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

Importance of Inter-residue Contacts for Understanding Protein Folding and Unfolding Rates, Remote Homology, and Drug Design

Inter-residue interactions in protein structures provide valuable insights into protein folding and stability. Understanding these interactions can be helpful in many crucial applications, including rational design of therapeutic small molecules and biologics, locating functional protein sites, and predicting protein-protein and protein-ligand interactions. The process of developing machine learning models incorporating inter-residue interactions has been improved recently. This review highlights the theoretical models incorporating inter-residue interactions in predicting folding and unfolding rates of proteins. Utilizing contact maps to depict inter-residue interactions aids researchers in developing computer models for detecting remote homologs and interface residues within protein-protein complexes which, in turn, enhances our knowledge of the relationship between sequence and structure of proteins. Further, the application of contact maps derived from inter-residue interactions is highlighted in the field of drug discovery. Overall, this review presents an extensive assessment of the significant models that use inter-residue interactions to investigate folding rates, unfolding rates, remote homology, and drug development, providing potential future advancements in constructing efficient computational models in structural biology.

Kinetics of Structural Transitions Induced by Sodium Dodecyl Sulfate in α-Chymotrypsin

The temporal changes in circular dichroism at 222 and 260 nm were recorded by using stopped-flow spectroscopy after mixing α-chymotrypsin solutions with sodium dodecyl sulfate solutions. Simultaneously with the circular dichroism signal, the fluorescence emission was recorded. Changes in the secondary and tertiary structures of chymotrypsin induced by sodium dodecyl sulfate are characterized by either three or four one-way reactions with relaxation amplitudes and times precisely determined by an advanced numerical procedure of Kuzmič. Quantitatively, transitions within the secondary and tertiary structures of the protein are significantly different. Moreover, changes in the tertiary structure depend on the type of recorded signal (either circular dichroism or fluorescence) and the wavelength of the incident radiation. The latter observation is particularly interesting as it indicates that the contributions of protein's different tryptophans to the total recorded fluorescence depend on the excitation wavelength. We present several results justifying this hypothesis.

Effects of Sodium Dodecyl Sulfate on the Enzyme Catalysis and Conformation of a Recombinant γ-Glutamyltranspeptidase from Bacillus licheniformis

The study of interactions between proteins and surfactants is of relevance in a diverse range of applications including food, enzymatic detergent formulation, and drug delivery. In spite of sodium dodecyl sulfate (SDS)-induced unfolding has been studied in detail at the protein level, deciphering the conformation-activity relationship of a recombinant γ-glutamyltranspeptidase (BlrGGT) from Bacillus licheniformis remains important to understand how the transpeptidase activity is related to its conformation. In this study, we examined the enzyme catalysis and conformational transition of BlrGGT in the presence of SDS. Enzymatic assays showed that the transpeptidase activity of BlrGGT was greatly affected by SDS in a concentration-dependent manner with approximately 90% inactivation at 6 mM. Native polyacrylamide gel electrophoresis of SDS-treated samples clearly revealed that the heterodimeric enzyme was apparently dissociated into two different subunits at concentrations above 2 mM. The study of enzyme kinetics showed that SDS can act as a mixed-type inhibitor to reduce the catalytic efficiency of BlrGGT. Moreover, the t_1/2 value of the enzyme at 55 °C was greatly reduced from 495.1 min to 7.4 min in the presence of 1 mM SDS. The I₃/I₁ ratio of pyrene excimer fluorescence emission changed around 3.7 mM SDS in the absence of BlrGGT and the inflection point of enzyme samples was reduced to less than 2.7 mM. The Far-UV CD spectrum of the native enzyme had two negative peaks at 208 and 222 nm, respectively; however, both negative peaks increased in magnitude with increasing SDS concentration and reached maximal values at above 4.0 mM. The intrinsic fluorescence spectra of tryptophan further demonstrated that the SDS-induced enzyme conformational transition occurred at approximately 5.1 mM. Tween 20 significantly suppressed the interaction of BlrGGT with SDS by forming mixed micelles at a molar ratio of 1.0. Taken together, this study definitely promotes our better understanding of the relationship between the conformation and catalysis of BlrGGT.

Brave new surfactant world revisited by thermoalkalophilic lipases: computational insights into the role of SDS as a substrate analog

Non-ionic surfactants were shown to stabilize the active conformation of thermoalkalophilic lipases by mimicking the lipid substrate while the catalytic interactions formed by anionic surfactants have not been well characterized. In this study, we combined μs-scale molecular dynamics (MD) simulations and lipase activity assays to analyze the effect of ionic surfactant, sodium dodecyl sulfate (SDS), on the structure and activity of thermoalkalophilic lipases. Both the open and closed lipase conformations that differ in geometry were recruited to the MD analysis to provide a broader understanding of the molecular effect of SDS on the lipase structure. Simulations at 298 K showed the potential of SDS for maintaining the active lipase through binding to the sn-1 acyl-chain binding pocket in the open conformation or transforming the closed conformation to an open-like state. Consistent with MD findings, experimental analysis showed increased lipase activity upon SDS incubation at ambient temperature. Notably, the lipase cores stayed intact throughout 2 μs regardless of an increase in the simulation temperature or SDS concentration. However, the surface structures were unfolded in the presence of SDS and at elevated temperature for both conformations. Simulations of the dimeric lipase were also carried out and showed reduced flexibility of the surface structures which were unfolded in the monomer, indicating the insulating role of dimer interactions against SDS. Taken together, this study provides insights into the possible substrate mimicry by the ionic surfactant SDS for the thermoalkalophilic lipases without temperature elevation, underscoring SDS's potential for interfacial activation at ambient temperatures.

Circular Dichroism Spectra of α-Chymotrypsin-SDS Solutions Depend on the Procedure of Their Preparation

We recorded the far- and near-UV circular dichroism (CD) spectra of solutions of α-chymotrypsin and sodium dodecyl sulfate (SDS) with the final surfactant concentration significantly above the critical micellization concentration. Solutions were prepared using three different procedures. The reference procedure was to mix the chymotrypsin solution with the SDS solution once, immediately achieving the final SDS concentration. In alternative procedures, the protein solutions initially contained some SDS and were mixed with pure SDS solutions at a concentration to provide the same final surfactant as the reference mixing. We demonstrate that the supplementation to the selected final concentration of SDS of the pure chymotrypsin solution leads to different CD spectra than the supplementation to this final concentration of SDS in the chymotrypsin solution containing a small concentration of a few millimolar SDS. These differences disappear when the initial concentration of SDS in the protein solution, which we then supplement to the indicated final concentration, is higher. This suggests the irreversibility of the processes caused by the addition of SDS to chymotrypsin and the influence of the initial amount of this surfactant on the processes occurring with its further addition to the solution. For quantitative analysis of far-UV CD spectra in terms of populations of protein secondary structure elements, we used four well-established software packages. All programs consistently indicate that the observed differences in the far-UV CD spectra can be explained by the differences in the increase in the population of helical forms in chymotrypsin under the influence of SDS.

Quantitating denaturation by formic acid: imperfect repeats are essential to the stability of the functional amyloid protein FapC

Bacterial functional amyloids are evolutionarily optimized to aggregate, so much so that the extreme robustness of functional amyloid makes it very difficult to examine their structure-function relationships in a detailed manner. Previous work has shown that functional amyloids are resistant to conventional chemical denaturants, but they dissolve in formic acid (FA) at high concentrations. However, systematic investigation requires a quantitative analysis of FA's ability to denature proteins. Amyloid formed by Pseudomonas sp. protein FapC provides an excellent model to investigate FA denaturation. It contains three imperfect repeats, and stepwise removal of these repeats slows fibrillation and increases fragmentation during aggregation. However, the link to stability is unclear. We first calibrated FA denaturation using three small, globular, and acid-resistant proteins. This revealed a linear relationship between the concentration of FA and the free energy of unfolding with a slope of mFA+pH (the combined contribution of FA and FA-induced lowering of pH), as well as a robust correlation between protein size and mFA+pH We then measured the solubilization of fibrils formed from different FapC variants with varying numbers of repeats as a function of the concentration of FA. This revealed a decline in the number of residues driving amyloid formation upon deleting at least two repeats. The midpoint of denaturation declined with the removal of repeats. Complete removal of all repeats led to fibrils that were solubilized at FA concentrations 2-3 orders of magnitude lower than the repeat-containing variants, showing that at least one repeat is required for the stability of functional amyloid.

SDS-induced multi-stage unfolding of a small globular protein through different denatured states revealed by single-molecule fluorescence

Ionic surfactants such as sodium dodecyl sulfate (SDS) unfold proteins in a much more diverse yet effective way than chemical denaturants such as guanidium chloride (GdmCl). But how these unfolding processes compare on a molecular level is poorly understood. Here, we address this question by scrutinising the unfolding pathway of the globular protein S6 in SDS and GdmCl with single-molecule Förster resonance energy transfer (smFRET) spectroscopy. We show that the unfolding mechanism in SDS is strikingly different and convoluted in comparison to denaturation in GdmCl. In contrast to the reversible two-state unfolding behaviour in GdmCl characterised by kinetics on the timescale of seconds, SDS demonstrated not one, but four distinct regimes of interactions with S6, dependent on the surfactant concentration. At ≤1 mM SDS, S6 and surfactant molecules form quasi-micelles on a minute timescale; at millimolar [SDS], the protein denatures through an unfolded/denatured ensemble of highly heterogeneous states on a multi-second timescale; at tens of millimolar of SDS, the protein unfolds into a micelle-packed conformation on the second timescale; and >50 mM SDS, the protein unfolds with millisecond timescale dynamics. We propose a detailed model for multi-stage unfolding of S6 in SDS, which involves at least three different types of denatured states with different level of compactness and dynamics and a continually changing landscape of interactions between protein and surfactant. Our results highlight the great potential of single-molecule fluorescence as a direct probe of nanoscale protein structure and dynamics in chemically complex surfactant environments.

Protein unfolding by SDS: the microscopic mechanisms and the properties of the SDS-protein assembly

The effects of detergent sodium dodecyl sulfate (SDS) on protein structure and dynamics are fundamental to the most common laboratory technique used to separate proteins and determine their molecular weights: polyacrylamide gel electrophoresis. However, the mechanism by which SDS induces protein unfolding and the microstructure of protein-SDS complexes remain largely unknown. Here, we report a detailed account of SDS-induced unfolding of two proteins-I27 domain of titin and β-amylase-obtained through all-atom molecular dynamics simulations. Both proteins were found to spontaneously unfold in the presence of SDS at boiling water temperature on the time scale of several microseconds. The protein unfolding was found to occur via two distinct mechanisms in which specific interactions of individual SDS molecules disrupt the protein's secondary structure. In the final state of the unfolding process, the proteins are found to wrap around SDS micelles in a fluid necklace-and-beads configuration, where the number and location of bound micelles changes dynamically. The global conformation of the protein was found to correlate with the number of SDS micelles bound to it, whereas the number of SDS molecules directly bound to the protein was found to define the relaxation time scale of the unfolded protein. Our microscopic characterization of SDS-protein interactions sets the stage for future refinement of SDS-enabled protein characterization methods, including protein fingerprinting and sequencing using a solid-state nanopore.

Molecular dynamics study of ACBP denaturation in alkyl sulfates demonstrates possible pathways of unfolding through fused surfactant clusters

Anionic surfactants denature proteins at low millimolar concentrations, yet little is known about the underlying molecular mechanisms. Here, we undertake 1-μs-long atomistic molecular dynamics simulations of the denaturation of acyl coenzyme A binding protein (ACBP) and compare our results with previously published and new experimental data. Since increasing surfactant chain length is known to lead to more rapid denaturation, we studied denaturation using both the medium-length alkyl chain surfactant sodium dodecyl sulfate (SDS) and the long alkyl chain surfactant sodium hexadecyl sulfate (SHS). In silico denaturation on the microsecond timescale was not achieved using preformed surfactant micelles but required ACBP to be exposed to monomeric surfactant molecules. Micellar self-assembly occurred together with protein denaturation. To validate our analyses, we calculated small-angle X-ray scattering spectra of snapshots from the simulations. These agreed well with experimental equilibrium spectra recorded on ACBP-SDS mixtures with similar compositions. Protein denaturation occurs through the binding of partial micelles to multiple preferred binding sites followed by the accretion of surfactant monomers until these partial micelles merge to form a mature micelle and the protein chain is left disordered on the surface of the micelle. While the two surfactants attack in a similar fashion, SHS’s longer alkyl chain leads to a more efficient denaturation through the formation of larger clusters that attack ACBP, a more rapid drop in native contacts, a greater expansion in size, as well as a more thorough rearrangement of hydrogen bonds and disruption of helices.

A complete picture of protein unfolding and refolding in surfactants

Interactions between proteins and surfactants are of relevance in many applications including food, washing powder formulations, and drug formulation. The anionic surfactant sodium dodecyl sulfate (SDS) is known to unfold globular proteins, while the non-ionic surfactant octaethyleneglycol monododecyl ether (C₁₂E₈) can be used to refold proteins from their SDS-denatured state. While unfolding have been studied in detail at the protein level, a complete picture of the interplay between protein and surfactant in these processes is lacking. This gap in our knowledge is addressed in the current work, using the β-sheet-rich globular protein β-lactoglobulin (bLG). We combined stopped-flow time-resolved SAXS, fluorescence, and circular dichroism, respectively, to provide an unprecedented in-depth picture of the different steps involved in both protein unfolding and refolding in the presence of SDS and C₁₂E₈. During unfolding, core-shell bLG-SDS complexes were formed within ∼10 ms. This involved an initial rapid process where protein and SDS formed aggregates, followed by two slower processes, where the complexes first disaggregated into single protein structures situated asymmetrically on the SDS micelles, followed by isotropic redistribution of the protein. Refolding kinetics (>100 s) were slower than unfolding (<30 s), and involved rearrangements within the mixing deadtime (∼5 ms) and transient accumulation of unfolded monomeric protein, differing in structure from the original bLG-SDS structure. Refolding of bLG involved two steps: extraction of most of the SDS from the complexes followed by protein refolding. These results reveal that surfactant-mediated unfolding and refolding of proteins are complex processes with rearrangements occurring on time scales from sub-milliseconds to minutes.

Production and Characterization of Extremophilic Proteinases From a New Enzyme Source, Barrientosiimonas sp. V9

Microbial proteases are widely used as commercial enzymes, which have an active role in several industrial processes. The aim of this study was to investigate the production and properties of extracellular proteases from Barrientosiimonas sp. strain V9. The cultivation conditions for protease production were studied using different carbon and nitrogen sources. Maximum protease production was obtained in medium containing 25 g L^-1 sucrose, 7 g L^-1 KNO₃, and initial pH 7.0 at 35 °C and 150 rpm during 72 h. Under these conditions, maximum proteolytic activity reached 1200 U mL^-1. The enzyme extract showed optimum activity at 60 °C, pH 9.0, and was stable from 30 to 50 °C within a pH range from 4.0 to 10.0 and NaCl concentration up to 2.5 M. The enzyme was stable in the presence of EDTA, urea, Triton X-100 and laundry detergent (sodium lauryl sulfate as main component). The addition of 1% sodium dodecyl sulfate, Tween-80, or Tween-20 increased the activity by 183% and 119% respectively, while 2-mercaptoethanol reduced the activity to 71%. Casein zymogram analysis revealed three hydrolysis zones suggesting that Barrientosiimonas sp. V9 expresses proteases with molecular weights about 60, 45, and 35 kDa, which were inhibited in the presence of phenylmethylsulfonyl fluoride. Barrientosiimonas sp. V9 produces halotolerant serine proteases with great biotechnological potential.

Peptide-surfactant interactions: A combined spectroscopic and molecular dynamics simulation approach

In the present contribution, we report a combined spectroscopic and computational approach aiming to unravel at atomic resolution the effect of the anionic SDS detergent on the structure of two model peptides, the α-helix TrpCage and the β-stranded TrpZip. A detailed characterization of the specific amino acids involved is performed. Monomeric (single molecules) and micellar SDS species differently interact with the α-helix and β-stranded peptides, emphasizing the different mechanisms occurring below and above the critical aggregation concentration (CAC). Below the CAC, the α-helix peptide is fully unfolded, losing its hydrophobic core and its Asp-Arg salt bridge, while the β-stranded peptide keeps its native structure with its four Trp well oriented. Above the CAC, the SDS micelles have the same effect on both peptides, that is, destabilizing the tertiary structure while keeping their secondary structure. Our studies will be helpful to deepen our understanding of the action of the denaturant SDS on peptides and proteins.

The Molecular Basis of the Sodium Dodecyl Sulfate Effect on Human Ubiquitin Structure: A Molecular Dynamics Simulation Study

To investigate the molecular interactions of sodium dodecyl sulfate (SDS) with human ubiquitin and its unfolding mechanisms, a comparative study was conducted on the interactions of the protein in the presence and absence of SDS at different temperatures using six independent 500 ns atomistic molecular dynamics (MD) simulations. Moreover, the effects of partial atomic charges on SDS aggregation and micellar structures were investigated at high SDS concentrations. The results demonstrated that human ubiquitin retains its native-like structure in the presence of SDS and pure water at 300 K, while the conformation adopts an unfolded state at a high temperature. In addition, it was found that both SDS self-assembly and the conformation of the resulting protein may have a significant effect of reducing the partial atomic charges. The simulations at 370 K provided evidence that the SDS molecules disrupted the first hydration shell and expanded the hydrophobic core of ubiquitin, resulting in complete protein unfolding. According to these results, SDS and temperature are both required to induce a completely unfolded state under ambient conditions. We believe that these findings could be useful in protein folding/unfolding studies and structural biology.

Disulfide bonds and disorder in granulin-3: An unusual handshake between structural stability and plasticity

Granulins (GRNs) are a family of small (∼6 kDa) proteins generated by the proteolytic processing of their precursor, progranulin (PGRN), in many cell types. Both PGRN and GRNs are implicated in a plethora of biological functions, often in opposing roles to each other. Lately, GRNs have generated significant attention due to their implicated roles in neurodegenerative disorders. Despite their physiological and pathological significance, the structure-function relationships of GRNs are poorly defined. GRNs contain 12 conserved cysteines forming six intramolecular disulfide bonds, making them rather exceptional, even among a few proteins with high disulfide bond density. Solution NMR investigations in the past have revealed a unique structure containing putative interdigitated disulfide bonds for several GRNs, but GRN-3 was unsolvable due to its heterogeneity and disorder. In our previous report, we showed that abrogation of disulfide bonds in GRN-3 renders the protein completely disordered (Ghag et al., Prot Eng Des Sel 2016). In this study, we report the cellular expression and biophysical analysis of fully oxidized, native GRN-3. Our results indicate that both E. coli and human embryonic kidney (HEK) cells do not exclusively make GRN-3 with homogenous disulfide bonds, likely due to the high cysteine density within the protein. Biophysical analysis suggests that GRN-3 structure is dominated by irregular loops held together only by disulfide bonds, which induced remarkable thermal stability to the protein despite the lack of regular secondary structure. This unusual handshake between disulfide bonds and disorder within GRN-3 could suggest a unique adaptation of intrinsically disordered proteins towards structural stability.

Enzymatic activity in the presence of surfactants commonly used in dissolution media, Part 1: Pepsin

The United States Pharmacopeia (USP) General Chapters Dissolution 〈711〉 and Disintegration and Dissolution of Dietary Supplements 〈2040〉 allows the use of enzymes in dissolution media when gelatin capsules do not conform to dissolution specifications due to cross linking. Possible interactions between enzymes and surfactants when used together in dissolution media could result in loss of the enzymatic activity. Pepsin is an enzyme commonly used in dissolution media, and in this work, the activity of pepsin was determined in the presence of different surfactants as usually found in case of dissolution tests of certain gelatin capsule formulations. Pepsin enzymatic activity was determined according to the Ninth Edition of the Food Chemicals Codex (FCC) 9 method, in dissolution conditions: simulated gastric fluid, 37 °C and 50 rpm. Sodium dodecyl sulfate (SDS), cetyltrimethyl ammonium bromide (CTAB), polysorbate 80 (Tween 80) and octoxynol 9 (Triton X100) in concentrations above and below their critical micellar concentrations were selected. Results showed a significant reduction in the activity of pepsin at all the concentrations of SDS assayed. On the contrary, CTAB, Tween 80, and Triton X100 did not alter the enzymatic activity at of pepsin any of the concentration assayed. This data demonstrates a rational selection of the surfactant to be used when pepsin is required in dissolution test.

Cation modulation of hemoglobin interaction with sodium n-dodecyl sulfate (SDS). III: Calcium interaction with R- and mixed spin states of hemoglobin S at pH 5.0: the musical chair paradox

We investigate the interaction of Ca(2+) (0-500 µM) and a membrane mimic (0.60 mM SDS) with both the R- and mixed spin states hemoglobin S (HbS) as a function of time. These interactions were carried out at pH 5.0. We aim at ascertaining if there is or are differences in the UV-Visible spectra of such interactions to account for the dynamics of calcium ion concentrations [Ca(2+)] in initiating structures which may ultimately suggest HbS polymerization and or resistance to Plasmodium attack. From our results, we conclude that (a) simultaneous interaction of 40 µM Ca(2+) and 0.60 mM SDS with the R state protein would promote structural formations that can "lock up" the protein for nucleation on the membranes and or become cytotoxic to the parasite; (b) simultaneous R state HbS-SDS or R state HbS-Ca(2+) would lead to enhanced hemin formation and less deoxyHb species. This condition is unlikely to precipitate polymerization in the HbS but the resulting hemin would poison the parasite; (c) the mixed spin state HbS-SDS and 40 µM Ca(2+) interaction yields more toxic products to that of the interaction of the mixed spin HbS-SDS with 500 µM Ca(2+) thus suggesting why the 40 µM Ca(2+) is important in parasite Hb proteolysis; and (d) pronounced structural changes on interaction with SDS and Ca(2+) are more in the R state to the mixed spin state.

Sorbitol counteracts temperature- and chemical-induced denaturation of a recombinant α-amylase from alkaliphilic Bacillus sp. TS-23

Enzymes are highly complex systems with a substantial degree of structural variability in their folded state. In the presence of cosolvents, fluctuations among vast numbers of folded and unfolded conformations occur via many different pathways; alternatively, certain conformations can be stabilized or destabilized. To understand the contribution of osmolytes to the stabilization of structural changes and enzymatic activity of a truncated Bacillus sp. TS-23 α-amylase (BACΔNC), we monitored amylolytic activity, circular dichroism, and fluorescence as a function of osmolytes. In the presence of trimethylamine N-oxide (TMAO) and sorbitol, BACΔNC activity was retained significantly at elevated temperatures. As compared to the control, the secondary structures of this enzyme were essentially conserved upon the addition of these two kinds of osmolytes. Fluorescence results revealed that the temperature-induced conformational change of BACΔNC was prevented by TMAO and sorbitol. However, glycerol did not provide profound protection against thermal denaturation of the enzyme. Sorbitol was further found to counteract guanidine hydrochloride- and SDS-induced denaturation of BACΔNC. Thus, some well-known naturally occurring osmolytes make a dominant contribution to the stabilization of BACΔNC.

Denaturation of proteins by SDS and tetraalkylammonium dodecyl sulfates

This article describes the use of capillary electrophoresis (CE) to examine the influence of different cations (C(+); C(+) = Na(+) and tetra-n-alkylammonium, NR(4)(+), where R = Me, Et, Pr, and Bu) on the rates of denaturation of bovine carbonic anhydrase II (BCA) in the presence of anionic surfactant dodecylsulfate (DS(-)). An analysis of the denaturation of BCA in solutions of Na(+)DS(-) and NR(4)(+)DS(-) (in Tris-Gly buffer) indicated that the rates of formation of complexes of denatured BCA with DS(-) (BCA(D)-DS(-)(n,sat)) are indistinguishable and independent of the cation below the critical micellar concentration (cmc) and independent of the total concentration of DS(-) above the cmc. At concentrations of C(+)DS(-) above the cmc, BCA denatured at rates that depended on the cation; the rates decreased by a factor >10(4) in the order of Na(+) ≈ NMe(4)(+) > NEt(4)(+) > NPr(4)(+) > NBu(4)(+), which is the same order as the values of the cmc (which decrease from 4.0 mM for Na(+)DS(-) to 0.9 mM for NBu(4)(+)DS(-) in Tris-Gly buffer). The relationship between the cmc values and the rates of formation of BCA(D)-DS(-)(n,sat()) suggested that the kinetics of denaturation of BCA involve the association of this protein with monomeric DS(-) rather than with micelles of (C(+)DS(-))(n). A less-detailed survey of seven other proteins (α-lactalbumin, β-lactoglobulin A, β-lactoglobulin B, carboxypeptidase B, creatine phosphokinase, myoglobin, and ubiquitin) showed that the difference between Na(+)DS(-) and NR(4)(+)DS(-) observed with BCA was not general. Instead, the influence of NR(4)(+) on the association of DS(-) with these proteins depended on the protein. The selection of the cation contributed to the properties (including the composition, electrophoretic mobility, and partitioning behavior in aqueous two-phase systems) of aggregates of denatured protein and DS(-). These results suggest that the variation in the behavior of NR(4)(+)DS(-) with changes in R may be exploited in methods used to analyze and separate mixtures of proteins.

PagP crystallized from SDS/cosolvent reveals the route for phospholipid access to the hydrocarbon ruler

Enzymatic reactions involving bilayer lipids occur in an environment with strict physical and topological constraints. The integral membrane enzyme PagP transfers a palmitoyl group from a phospholipid to lipid A in order to assist Escherichia coli in evading host immune defenses during infection. PagP measures the palmitoyl group with an internal hydrocarbon ruler that is formed in the interior of the eight-stranded antiparallel β barrel. The access and egress of the palmitoyl group is thought to take a lateral route from the bilayer phase to the barrel interior. Molecular dynamics, mutagenesis, and a 1.4 A crystal structure of PagP in an SDS / 2-methyl-2,4-pentanediol (MPD) cosolvent system reveal that phospholipid access occurs at the crenel present between strands F and G of PagP. In this way, the phospholipid head group can remain exposed to the cell exterior while the lipid acyl chain remains in a predominantly hydrophobic environment as it translocates to the protein interior.

Ionic strength-dependent denaturation of Thermomyces lanuginosus lipase induced by SDS

Triglyceride lipase from Thermomyces lanuginosus (TlL) has been reported to be resistant to denaturation by sodium dodecyl sulfate (SDS). We have found that at neutral pH, structural integrity is strongly dependent on ionic strength. In 10mM phosphate buffer and SDS, the lipase exhibits a far-UV CD spectrum similar to other proteins denatured in this surfactant while the near-UV CD spectrum shows a complete loss of tertiary structure, observations supported by steady state fluorescence spectroscopy. However, when increasing the ionic strength by the addition of NaCl, the lipase was rendered resistant towards SDS denaturation, as observed by all techniques employed. The effect of salt on the critical micelle concentration (CMC) of SDS was observed to correlate with the effect on the degree of SDS-induced denaturation. This finding is compatible with the notion that the concentration of SDS monomers is a crucial factor for SDS-lipase interactions. The presented results are important for the understanding and improvement of protein stability in surfactant systems.

Characterization of membrane protein non-native states. 2. The SDS-unfolded states of rhodopsin

Little is known about the molecular nature of residual structure in unfolded states of membrane proteins. A screen of chemical denaturants to maximally unfold the mammalian membrane protein and prototypic G protein coupled receptor rhodopsin, without interference from aggregation, described in an accompanying paper (DOI 10.1021/bi100338e ), identified sodium dodecyl sulfate (SDS), alone or in combination with other chemicals, as the most suitable denaturant. Here, we initiate the biophysical characterization of SDS-denatured states of rhodopsin. Using absorption, steady-state and time-resolved fluorescence spectroscopy, dynamic light scattering, and cysteine accessibility studies, tertiary structure of denatured states was characterized. In agreement with the pattern of secondary structure changes detected by circular dichroism described in the accompanying paper (DOI 10.1021/bi100338e ), tertiary structure changes are distinct over four SDS concentration ranges based on the expected predominant micellar structures. Dodecyl maltoside (DM)/SDS mixed micelle spheres (0.05-0.3% SDS) turn into SDS spheres (0.3-3% SDS) that gradually (3-15% SDS) become cylindrical (above 15% SDS). Denatured states in SDS spheres and cylinders show a relatively greater burial of cysteine and tryptophan residues and are more compact as compared to the states observed in mixed micellar structures. Protein structural changes at the membrane/water interface region are most prominent at very low SDS concentrations but reach transient stability in the compact conformations in SDS spheres. This is the first experimental evidence for the formation of a compact unfolding intermediate state with flexible surface elements in a membrane protein.

Characterization of membrane protein non-native states. 1. Extent of unfolding and aggregation of rhodopsin in the presence of chemical denaturants

Little is known about the general folding mechanisms of helical membrane proteins. Unfolded, i.e., non-native states, in particular, have not yet been characterized in detail. Here, we establish conditions under which denatured states of the mammalian membrane protein rhodopsin, a prototypic G protein coupled receptor with primary function in vision, can be studied. We investigated the effects of the chemical denaturants sodium dodecyl sulfate (SDS), urea, guanidine hydrochloride (GuHCl), and trifluoroacetic acid (TFA) on rhodopsin's secondary structure and propensity for aggregation. Ellipticity at 222 nm decreases in the presence of maximum concentrations of denaturants in the order TFA > GuHCl > urea > SDS + urea > SDS. Interpretation of these changes in ellipticity in terms of helix loss is challenged because the addition of some denaturants leads to aggregation. Through a combination of SDS-PAGE, dependence of ellipticity on protein concentration, and 1D (1)H NMR we show that aggregates form in the presence of GuHCl, TFA, and urea but not in any concentration of SDS, added over a range of 0.05%-30%. Mixed denaturant conditions consisting of 3% SDS and 8 M urea, added in this order, also did not result in aggregation. We conclude that SDS is able to prevent the exposure of large hydrophobic regions present in membrane proteins which otherwise leads to aggregation. Thus, 30% SDS and 3% SDS + 8 M urea are the denaturing conditions of choice to study maximally unfolded rhodopsin without aggregation.

On the mechanism of SDS-induced protein denaturation

To understand the mechanism of ionic detergent-induced protein denaturation, this study examines the action of sodium dodecyl sulfate on ferrocytochrome c conformation under neutral and strongly alkaline conditions. Equilibrium and stopped-flow kinetic results consistently suggest that tertiary structure unfolding in the submicellar and chain expansion in the micellar range of SDS concentrations are the two major and discrete events in the perturbation of protein structure. The nature of interaction between the detergent and the protein is predominantly hydrophobic in the submicellar and exclusively hydrophobic at micellar levels of SDS concentration. The observation that SDS also interacts with a highly denatured and negatively charged form of ferrocytochrome c suggests that the interaction is independent of structure, conformation, and ionization state of the protein. The expansion of the protein chain at micellar concentration of SDS is driven by coulombic repulsion between the protein-bound micelles, and the micelles and anionic amino acid side chains.

Pathway for unfolding of ubiquitin in sodium dodecyl sulfate, studied by capillary electrophoresis

This paper characterizes the complexes formed by a small protein, ubiquitin (UBI), and a negatively charged surfactant, sodium dodecyl sulfate (SDS), using capillary electrophoresis (CE), circular dichroism (CD), and amide hydrogen-deuterium exchange (HDX; as monitored by mass spectroscopy, MS). Capillary electrophoresis of complexes of UBI and SDS, at apparent equilibrium, at concentrations of SDS ranging from sub-micellar and sub-denaturing to micellar and denaturing, revealed multiple complexes of UBI and SDS of the general composition UBI-SDS(n). Examination of electrophoretic mobilities of complexes of UBI and SDS as a function of the concentration of SDS provided a new way to characterize the interaction of this protein with SDS and established key characteristics of this system: e.g., the reversibility of the formation of the complexes, their approximate chemical compositions, and the pathway of SDS binding to UBI. The work identified, in addition to SDS-saturated UBI, at least six groups of complexes of UBI with SDS, within which four groups were populated with complexes of distinct stoichiometries: UBI-SDS(approximately 11), UBI-SDS(approximately 25), UBI-SDS(approximately 33), and UBI-SDS(approximately 42). CD spectroscopy and amide HDX of the UBI-SDS(n) complexes suggested that many of the UBI-SDS(n) complexes (n > 11) have greater alpha-helical content than native UBI. Capillary electrophoresis provides a level of detail about interactions of proteins and SDS that has not previously been accessible, and CE is an analytical and biophysical method for studies of interactions of proteins and surfactants that is both convenient and practical. This study sheds light on the formation of the enigmatic protein-SDS complexes formed during SDS polyacrylamide gel electrophoresis and brings a new tool to the study of proteins and detergents.

The role of decorated SDS micelles in sub-CMC protein denaturation and association

We have combined spectroscopy, chromatography, calorimetry, and small-angle X-ray scattering (SAXS) to provide a comprehensive structural and stoichiometric description of the sodium dodecyl sulfate (SDS)-induced denaturation of the 86-residue alpha-helical bovine acyl-coenzyme-A-binding protein (ACBP). Denaturation is a multistep process. Initial weak binding of 1-3 SDS molecules per protein molecule below 1.3 mM does not perturb the tertiary structure. Subsequent binding of approximately 13 SDS molecules per ACBP molecule leads to the formation of SDS aggregates on the protein and changes in both tertiary and secondary structures. SAXS data show that, at this stage, a decorated micelle links two ACBP molecules together, leaving about half of the polypeptide chain as a disordered region protruding into the solvent. Further titration with SDS leads to the additional uptake of 26 SDS molecules, which, according to SAXS, forms a larger decorated micelle bound to a single ACBP molecule. At the critical micelle concentration, we conclude from reduced mobility and increased fluorescence anisotropy that each ACBP molecule becomes associated with more than one micelle. At this point, 56-60 SDS molecules are bound per ACBP molecule. Our data provide key structural insights into decorated micelle complexes with proteins, revealing a remarkable diversity in the different conformations they can stabilize. The data highlight that a minimum decorated micelle size, which may be a key driving force for intermolecular protein association, exists. This may also provide a structural basis for the known ability of submicellar surfactant concentrations to induce protein aggregation and fibrillation.

The bacteriorhodopsin carboxyl-terminus contributes to proton recruitment and protein stability

We examined functional and structural roles for the bacteriorhodopsin (bR) carboxyl-terminus. The extramembranous and intracellular carboxyl-terminus was deleted by insertion of premature translation stop codons. Deletion of the carboxyl-terminus had no effect on purple membrane (PM) lattice dimensions, sheet size, or the electrogenic environment of the ground-state chromophore. Removal of the distal half of the carboxyl-terminus had no effect on light-activated proton pumping, however, truncation of the entire carboxyl-terminus accelerated the rates of M-state decay and proton uptake approximately 3.7-fold and severely distorted the kinetics of proton uptake. Differential scanning calorimetry (DSC) and SDS denaturation demonstrated that removal of the carboxyl-terminus decreased protein stability. The DSC melting temperature was lowered by 6 degrees C and the calorimetric enthalpy reduced by 50% following removal of the carboxyl-terminus. Over the time range of milliseconds to hours at least 3 phases were required to describe the SDS denaturation kinetics for each bR construction. The fastest phases were indistinguishable for all bR's, and reflected PM solubilization. At pH 7.4, 20 degrees C, and in 0.3% SDS (w/v) the half-times of bR denaturation were 19.2 min for the wild-type, 12.0 min for the half-truncation and 3.6 min for the full-truncation. Taken together the results of this study suggest that the bR ground state exhibits two "domains" of stability: (1) a core chromophore binding pocket domain that is insensitive to carboxyl-terminal interactions and (2) the surrounding helical bundle whose contributions to protein stability and proton pumping are influenced by long-range interactions with the extramembranous carboxyl-terminus.

Interaction of giant extracellular Glossoscolex paulistus hemoglobin (HbGp) with zwitterionic surfactant N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS): Effects of oligomeric dissociation

The present work focuses on the interaction between the zwitterionic surfactant N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) and the giant extracellular hemoglobin of Glossoscolex paulistus (HbGp). Electronic optical absorption, fluorescence emission and circular dichroism spectroscopy techniques, together with Gel-filtration chromatography, were used in order to evaluate the oligomeric dissociation as well as the autoxidation of HbGp as a function of the interaction with HPS. A peculiar behavior was observed for the HPS-HbGp interaction: a complex ferric species formation equilibrium was promoted, as a consequence of the autoxidation and oligomeric dissociation processes. At pH 7.0, HPS is more effective up to 1mM while at pH 9.0 the surfactant effect is more intense above 1mM. Furthermore, the interaction of HPS with HbGp was clearly less intense than the interaction of this hemoglobin with cationic (CTAC) and anionic (SDS) surfactants. Probably, this lower interaction with HPS is due to two factors: (i) the lower electrostatic attraction between the HPS surfactant and the protein surface ionic sites when compared to the electrostatic interaction between HbGp and cationic and anionic surfactants, and (ii) the low cmc of HPS, which probably reduces the interaction of the surfactant in the monomeric form with the protein. The present work emphasizes the importance of the electrostatic contribution in the interaction between ionic surfactants and HbGp. Furthermore, in the whole HPS concentration range used in this study, no folding and autoxidation decrease induced by this surfactant were observed. This is quite different from the literature data on the interaction between surfactants and tetrameric hemoglobins, that supports the occurrence of this behavior for the intracellular hemoglobins at low surfactant concentration range. Spectroscopic data are discussed and compared with the literature in order to improve the understanding of hemoglobin-surfactant interaction as well as the acid isoelectric point (pI) influence of the giant extracellular hemoglobins on their structure-activity relationship.

Global study of myoglobin-surfactant interactions

Surfactants interact with proteins in multifarious ways which depend on surfactant concentration and structure. To obtain a global overview of this process, we have analyzed the interaction of horse myoglobin (Mb) with an anionic (SDS) and cationic (CTAC) surfactant, using both equilibrium titration techniques and stopped-flow kinetics. Binding and kinetics of conformational changes can be divided into a number of different regions (five below the cmc and one above) with very distinct features (broadly similar between the two surfactants, despite their difference in head group and chain length), which nuance the classical view of biphasic binding prior to micellization. In stage A, fairly weak interactions lead to a linear decrease in thermal stability. This gives way to a more cooperative process in stage B, where aggregates (presumably hemimicelles) start to form on the protein surface, leading to global denaturation (loss of a thermal transition) and biphasic unfolding kinetics. This is consolidated in stage C with titratable surfactant adsorption. Adsorption of this surfactant species leads to significant changes in kinetics, namely, inhibition of unfolding kinetics in CTAC and altered unfolding amplitudes in SDS, though the process is still biphasic in both surfactants. Stage D commences the reduction in exothermic binding signals, leading to further uptake of 5 (SDS) or 31 (CTAC) surfactant molecules without any major changes in protein conformation. In stage E many more surfactant molecules (46 SDS and 39 CTAC) are bound, presumably as quasi-micellar structures, and we observe a very slow unfolding phase in SDS, which disappears as we reach the cmc. Above the cmc, the unfolding rates remain essentially constant in SDS, but increase significantly in CTAC, possibly because binding of bulk micelles removes the inhibition by hemimicellar aggregates. Our work highlights the fascinating richness of conformational changes that proteins can undergo in the presence of molecules with self-assembling properties.

A comparative study on two fungal lipases from Thermomyces lanuginosus and Yarrowia lipolytica shows the combined effects of detergents and pH on lipase adsorption and activity

The effects of various detergents and pH on the interfacial binding and activity of two fungal lipases from Yarrowia lipolytica (YLLIP2) and Thermomyces lanuginosus (TLL) were investigated using trioctanoin emulsions as well as monomolecular films spread at the air-water interface. Contrary to TLL, YLLIP2 was found to be more sensitive than TLL to interfacial denaturation but it was protected by detergent monomers and lowering the temperature. At pH 7.0, both the interfacial binding and the activities on trioctanoin of YLLIP2 and TLL were inhibited by sodium taurodeoxycholate (NaTDC). At pH 6.0, however, YLLIP2 remained active on trioctanoin in the presence of NaTDC, whereas TLL did not. YLLIP2 activity on trioctanoin was associated with strong interfacial binding of the enzyme to trioctanoin emulsion, whereas TLL was mostly detected in the water phase. The combined effects of bile salts and pH on lipase activity were therefore enzyme-dependent. YLLIP2 binds more strongly than TLL at oil-water interfaces at low pH when detergents are present. These findings are particularly important for lipase applications, in particular for enzyme replacement therapy in patients with pancreatic enzyme insufficiency since high detergent concentrations and highly variable pH values can be encountered in the GI tract.

Glycoprotein-surfactant interactions: A calorimetric and spectroscopic investigation of the phytase-SDS system

The interactions of sodium dodecyl sulfate (SDS) and two glyco-variants of the enzyme phytase from Peniophora lycii were investigated. One variant (Phy) was heavily glycosylated while the other (dgPhy) was enzymatically deglycosylated. Effects at 24 degrees C of titrating SDS to Phy and dgPhy were studied by Isothermal Titration Calorimetry (ITC) and Synchrotron Radiation Circular Dichroism (SRCD) spectroscopy. Comparisons of results for the two variants were used to elucidate glycan-surfactant interrelationships. The CD spectra suggested that both the native and the SDS-denatured states of the two variants were mutually similar, and hence that the denaturation process was structurally equivalent for the two glyco-variants. The denatured state was far from fully unfolded and probably retained a substantial content of native-like structure. Furthermore, it was found that the glycans brought about only a small increase in the resistance towards SDS induced denaturation. The SDS concentration required to denature half of the protein molecules differed less than 1 mM for the two variants. The affinity for SDS of both variants was unusually low. The amount of bound SDS (w/w) at different stages of the binding isotherm was 3-10 times lower than that reported for the most previously investigated globular proteins. Analysis of the relative affinity of the glycan and peptide moieties suggested that the carbohydrates bind much less surfactant. At saturation, glycans adsorbed about half as much SDS (in g/g) as the peptide moiety of Phy and about five times less than average proteins.

Unfolding kinetics of beta-lactoglobulin induced by surfactant and denaturant: a stopped-flow/fluorescence study

The beta-->alpha transition of beta-lactoglobulin, a globular protein abundant in the milk of several mammals, is investigated in this work. This transition, induced by the cationic surfactant dodecyltrimethylammonium chloride (DTAC), is accompanied by partial unfolding of the protein. In this work, unfolding of bovine beta-lactoglobulin in DTAC is compared with its unfolding induced by the chemical denaturant guanidine hydrochloride (GnHCl). The final protein states attained in the two media have quite different secondary structure: in DTAC the alpha-helical content increases, leading to the so-called alpha-state; in GnHCl the amount of ordered secondary-structure decreases, resulting in a random coil-rich final state (denatured, or D, state). To obtain information on both mechanistic routes, in DTAC and GnHCl, and to characterize intermediates, the kinetics of unfolding were investigated in the two media. Equilibrium and kinetic data show the partial accumulation of an on-pathway intermediate in each unfolding route: in DTAC, an intermediate (I(1)) with mostly native secondary structure but loose tertiary structure appears between the native (beta) and alpha-states; in GnHCl, another intermediate (I(2)) appears between states beta and D. Kinetic rate constants follow a linear Chevron-plot representation in GnHCl, but show a more complex mechanism in DTAC, which acts like a stronger binding species.

Conformation and lipid binding of a C-terminal (198-243) peptide of human apolipoprotein A-I

Human apolipoprotein A-I (apoA-I) is the principle apolipoprotein of high-density lipoproteins that are critically involved in reverse cholesterol transport. The intrinsically flexibility of apoA-I has hindered studies of the structural and functional details of the protein. Our strategy is to study peptide models representing different regions of apoA-I. Our previous report on [1-44]apoA-I demonstrated that this N-terminal region is unstructured and folds into approximately 60% alpha-helix with a moderate lipid binding affinity. We now present details of the conformation and lipid interaction of a C-terminal 46-residue peptide, [198-243]apoA-I, encompassing putative helix repeats 10 and 9 and the second half of repeat 8 from the C-terminus of apoA-I. Far-ultraviolet circular dichroism spectra show that [198-243]apoA-I is also unfolded in aqueous solution. However, self-association induces approximately 50% alpha-helix in the peptide. The self-associated peptide exists mainly as a tetramer, as determined by native electrophoresis, cross-linking with glutaraldehyde, and unfolding data from circular dichroism (CD) and differential scanning calorimetry (DSC). In the presence of a number of lipid-mimicking detergents, above their CMC, approximately 60% alpha-helix was induced in the peptide. In contrast, SDS, an anionic lipid-mimicking detergent, induced helical folding in the peptide at a concentration of approximately 0.003% (approximately 100 microM), approximately 70-fold below its typical CMC (0.17-0.23% or 6-8 mM). Both monomeric and tetrameric peptide can solubilize dimyristoylphosphatidylcholine (DMPC) liposomes and fold into approximately 60% alpha-helix. Fractionation by density gradient ultracentrifugation and visualization by negative staining electromicroscopy demonstrated that the peptide binds to DMPC with a high affinity to form at least two sizes of relatively homogeneous discoidal HDL-like particles depending on the initial lipid:peptide ratio. The characteristics (lipid:peptide weight ratio, diameter, and density) of both complexes are similar to those of plasma A-I/DMPC complexes formed under similar conditions: small discoidal complexes (approximately 3:1 weight ratio, approximately 110 A, and approximately 1.10 g/cm3) formed at an initial 1:1 weight ratio and larger discoidal complexes (approximately 4.6:1 weight ratio, approximately 165 A, and approximately 1.085 g/cm3) formed at initial 4:1 weight ratio. The cross-linking data for the peptide on the complexes of two sizes is consistent with the calculated peptide numbers per particle. Compared to the approximately 100 A disk-like complex formed by the N-terminal peptide in which helical structure was insufficient to cover the disk edge by a single belt, the compositions of these two types of complexes formed by the C-terminal peptide are more consistent with a "double belt" model, similar to that proposed for full-length apoA-I. Thus, our data provide direct evidence that this C-terminal region of apoA-I is responsible for the self-association of apoA-I, and this C-terminal peptide model can mimic the interaction with the phospholipid of plasma apoA-I to form two sizes of homogeneous discoidal complexes and thus may be responsible for apoA-I function in the formation and maintenance of HDL subspecies in plasma.

Giant extracellular Glossoscolex paulistus Hemoglobin (HbGp) upon interaction with cethyltrimethylammonium chloride (CTAC) and sodium dodecyl sulphate (SDS) surfactants: Dissociation of oligomeric structure and autoxidation

The effects of two ionic surfactants on the oligomeric structure of the giant extracellular hemoglobin of Glossoscolex paulistus (HbGp) in the oxy - form have been studied through the use of several spectroscopic techniques such as electronic optical absorption, fluorescence emission, light scattering, and circular dichroism. The use of anionic sodium dodecyl sulphate (SDS) and cationic cethyltrimethyl ammonium chloride (CTAC) has allowed to differentiate the effects of opposite headgroup charges on the oligomeric structure dissociation and hemoglobin autoxidation. At pH 7.0, both surfactants induce the protein dissociation and a significant oxidation. Spectral changes occur at very low CTAC concentrations suggesting a significant electrostatic contribution to the protein-surfactant interaction. At low protein concentration, 0.08 mg/ml, some light scattering within a narrow CTAC concentration range occurs due to protein-surfactant precipitation. Light scattering experiments showed the dissociation of the oligomeric structure by SDS and CTAC, and the effect of precipitation induced by CTAC. At higher protein concentrations, 3.0 mg/ml, a precipitation was observed due to the intense charge neutralization upon formation of ion pair in the protein-surfactant precipitate. The spectral changes are spread over a much wider SDS concentration range, implying a smaller electrostatic contribution to the protein-surfactant interactions. The observed effects are consistent with the acid isoelectric point (pI) of this class of hemoglobins, which favors the intense interaction of HbGp with the cationic surfactant due to the existence of excess acid anionic residues at the protein surface. Protein secondary structure changes are significant for CTAC at low concentrations while they occur at significantly higher concentrations for SDS. In summary, the cationic surfactant seems to interact more strongly with the protein producing more dramatic spectral changes as compared to the anionic one. This is opposite as observed for several other hemoproteins. The surfactants at low concentrations produce the oligomeric dissociation, which facilitates the iron oxidation, an important factor modulating further oligomeric protein dissociation.

α-Synuclein Multistate Folding Thermodynamics: Implications for Protein Misfolding and Aggregation

Alpha-synuclein aggregation has been tightly linked with the pathogenesis of Parkinson's disease and other neurodegenerative disorders. Despite the protein's putative function in presynaptic vesicle regulation, the roles of lipid binding in modulating alpha-synuclein conformations and the aggregation process remain to be fully understood. This study focuses on a detailed thermodynamic characterization of monomeric alpha-synuclein folding in the presence of SDS, a well-studied lipid mimetic. Far-UV CD spectroscopy was employed for detection of conformational transitions induced by SDS, temperature, and pH. The data we present here clearly demonstrate the multistate nature of alpha-synuclein folding, which involves two predominantly alpha-helical partially folded thermodynamic intermediates that we designate as F (most folded) and I (intermediately folded) states. Likely structures of these alpha-synuclein conformational states are also discussed. These partially folded forms can exist in the presence of either monomeric or micellar forms of SDS, which suggests that alpha-synuclein has an intrinsic propensity for adopting multiple alpha-helical structures even in the absence of micelle or membrane binding, a feature that may have implications for its biological activity and toxicity. Additionally, we discuss the relation between alpha-synuclein three-state folding and its aggregation, within the context of isothermal titration calorimetry and transmission electron microscopy measurements of SDS-initiated oligomer formation.