How do chemical denaturants affect the mechanical folding and unfolding of proteins?

We present the first single-molecule atomic force microscopy study on the effect of chemical denaturants on the mechanical folding/unfolding kinetics of a small protein GB1 (the B1 immunoglobulin-binding domain of protein G from Streptococcus). Upon increasing the concentration of the chemical denaturant guanidinium chloride (GdmCl), we observed a systematic decrease in the mechanical stability of GB1, indicating the softening effect of the chemical denaturant on the mechanical stability of proteins. This mechanical softening effect originates from the reduced free-energy barrier between the folded state and the unfolding transition state, which decreases linearly as a function of the denaturant concentration. Chemical denaturants, however, do not alter the mechanical unfolding pathway or shift the position of the transition state for mechanical unfolding. We also found that the folding rate constant of GB1 is slowed down by GdmCl in mechanical folding experiments. By combining the mechanical folding/unfolding kinetics of GB1 in GdmCl solution, we developed the “mechanical chevron plot” as a general tool to understand how chemical denaturants influence the mechanical folding/unfolding kinetics and free-energy diagram in a quantitative fashion. This study demonstrates great potential in combining chemical denaturation with single-molecule atomic force microscopy techniques to reveal invaluable information on the energy landscape underlying protein folding/unfolding reactions.     

Molecular basis of cooperativity in protein folding IV. Core: A general cooperative folding model

The cooperative nature of the protein folding process is independent of the characteristic fold and the specific secondary structure attributes of a globular protein. A general folding/unfolding model should, therefore, be based upon structural features that transcend the peculiarities of α-helices, β-sheets, and other structural motifs found in proteins. The studies presented in this paper suggest that a single structural characteristic common to all globular proteins is essential for cooperative folding. The formation of a partly folded state from the native state results in the exposure to solvent of two distinct regions: (1) the portions of the protein that are unfolded; and (2) the “complementary surfaces,” located in the regions of the protein that remain folded. The cooperative character of the folding/unfolding transition is determined largely by the energetics of exposing complementary surface regions to the solvent. By definition, complementary regions are present only in partly folded states; they are absent from the native and unfolded states. An unfavorable free energy lowers the probability of partly folded states and increases the cooperativity of the transition. In this paper we present a mathematical formulation of this behavior and develop a general cooperative folding/unfolding model, termed the “complementary region” (CORE) model. This model successfully reproduces the main properties of folding/unfolding transitions without limiting the number of partly folded states accessible to the protein, thereby permitting a systematic examination of the structural and solvent conditions under which intermediates become populated. It is shown that the CORE model predicts two-state folding/unfolding behavior, even though the two-state character is not assumed in the model. © 1993 Wiley-Liss, Inc.     

Evidence for sequential barriers and obligatory intermediates in apparent two-state protein folding

Many small proteins fold fast and without detectable intermediates. This is frequently taken as evidence against the importance of partially folded states, which often transiently accumulate during folding of larger proteins. To get insight into the properties of free energy barriers in protein folding we analyzed experimental data from 23 proteins that were reported to show non-linear activation free-energy relationships. These non-linearities are generally interpreted in terms of broad transition barrier regions with a large number of energetically similar states. Our results argue against the presence of a single broad barrier region. They rather indicate that the non-linearities are caused by sequential folding pathways with consecutive distinct barriers and a few obligatory high-energy intermediates. In contrast to a broad barrier model the sequential model gives a consistent picture of the folding barriers for different variants of the same protein and when folding of a single protein is analyzed under different solvent conditions. The sequential model is also able to explain changes from linear to non-linear free energy relationships and from apparent two-state folding to folding through populated intermediates upon single point mutations or changes in the experimental conditions. These results suggest that the apparent discrepancy between two-state and multi-state folding originates in the relative stability of the intermediates, which argues for the importance of partially folded states in protein folding.     

The native conformation of plasmepsin II is kinetically trapped at neutral pH

Plasmepsin II (PMII), an aspartic protease from the malarial parasite Plasmodium falciparum, represents a model for understanding protease structure/function relationships due to its unique structure and properties. The present study undertook a thermodynamic and kinetic analysis of the PMII folding mechanism and a pH stability profile. Differential scanning calorimetry revealed that the native state of PMII (Np) was irreversibly unfolded, and in the pH range of 6.5–8.0, PMII refolds to a denatured state (Rp) with higher thermal stability than Np. Rp could also be formed upon partially unfolding PMII at pH 11.0 and 37 °C for 2 h, followed by adjustment to a pH in the range of 6.5–8.0. While Rp could be folded/unfolded reversibly, Np was shown to exist as a kinetically trapped state. By examining the unfolding kinetics of Np and the kinetics of Rp folding to Np at 25 °C, it was found that Np is kinetically trapped by an unfolding barrier of 25.5 kcal/mol, and yet once unfolded, is prevented from folding by a comparable folding barrier. The folding mechanism of PMII is similar to that reported for pepsin. It is hypothesized that the PMII zymogen also utilizes a prosegment-catalyzed folding mechanism.     

50+ Years of Protein Folding

The ability of proteins to spontaneously form their spatial structures is a long-standing puzzle in molecular biology. Experimentally measured rates of spontaneous folding of single-domain globular proteins range from microseconds to hours: the difference–10-11 orders of magnitude–is the same as between the lifespan of a mosquito and the age of the Universe. This review (based on the literature and some personal recollections) describes a winding road to understanding spontaneous folding of protein structure. The main attention is given to the free-energy landscape of conformations of a protein chain–especially to the barrier separating its unfolded (U) and the natively folded (N) states–and to physical the-ories of rates of crossing this barrier in both directions: from U to N, and from N to U. It is shown that theories of both these processes come to essentially the same result and outline the observed range of folding and unfolding rates for single-domain globular proteins. In addition, they predict the maximal size of protein domains that fold under solely thermodynamic (rather than kinetic) control, and explain the observed maximal size of “foldable” protein domains.     

MD simulations of Mistic: conformational stability in detergent micelles and water

Mistic is an unusual membrane protein from Bacillus subtilis. It appears to fold and insert autonomously into a lipid bilayer and has been suggested as a tool that aids the targeting of eukaryotic membrane proteins to bacterial membranes. The NMR structure of Mistic in detergent (LDAO) micelles has revealed it to be a four alpha-helix bundle. From a structural perspective, Mistic does not resemble other membrane proteins. Its external surface is not very hydrophobic, and standard methods do not predict any of its helices to be in the transmembrane orientation. Molecular dynamics simulations (simulation times approximately 30 ns) in water and in detergent micelles have been used to explore the conformational stability of Mistic as a function of its environment. In water, the protein is stable, exhibiting no significant change in fold on a 30 ns time scale. In contrast, in three simulations in detergent micelles, the partial unfolding of Mistic occurred, whereby the H4 helix drifted away from the H1-H3 core. This was due to the penetration of detergent molecules between H4 and the remainder of the protein. This is unlike the behavior of several other membrane proteins, both alpha-helix bundles and beta-barrels, in comparable detergent micelle simulations. The unfolding of H4 from the H1-H3 core of Mistic could be partially reversed by a simulation in which the detergent molecules were removed, and the unfolded protein was simulated in water. These results suggest that Mistic may not be a stable integrated membrane protein but rather that it may undergo a conformational change upon interaction with a membrane or membrane-like environment.     

On the Prediction of Folding Nuclei in Globular Proteins

An approach to predicting folding nuclei in globular proteins with known three-dimensional structures is proposed. This approach is based on the pinpointing of the lowest saddle points on the barrier between the unfolded state and native structure on the free-energy landscape of a protein chain; the proposed technique uses the dynamic programming method. A comparison of calculation results with experimental data on the folding nuclei of 21 proteins shows that the model provides good Φ value predictions for protein structures determined by X-ray analysis and, less successfully, in structures determined by nuclear magnetic resonance. Consideration of the whole ensemble of transition states provides a better prediction of folding nuclei than consideration of only transition states with lowest free energies. In addition, we predict the location of folding nuclei in three-dimensional structures of some proteins whose folding kinetics is being studied, but there is no experimental evidence concerning their folding nuclei.     

Some recommendations for the practitioner to improve the precision of experimentally determined protein folding rates and phi values

The mechanism by which proteins fold from an initially random conformation into a functional, native structure remains a major unsolved question in molecular biology. Of particular interest to the protein folding community is the structure that the protein adopts in the folding transition state (the highest free energy state on the pathway from unfolded to folded), as that state forms the barrier that defines the folding pathway. Unfortunately, however, unlike those of the initial, unfolded state and the final, folded state of the protein, the structure in the transition state cannot be directly assessed via experiment. Instead, experimentalists infer the structure of the transition state, often by estimating changes in its free energy by measuring the effects of amino acid substitutions on folding and unfolding rates (Phi-value analysis). In this article we show how to obtain more efficient estimates of these important quantities via improved experimental designs, and how to avoid common pitfalls in the analysis of kinetic data during the extraction of these parameters.     

Cotranslational Folding of a Pentarepeat β-Helix Protein

It is becoming increasingly clear that many proteins start to fold cotranslationally before the entire polypeptide chain has been synthesized on the ribosome. One class of proteins that a priori would seem particularly prone to cotranslational folding is repeat proteins, that is, proteins that are built from an array of nearly identical sequence repeats. However, while the folding of repeat proteins has been studied extensively in vitro with purified proteins, only a handful of studies have addressed the issue of cotranslational folding of repeat proteins. Here, we have determined the structure and studied the cotranslational folding of a β-helix pentarepeat protein from the human pathogen Clostridium botulinum—a homolog of the fluoroquinolone resistance protein MfpA—using an assay in which the SecM translational arrest peptide serves as a force sensor to detect folding events. We find that cotranslational folding of a segment corresponding to the first four of the eight β-helix coils in the protein produces enough force to release ribosome stalling and that folding starts when this unit is ~ 35 residues away from the P-site, near the distal end of the ribosome exit tunnel. An additional folding transition is seen when the whole PENT moiety emerges from the exit tunnel. The early cotranslational formation of a folded unit may be important to avoid misfolding events in vivo and may reflect the minimal size of a stable β-helix since it is structurally homologous to the smallest known β-helix protein, a four-coil protein that is stable in solution.     

Folding and aggregation of a multi-domain engineered immunotoxin

Engineered immunotoxins with specific targeting mechanisms have potential applications for the treatment of cancer and other diseases; however, their folding behavior is often poorly understood and this presents challenges during process development, manufacturing, and formulation. Folding thermodynamics of an antibody variable domain (V_H/V_L) genetically fused to a biological toxin payload were characterized at pH 6.0 and pH 8.0 in order to assess the relative domain stabilities, along with time scales on which they fold, and the competition between aggregation and folding. The toxin and V_H/V_L domains had considerably different unfolding free energies (ΔG_UNF), leading to a thermodynamically-distinct intermediate species, with the toxin domain unfolded and the V_H/V_L folded. The intermediate is the majority species over a range of denaturant concentrations (∼4–6 M urea; ∼2–4 M guanidine HCl). Thermal unfolding resulted in reversible unfolding of the toxin domain at pH 8, but at pH 6 thermal unfolding was convoluted with aggregation due to irreversible unfolding and aggregation for the V_H/V_L domain. Chemical unfolding of both domains was more easily reversible, provided that the refold was done stepwise, allowing the antibody domain to fold first at intermediate denaturant concentration, as folding of the V_H/V_L domain played a key role in aggregation of this antibody fusion protein.     

The interplay between protein stability and dynamics in conformational diseases: The case of hPGK1 deficiency

Conformational diseases often show defective protein folding efficiency in vivo upon mutation, affecting protein properties such as thermodynamic stability and folding/unfolding/misfolding kinetics as well as the interactions of the protein with the protein homeostasis network. Human phosphoglycerate kinase 1 (hPGK1) deficiency is a rare inherited disease caused by mutations in hPGK1 that lead to loss-of-function. This disease offers an excellent opportunity to explore the complex relationships between protein stability and dynamics because of the different unfolding mechanisms displayed towards chemical and thermal denaturation. This work explores these relationships using two thermostable mutants (p.E252A and p.T378P) causing hPGK1 deficiency and WT hPGK1 using proteolysis and chemical denaturation. p.T378P is degraded ~ 30-fold faster at low protease concentrations (here, the proteolysis step is rate-limiting) and ~ 3-fold faster at high protease concentrations (where unfolding kinetics is rate-limiting) than WT and p.E252A, indicating that p.T378P is thermodynamically and kinetically destabilized. Urea denaturation studies support the decrease in thermodynamic stability and folding cooperativity for p.T378P, as well as changes in folding/unfolding kinetics. The present study reveals changes in the folding landscape of hPGK1 upon mutation that may affect protein folding efficiency and stability in vivo, also suggesting that native state stabilizers and protein homeostasis modulators may help to correct folding defects in hPGK1 deficiency. Moreover, detailed kinetic proteolysis studies are shown to be powerful and simple tools to provide deep insight into mutational effects on protein folding and stability in conformational diseases.     

Topological frustration and the folding of interleukin-1 beta

The cytokine, interleukin-1beta (IL-1beta), adopts a beta-trefoil fold. It is known to be much slower folding than similarly sized proteins, despite having a low contact order. Proteins are sufficiently well designed that their folding is not dominated by local energetic traps. Therefore, protein models that encode only the folded structure and are energetically unfrustrated (Gō-type), can capture the essentials of the folding routes. We investigate the folding thermodynamics of IL-1beta using such a model and molecular dynamics (MD) simulations. We develop an enhanced sampling technique (a modified multicanonical method) to overcome the sampling problem caused by the slow folding. We find that IL-1beta has a broad and high free energy barrier. In addition, the protein fold causes intermediate unfolding and refolding of some native contacts within the protein along the folding trajectory. This "backtracking" occurs around the barrier region. Complex folds like the beta-trefoil fold and functional loops like the beta-bulge of IL-1beta can make some of the configuration space unavailable to the protein and cause topological frustration.     

Theoretical investigation of the photoinitiated folding of HP-36

A computational model was developed to examine the phototriggered folding of a caged protein, a protein modified with an organic photolabile cross-linker. Molecular dynamics simulations of the modified 36-residue fragment of subdomain B of chicken villin head piece with a photolabile linker were performed, starting from both the caged and the uncaged structures. Construction of a free-energy landscape, based on principal components as well as on radius of gyration versus root-mean-square deviation, and circular dichroism calculations were employed to characterize folding behavior and structures. The folded structures observed in the molecular dynamics trajectories were found to be similar to that of the wild-type protein, in agreement with the published experimental results. The free-energy landscapes of the modified and wild-type proteins have similar topology, suggesting common thermodynamic/kinetic behavior. The existence of small differences in the free-energy surface of the modified protein from that of the native protein, however, indicates subtle differences in the folding behavior.     

Rapid unfolding of a domain populates an aggregation-prone intermediate that can be recognized by GroEL

Some amino acid substitutions in phage P22 coat protein cause a temperature-sensitive folding (tsf) phenotype. In vivo, these tsf amino acid substitutions cause coat protein to aggregate and form intracellular inclusion bodies when folded at high temperatures, but at low temperatures the proteins fold properly. Here the effects of tsf amino acid substitutions on folding and unfolding kinetics and the stability of coat protein in vitro have been investigated to determine how the substitutions change the ability of coat protein to fold properly. The equilibrium unfolding transitions of the tsf variants were best fit to a three-state model, N if I if U, where all species concerned were monomeric, a result confirmed by velocity sedimentation analytical ultracentrifugation. The primary effect of the tsf amino acid substitutions on the equilibrium unfolding pathway was to decrease the stability (DeltaG) and the solvent accessibility (m-value) of the N if I transition. The kinetics of folding and unfolding of the tsf coat proteins were investigated using tryptophan fluorescence and circular dichroism (CD) at 222 nm. The tsf amino acid substitutions increased the rate of unfolding by 8-14-fold, with little effect on the rate of folding, when monitored by tryptophan fluorescence. In contrast, when folding or unfolding reactions were monitored by CD, the reactions were too fast to be observed. The tsf coat proteins are natural substrates for the molecular chaperones, GroEL/S. When native tsf coat protein monomers were incubated with GroEL, they bound efficiently, indicating that a folding intermediate was significantly populated even without denaturant. Thus, the tsf coat proteins aggregate in vivo because of an increased propensity to populate this unfolding intermediate.     

Bacteriorhodopsin folds into the membrane against an external force

Despite their crucial importance for cellular function, little is known about the folding mechanisms of membrane proteins. Recently details of the folding energy landscape were elucidated by atomic force microscope (AFM)-based single molecule force spectroscopy. Upon unfolding and extraction of individual membrane proteins energy barriers in structural elements such as loops and helices were mapped and quantified with the precision of a few amino acids. Here we report on the next logical step: controlled refolding of single proteins into the membrane. First individual bacteriorhodopsin monomers were partially unfolded and extracted from the purple membrane by pulling at the C-terminal end with an AFM tip. Then by gradually lowering the tip, the protein was allowed to refold into the membrane while the folding force was recorded. We discovered that upon refolding certain helices are pulled into the membrane against a sizable external force of several tens of picoNewton. From the mechanical work, which the helix performs on the AFM cantilever, we derive an upper limit for the Gibbs free folding energy. Subsequent unfolding allowed us to analyze the pattern of unfolding barriers and corroborate that the protein had refolded into the native state.     

Burst-phase expansion of native protein prior to global unfolding in SDS

Although numerous studies have been directed at understanding early folding events through the characterization of folding intermediates, there are few reports on the very late folding events, i.e. on the events taking place on the native side of the folding barrier and on alternative conformations of the folded state. To shed further light on these issues, we have characterized by protein engineering the structure of an expanded but native-like intermediate that accumulates transiently in the unfolding reaction of the small protein S6 in the presence of SDS. The results show that the SDS micelles attack the native protein in the dead-time of the denaturation experiment, causing an expansion of the hydrophobic core prior to the major unfolding transition. We distinguish two forms of the unfolding intermediate that are correlated with the micellar structure. With spherical micelles, the expansion is seen mainly as a weakening of the interactions which anchor the two alpha-helices to the core of the S6 structure. With cylindrical micelles, prevalent at higher SDS concentrations, the expansion is more global and produces a species which closely resembles the transition-state structure for unfolding in GdmCl. Despite the highly weakened core, the micelle-associated intermediate displays cooperative unfolding, indicating a significant structural plasticity of the species on the native side of the folding barrier in the presence of SDS.     

Thermodynamics and Kinetics of Single-Chain Monellin Folding with Structural Insights into Specific Collapse in the Denatured State Ensemble

Proteins, which behave as random coils in high denaturant concentrations undergo collapse transition similar to polymers on denaturant dilution. We study collapse in the denatured ensemble of single-chain monellin (MNEI) using a coarse-grained protein model and molecular dynamics simulations. The model is validated by quantitatively comparing the computed guanidinium chloride and pH-dependent thermodynamic properties of MNEI folding with the experiments. The computed properties such as the fraction of the protein in the folded state and radius of gyration (R_g) as function of [GuHCl] are in good agreement with the experiments. The folded state of MNEI is destabilized with an increase in pH due to the deprotonation of the residues Glu24 and Cys42. On decreasing [GuHCl], the protein in the unfolded ensemble showed specific compaction. The R_g of the protein decreased steadily with [GuHCl] dilution due to increase in the number of native contacts in all the secondary structural elements present in the protein. MNEI folding kinetics is complex with multiple folding pathways and transiently stable intermediates are populated in these pathways. In strong stabilizing conditions, the protein in the unfolded ensemble showed transition to a more compact unfolded state where R_g decreased by ≈ 17% due to the formation of specific native contacts in the protein. The intermediate populated in the dominant MNEI folding pathway satisfies the structural features of the dry molten globule inferred from experiments.     

Predicting repeat protein folding kinetics from an experimentally determined folding energy landscape

The Notch ankyrin domain is a repeat protein whose folding has been characterized through equilibrium and kinetic measurements. In previous work, equilibrium folding free energies of truncated constructs were used to generate an experimentally determined folding energy landscape (Mello and Barrick, Proc Natl Acad Sci USA 2004;101:14102–14107). Here, this folding energy landscape is used to parameterize a kinetic model in which local transition probabilities between partly folded states are based on energy values from the landscape. The landscape‐based model correctly predicts highly diverse experimentally determined folding kinetics of the Notch ankyrin domain and sequence variants. These predictions include monophasic folding and biphasic unfolding, curvature in the unfolding limb of the chevron plot, population of a transient unfolding intermediate, relative folding rates of 19 variants spanning three orders of magnitude, and a change in the folding pathway that results from C‐terminal stabilization. These findings indicate that the folding pathway(s) of the Notch ankyrin domain are thermodynamically selected: the primary determinants of kinetic behavior can be simply deduced from the local stability of individual repeats.     

Structure of a partially unfolded form of Escherichia coli dihydrofolate reductase provides insight into its folding pathway

Proteins frequently fold via folding intermediates that correspond to local minima on the conformational energy landscape. Probing the structure of the partially unfolded forms in equilibrium under native conditions can provide insight into the properties of folding intermediates. To elucidate the structures of folding intermediates of Escherichia coli dihydrofolate reductase (DHFR), we investigated transient partial unfolding of DHFR under native conditions. We probed the structure of a high‐energy conformation susceptible to proteolysis (cleavable form) using native‐state proteolysis. The free energy for unfolding to the cleavable form is clearly less than that for global unfolding. The dependence of the free energy on urea concentration (m‐value) also confirmed that the cleavable form is a partially unfolded form. By assessing the effect of mutations on the stability of the partially unfolded form, we found that native contacts in a hydrophobic cluster formed by the F‐G and Met‐20 loops on one face of the central β‐sheet are mostly lost in the partially unfolded form. Also, the folded region of the partially unfolded form is likely to have some degree of structural heterogeneity. The structure of the partially unfolded form is fully consistent with spectroscopic properties of the near‐native kinetic intermediate observed in previous folding studies of DHFR. The findings suggest that the last step of the folding of DHFR involves organization in the structure of two large loops, the F‐G and Met‐20 loops, which is coupled with compaction of the rest of the protein.     

How cooperative are protein folding and unfolding transitions?

A thermodynamically and kinetically simple picture of protein folding envisages only two states, native (N) and unfolded (U), separated by a single activation free energy barrier, and interconverting by cooperative two‐state transitions. The folding/unfolding transitions of many proteins occur, however, in multiple discrete steps associated with the formation of intermediates, which is indicative of reduced cooperativity. Furthermore, much advancement in experimental and computational approaches has demonstrated entirely non‐cooperative (gradual) transitions via a continuum of states and a multitude of small energetic barriers between the N and U states of some proteins. These findings have been instrumental towards providing a structural rationale for cooperative versus noncooperative transitions, based on the coupling between interaction networks in proteins. The cooperativity inherent in a folding/unfolding reaction appears to be context dependent, and can be tuned via experimental conditions which change the stabilities of N and U. The evolution of cooperativity in protein folding transitions is linked closely to the evolution of function as well as the aggregation propensity of the protein. A large activation energy barrier in a fully cooperative transition can provide the kinetic control required to prevent the accumulation of partially unfolded forms, which may promote aggregation. Nevertheless, increasing evidence for barrier‐less “downhill” folding, as well as for continuous “uphill” unfolding transitions, indicate that gradual non‐cooperative processes may be ubiquitous features on the free energy landscape of protein folding.