Demonstration by NMR of folding domains in lysozyme

Prediction of protein folding pathways

Recent 1H nuclear magnetic resonance (n.m.r.) hydrogen exchange experiments on five different proteins have delineated the secondary structures formed in trapped, partially folded intermediates. The early forming structural elements are identifiable through a technique described in this work to predict folding pathways. The method assumes that the sequential selection of structural fragments such as alpha-helices and beta-strands involved in the folding process is founded upon the maximal burial of solvent accessible surface from both the formation of internal structure and substructure association. The substructural elements were defined objectively by major changes in main-chain direction. The predicted folding pathways are in complete correspondence with the n.m.r. results in that the formed structural fragments found in the folding intermediates are those predicted earliest in the pathways. The technique was also applied to proteins of known tertiary structure and with fold similar to one of the five proteins examined by 1H n.m.r. The pathways for these structures also showed general consistency with the n.m.r. observations, suggesting conservation of a secondary structural framework or molten globule about which folding nucleates and proceeds.

Protein folding in vitro

It is becoming increasingly evident that intermediates observed in protein folding in vitro may be closely related to conformational states that are important in various intracellular processes. This review focuses on recent advances in in vitro protein-folding studies with particular reference to the molten globule state, which is purported to be a common and distinct intermediate of protein folding.

Hydrogen exchange in native and denatured states of hen egg-white lysozyme

The hydrogen exchange kinetics of 68 individual amide protons in the native state of hen lysozyme have been measured at pH 7.5 and 30 degrees C by 2D NMR methods. These constitute the most protected subset of amides, with exchange half lives some 10(5)-10(7) times longer than anticipated from studies of small model peptides. The observed distribution of rates under these conditions can be rationalized to a large extent in terms of the hydrogen bonding of individual amides and their burial from bulk solvent. Exchange rates have also been measured in a reversibly denatured state of lysozyme; this was made possible under very mild conditions, pH 2.0 35 degrees C, by lowering the stability of the native state through selective cleavage of the Cys-6-Cys-127 disulfide cross-link (CM6-127 lysozyme). In this state the exchange rates for the majority of amides approach, within a factor of 5, the values anticipated from small model peptides. For a few amides, however, there is evidence for significant retardation (up to nearly 20-fold) relative to the predicted rates. The pattern of protection observed under these conditions does not reflect the behavior of the protein under strongly native conditions, suggesting that regions of native-like structure do not persist significantly in the denatured state of CM6-127 lysozyme. The pattern of exchange rates from the native protein at high temperature, pH 3.8 69 degrees C, resembles that of the acid-denatured state, suggesting that under these conditions the exchange kinetics are dominated by transient global unfolding. The rates of folding and unfolding under these conditions were determined independently by magnetization transfer NMR methods, enabling the intrinsic exchange rates from the denatured state to be deduced on the basis of this model, under conditions where the predominant equilibrium species is the native state. Again, in the case of most amides these rates showed only limited deviation from those predicted by a simple random coil model. This reinforces the view that these denatured states of lysozyme have little persistent residual order and contrasts with the behavior found for compact partially folded states of proteins, including an intermediate detected transiently during the refolding of hen lysozyme.

1H nuclear magnetic resonance studies of the interaction of urea with hen lysozyme. Origins of the conformational change induced in hen lysozyme by N-acetylglucosamine oligosaccharides

The interaction between hen lysozyme and urea has been investigated using 1H nuclear magnetic resonance spectroscopy. Chemical shift changes for resonances of a number of residues in the vicinity of the active site of the protein have been observed in the presence of urea prior to denaturation. These shifts are similar to those induced in the hen lysozyme spectrum by the specific binding of N-acetylglucosamine (GlcNAc) in site C of the active site cleft, indicating that urea and GlcNAc induce a similar conformational change in the enzyme. This implies that the conformational changes experienced by the enzyme on the binding of GlcNAc oligosaccharides are the consequence of interactions, possibly hydrogen bonding, involving the N-acetyl group of the sugar residue bound in site C, rather than the result of contacts between the protein and the pyranose rings of the oligosaccharides. This suggests that hen lysozyme employs an induced fit type mechanism to discriminate for N-acetylated saccharides as substrates.

Simulation of the thermal denaturation of hen egg white lysozyme: trapping the molten globule state

In the study of protein folding, much attention has focused on the characterization of folding intermediates. We report here molecular dynamics simulations in which the initial stages of the thermal denaturation of hen egg white lysozyme in aqueous solution are examined in detail. It is found that lysozyme unfolds in a two-stage process with the initial formation a quasi-stable state in which significant rearrangement of the secondary structure takes place. No evidence for distinct folding domains was found. The simulations suggest that the formation of well-defined secondary structure occurs after the initial collapse of the peptide chain and thus tend against the framework model of protein folding.

Folding of peptide fragments comprising the complete sequence of proteins. Models for initiation of protein folding. II. Plastocyanin

In an attempt to understand the earliest events in the protein folding pathway, the complete sequence of French bean plastocyanin has been synthesized as a series of short peptide fragments, and the conformational preferences of each peptide examined in aqueous solution using proton n.m.r. methods. Plastocyanin consists largely of beta-sheet, with reverse turns and loops between the strands of the sheet, and one short helix. The n.m.r. experiments indicate that most of the peptides derived from the plastocyanin sequence have remarkably little propensity to adopt folded conformations in aqueous solution, in marked contrast to the peptides derived from the helical protein, myohemerythrin (accompanying paper). For most plastocyanin peptides, the backbone dihedral angles are predominantly in the beta-region of conformational space. Some of the peptides show weak NOE connectivities between adjacent amide protons, indicative of small local populations of backbone conformations in the a region of (phi,psi) space. A conformational preference for a reverse turn is seen in the sequence Ala65-Pro-Gly-Glu68, where a turn structure is found in the folded protein. Significantly, the peptide sequences that populate the alpha-region of (phi,psi) space are mostly derived from turn and loop regions in the protein. The addition of trifluoroethanol does not drive the peptides into helical conformations. In one region of the sequence, the n.m.r. spectra provide evidence of the formation of a hydrophobic cluster involving aromatic and aliphatic side-chains. These results have significance for understanding the initiation of protein folding. From these studies of the fragments of plastocyanin (this paper) and myohemerythrin (accompanying paper), it appears that there is a pre-partitioning of the conformational space sampled by the polypeptide backbone that is related to the secondary structure in the final folded state.

Folding of peptide fragments comprising the complete sequence of proteins. Models for initiation of protein folding. I. Myohemerythrin

In an attempt to delineate potential folding initiation sites for different protein structural motifs, we have synthesized series of peptides that span the entire length of the polypeptide chain of two proteins, and examined their conformational preferences in aqueous solution using proton nuclear magnetic resonance and circular dichroism spectroscopy. We describe here the behavior of peptides derived from a simple four-helix bundle protein, myohemerythrin. The peptides correspond to the sequences of the four long helices (the A, B, C and D helices), the N- and C-terminal loops and the connecting sequences between the helices. The peptides corresponding to the helices of the folded protein all exhibit preferences for helix-like conformations in solution. The conformational ensembles of the A- and D-helix peptides contain ordered helical forms, as shown by extensive series of medium-range nuclear Overhauser effect connectivities, while the B- and C-helix peptides exhibit conformational preferences for nascent helix. All four peptides adopt ordered helical conformations in mixtures of trifluoroethanol and water. The terminal and interconnecting loop peptides also appear to contain appreciable populations of conformers with backbone phi and psi angles in the alpha-region and include highly populated hydrophobic cluster and/or turn conformations in some cases. Trifluoroethanol is unable to drive these peptides towards helical conformations. Overall, the peptide fragments of myohemerythrin have a marked preference towards secondary structure formation in aqueous solution. In contrast, peptide fragments derived from the beta-sandwich protein plastocyanin are relatively devoid of secondary structure in aqueous solution (see accompanying paper). These results suggest that the two different protein structural motifs may require different propensities for formation of local elements of secondary structure to initiate folding, and that there is a prepartitioning of conformational space determined by the local amino acid sequence that is different for the helical and beta-sandwich structural motifs.

Dynamic NMR spectral analysis and protein folding: identification of a highly populated folding intermediate of rat intestinal fatty acid-binding protein by 19F NMR

The folding of intestinal fatty-acid binding protein has been monitored by 19F NMR after incorporation of 6-fluorotryptophan into the protein. The two resonances resulting from the two tryptophans of this protein showed different dependencies on denaturant concentration. One of the resonances was in slow chemical exchange between two resonance frequencies, native and completely unfolded. The changes for this resonance occurred over a denaturant concentration range identical to that monitored by circular dichroism or fluorescence during unfolding. The other resonance continued to show changes at concentrations of denaturant well above that needed to complete the unfolding transition as monitored by optical techniques. Site directed mutagenesis showed that tryptophan-82 was the residue responsible for the unexpected behavior. We conclude, based on complete line-shape analysis, that there are significant concentrations of one or more intermediates in equilibrium with the native and unfolded forms. The structure of the intermediate(s) is more similar to the completely unfolded form of the protein than to the native structure, since little if any secondary structure is present. Further, these structure(s) persist at high denaturant concentrations and may represent local initiating sites in the folding of this beta-sheet protein.

The folding of hen lysozyme involves partially structured intermediates and multiple pathways

Analysis of the folding of hen lysozyme shows that the protein does not become organized in a single cooperative event but that different parts of the structure become stabilized with very different kinetics. In particular, in most molecules the alpha-helical domain folds faster than the beta-sheet domain. Furthermore, different populations of molecules fold by kinetically distinct pathways. Thus, folding is not a simple sequential assembly process but involves parallel alternative pathways, some of which may involve substantial reorganization steps.

Protein surface topology-probing by selective chemical modification and mass spectrometric peptide mapping

Aminoacetylation of lysine residues and the modification of arginine by 1,2-cyclohexanedione to N7,N8-(dihydroxy-1,2-cyclohexylidene)arginine were used for probing the surface topology of hen-eggwhite lysozyme as a model protein. The molecular identification of lysine and arginine modification sites was provided by molecular weight determinations of modified and unmodified tryptic peptide mixtures (peptide mapping) using 252Cf plasma desorption mass spectrometry. At conditions of limited chemical modification, mass-spectrometric peptide-mapping analyses of lysozyme derivatives enabled the direct assignment of relative reactivities of lysine and arginine residues at different reaction times and reagent concentrations. The relative reactivities of lysine residues showed a direct correlation with their surface accessibilities from x-ray structure data. For the reaction with 1,2-cyclohexanedione, a selective modification at Arg-5, -125, -112, and -73 was identified, and an inverse correlation of relative reactivities with the surface accessibility ratios of the N7- and the N8-guanidino functions was obtained. By examination of the x-ray structural data of lysozyme, this selective modification was attributed to intramolecular catalysis because of the presence of neighboring proton acceptor groups, such as the Asp-119 carboxylate group for Arg-125 and the Trp-123 and Arg-125 carbonyl groups for Arg-5.

1H, 15N and 13C NMR assignments of the 434 repressor fragments 1-63 and 44-63 unfolded in 7 M urea

An E. coli overexpression system for the N-terminal domain of the 434 repressor with residues 1-63 (434 repressor(1-63)) was constructed and used to produce this polypeptide with uniform 15N-labeling, and with 13C-labeling of the methyl groups of valine and leucine. Using these protein preparations almost complete sequence-specific resonance assignments were obtained for the urea-unfolded form of the 434 repressor(1-63). In addition, the isotope-labeled tryptic peptide, 44-63, was produced by enzymatic cleavage of the recombinant 434 repressor(1-63), and its NMR spectrum was assigned. Corresponding residues in 434 repressor(1-63) and 434 repressor(44-63) in 7 M urea were found to have nearly identical chemical shifts, and in both species similar deviations from 1H random coil shifts were found as previously in 434 repressor(1-69). These indicate the presence of residual non-random structure in the polypeptide segment 50-60. The present NMR assignments, which include stereospecific assignments for the diastereotopic methyl groups of Val and Leu, are the basis for detailed studies of this residual structure in the urea-unfolded form of the 434 repressor.

Detection and characterization of an early folding intermediate of T4 lysozyme using pulsed hydrogen exchange and two-dimensional NMR

Two-dimensional 1H-15N NMR techniques combined with pulsed hydrogen-deuterium exchange have been used to characterize the folding pathway of T4 lysozyme. In the unfolded state, there is little differential protection of the various amides from hydrogen exchange. In the native folded structure, 84 amides of the 164 residues are sufficiently spectrally resolved and protected from solvent exchange to serve as probes of the folding pathway. These probes are located in both the N-terminal and C-terminal domains of the native folded structure of the protein. The studies described here show that at least one intermediate is formed early during refolding at low denaturant concentrations. This intermediate (or intermediates) forms very rapidly (within the 10-ms temporal resolution of our mixing device) under the conditions used and is completed at least 10 times faster than the overall folding event. The intermediate(s) protect(s) from exchange a subset of amides in the N-terminal and C-terminal regions of the protein. In the final folded states these protected regions correspond to two alpha-helices and a beta-sheet region. These amides are protected from exchange by factors between 20 and 200 as compared to the fully unfolded protein. Protection of this magnitude is consistent with the formation of somewhat exposed secondary structure in these regions and could represent a "molten globule"-like or a "framework"-like structure for the intermediate(s) in which specific parts of the sequence form isolated secondary structures that are not stabilized by extensive tertiary interactions.

Structural domains of phytochrome deduced from homologies in amino acid sequences

A method of semiempirical identification of structural domains is proposed. The procedure is based on the comparison of amino acid sequences in groups of homologous proteins. This approach was tested using 32 known protein sequences from different cytochrome b5, cytochrome c, lysozyme, hemoglobin, and myoglobin proteins. The method presented was able to identify all structural domains of these reference proteins. A consensus secondary structure provided information on structural content of these domains predicting correctly 21 of 23 (91%) of alpha-helices. We applied this method to six homologous phytochrome sequences from Avena, Arabadopsis, Cucurbita, Maize, Oryza, and Pisum. Some of the identified domains can be assigned to the known tertiary structure categories. For example, an alpha/beta domain is localized in the region known to stabilize the phytochrome chromophore in the red light absorbing form (Pr). One alpha-helical and one alpha/beta domains are localized in regions important for the chromophore stabilization in the far-red absorbing form (Pfr). From an analysis of noncovalent interaction patterns in another domain it is proposed that a phytochrome dimer contact involves two segments localized between residues 730 and 821 (using numbering of aligned sequences). Also, a possible antiparallel beta-sheet structure of this region has been suggested. According to this model, the long axis of the interacting structures is perpendicular to a twofold symmetry axis of the phytochrome dimer.

On the interpretation of biochemical data by molecular dynamics computer simulation

The application of computer simulation to molecular systems of biochemical interest is reviewed. It is shown that computer simulation is a tool complementary to experimental methods, which can be used to access atomic details inaccessible to experimental probes. Examples are given in which computer simulation augments the experimental information by providing an atomic picture of high resolution with respect to space, energy or time. The usefulness of a computer simulation largely depends on its quality. The most important factors that limit the accuracy of simulated results are discussed. The accuracy of different simulation studies can differ by orders of magnitude. The accuracy will depend on the type of biomolecular system and process studied. It will also depend on the choice of force field, the simulation set-up and the protocol that is used. A list of quality-determining factors is given, which may be useful when interpreting simulation studies appearing in the literature.

Folding kinetics of T4 lysozyme and nine mutants at 12 degrees C

The kinetics of unfolding and refolding of T4 lysozyme and nine of its mutants have been investigated as a function of guanidinium chloride concentration at 12 degrees C. All show simple two-state, first-order kinetics. Two types of mutants were studied: proline-alanine interchanges and substitutions at position 3 with side chains of varying hydrophobicity. Crystal structures are available for seven of the ten proteins. The effect of mutations on the folding kinetics is more pronounced and complex than on equilibrium thermodynamics. The proteins fall into two broad kinetic classes with one class rather close to the wild type. P86A is a mutant with marked changes in kinetics but only a very small change in stability. Since the 86 position is in the middle of an alpha-helix, the indications are that the helix containing an A residue is more stable in the transition state than one containing a P residue. The other mutants are more complicated, with the refolding and unfolding rates unequally affected by the mutations. On the basis of comparisons with other investigations, we conclude that the rate-determining step in the presence of guanidinium chloride is not the same as in aqueous solution and that it most likely precedes it. The indications are that we are studying the formation of a transition intermediate which is destabilized by the denaturant and which resembles the A intermediate of the framework or molten globule models for protein folding.

Kinetic study of the helix to coil dark reaction of poly(spiropyran-L-glutamate)

An investigation of kinetics of the helix to coil dark reaction of light adapted poly(spiropyran-L-glutamic acid) (PSLG) dissolved in hexafluoroisopropanol was performed. The reaction was associated with the spiropyran (SP) to merocyanine (MC) ring opening. The ring opening reaction monitored with UV/VIS spectroscopy showed first order kinetics. Chromophore and polypeptide backbone circular dichroism data fit to an expression consistent with a single intermediate series mechanism. By FTIR, we monitored the polypeptide alpha-helix amide I, the MC chromophore--C = C--stretch and the protonated unmodified carboxylate C = O stretch bands. During the first step of the series mechanism, changes in the hydrogen bonding of the unmodified carboxylate groups occurred, suggesting breakup of polypeptide aggregates. The second step of the proposed series mechanism was dominated by the helix to coil transition and the ring opening of SP to MC. The CD spectrum of MC in the dark adapted PSLG was red shifted and had a narrower bandwidth than the UV/VIS spectrum. The kinetic and spectroscopic data suggested that a fraction (population I) of the MC chromophores experienced optical activity induced by the chiral polypeptide environment, while the remainder of the MC chromophores (population II) were solvated and enantiomeric.

High resolution structure of an oligomeric eye lens beta-crystallin. Loops, arches, linkers and interfaces in beta B2 dimer compared to a monomeric gamma-crystallin

beta-Crystallins are polydisperse, oligomeric structural proteins that have a major role in forming the high refractive index of the eye lens. Using single crystal X-ray crystallography with molecular replacement, the structure of beta B2 dimer has been solved at 2.1 A resolution. Each subunit comprises an N and C-terminal domain that are very similar and each domain is formed from two similar "Greek key" motifs related by a local dyad. Sequence differences in the internally quadruplicated molecules, analysed in terms of their beta-sheets, hairpins and arches, give rise to structural differences in the motifs. Whereas the related family of gamma-crystallins are monomers, beta-crystallins are always oligomers. In the beta B2 subunit, the domains, each comprising two motifs, are separated by an extended linking peptide. A crystallographic 2-fold axis relates the two subunits of the dimer so that the N-terminal domain of one subunit of beta B2 and the C-terminal domain of the symmetry-related subunit are topologically equivalent to the two covalently connected domains of gamma B-crystallin. The intersubunit domain interface is very similar to the intradomain interface of gamma B, although many sequence differences have resulted in an increase in polar interactions between domains in beta B2. Comparison of the structures of beta B2 and gamma B-crystallins shows that the two families differ largely in the conformation of their connecting peptides. A further extensive lattice contact indicates a tetramer with 222 symmetry. The ways in which insertions and extensions in the beta-crystallin effect oligomer interactions are described. The two kinds of crystallin are analysed for structural features that account for their different stabilities. These studies are a basis for understanding formation of higher aggregates in the lens.

Role of electrostatic repulsion in the acidic molten globule of cytochrome c

The molten globule has been assumed to be a major intermediate state of protein folding. To extend our understanding of protein folding it is important to elucidate the thermodynamic mechanism of conformational stability of the molten globule. To clarify the role of electrostatic charge repulsion in the stability of the acidic molten globule state, we prepared a series of acetylated horse ferricytochrome c species with various degrees of charge repulsion. On the basis of circular dichroism measurement, we show that the stability of the acidic molten globule is determined by a balance of electrostatic repulsions between positive residues, which favor the extended conformation, and the opposing forces, which stabilize the molten globule. These results provide a clear example of charge repulsions producing unfolding of the compact protein structure, and suggest that the reversibly denatured conformation of ferricytochrome c under physiological conditions (i.e. neutral pH, ambient temperature and no denaturant) is the molten globule.

Antigen mobility in the combining site of an anti-peptide antibody

The interaction between a high-affinity antibody, raised against a peptide incorporating the loop region of hen egg lysozyme (residues 57-84), and a peptide antigen corresponding to this sequence, has been probed by proton NMR. The two-dimensional correlated spectroscopy spectrum of the antibody-antigen complex shows sharp, well-resolved resonances from at least half of the bound peptide residues, indicating that the peptide retains considerable mobility when bound to the antibody. The strongly immobilized residues (which include Arg-61, Trp-62, Trp-63, and Ile-78) do not correspond to a contiguous region in the sequence of the peptide. Examination of the crystal structure of the protein shows that these residues, although remote in sequence, are grouped together in the protein structure, forming a hydrophobic projection on the surface of the molecule. The antibody binds hen egg lysozyme with only a 10-fold lower affinity than the peptide antigen. We propose that the peptide could bind to the antibody in a conformation that brings these groups together in a manner related to that found in the native protein, accounting for the high crossreactivity.

New nuclear magnetic resonance structures and structural methodology

Progress in the field of protein nuclear magnetic resonance spectroscopy during the past year has included the elucidation of a number of new structures. In addition, several critical developments in the experimental methodology have opened up the potential for applying the nuclear magnetic resonance-based approach to structure determination in solution of recombinant proteins in excess of 15 kD.

The chaperonin GroEL binds a polypeptide in an alpha-helical conformation

Chaperones facilitate folding and assembly of nascent polypeptides in vivo and prevent aggregation in refolding assays in vitro. A given chaperone acts on a number of different proteins. Thus, chaperones must recognize features present in incompletely folded polypeptide chains and not strictly dependent on primary structural information. We have used transferred nuclear Overhauser effects to demonstrate that the Escherichia coli chaperonin GroEL binds to a peptide corresponding to the N-terminal alpha-helix in rhodanese, a mitochondrial protein whose in vitro refolding is facilitated by addition of GroEL, GroES, and ATP. Furthermore, the peptide, which is unstructured when free in aqueous solution, adopts an alpha-helical conformation upon binding to GroEL. Modification of the peptide to reduce its intrinsic propensity to take up alpha-helical structure lowered its affinity for GroEL, but, nonetheless, it could be bound and took up a helical conformation when bound. We propose that GroEL interacts with sequences in an incompletely folded chain that have the potential to adopt an amphipathic alpha-helix and that the chaperonin binding site promotes formation of a helix.

Molecular biology and pathology of scrapie and the prion diseases of humans

Scrapie and bovine spongiform encephalopathy of animals and Creutzfeldt-Jakob and Gerstmann-Sträussler-Scheinker diseases of humans are transmissible and genetic neurodegenerative diseases caused by prions. Infectious prion particles are composed largely, if not entirely, of an abnormal isoform of the prion protein which is encoded by a chromosomal gene. An as yet unidentified post-translational process converts the cellular prion protein into an abnormal isoform. Scrapie neuropathology, incubation times, and prion synthesis in transgenic mice are controlled by the prion protein gene. Point mutations in the prion protein genes of animals and humans are genetically linked to development of neurodegeneration. Transgenic mice expressing mutant prion proteins spontaneously develop neurologic dysfunction and spongiform neuropathology. Studies of prion diseases may advance investigations of other neurodegenerative disorders and of how neurons differentiate, function for decades and grow senescent.

Molecular biology of prion diseases

Prions cause transmissible and genetic neurodegenerative diseases, including scrapie and bovine spongiform encephalopathy of animals and Creutzfeldt-Jakob and Gerstmann-Sträussler-Scheinker diseases of humans. Infectious prion particles are composed largely, if not entirely, of an abnormal isoform of the prion protein, which is encoded by a chromosomal gene. A posttranslational process, as yet unidentified, converts the cellular prion protein into an abnormal isoform. Scrapie incubation times, neuropathology, and prion synthesis in transgenic mice are controlled by the prion protein gene. Point mutations in the prion protein genes of animals and humans are genetically linked to development of neuro-degeneration. Transgenic mice expressing mutant prion proteins spontaneously develop neurologic dysfunction and spongiform neuropathology. Understanding prion diseases may advance investigations of other neurodegenerative disorders and of the processes by which neurons differentiate, function for decades, and then grow senescent.

Molten globule intermediates and protein folding

The background to the concept of the term "molten globule" as a description of intermediates observed in the folding of globular proteins is discussed. These compact intermediates are characterised by certain properties including the presence of secondary structure and considerable conformational mobility compared to the native, functional state. Those intermediates that are thermodynamically stable under mild denaturing conditions have many features in common with the transient intermediates that accumulate significantly during the process of folding. Attention is drawn to cases where the two types are however distinguished on grounds of their Stokes radius, in which cases there is currently no direct evidence for the involvement of the stable intermediates on the folding pathway. Experimental evidence relating to the early stages in folding is reviewed and compared, highlighting the temporal relationship between general collapse of the polypeptide chain and the formation of secondary structure. The continued use of the term "molten globule" is recommended where the minimum essential structural criteria for this state are met.