Tertiary interactions in the folding pathway of hen lysozyme: kinetic studies using fluorescent probes

Molecular insights into the phase transition of lysozyme into amyloid nanostructures: Implications of therapeutic strategies in diverse pathological conditions

Lysozyme, a well-known bacteriolytic enzyme, exhibits a fascinating yet complex behavior when it comes to protein aggregation. Under certain conditions, this enzyme undergoes flexible transformation, transitioning from partially unfolded intermediate units of native conformers into complex cross-β-rich nano fibrillar amyloid architectures. Formation of such lysozyme amyloids has been implicated in a multitude of pathological and medical severities, like hepatic dysfunction, hepatomegaly, splenic rupture as well as spleen dysfunction, nephropathy, sicca syndrome, renal dysfunction, renal amyloidosis, and systemic amyloidosis. In this comprehensive review, we have attempted to provide in-depth insights into the aggregating behavior of lysozyme across a spectrum of variables, including concentrations, temperatures, pH levels, and mutations. Our objective is to elucidate the underlying mechanisms that govern lysozyme's aggregation process and to unravel the complex interplay between its structural attributes. Moreover, this work has critically examined the latest advancements in the field, focusing specifically on novel strategies and systems, that have been implemented to delay or inhibit the lysozyme amyloidogenesis. Apart from this, we have tried to explore and advance our fundamental understanding of the complex processes involved in lysozyme aggregation. This will help the research community to lay a robust foundation for screening, designing, and formulating targeted anti-amyloid therapeutics offering improved treatment modalities and interventions not only for lysozyme-linked amyloidopathy but for a wide range of amyloid-related disorders.

The Right-Handed Parallel β-Helix Topology of Erwinia chrysanthemi Pectin Methylesterase Is Intimately Associated with Both Sequential Folding and Resistance to High Pressure

The complex topologies of large multi-domain globular proteins make the study of their folding and assembly particularly demanding. It is often characterized by complex kinetics and undesired side reactions, such as aggregation. The structural simplicity of tandem-repeat proteins, which are characterized by the repetition of a basic structural motif and are stabilized exclusively by sequentially localized contacts, has provided opportunities for dissecting their folding landscapes. In this study, we focus on the Erwinia chrysanthemi pectin methylesterase (342 residues), an all-β pectinolytic enzyme with a right-handed parallel β-helix structure. Chemicals and pressure were chosen as denaturants and a variety of optical techniques were used in conjunction with stopped-flow equipment to investigate the folding mechanism of the enzyme at 25 °C. Under equilibrium conditions, both chemical- and pressure-induced unfolding show two-state transitions, with average conformational stability (ΔG° = 35 ± 5 kJ·mol⁻¹) but exceptionally high resistance to pressure (P_m = 800 ± 7 MPa). Stopped-flow kinetic experiments revealed a very rapid (τ < 1 ms) hydrophobic collapse accompanied by the formation of an extended secondary structure but did not reveal stable tertiary contacts. This is followed by three distinct cooperative phases and the significant population of two intermediate species. The kinetics followed by intrinsic fluorescence shows a lag phase, strongly indicating that these intermediates are productive species on a sequential folding pathway, for which we propose a plausible model. These combined data demonstrate that even a large repeat protein can fold in a highly cooperative manner.

An experimental investigation on the influence of various buffer concentrations, osmolytes and gold nanoparticles on lysozyme: Spectroscopic and calorimetric study

Considering importance and several industrial applications of lysozyme, including natural antibiotic and preservative, identifier for the diagnosis of diseases, and extraction purposes, its reversibility and stability studies can be very important. In this paper, the role that buffer and osmolytes concentrations play on the thermodynamic stability of lysozyme denaturation process, that is a new simple and inexpensive method, was evaluated by Nano-DSC III, far- and near-UV CD and fluorescence techniques. In thermal denaturation study, RI and ΔG of protein increased from 25.62% to 58.82% and 48.87 to 63.63 kJ mol^-1 with the increment of buffer and osmolytes concentrations, respectively. These changes showed a significant increase of 129.59% in RI and 28.16% in ΔG. The effect of buffer and osmolytes concentrations on the secondary and tertiary structures of protein was also investigated. The results indicated that increment of buffer and osmolytes concentrations increase rigidity and thermodynamic stability of protein. Also, structure of protein may be changed by its interaction with GNPs. Hence, interaction of lysozyme with GNPs was studied at the buffer and osmolytes concentrations that gives the maximum RI and ΔG, respectively. The results showed that molten globule-like state was formed by lysozyme in the presence of GNPs.

Effect of the concentration of protein and nanoparticles on the structure of biohybrid nanocomposites

We present colloidal nanocomposites formed by incorporating magnetite Fe₃ O₄ nanoparticles (MNPs) with lysozyme amyloid fibrils (LAFs). Preparation of two types of solutions, with and without addition of salt, was carried out to elucidate the structure of MNPs-incorporated fibrillary nanocomposites and to study the effect of the presence of salt on the stability of the nanocomposites. The structural morphology of the LAFs and their interaction with MNPs were analyzed by atomic force microscopy and small-angle x-ray scattering measurements. The results indicate that conformational properties of the fibrils are dependent on the concentration of protein, and the precise ratio of the concentration of the protein and MNPs is crucially important for the stability of the fibrillary nanocomposites. Our results confirm that despite the change in fibrillary morphology induced by the varying concentration of the protein, the adsorption of MNPs on the surface of LAF is morphologically independent. Moreover, most importantly, the samples containing salt have excellent stability for up to 1 year of shelf-life.

Comparative Study of Protein Aggregation Arrest by Zwitterionic Polysulfobetaines: Using Contrasting Raft Agents

Protein aggregation has caused limitations in the study and development of protein-based biopharmaceuticals. We prepared different polysulfobetaine (poly-SPB) polymers via reversible addition fragmentation chain transfer (RAFT) polymerization. These polymers exhibited high efficiency in modulation of protein aggregation. We synthesized polysulfobetaines using two different RAFT agents, and analyzed the aggregation profile of lysozyme and insulin. In poly-SPBs, existence of a hydrophobic RAFT agent resulted in visible enhancement of the residual enzymatic activity of lysozyme, whereas it remained unaffected by the hydrophilic RAFT agent. In addition, these polymers resulted in significant suppression in the aggregation of insulin. Increase in the molecular weight of the polymer caused higher efficiency to perpetuate enzymatic activity of lysozyme upon thermal denaturation. The polymers arrested the formation of amyloid like fibrils of lysozyme and insulin, thus indicating their potential to inhibit aggregation. The results unambiguously demonstrate the importance of polysulfobetaine moiety and hydrophobicity in protein aggregation inhibition. This study gives insight into the protein aggregation inhibition by zwitterionic polymers, which have a potential to be developed as aggregation inhibitors in the future.

Mechanical Softening of a Small Ubiquitin-Related Modifier Protein Due to Temperature Induced Flexibility at the Core

Despite the growing interest in the thermal softening of proteins, the mechanistic details of it are far from understood. β-Grasp proteins have globular shape with compact structure and they are mechanically resilient. The β-clamp or mechanical clamp in them formed by the interactions between the terminal β-strands is generally associated with the protein mechanical resistance. Although previous studies showed that temperature can perturb the protein mechanical stability, the structural changes leading to the lowered mechanical resistance are not known. Here, we investigated the temperature dependent mechanical stability of small ubiquitin-related modifier 2 (SUMO2) using single-molecule force spectroscopy (SMFS) and the corresponding conformational changes using ensemble experiments. SMFS studies on the polyprotein of SUMO2 estimate a decrease in the spring constant of the protein from 4.50 to 1.35 N/m upon increasing the temperature from 5 to 45 °C. Interestingly, near-UV circular dichroism spectroscopy reveals a decrease in tertiary structure content while the overall secondary structure of the protein remains unchanged. Steady-state fluorescence and quenching studies on SUMO2 with a tryptophan mutation at the core (F60W) show that the nonpolar environment of the tryptophan is unchanged and the protein core is inaccessible to the bulk solvent, in the same temperature range. We attribute the thermal softening observed in atomic force microscopy (AFM) experiments to the reduction in tertiary structure of SUMO2. Our results provide evidence for the importance of the intramolecular interactions at the protein core along with the β-clamp or mechanical clamp in providing the mechanical resistance as well as in modulating the protein stiffness.

Interaction of nonionic detergents with the specific sites of lysozyme amyloidogenic region - inhibition of amyloid fibrillization

Two nonionic detergents, Triton X-100 (TX-100) and n-dodecyl-β-d-maltoside (DDM) were tested for their ability to affect lysozyme amyloid aggregation. We have demonstrated that fibrillization of lysozyme is completely inhibited by low sub-micellar concentrations of both of these detergents. The apparent IC₅₀ values were calculated to be 22μM and 26μM for TX-100 and DDM, respectively. The detergent/protein ratio is not the only parameter controlling inhibition. The precise timing of the detergent addition was found to be also crucial. It appears that the primary inhibitory activity of detergents resulted from inhibition of nuclei formation, in addition to inhibition of fibril polymerization at the early stage of protofibrils growth. The docking study revealed that Asn-59, Trp-63 and Ala-107, all present within the lysozyme amyloidogenic region, were involved in the interaction with both detergents. In addition, TX-100 also interacted with Gln-57 and Asp-103 within lysozyme. Moreover, based on our computational results, TX-100 bridges the Gln-57 and Ala-107 amino acids of the amyloidogenic segment of lysozyme and therefore inhibits more effectively the amyloid fibril formation. Along these lines, the knowledge gained from our study indicates that the detergents or their derivatives may be applicable as a promising strategy for the modulation of lysozyme protein aggregation.

ReAsH as a Quantitative Probe of In-Cell Protein Dynamics

The tetracysteine (tc) tag/biarsenical dye system (FlAsH or ReAsH) promises to combine the flexibility of fluorescent protein tags with the small size of dye labels, allowing in-cell study of target proteins that are perturbed by large protein tags. Quantitative thermodynamic and kinetic studies in-cell using FlAsH and ReAsH have been hampered by methodological complexities presented by the fluorescence properties of the tag-dye complex probed by either Förster resonance energy transfer (FRET) or direct excitation. We label the model protein phosphoglycerate kinase (PGK) with AcGFP1 and ReAsH for direct comparison with AcGFP1/mCherry-labeled PGK. We find that fast relaxation imaging (FReI), combining millisecond temperature jump kinetics with fluorescence microscopy detection, circumvents many of the difficulties encountered working with the ReAsH system, allowing us to obtain quantitative FRET measurements of protein stability and kinetics both in vitro and in cells. We also demonstrate the to us surprising result that fluorescence from directly excited, unburied ReAsH at the C-terminus of the model protein also reports on folding in vitro and in cells. Comparing the ReAsH-labeled protein to a construct labeled with two fluorescent protein tags allows us to evaluate how a bulkier protein tag affects protein dynamics in cells and in vitro. We find that the average folding rate in the cell is closer to the in vitro rate with the smaller tag, highlighting the effect of tags on quantitative in-cell measurements.

Preferential Interactions and the Effect of Protein PEGylation

Background

PEGylation is a strategy used by the pharmaceutical industry to prolong systemic circulation of protein drugs, whereas formulation excipients are used for stabilization of proteins during storage. Here we investigate the role of PEGylation in protein stabilization by formulation excipients that preferentially interact with the protein.

Methodology/Principal Findings

The model protein hen egg white lysozyme was doubly PEGylated on two lysines with 5 kDa linear PEGs (mPEG-succinimidyl valerate, MW 5000) and studied in the absence and presence of preferentially excluded sucrose and preferentially bound guanine hydrochloride. Structural characterization by far- and near-UV circular dichroism spectroscopy was supplemented by investigation of protein thermal stability with the use of differential scanning calorimetry, far and near-UV circular dichroism and fluorescence spectroscopy. It was found that PEGylated lysozyme was stabilized by the preferentially excluded excipient and destabilized by the preferentially bound excipient in a similar manner as lysozyme. However, compared to lysozyme in all cases the melting transition was lower by up to a few degrees and the calorimetric melting enthalpy was decreased to half the value for PEGylated lysozyme. The ratio between calorimetric and van’t Hoff enthalpy suggests that our PEGylated lysozyme is a dimer.

Conclusion/Significance

The PEGylated model protein displayed similar stability responses to the addition of preferentially active excipients. This suggests that formulation principles using preferentially interacting excipients are similar for PEGylated and non-PEGylated proteins.

Unfolding of a small protein proceeds via dry and wet globules and a solvated transition state

Dissecting a protein unfolding process into individual steps can provide valuable information on the forces that maintain the integrity of the folded structure. Solvation of the protein core determines stability, but it is not clear when such solvation occurs during unfolding. In this study, far-UV circular dichroism measurements suggest a simplistic two-state view of the unfolding of barstar, but the use of multiple other probes brings out the complexity of the unfolding reaction. Near-UV circular dichroism measurements show that unfolding commences with the loosening of tertiary interactions in a native-like intermediate, N(∗). Fluorescence resonance energy transfer measurements show that N(∗) then expands rapidly but partially to form an early unfolding intermediate IE. Fluorescence spectral measurements indicate that both N(∗) and IE have retained native-like solvent accessibility of the core, suggesting that they are dry molten globules. Dynamic quenching measurements at the single tryptophan buried in the core suggest that the core becomes solvated only later in a late wet molten globule, IL, which precedes the unfolded form. Fluorescence anisotropy decay measurements show that tight packing around the core tryptophan is lost when IL forms. Of importance, the slowest step is unfolding of the wet molten globule and involves a solvated transition state.

Development and characterization of an automated high throughput screening method for optimization of protein refolding processes

Optimization of protein refolding parameters by automated, miniaturized, and parallelized high throughput screening is a powerful approach to meet the demand for fast process development with low material consumption. In this study, we validated methods applicable on a standard liquid handling robot for screening of refolding process parameters by dilution of denatured lysozyme in refolding buffer systems. Different approaches for the estimation of protein solubility and folding were validated concerning resolution and compatibility with the robotic system and with the complex buffer and protein structure composition. We established an indirect method to assess soluble lysozyme concentration independent of matrix effects and protein structure varieties by automated separation of aggregated protein, resolubilization, and measurement of absorption at 280 nm. Using this nonspecific solubility assays the correlation between favorable parameters for high active and soluble lysozyme yields were evaluated. An overlap of good refolding buffer compositions was found provided that the redox environment was controlled with redox reagents. In addition, the need to control unfolding conditions like time, temperature, lysozyme, and dithiothreitol concentration was pointed out as different feedstocks resulted in different refolding yields.

Three types of induced tryptophan optical activity compared in model dipeptides: theory and experiment

The tryptophan (Trp) aromatic residue in chiral matrices often exhibits a large optical activity and thus provides valuable structural information. However, it can also obscure spectral contributions from other peptide parts. To better understand the induced chirality, electronic circular dichroism (ECD), vibrational circular dichroism (VCD), and Raman optical activity (ROA) spectra of Trp-containing cyclic dipeptides c-(Trp-X) (where X = Gly, Ala, Trp, Leu, nLeu, and Pro) are analyzed on the basis of experimental spectra and density functional theory (DFT) computations. The results provide valuable insight into the molecular conformational and spectroscopic behavior of Trp. Whereas the ECD is dominated by Trp π-π* transitions, VCD is dominated by the amide modes, well separated from minor Trp contributions. The ROA signal is the most complex. However, an ROA marker band at 1554 cm(-1) indicates the local χ(2) angle value in this residue, in accordance with previous theoretical predictions. The spectra and computations also indicate that the peptide ring is nonplanar, with a shallow potential so that the nonplanarity is primarily induced by the side chains. Dispersion-corrected DFT calculations provide better results than plain DFT, but comparison with experiment suggests that they overestimate the stability of the folded conformers. Molecular dynamics simulations and NMR results also confirm a limited accuracy of the dispersion-DFT model in nonaqueous solvents. Combination of chiral spectroscopies with theoretical analysis thus significantly enhances the information that can be obtained from the induced chirality of the Trp aromatic residue.

Newly folded substrates inside the molecular cage of the HtrA chaperone DegQ

The HtrA protein family combines chaperone and protease activities and is essential for protein quality control in many organisms. Whereas the mechanisms underlying the proteolytic function of HtrA proteins are well characterized, their chaperone activity remains poorly understood. Here we describe cryo-EM structures of Escherichia coli DegQ in its 12- and 24-mer states in complex with model substrates, providing a structural model of HtrA chaperone action. Up to six lysozyme substrates bind inside the DegQ 12-mer cage and are visualized in a close-to-native state. An asymmetric reconstruction reveals the binding of a well-ordered lysozyme to four DegQ protomers. DegQ PDZ domains are located adjacent to substrate density and their presence is required for chaperone activity. The substrate-interacting regions appear conserved in 12- and 24-mer cages, suggesting a common mechanism of chaperone function.

Design and characterization of protein-quercetin bioactive nanoparticles

Background

The synthesis of bioactive nanoparticles with precise molecular level control is a major challenge in bionanotechnology. Understanding the nature of the interactions between the active components and transport biomaterials is thus essential for the rational formulation of bio-nanocarriers. The current study presents a single molecule of bovine serum albumin (BSA), lysozyme (Lys), or myoglobin (Mb) used to load hydrophobic drugs such as quercetin (Q) and other flavonoids.

Results

Induced by dimethyl sulfoxide (DMSO), BSA, Lys, and Mb formed spherical nanocarriers with sizes less than 70 nm. After loading Q, the size was further reduced by 30%. The adsorption of Q on protein is mainly hydrophobic, and is related to the synergy of Trp residues with the molecular environment of the proteins. Seven Q molecules could be entrapped by one Lys molecule, 9 by one Mb, and 11 by one BSA. The controlled releasing measurements indicate that these bioactive nanoparticles have long-term antioxidant protection effects on the activity of Q in both acidic and neutral conditions. The antioxidant activity evaluation indicates that the activity of Q is not hindered by the formation of protein nanoparticles. Other flavonoids, such as kaempferol and rutin, were also investigated.

Conclusions

BSA exhibits the most remarkable abilities of loading, controlled release, and antioxidant protection of active drugs, indicating that such type of bionanoparticles is very promising in the field of bionanotechnology.

Potentially amyloidogenic conformational intermediates populate the unfolding landscape of transthyretin: insights from molecular dynamics simulations

Protein aggregation into insoluble fibrillar structures known as amyloid characterizes several neurodegenerative diseases, including Alzheimer's, Huntington's and Creutzfeldt-Jakob. Transthyretin (TTR), a homotetrameric plasma protein, is known to be the causative agent of amyloid pathologies such as FAP (familial amyloid polyneuropathy), FAC (familial amyloid cardiomiopathy) and SSA (senile systemic amyloidosis). It is generally accepted that TTR tetramer dissociation and monomer partial unfolding precedes amyloid fibril formation. To explore the TTR unfolding landscape and to identify potential intermediate conformations with high tendency for amyloid formation, we have performed molecular dynamics unfolding simulations of WT-TTR and L55P-TTR, a highly amyloidogenic TTR variant. Our simulations in explicit water allow the identification of events that clearly discriminate the unfolding behavior of WT and L55P-TTR. Analysis of the simulation trajectories show that (i) the L55P monomers unfold earlier and to a larger extent than the WT; (ii) the single alpha-helix in the TTR monomer completely unfolds in most of the L55P simulations while remain folded in WT simulations; (iii) L55P forms, early in the simulations, aggregation-prone conformations characterized by full displacement of strands C and D from the main beta-sandwich core of the monomer; (iv) L55P shows, late in the simulations, severe loss of the H-bond network and consequent destabilization of the CBEF beta-sheet of the beta-sandwich; (v) WT forms aggregation-compatible conformations only late in the simulations and upon extensive unfolding of the monomer. These results clearly show that, in comparison with WT, L55P-TTR does present a much higher probability of forming transient conformations compatible with aggregation and amyloid formation.

Fast amide proton exchange reveals close relation between native-state dynamics and unfolding kinetics

It has long been recognized that many proteins fold and unfold via partially structured intermediates, but it is still unclear why some proteins unfold in a two-state fashion while others do not. Here we compare the unfolding pathway of the small one-domain protein barstar with its dynamics under native conditions. Using very fast proton-exchange experiments, extensive dynamic heterogeneity within the native-state ensemble could be identified. Especially the dynamics of helix 3, covering the hydrophobic core of the molecule, is found to be clearly cooperative but decoupled from the global dynamics. Moreover, an initial unfolding of this helix followed by the breakdown of the remaining tertiary structure can be concluded from the comparison of the proton exchange experiments with unfolding kinetics detected by stopped-flow fluorescence. We infer that the unfolding pathway of barstar is closely coupled to its native-state dynamics.

An Integral Equation Study of the Hydrophobic Interaction between Graphene Plates

The hydrophobic association of two parallel graphene sheets is studied using the 3D-RISM HNC integral equations with several theoretical methods for the solvent distribution functions. The potential of mean force is calculated to study the effects of the aqueous solvent models and methods on the plates as a function of distance. The results of several integral equations (IE) are compared to MD simulations for the same model. The 3D-IEs are able to qualitatively reproduce the nature of the solvent effects on the potential of mean force but not quantitatively. The local minima in the potential of mean force occur at distances allowing well defined layers of solvent between the plates but are not coincident with those found in simulation of the same potential regardless of the theoretical methods tested here. The dewetting or drying transition between the plates is generally incorrectly dependent on steric effects with these methods even for very hydrophobic systems without solute-solvent attractions, in contradiction with simulation.

Thermodynamically stable aggregation-resistant antibody domains through directed evolution

Protein aggregates are usually formed by interactions between unfolded or partially unfolded species, and often occur when a protein is denatured by, for example, heat or low pH. In earlier work, we used a Darwinian selection strategy to create human antibody variable domains that resisted heat aggregation. The repertoires of domains were displayed on filamentous phage and denatured (at 80 degrees C in pH 7.4), and folded domains were selected by binding to a generic ligand after cooling. This process appeared to select for domains with denatured states that resisted aggregation, but the domains only had low free energies of folding (Delta G(N-D)(o)=15-20 kJ/mol at 25 degrees C in pH 7.4). Here, using the same phage repertoire, we have extended the method to the selection of domains resistant to acid aggregation. In this case, however, the thermodynamic stabilities of selected domains were higher than those selected by thermal denaturation (under both neutral and acidic conditions; Delta G(N-D)(o)=26-47 kJ/mol at 25 degrees C in pH 7.4, or Delta G(N-D)(o)=27-34 kJ/mol in pH 3.2). Furthermore, we identified a key determinant (Arg28) that increased the aggregation resistance of the denatured states of the domains at low pH without compromising their thermodynamic stabilities. Thus, the selection process yielded domains that combined thermodynamic stability and aggregation-resistant unfolded states. We suggest that changes to these properties are controlled by the extent to which the folding equilibrium is displaced during the process of selection.

Role of a conserved salt bridge between the PAS core and the N-terminal domain in the activation of the photoreceptor photoactive yellow protein

The effect of ionic strength on the conformational equilibrium between the I(2) intermediate and the signaling state I(2)' of the photoreceptor PYP and on the rate of recovery to the dark state were investigated by time-resolved absorption and fluorescence spectroscopy. With increasing salt concentration up to approximately 600 mM, the recovery rate k(3) decreases and the I(2)/I(2)' equilibrium (K) shifts in the direction of I(2)'. At higher ionic strength both effects reverse. Experiments with mono-(KCl, NaBr) and divalent (MgCl(2), MgSO(4)) salts show that the low salt effect depends on the ionic strength and not on the cation or anion species. These observations can be described over the entire ionic strength range by considering the activity coefficients of an interdomain salt bridge. At low ionic strength the activity coefficient decreases due to counterion screening whereas at high ionic strength binding of water by the salt leads to an increase in the activity coefficient. From the initial slopes of the plots of log k(3) and log K versus the square root of the ionic strength, the product of the charges of the interacting groups was found to be -1.3 +/- 0.2, suggesting a monovalent ion pair. The conserved salt bridge K110/E12 connecting the beta-sheet of the PAS core and the N-terminal domain is a prime candidate for this ion pair. To test this hypothesis, the mutants K110A and E12A were prepared. In K110A the salt dependence of the I(2)/I(2)' equilibrium was eliminated and of the recovery rate was greatly reduced below approximately 600 mM. Moreover, at low salt the recovery rate was six times slower than in wild-type. In E12A significant salt dependence remained, which is attributed to the formation of a novel salt bridge between K110 and E9. At high salt reversal occurs in both mutants suggesting that salting out stabilizes the more compact I(2) structure. However, chaotropic anions like SCN shift the I(2)/I(2)' equilibrium toward the partially unfolded I(2)' form. The salt linkage K110/E12 stabilizes the photoreceptor in the inactive state in the dark and is broken in the light-induced formation of the signaling state, allowing the N-terminal domain to detach from the beta-scaffold PAS core.

A unified mechanism for protein folding: predetermined pathways with optional errors

There is a fundamental conflict between two different views of how proteins fold. Kinetic experiments and theoretical calculations are often interpreted in terms of different population fractions folding through different intermediates in independent unrelated pathways (IUP model). However, detailed structural information indicates that all of the protein population folds through a sequence of intermediates predetermined by the foldon substructure of the target protein and a sequential stabilization principle. These contrary views can be resolved by a predetermined pathway--optional error (PPOE) hypothesis. The hypothesis is that any pathway intermediate can incorporate a chance misfolding error that blocks folding and must be reversed for productive folding to continue. Different fractions of the protein population will then block at different steps, populate different intermediates, and fold at different rates, giving the appearance of multiple unrelated pathways. A test of the hypothesis matches the two models against extensive kinetic folding results for hen lysozyme which have been widely cited in support of independent parallel pathways. The PPOE model succeeds with fewer fitting constants. The fitted PPOE reaction scheme leads to known folding behavior, whereas the IUP properties are contradicted by experiment. The appearance of a conflict with multipath theoretical models seems to be due to their different focus, namely on multitrack microscopic behavior versus cooperative macroscopic behavior. The integration of three well-documented principles in the PPOE model (cooperative foldons, sequential stabilization, optional errors) provides a unifying explanation for how proteins fold and why they fold in that way.

Comparison of the effects of 2,2,2-trifluoroethanol on peptide and protein structure and function

The co-solvent 2,2,2-trifluoroethanol (TFE) has been often used to aid formation of secondary structure in solution peptides or alternately as a denaturant within protein folding studies. Hen egg white lysozyme (HEWL) and a synthetic model peptide defining HEWL helix-4 were used as comparative model systems to systematically investigate the effect of increasing TFE concentrations on the structure of proteins and peptides. HEWL was analyzed using NMR, far-UV CD and fluorescence spectroscopy; with correlation of these results towards changes in enzymatic activity and the helix-4 peptide was analysed using NMR. Data illustrates two conflicting modes of interaction: Low TFE concentrations stabilize tertiary structure, observed from an increase in the number of NMR NOE contacts. Higher TFE concentrations denatured HEWL with the loss of lysozyme tertiary structure. The effects of TFE upon secondary structural elements within HEWL are distinct from those observed for the helix-4 peptide. This illustrates a dissimilar interaction of TFE towards both protein and peptide at equivalent TFE concentrations. The concentration that TFE promotes stabilization over denaturation is likely to be protein dependent although the structural action can be extrapolated to other protein systems with implications for the use of TFE in structural stability studies.

Hofmeister effects in protein unfolding kinetics: Estimation of changes in surface area upon formation of the transition state

We studied the effect of three electrolytes (LiCl, Na(2)SO(4), GuHCl) on the unfolding reaction of chymopapain, a two-domain protein belonging in the papain family of cysteine proteinases. Due to methodological reasons, these studies were carried out at pH 1.5 where the protein unfolds following biphasic kinetics. We have observed the presence of two different effects of electrolyte concentration on the unfolding reactions. At low ionic strength, the ionic atmosphere brought about an increase in reaction rates, regardless of the type of ions being present; this effect is attributed to a general "electrostatic screening" of charge-charge interactions in the macromolecule. At high ionic strength, each electrolyte exerted a distinctively different effect: both rate constants were largely increased by GuHCl (a well-known protein denaturant), but only slightly by LiCl; in contrast, Na(2)SO(4) (a good precipitant) decreased the value of both unfolding rates. These ion-specific (Hofmeister) effects were further used to estimate changes in accessible surface area (DeltaASA) upon formation of the transition states (TS) for unfolding. Results obtained with LiCl and Na(2)SO(4), which we analyzed by means of a parameterization derived from published solubility data of amino acid derivatives, are consistent with DeltaASA increments (for each phase) of about 8.0% of the total theoretical DeltaASA for complete unfolding of the chymopapain molecule. Results in the presence of GuHCl, which were analyzed by using a previous parameterization of protein unfolding data, gave larger DeltaASAs of activation, equivalent to 13 and 16% of the total unfolding DeltaASA.

Effect of ethanol on folding of hen egg-white lysozyme under acidic condition

The equilibrium and kinetics of folding of hen egg-white lysozyme were studied by means of CD spectroscopy in the presence of varying concentrations of ethanol under acidic condition. The equilibrium transition curves of guanidine hydrochloride-induced unfolding in 13 and 26% (v/v) ethanol have shown that the unfolding significantly deviates from a two-state mechanism. The kinetics of denaturant-induced refolding and unfolding of hen egg-white lysozyme were investigated by stopped-flow CD at three ethanol concentrations: 0, 13, and 26% (v/v). Immediately after dilution of the denaturant, the refolding curves showed a biphasic time course in the far-UV region, with a burst phase with a significant secondary structure and a slower observable phase. However, when monitored by the near-UV CD, the burst phase was not observed and all refolding kinetics were monophasic. To clarify the effect of nonnative secondary structure induced by the addition of ethanol on the folding/unfolding kinetics, the kinetic m values were estimated from the chevron plots obtained for the three ethanol concentrations. The data indicated that the folding/unfolding kinetics of hen lysozyme in the presence of varying concentrations of ethanol under acidic condition is explained by a model with both on-pathway and off-pathway intermediates of protein folding.

The Folding Mechanism of a Dimeric β-Barrel Domain

The dimeric beta-barrel domain is an unusual topology, shared only by two viral origin binding proteins, where secondary, tertiary and quaternary structure are coupled, and where the dimerization interface is composed of two four-stranded half-beta-barrels. The folding of the DNA binding domain of the E2 transcriptional regulator from human papillomavirus, strain-16, takes place through a stable and compact monomeric intermediate, with 31% the stability of the folded dimeric domain. Double jump multiple wavelength experiments allowed the reconstruction of the fluorescence spectrum of the monomeric intermediate at 100 milliseconds, indicating that tryptophan residues, otherwise buried in the folded state, are accessible to the solvent. Burial of surface area as well as differential behavior to ionic strength and pH with respect to the native ground state, plus the impossibility of having over 2500 A2 of surface area of the half-barrel exposed to the solvent, indicates that the formation of a non-native compact tertiary structure precedes the assembly of native quaternary structure. The monomeric intermediate can dimerize, albeit with a weaker affinity (approximately 1 microM), to yield a non-native dimeric intermediate, which rearranges to the native dimer through a parallel folding channel, with a unimolecular rate-limiting step. Folding pathways from either acid or urea unfolded states are identical, making the folding model robust. Unfolding takes place through a major phase accounting for apparently all the secondary structure change, with identical rate constant to that of the fluorescence unfolding experiment. In contrast to the folding direction, no unfolding intermediate was found.

Thermally induced fibrillar aggregation of hen egg white lysozyme

We study the effect of pH and temperature on fibril formation from hen egg white lysozyme. Fibril formation is promoted by low pH and temperatures close to the midpoint temperature for protein unfolding (detected using far-ultraviolet circular dichroism). At the optimal conditions for fibril formation (pH 2.0, T = 57 degrees C), on-line static light-scattering shows the formation of fibrils after a concentration-independent lag time of approximately 48 h. Nucleation presumably involves a change in the conformation of individual lysozyme molecules. Indeed, long-term circular dichroism measurements at pH 2.0, T = 57 degrees C show a marked change of the secondary structure of lysozyme molecules after approximately 48 h of heating. From atomic force microscopy we find that most of the fibrils have a thickness of approximately 4 nm. These fibrils have a coiled structure with a periodicity of approximately 30 nm and show characteristic defects after every four or five turns.

Rapid formation of non-native contacts during the folding of HPr revealed by real-time photo-CIDNP NMR and stopped-flow fluorescence experiments

We report the combined use of real-time photo-CIDNP NMR and stopped-flow fluorescence techniques to study the kinetic refolding of a set of mutants of a small globular protein, HPr, in which each of the four phenylalanine residues has in turn been replaced by a tryptophan residue. The results indicate that after refolding is initiated, the protein collapses around at least three, and possibly all four, of the side-chains of these residues, as (i) the observation of transient histidine photo-CIDNP signals during refolding of three of the mutants (F2W, F29W, and F48W) indicates a strong decrease in tryptophan accessibility to the flavin dye; (ii) iodide quenching experiments show that the quenching of the fluorescence of F48W is less efficient for the species formed during the dead-time of the stopped-flow experiment than for the fully native state; and (iii) kinetic fluorescence anisotropy measurements show that the tryptophan side-chain of F48W has lower mobility in the dead-time intermediate state than in both the fully denatured and fully native states. The hydrophobic collapse observed for HPr during the early stages of its folding appears to act primarily to bury hydrophobic residues. This process may be important in preventing the protein from aggregating prior to the acquisition of native-like structure in which hydrophobic residues are exposed in order to play their role in the function of the protein. The phenylalanine residue at position 48 is likely to be of particular interest in this regard as it is involved in the binding to enzymes I and II that mediates the transfer of a phosphoryl group between the two enzymes.

The DCX-domain tandems of doublecortin and doublecortin-like kinase

The doublecortin-like domains (DCX), which typically occur in tandem, are novel microtubule-binding modules. DCX tandems are found in doublecortin, a 360-residue protein expressed in migrating neurons; the doublecortin-like kinase (DCLK); the product of the RP1 gene that is responsible for a form of inherited blindness; and several other proteins. Mutations in the gene encoding doublecortin cause lissencephaly in males and the 'double-cortex syndrome' in females. We here report a solution structure of the N-terminal DCX domain of human doublecortin and a 1.5 A resolution crystal structure of the equivalent domain from human DCLK. Both show a stable, ubiquitin-like tertiary fold with distinct structural similarities to GTPase-binding domains. We also show that the C-terminal DCX domains of both proteins are only partially folded. In functional assays, the N-terminal DCX domain of doublecortin binds only to assembled microtubules, whereas the C-terminal domain binds to both microtubules and unpolymerized tubulin.

Design and characterization of a hyperstable p16INK4a that restores Cdk4 binding activity when combined with oncogenic mutations

Cyclin-dependent kinase inhibitor p16(INK4a) is the founding member of the INK4 family of tumor suppressors capable of arresting mammalian cell division. Missense mutations in the p16(INK4a) gene (INK4a/CDKN2A/MTS1) are strongly linked to several types of human cancer. These mutations are evenly distributed throughout this small, ankyrin repeat protein and the majority of them disrupt the native secondary and/or tertiary structure, leading to protein unfolding, aggregation and loss of function. We report here the use of multiple stabilizing substitutions to increase the stability of p16(INK4a) and furthermore, to restore Cdk4 binding activity of several defective, cancer-related mutant proteins. Stabilizing substitutions were predicted using four different techniques. The three most effective substitutions were combined to create a hyperstable p16(INK4a) variant that is 1.4 kcal/mol more stable than wild-type. This engineered construct is monomeric in solution with wild-type-like secondary and tertiary structure and cyclin-dependent kinase 4 binding activity. Interestingly, these hyperstable substitutions, when combined with oncogenic mutations R24P, P81L or V126D, can significantly restore Cdk4 binding activity, despite the divergent features of each destabilizing mutation. Extensive biophysical studies indicate that the hyperstable substitutions enhance the binding activity of mutant p16 through several different mechanisms, including an increased amount of secondary structure and thermostability, reduction in exposed hydrophobic surface(s) and/or a reduced tendency to aggregate. This apparent global suppressor effect suggests that increasing the thermodynamic stability of p16 can be used as a general strategy to restore the biological activity to defective mutants of this important tumor suppressor protein.

Equilibrium and kinetic studies on folding of canine milk lysozyme

Thermal and chemical unfolding studies of the calcium-binding canine lysozyme (CL) by fluorescence and circular dichroism spectroscopy show that, upon unfolding in the absence of calcium ions, a very stable equilibrium intermediate state is formed. At room temperature and pH 7.5, for example, a stable molten globule state is attained in 3 M GdnHCl. The existence of such a pure and stable intermediate state allowed us to extend classical stopped-flow fluorescence measurements that describe the transition from the native to the unfolded form, with kinetic experiments that monitor separately the transition from the unfolded to the intermediate state and from the intermediate to the native state, respectively. The overall refolding kinetics of apo-canine lysozyme are characterized by a significant drop in the fluorescence intensity during the dead time, followed by a monoexponential increase of the fluorescence with k = 3.6 s(-1). Furthermore, the results show that, unlike its drastic effect on the stability, Ca(2+)-binding only marginally affects the refolding kinetics. During the refolding process of apo-CL non-native interactions, comparable to those observed in hen egg white lysozyme, are revealed by a substantial quenching of tryptophan fluorescence. The dissection of the refolding process in two distinct steps shows that these non-native interactions only occur in the final stage of the refolding process in which the two domains match to form the native conformation.

Equilibrium and kinetic folding of hen egg-white lysozyme under acidic conditions

The equilibrium and kinetic folding of hen egg-white lysozyme was studied by means of circular dichroism spectra in the far- and near-ultraviolet (UV) regions at 25 degrees C under the acidic pH conditions. In equilibrium condition at pH 2.2, hen lysozyme shows a single cooperative transition in the GdnCl-induced unfolding experiment. However, in the GdnCl-induced unfolding process at lower pH 0.9, a distinct intermediate state with molten globule characteristics was observed. The time-dependent unfolding and refolding of the protein were induced by concentration jumps of the denaturant and measured by using stopped-flow circular dichroism at pH 2.2. Immediately after the dilution of denaturant, the kinetics of refolding shows evidence of a major unresolved far-UV CD change during the dead time (<10 ms) of the stopped-flow experiment (burst phase). The observed refolding and unfolding curves were both fitted well to a single-exponential function, and the rate constants obtained in the far- and near-UV regions coincided with each other. The dependence on denaturant concentration of amplitudes of burst phase and both rate constants was modeled quantitatively by a sequential three-state mechanism, U<-->I<-->N, in which the burst-phase intermediate (I) in rapid equilibrium with the unfolded state (U) precedes the rate-determining formation of the native state (N). The role of folding intermediate state of hen lysozyme was discussed.

Test for cooperativity in the early kinetic intermediate in lysozyme folding

During the folding of many proteins, collapsed globular states are formed prior to the native structure. The role of these states for the folding process has been widely discussed. Comparison with properties of synthetic homo and heteropolymers had suggested that the initial collapse represented a shift of the ensemble of unfolded conformations to more compact states without major energy barriers. We investigated the folding/unfolding transition of a collapsed state, which transiently populates early in lysozyme folding. This state forms within the dead-time of stopped-flow mixing and it has been shown to be significantly more compact and globular than the denaturant-induced unfolded state. We used the GdmCl-dependence of the dead-time signal change to characterize the unfolding transition of the burst phase intermediate. Fluorescence and far-UV CD give identical unfolding curves, arguing for a cooperative two-state folding/unfolding transition between unfolded and collapsed lysozyme. These results show that collapse leads to a distinct state in the folding process, which is separated from the ensemble of unfolded molecules by a significant energy barrier. NMR, fluorescence and small angle X-ray scattering data further show that some local interactions in unfolded lysozyme exist at denaturant concentrations above the coil-collapse transition. These interactions might play a crucial role in the kinetic partitioning between fast and slow folding pathways.

Role of individual disulfide bonds in hen lysozyme early folding steps

To probe the role of individual disulfide bonds in the folding kinetics of hen lysozyme, the variants with two mutations, C30A,C115A, C64A,C80A, and C76A,C94A, were constructed. The corresponding proteins, each lacking one disulfide bond, were produced in Escherichia coli as inclusion bodies and solubilized, purified, and renatured/oxidized using original protocols. Their enzymatic, spectral, and hydrodynamic characteristics confirmed that their conformations were very similar to that of native wild-type (WT) lysozyme. Stopped-flow studies on the renaturation of these guanidine-unfolded proteins with their three disulfides intact showed that, for the three variants, the native far-UV ellipticity was regained in a burst phase within the 4-ms instrument dead-time. The transient overshoots of far-UV ellipticity and tryptophan fluorescence that follow the burst phase, as well as the kinetics of transient 8-anilino-1-naphthalene-sulfonic acid (ANS) binding, were diversely affected depending on the variant. Together with previous reports on the folding kinetics of WT lysozyme carboxymethylated on cysteines 6 and 127, detailed analysis of the kinetics showed that (1) none of the disulfide bonds were indispensable for the rapid formation (<4 ms) of the native-like secondary structure; (2) the two intra-alpha-domain disulfides (C6-C127 and C30-C115) must be simultaneously present to generate the trapped intermediate responsible for the slow folding population observed in WT lysozyme; and (3) the intra-beta-domain (C64-C80) and the inter-alphabeta-domains (C76-C94) disulfides do not affect the kinetics of formation of the trapped intermediate but are involved in its stability.

Use of a hydrophobic dye to indirectly probe the structural organization and conformational plasticity of molecules in amorphous aggregates of carbonic anhydrase

Understanding protein aggregation may hold important clues to understanding what goes wrong with protein folding in neurodegenerative disorders and in bioreactors in which proteins are overexpressed. Unfortunately, aggregates tend to be intractable to most standard methods of biochemical investigation. Thus, relatively little is even now known about the micro- and macro-structural features of aggregates. To gain insights into the thermal aggregation of a model globular protein [bovine carbonic anhydrase (BCA)], we have used spectrofluorimetry to examine the binding of a hydrophobic dye, 8-anilinonaphthalene sulfonate (ANS), to hydrophobic clusters on the protein's surface both before and after heat-induced aggregation and upon cooling. Whereas native BCA shows no surface hydrophobicity, thermally aggregated BCA displays significant hydrophobicity both in the heated state and upon cooling. The timing of the addition of ANS in the course of aggregation makes no net difference to the ANS bound; we argue that this suggests that aggregates are essentially porous. Cooling of aggregates results in a dramatic, fully reversible increase in ANS binding that cannot be explained by the temperature dependence of fluorescence quantum yield alone; we argue that the enhancement of fluorescence upon cooling indicates possible structural consolidation of unfolded regions within aggregates (akin to refolding), with the required structural reorganization being facilitated by porosity. Finally, implications of porosity in aggregates are discussed, in particular, for the possible immobilization of enzymes through fusion with aggregation-prone protein domains.

Single-domain antibody fragments with high conformational stability

A variety of techniques, including high-pressure unfolding monitored by Fourier transform infrared spectroscopy, fluorescence, circular dichroism, and surface plasmon resonance spectroscopy, have been used to investigate the equilibrium folding properties of six single-domain antigen binders derived from camelid heavy-chain antibodies with specificities for lysozymes, beta-lactamases, and a dye (RR6). Various denaturing conditions (guanidinium chloride, urea, temperature, and pressure) provided complementary and independent methods for characterizing the stability and unfolding properties of the antibody fragments. With all binders, complete recovery of the biological activity after renaturation demonstrates that chemical-induced unfolding is fully reversible. Furthermore, denaturation experiments followed by optical spectroscopic methods and affinity measurements indicate that the antibody fragments are unfolded cooperatively in a single transition. Thus, unfolding/refolding equilibrium proceeds via a simple two-state mechanism (N <--> U), where only the native and the denatured states are significantly populated. Thermally-induced denaturation, however, is not completely reversible, and the partial loss of binding capacity might be due, at least in part, to incorrect refolding of the long loops (CDRs), which are responsible for antigen recognition. Most interestingly, all the fragments are rather resistant to heat-induced denaturation (apparent T(m) = 60-80 degrees C), and display high conformational stabilities (DeltaG(H(2)O) = 30-60 kJ mole(-1)). Such high thermodynamic stability has never been reported for any functional conventional antibody fragment, even when engineered antigen binders are considered. Hence, the reduced size, improved solubility, and higher stability of the camelid heavy-chain antibody fragments are of special interest for biotechnological and medical applications.

Cooperative folding of the isolated alpha-helical domain of hen egg-white lysozyme

Proteins in the alpha-lactalbumin and c-type lysozyme family have been studied extensively as model systems in protein folding. Early formation of the alpha-helical domain is observed in both alpha-lactalbumin and c-type lysozyme; however, the details of the kinetic folding pathways are significantly different. The major folding intermediate of hen egg-white lysozyme has a cooperatively formed tertiary structure, whereas the intermediate of alpha-lactalbumin exhibits the characteristics of a molten globule. In this study, we have designed and constructed an isolated alpha-helical domain of hen egg-white lysozyme, called Lyso-alpha, as a model of the lysozyme folding intermediate that is stable at equilibrium. Disulfide-exchange studies show that under native conditions, the cysteine residues in Lyso-alpha prefer to form the same set of disulfide bonds as in the alpha-helical domain of full-length lysozyme. Under denaturing conditions, formation of the nearest-neighbor disulfide bonds is strongly preferred. In contrast to the isolated alpha-helical domain of alpha-lactalbumin, Lyso-alpha with two native disulfide bonds exhibits a well-defined tertiary structure, as indicated by cooperative thermal unfolding and a well-dispersed NMR spectrum. Thus, the determinants for formation of the cooperative side-chain interactions are located mainly in the alpha-helical domain. Our studies suggest that the difference in kinetic folding pathways between alpha-lactalbumin and lysozyme can be explained by the difference in packing density between secondary structural elements and support the hypothesis that the structured regions in a protein folding intermediate may correspond to regions that can fold independently.

A simple way to measure protein refolding rates in water

Refolding of proteins is traditionally carried out either by diluting the denaturant-unfolded protein into buffer (GdmCl-jump) or by mixing the acid-denatured protein with strong buffer (pH-jump). The first method does not allow direct measurement of folding rates in water since the GdmCl cannot be infinitely diluted, and the second method suffers from the limitation that many proteins cannot be pH-denatured. Further, some proteins do not refold reversibly from low pH where they get trapped as aggregation prone intermediates. Here, we present an alternative approach for direct measurement of refolding rates in water, which does not rely on extrapolation. The protein is denatured in SDS, and is then mixed with alpha-cyclodextrin, which rapidly strips SDS molecules from the protein, leaving the naked unfolded protein to refold.

Origin of apparent fast and non-exponential kinetics of lysozyme folding measured in pulsed hydrogen exchange experiments

Folding of lysozyme at pH 5.2 is a complex processes. After rapid collapse (<1 ms) kinetic partitioning into a slow and fast folding pathway occurs. The fast pathway leads directly to the native structure (N), whereas the slow pathway goes through a partially folded intermediate (I(1)) with native-like secondary structure in the alpha-domain. This mechanism is in agreement with data from a large number of spectroscopic probes, from changes in the radius of gyration and from measurements on the time-course of the populations of the different species. Results from pulsed hydrogen exchange experiments, in contrast, revealed that the secondary structure of I(1) and of N is formed significantly faster than changes in spectroscopic properties occur and showed large variations in the protection kinetics of individual amide sites. We investigated the molecular origin of the rapid amide protection by quantitatively simulating all kinetic processes during the pulse-labeling experiments. Absorbance and fluorescence-detected folding kinetics showed that the early events in lysozyme folding are accelerated under exchange conditions (pH 9.2) and that a change in folding mechanism occurs due to the transient population of an additional intermediate (I(2)). This leads to kinetic competition between exchange and folding during the exchange pulse and to incomplete labeling of amide sites with slow intrinsic exchange rates. As a result, apparently faster and non-exponential kinetics of amide protection are measured in the labeling experiments. Our results further suggest that collapsed lysozyme (C) and I(1) have five and ten-times reduced free exchange rates, respectively, due to limited solvent accessibility.

An electrospray ionization mass spectrometry investigation of 1-anilino-8-naphthalene-sulfonate (ANS) binding to proteins

The binding of 1-anilino-8-naphthalene-sulfonic acid (ANS) to various globular proteins at acidic pH has been investigated by electrospray ionization mass spectrometry (ESI-MS). Maximal ANS binding is observed in the pH range 3-5. As many as seven species of dye-bound complexes are detected for myoglobin. Similar studies were carried out with cytochrome c, carbonic anhydrase, triosephosphate isomerase, lysozyme, alpha-lactalbumin, and bovine pancreatic trypsin inhibitor (BPTI). Strong ANS binding was observed wherever molten globule states were postulated in solution. ANS binding is not observed for lysozyme and BPTI, which have tightly folded structures in the native form. Alpha-lactalbumin, which is structurally related to lysozyme but forms a molten globule at acidic pH, exhibited ANS binding. Reduction of disulfide bonds in these proteins leads to the detection of ANS binding even at neutral pH. Binding was suppressed at very low pH (<2.5), presumably due to neutralization of the charge on the sulfonate moiety. The distribution of the relative intensities of the protein bound ANS species varies with the charge state, suggesting heterogeneity of gas phase conformations. The binding strength of these complexes was qualitatively estimated by dissociating them using enhanced nozzle skimmer potentials. The skimmer voltages also affected the lower and higher charge states of these complexes in a different manner.

High-sensitivity fluorescence anisotropy detection of protein-folding events: application to alpha-lactalbumin

An experimental procedure has been devised to record simultaneously fluorescence intensity and fluorescence anisotropy. A photoelastic modulator on the excitation beam enables the anisotropy signal to be recorded in one pass using a single photomultiplier tube and eliminates the need for a polarizer on the emission path. In conjunction with a stopped-flow mixer, providing a time-resolved capability, this procedure was used to study the refolding of apo alpha-lactalbumin following dilution from guanidinium chloride. Although the fluorescence intensity does not change detectably, the fluorescence anisotropy was found to resolve the conformational changes occurring between the initial unfolded state and the molten globule state formed either kinetically during refolding at pH 7.0 or at equilibrium at pH 2.0 (A-state). This result provides further evidence that fluorescence anisotropy is a valuable probe of protein structural transitions and that the information it provides concerning the rotational mobility of a fluorophore can be complementary to the information about the local environment provided by fluorescence intensity.