100Ala mutation as a step towards designing a more robust and thermostable SazCA.Communicated by Ramaswamy H. Sarma.

Molecular dynamics simulations identify the regions of compromised thermostability in SazCA

The present study examined the structure and dynamics of the most active and thermostable carbonic anhydrase, SazCA, probed using molecular dynamics simulations. The molecular system was described by widely used biological force-fields (AMBER, CHARMM22, CHARMM36, and OPLS-AA) in conjunction with TIP3P water model. The comparison of molecular dynamics simulation results suggested AMBER to be a suitable choice to describe the structure and dynamics of SazCA. In addition to this, we also addressed the effect of temperature on the stability of SazCA. We performed molecular dynamics simulations at 313, 333, 353, 373, and 393 K to study the relationship between thermostability and flexibility in SazCA. The amino acid residues VAL98, ASN99, GLY100, LYS101, GLU145, and HIS207 were identified as the most flexible residues from root-mean-square fluctuations. The salt bridge analysis showed that ion-pairs ASP113-LYS81, ASP115-LYS81, ASP115-LYS114, GLU144-LYS143, and GLU144-LYS206, were responsible for the compromised thermal stability of SazCA.

Impact of hydrodynamic interactions on protein folding rates depends on temperature

We investigated the impact of hydrodynamic interactions (HI) on protein folding using a coarse-grained model. The extent of the impact of hydrodynamic interactions, whether it accelerates, retards, or has no effect on protein folding, has been controversial. Together with a theoretical framework of the energy landscape theory (ELT) for protein folding that describes the dynamics of the collective motion with a single reaction coordinate across a folding barrier, we compared the kinetic effects of HI on the folding rates of two protein models that use a chain of single beads with distinctive topologies: a 64-residue α/β chymotrypsin inhibitor 2 (CI2) protein, and a 57-residue β-barrel α-spectrin Src-homology 3 domain (SH3) protein. When comparing the protein folding kinetics simulated with Brownian dynamics in the presence of HI to that in the absence of HI, we find that the effect of HI on protein folding appears to have a "crossover" behavior about the folding temperature. This means that at a temperature greater than the folding temperature, the enhanced friction from the hydrodynamic solvents between the beads in an unfolded configuration results in lowered folding rate; conversely, at a temperature lower than the folding temperature, HI accelerates folding by the backflow of solvent toward the folded configuration of a protein. Additionally, the extent of acceleration depends on the topology of a protein: for a protein like CI2, where its folding nucleus is rather diffuse in a transition state, HI channels the formation of contacts by favoring a major folding pathway in a complex free energy landscape, thus accelerating folding. For a protein like SH3, where its folding nucleus is already specific and less diffuse, HI matters less at a temperature lower than the folding temperature. Our findings provide further theoretical insight to protein folding kinetic experiments and simulations.

Conformational dynamics is more important than helical propensity for the folding of the all α-helical protein Im7

Im7 folds via an on-pathway intermediate that contains three of the four native α-helices. The missing helix, helix III, is the shortest and its failure to be formed until late in the pathway is related to frustration in the structure. Im7H3M3, a 94-residue variant of the 87-residue Im7 in which helix III is the longest of the four native helices, also folds via an intermediate. To investigate the structural basis for this we calculated the frustration in the structure of Im7H3M3 and used NMR to investigate its dynamics. We found that the native state of Im7H3M3 is highly frustrated and in equilibrium with an intermediate state that lacks helix III, similar to Im7. Model-free analysis identified residues with chemical exchange contributions to their relaxation that aligned with the residues predicted to have highly frustrated interactions, also like Im7. Finally, we determined properties of urea-denatured Im7H3M3 and identified four clusters of interacting residues that corresponded to the α-helices of the native protein. In Im7 the cluster sizes were related to the lengths of the α-helices with cluster III being the smallest but in Im7H3M3 cluster III was also the smallest, despite this region forming the longest helix in the native state. These results suggest that the conformational properties of the urea-denatured states promote formation of a three-helix intermediate in which the residues that form helix III remain non-helical. Thus it appears that features of the native structure are formed early in folding linked to collapse of the unfolded state.

α-helix to β-hairpin transition of human amylin monomer

The human islet amylin polypeptide is produced along with insulin by pancreatic islets. Under some circumstances, amylin can aggregate to form amyloid fibrils, whose presence in pancreatic cells is a common pathological feature of Type II diabetes. A growing body of evidence indicates that small, early stage aggregates of amylin are cytotoxic. A better understanding of the early stages of the amylin aggregation process and, in particular, of the nucleation events leading to fibril growth could help identify therapeutic strategies. Recent studies have shown that, in dilute solution, human amylin can adopt an α-helical conformation, a β-hairpin conformation, or an unstructured coil conformation. While such states have comparable free energies, the β-hairpin state exhibits a large propensity towards aggregation. In this work, we present a detailed computational analysis of the folding pathways that arise between the various conformational states of human amylin in water. A free energy surface for amylin in explicit water is first constructed by resorting to advanced sampling techniques. Extensive transition path sampling simulations are then employed to identify the preferred folding mechanisms between distinct minima on that surface. Our results reveal that the α-helical conformer of amylin undergoes a transformation into the β-hairpin monomer through one of two mechanisms. In the first, misfolding begins through formation of specific contacts near the turn region, and proceeds via a zipping mechanism. In the second, misfolding occurs through an unstructured coil intermediate. The transition states for these processes are identified. Taken together, the findings presented in this work suggest that the inter-conversion of amylin between an α-helix and a β-hairpin is an activated process and could constitute the nucleation event for fibril growth.

Diverse pathways of oxidative folding of disulfide proteins: underlying causes and folding models

The pathway of oxidative folding of disulfide proteins exhibits a high degree of diversity, which is manifested mainly by distinct structural heterogeneity and diverse rearrangement pathways of folding intermediates. During the past two decades, the scope of this diversity has widened through studies of more than 30 disulfide-rich proteins by various laboratories. A more comprehensive landscape of the mechanism of protein oxidative folding has emerged. This review will cover three themes. (1) Elaboration of the scope of diversity of disulfide folding pathways, including the two opposite extreme models, represented by bovine pancreatic trypsin inhibitor (BPTI) and hirudin. (2) Demonstration of experimental evidence accounting for the underlying mechanism of the folding diversity. (3) Discussion of the convergence between the extreme models of oxidative folding and models of conventional conformational folding (framework model, hydrophobic collapse model).

Studying submicrosecond protein folding kinetics using a photolabile caging strategy and time-resolved photoacoustic calorimetry

Kinetic measurement of protein folding is limited by the method used to trigger folding. Traditional methods, such as stopped flow, have a long mixing dead time and cannot be used to monitor fast folding processes. Here, we report a compound, 4-(bromomethyl)-6,7-dimethoxycoumarin, that can be used as a "photolabile cage" to study the early stages of protein folding. The folding process of a protein, RD1, including kinetics, enthalpy, and volume change, was studied by the combined use of a phototriggered caging strategy and time-resolved photoacoustic calorimetry. The cage caused unfolding of the photolabile protein, and then a pulse UV laser (∼10(-9) s) was used to break the cage, leaving the protein free to refold and allowing the resolving of two folding events on a nanosecond time scale. This strategy is especially good for monitoring fast folding proteins that cannot be studied by traditional methods.

Metadynamics as a tool for mapping the conformational and free-energy space of peptides--the alanine dipeptide case study

There is a need for a fast, accurate, and reliable method of sampling the conformational space of peptides and proteins in order to obtain a balanced free-energy profile which can lead to our understanding of protein structure. We have utilized metadynamics for the conformational study of the solvated alanine dipeptide molecule, and our results show that the method has proven to be competent as a fast, robust, and reliable method for the conformation free-energy calculations of peptides in an explicit solvent, surpassing traditional methods such as umbrella sampling. We have also addressed the issue of the influence of different water models on the resulting free-energy profile in order to consistently decompose the setting of our simulation. All of the explicit water models for the simulation of biomolecules TIP3P, TIP4P, TIP4P/Ew, TIP5P, and SPCE have exhibited similar effects on the conformational preferences of alanine dipeptide with no significant differences. On the other hand, by comparing the potential energy surface in the gas phase and the free-energy surface in a water environment, we have shown that the interaction with water molecules is one of the most important structure-driving elements, with a great influence on the free-energy surface (FES) of the solvated peptide and the conformational preferences of the peptide backbone. All of the tested force fields (ff03, ff99SB, opls-aa, and charmm27) appreciably differ in the population of the individual conformers and the barriers between them. Significant divergence was found on both the potential energy surface (PES) as well as free-energy surface (FES) calculated by charmm27. We have therefore concluded that the differences originate dominantly from the parametrization of the peptide backbone in the given force field rather than from a noncovalent interaction with water molecules.

Unfolded-state dynamics and structure of protein L characterized by simulation and experiment

While several experimental techniques now exist for characterizing protein unfolded states, all-atom simulation of unfolded states has been challenging due to the long time scales and conformational sampling required. We address this problem by using a combination of accelerated calculations on graphics processor units and distributed computing to simulate tens of thousands of molecular dynamics trajectories each up to approximately 10 mus (for a total aggregate simulation time of 127 ms). We used this approach in conjunction with Trp-Cys contact quenching experiments to characterize the unfolded structure and dynamics of protein L. We employed a polymer theory method to make quantitative comparisons between high-temperature simulated and chemically denatured experimental ensembles and find that reaction-limited quenching rates calculated from simulation agree remarkably well with experiment. In both experiment and simulation, we find that unfolded-state intramolecular diffusion rates are very slow compared to highly denatured chains and that a single-residue mutation can significantly alter unfolded-state dynamics and structure. This work suggests a view of the unfolded state in which surprisingly low diffusion rates could limit folding and opens the door for all-atom molecular simulation to be a useful predictive tool for characterizing protein unfolded states along with experiments that directly measure intramolecular diffusion.

Structural heterogeneity of 6 M GdmCl-denatured proteins: implications for the mechanism of protein folding

An in vitro experiment with protein folding is typically initiated with 6 M GdmCl-denatured proteins, which are generally considered fully unfolded. However, studies conducted by various laboratories have shown that many 6 M GdmCl-denatured proteins are structurally heterogeneous and still retain nativelike residual structures. The extent of conformational heterogeneity of the 6 M GdmCl-denatured protein has significant implications for the folding landscape as well as the interpretation of the observed early stage folding mechanism. Using the method of disulfide scrambling, we are able to gain rough insight into the diverse structural properties of 6 M GdmCl-denatured proteins. It demonstrates that most 6 M GdmCl-denatured proteins are approximately fully denatured, but partially unfolded. Most of them comprise diverse conformational isomers. We review here the cumulative evidence obtained from various laboratories and also provide experimental data obtained in our laboratory.