A study of renaturation of reduced hen egg white lysozyme. Enzymically active intermediates formed during oxidation of the reduced protein

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.

Native-like tertiary structure formation in the alpha-domain of a hen lysozyme two-disulfide variant

Structure formation in two species of the two-disulfide variant of hen lysozyme was investigated by means of CD spectroscopy, disulfide exchange measurement, and 1H-NMR spectroscopy. One species, 2SS [6-127, 30-115], which contained the two disulfide bonds found in the alpha-domain of authentic lysozyme, had amounts of secondary and tertiary structures, and bacteriolytic activity comparable to those of authentic lysozyme, and showed a cooperative thermal unfolding. By contrast, the other species, 2SS [64-80, 76-94], which contained the beta-domain disulfide bond as well as the inter-domain one, had a limited amount of secondary structure and little tertiary structure. Disulfide-exchange did not occur for 2SS [6-127, 30-115], whereas it occurred for 2SS [64-80, 76-94], indicating that the protein main-chain fold coupled with the formation of two disulfide bonds is relatively stable for the former variant, while unstable for the latter. 1H-NMR spectra of 2SS [6-127, 30-115] showed that native-like local environment is present within the region that corresponds to the alpha-domain, while it is absent within the region that corresponds to the beta or inter-domain. These results indicate that the alpha-domain of hen lysozyme can be an independent folding domain at equilibrium. Although the bipartite nature in the structure formation of hen lysozyme is similar to that reported for alpha-lactalbumin, differences exist between the disulfide-intermediates of the two proteins in terms of the structural domain that accomplishes tertiary structure.

Comparison of the kinetics of S-S bond, secondary structure, and active site formation during refolding of reduced denatured hen egg white lysozyme

To investigate the role of some tertiary interactions, the disulfide bonds, in the early stages of refolding of hen lysozyme, we report the kinetics of reoxidation of denatured and reduced lysozyme under the same refolding conditions as those previously used to investigate the kinetics of regain of its circular dichroism (CD), fluorescence, and activity. At different stages of the refolding, the oxidation of the protein was blocked by alkylation of the free cysteines with iodoacetamide and the various oxidation states present in the samples were identified by electrospray-mass spectrometry. Thus, it was possible to monitor the appearance and/or disappearance of the species with 0 to 4 disulfide bonds. Using a simulation program, these kinetics were compared with those of regain of far-UV CD, fluorescence, and enzymatic activity and were discussed in terms of a refined model for the refolding of reduced hen egg white lysozyme.

Kinetics of secondary structure recovery during the refolding of reduced hen egg white lysozyme

We have shown previously that, in less than 4 ms, the unfolded/oxidized hen lysozyme recovered its native secondary structure, while the reduced protein remained fully unfolded. To investigate the role played by disulfide bridges in the acquisition of the secondary structure at later stages of the renaturation/oxidation, the complete refolding of reduced lysozyme was studied. This was done in a renaturation buffer containing 0.5 M guanidinium chloride, 60 microM oxidized glutathione, and 20 microM reduced dithiothreitol, in which the aggregation of lysozyme was minimized and where a renaturation yield of 80% was obtained. The refolded protein could not be distinguished from the native lysozyme by activity, compactness, stability, and several spectroscopic measurements. The kinetics of renaturation were then studied by following the reactivation and the changes in fluorescence and circular dichroism signals. When bi- or triphasic sequential models were fitted to the experimental data, the first two phases had the same calculated rate constants for all the signals showing that, within the time resolution of these experiments, the folding/oxidation of hen lysozyme is highly cooperative, with the secondary structure, the tertiary structure, and the integrity of the active site appearing simultaneously.

Artificial chaperone-assisted refolding of denatured-reduced lysozyme: modulation of the competition between renaturation and aggregation

Conditions that promote renaturation of an unfolded protein also promote protein aggregation, in many cases, because these competing intramolecular and intermolecular processes are driven by similar networks of noncovalent interactions. The GroEL/GroES system and related biological chaperones facilitate the renaturation of substrate proteins by minimizing the aggregation pathway. We have devised a two-step method in which small molecules, "artificial chaperones," facilitate protein refolding from a chemically denatured state. In the first step, the protein is captured by a detergent as guanidinium chloride is diluted to a non-denaturing concentration; formation of a protein-detergent complex prevents both protein aggregation and proper refolding. In the second step, a cyclodextrin strips detergent from the protein, allowing the protein to refold. Here we describe the first application of this method to a protein that must form disulfides in the native state. Lysozyme (hen egg white) can be refolded from the Gdm-denatured, DTT-reduced state in good yields at final protein concentrations as high as 1 mg/mL with the artificial chaperone method. Several mechanistic aspects of artificial chaperone-assisted refolding have been probed, and a detailed mechanism for the kinetically controlled stripping step is proposed.

Folding of lysozyme

Due to the detailed knowledge of the three-dimensional structure, chemistry and catalytic mechanism of hen egg white lysozyme, this enzyme has become a major model for the analysis of the folding pathway of globular proteins. Unfolding and folding of lysozyme are reversible processes. Unfolding is a highly cooperative event; under physiological conditions only the native and the unfolded states are stable. Folding of lysozyme involves both a cooperative and a parallel pathway. The complexities in the folding pathway arise from the collapsed state which is formed within a burst-phase in the first milliseconds of folding. In a second, fast folding phase, major parts of the secondary structures both in the alpha-domain and the beta-domain are formed. During the slow folding phase, formation of secondary structure is completed and native tertiary structure is formed in less than 1 second. Folding of reduced lysozyme combines both secondary and tertiary structure organization, as well as formation of four disulphide bonds. Analysis of formation of disulphide bonds showed that there exists a restricted search of structures in the formation of the native conformation and a nucleation in the folding pathway. The transition from a two-disulphide bond intermediate to a three-disulphide bond form appears to be the rate-limiting step in this pathway. Native-like catalytic properties depend on the correct generation of all four disulphide bonds. Folding of both denatured and denatured/reduced lysozyme is characterized by transient folding species possessing structural properties of the molten globule state: high content of secondary structure, no tertiary fold, and the appearance of hydrophobic structures.

Antibody responses raised against a conformational V3 loop peptide of HIV-1

The amino acid sequence of the principal neutralizing determinant (PND) of 224 cases of human immunodeficiency virus type 1 (HIV-1) was determined and the most frequently occurring sequence was used as a peptide antigen for studying virus-specific antibody responses. In our present study, a linear peptide of the most frequent PND was first synthesized and then oxidized to create a disulfide-bridged loop conformation. Then, in order to construct a macromolecular structure for the purpose of increasing antigenicity, the synthetic peptide was conjugated to a core peptide. We compared the immunogenicity of the disulfide-bridged loop PND peptide antigen (AG4) and the linear PND peptide antigen (AG5). After immunizing rabbits 5 and 6 times with both peptides, the results obtained using ELISA revealed that AG4 (conformational-loop type) was more capable of inducing a high titer of antigen-specific antibodies than was AG5 (linear type). Despite an amino acid sequence homology of 72%, a 1:8 dilution of serum raised against AG4 inhibited 81.9% of HIV-1IIIB-mediated cell fusion, suggesting that conformational V3 loop peptide is able to elicit an antibody response which is strongly HIV-1-specific.

Renaturation of recombinant proteins produced as inclusion bodies

Expression of recombinant proteins in Escherichia coli often results in the formation of insoluble inclusion bodies. Within the last few years specific methods and strategies have been developed to prepare active proteins from these inclusion bodies. These methods include (i) isolation of inclusion bodies after disintegration of cells by mechanical forces and purification by washing with detergent solutions or low concentrations of denaturant, (ii) solubilization of inclusion bodies with high concentrations of urea or guanidine-hydrochloride in combination with reducing reagents, and (iii) renaturation of the proteins including formation of native disulphide bonds. Renatured and native disulphide bond formation are accomplished by (a) either air oxidation, (b) glutathione reoxidation starting from reduced material, or (c) disulphide interchange starting from mixed disulphides containing peptides. The final yield of renatured proteins can be increased by adding low concentrations of denaturant during renaturation.

Analysis of catalytic properties of hen egg white lysozyme during renaturation from denatured and reduced material

Catalytic properties of hen egg white lysozyme were analyzed during the renaturation of the enzyme from completely reduced and denatured material. The formation of intermediate folding products and the generation of native lysozyme was monitored by acetic acid/urea electrophoresis. The results showed that during the beginning of renaturation almost all reduced and denatured lysozyme is converted to forms possessing lower compactness than native lysozyme, probably as a result of formation of only one or two disulfide bonds. Kinetic analysis of lysozyme during renaturation showed that the generation of lysozyme with four disulfide bonds was not necessarily equivalent to the formation lysozyme with native-like catalytic properties. It appeared that the formation rate of the structures of the structures of the substrate binding site and of the catalytic site were limited by the generation of four disulfide bonds containing lysozyme. The catalytic properties of intermediate folding products made it evident that the final structures of the substrate binding site and of the catalytic site were formed after the generation of all disulfide bonds.

Reduced lysozyme in solution and its interaction with non-ionic surfactants

Reduced lysozyme at pH 2.5 bound poly(oxyethylene) alkylethers in two steps and the maximum bound amount Qmax of the surfactant reached as large as 0.5-0.7 mole per mole amino acid residue in the cooperative binding step. Binding isotherms were well superimposed when surfactant concentrations were normalized by respective values of the critical micelle concentration, cmc. In terms of the onset concentrations of the cooperative binding C*, hydrophobicity of reduced lysozyme was quantitatively defined as RT In (cmc/C*) which amounted to 670 J per mole surfactant and was unique to the protein irrespective of the kind of surfactant. Qmax could be used as another measure of the hydrophobicity of the protein. The binding isotherms were evaluated by two methods: equilibrium dialysis and surface tension. Their results were consistent with each other and rather complementary. Reduced lysozymes were molecularly dispersed at pH below 2.5 in 0.01 M NaCl but aggregation took place as pH increased. The aggregates could not be dissociated on dilution nor by the addition of nonionic surfactants but by lowering pH. The irreversible nature of the aggregation was reasonably interpreted with a model based on the 'entangled' arrangement of the beta-sheets, which could account for the irreversible aggregation of unfolded proteins in general.

Efficient in vitro folding of the three-disulfide derivatives of hen lysozyme in the presence of glycerol

Four derivatives of hen lysozyme, each lacking one native disulfide bond of the four in authentic lysozyme, were produced in Escherichia coli by expressing synthetic mutant genes. In the reoxidation reaction of the reduced derivatives purified from inclusion bodies, the addition of glycerol significantly enhanced the efficiency of folding and 'correct' disulfide bond formation. This enabled simple chromatographical purification of refolded materials. Purified 3SS-derivatives all showed lytic activities and secondary structures comparable to authentic lysozyme, which directly showed that none of the four native disulfide bonds is a prerequisite for 'correct' in vitro folding.

A kinetic study of the competition between renaturation and aggregation during the refolding of denatured-reduced egg white lysozyme

The recovery of proteins following denaturation is optimal at low protein concentrations. The decrease in yield at high concentrations has been explained by the kinetic competition of folding and "wrong aggregation". In the present study, the renaturation-reoxidation of hen and turkey egg white lysozyme was used as a model system to analyze the committed step in aggregate formation. The yield of renatured protein for both enzymes decreased with increasing concentration in the folding process. In addition, the yield decreased with increasing concentrations of the enzyme in the denatured state (i.e., prior to its dilution in the renaturation buffer). The kinetics of renaturation of turkey lysozyme were shown to be very similar to those of hen lysozyme, with a half-time of about 4.5 min at 20 degrees C. The rate of formation of molecular species that lead to formation of aggregates (and therefore fail to renature) was shown to be rapid. Most of the reaction occurred in less than 5 s after the transfer to renaturation buffer, and after 1 min, the reaction was essentially completed. Yet, by observing the effects of the delayed addition of denatured hen lysozyme to refolding turkey lysozyme, it was shown that folding intermediates become resistant to aggregation only much more slowly, with kinetics indistinguishable from those observed for the appearance of native molecules. The interactions leading to the formation of aggregates were nonspecific and do not involve disulfide bonds. These observations are discussed in terms of possible kinetic and structural aspects of the folding pathway.

Refolding human serum albumin at relatively high protein concentration

The conditions for refolding reduced and denatured human serum albumin (HSA) were investigated with a view to maximising the yield of native monomeric albumin. Refolding by dialysis was found to be preferable to dilution as a means of chaotrope (urea) and reductant (2-mercaptoethanol) removal. Dialysis of denatured HSA solutions containing 4-8 M urea and 14 mM 2-mercaptoethanol at pH 10.0 was found to be optimal for HSA refolding. The yield of monomeric HSA was maximal (94%) for dialysis in the presence of EDTA (1 mM) and sodium palmitate (20 microM). Using this protocol it was possible to refold HSA at concentrations in excess of 5 mg.ml-1 whilst maintaining a high recovery of native monomer. These results represent a considerable improvement on established methods of HSA refolding.

Detection of intermediate species in the refolding of bovine trypsinogen

The mixed disulfide of bovine trypsinogen and glutathione was refolded at pH 8.6 and 4 degrees C with a mixture of 3 mM cysteine and 1 mM cystine catalyzing disulfide interchange. The folding process was monitored by analysis of quenched samples with isoelectric focusing and size-exclusion chromatography. Isoelectric focusing showed a progressive change from a pI of 5.2 for the mixed disulfide derivative to a pI of 9.3 for native trypsinogen. A number of principal intermediates were detected as a function of the refolding time. These intermediates were also separated and further characterized by size-exclusion chromatography on columns of TSK G2000 SW operated in the high-performance liquid chromatographic mode. Rechromatography of a series of sequential fractions taken from the parental peak was necessary to resolve and characterize the principal intermediates. The loss of glutathione moieties produced a partly folded structure with an apparent hydrodynamic volume (Stokes radius, Rs) of 33.9 A. These structures became compact with time, and more intermediates were detected between 33.9 and 29.2 A. Finally, a change in conformation, resembling a two-state transition, changed the molecules of Rs 29.2 to the compact structure of native trypsinogen (22.4 A). The rate of formation of the native structure was determined from the progress curves derived from isoelectric focusing and size-exclusion chromatography.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction and reoxidation of the neurotoxin II from the scorpion Androctonus australis Hector

Reoxidation of the totally reduced scorpion neurotoxin II from Androctonus australis Hector (four disulfide bridges) has been investigated. The totally reduced toxin was highly insoluble in neutral and alkaline conditions, which prevented the use of the usual air oxidation process for renaturation. We tested a new method in which the reduced molecules were first solubilized in 10% (v/v) acetic acid and then oxidized by air through dialysis against a series of buffers with a slow pH gradient from 2.2 to 7.0 or 8.0. In this system, up to 95% of the protein was recovered in solution. Addition of reduced and oxidized glutathione accelerated refolding and also permitted a better recovery of fully active peptide as measured by both toxicity to mice and ability to displace 125I radiolabeled toxin II from its binding site on rat brain synaptosomal fractions. The reoxidation reaction could also be monitored directly by high pressure liquid chromatography. A strong effect of guanidine hydrochloride concentration as well as the temperature was observed both on the solubility of the reoxidation intermediates and on the refolding pathway. Finally, the method used, i.e. dialysis reoxidation with a pH gradient from 2.2 to 8.0 in 0.1 M sodium phosphate, 0.1 M sodium chloride, 20 mM guanidine hydrochloride, 1 mM oxidized and reduced glutathione allowed regeneration in high yield (70%) of a reoxidized toxin form indistinguishable from the native toxin. A minor stable and inactive molecular species (about 30%) showing a difference in mobility by electrophoresis was also detected.

Conformations of intermediates in the folding of the pancreatic trypsin inhibitor

Intermediates in the folding pathway of the bovine pancreatic trypsin inhibitor (PTI) have been examined by 1H nuclear magnetic resonance (n.m.r.). The intermediates were trapped during the reoxidation and consequent refolding of reduced PTI by alkylating free thiols; each intermediate contained different disulphide linkages. The n.m.r. spectra reveal that conformational features of the native protein are present in the intermediate containing just one of the three normal disulphide linkages (30-51). As additional normal disulphide bonds are formed, the conformation becomes more similar to that of the native protein. Introduction of additional but incorrect disulphide bonds does not lead to an increase in observable globular structure. A description of the folding process in terms of the conformations of the different intermediates is proposed. The significance of these results for the general mechanism of protein folding is outlined.

Renaturation of neurotoxin B-IV from the heteronemertine Cerebratulus lacteus

The heteronemertine Cerebratulus lacteus synthesizes a family of four structurally homologous, crustacean-selective polypeptide toxins (B-toxins) which affect action potential generation in nervous issue. The most abundant homolog, toxin B-IV, is completely inactivated by reduction of its four disulfide bands. Reduction is also accompanied by loss of secondary structure. Reoxidation of the reduced protein may be catalyzed by the oxidized forms of glutathione or dithiothreitol. Secondary structure and toxicity are recovered in parallel; the terminally reoxidized protein regains approximately 75% of both the neurotoxicity and the secondary structure of native B-IV.

Aggregation of bovine serum albumin upon cleavage of its disulfide bonds, studied by the time-resolved small-angle X-ray scattering technique with synchrotron radiation

A rapid mixing system of the stopped-flow type, used with small-angle X-ray scattering equipment using synchrotron radiation, is described. The process of aggregation of bovine serum albumin was traced with a time interval of 50 s, initiated upon cleavage of its disulfide bonds by reduction with dithiothreitol. The results indicate that a 218-fold molar excess of dithiothreitol over the number of moles of disulfide bonds in bovine serum albumin is sufficient to initiate the reaction immediately after mixing, which reaches equilibrium in about 15 min. On the other hand, half this amount is not sufficient to initiate the reaction, so that the reaction is delayed by about 150 s. Such a single-shot time-resolved experiment showed that experiments with a time interval of 100 ms are possible with repeated multi-shot runs.

Human leukocyte cathepsin G. Subsite mapping with 4-nitroanilides, chemical modification, and effect of possible cofactors

The extended substrate binding site of cathepsin G from human leukocytes has been mapped by using a series of peptide 4-nitroanilide substrates. The enzyme has a significant preference for substrates with a P1 Phe over those with the other aromatic amino acids Tyr and Trp. The S2 subsite was mapped with the substrates Suc-Phe-AA-Phe-NA where AA was 13 of the 20 amino acid residues commonly found in proteins. The best residues were Pro and Met. The S3 subsite was mapped with the sequence Suc-AA-Pro-Phe-NA by using 14 different amino acid residues for AA. The two best residues were the isosteric Val and Thr. No significant improvement in reactivity was obtained by extending the substrate to include seven different P4 residues. The kinetic parameters for cathepsin G are significantly slower than those for many other serine proteases. Changes in the reaction conditions and addition of possible cofactors or ligands were in general found to have little effect on the enzymatic activity, while chemical modifications and proteolysis destroyed the activity of cathepsin G. Cathepsin G hydrolyzed peptides containing model desmosine residues and prefers the hydrophobic picolinoyllysine derivative over lysine by substantial margins at both the S4 and S2 subsites but will not tolerate it at S3. Substrates with sequences related to the cathepsin G cleavage site in angiotensin I and angiotensinogen, and the reactive site of alpha 1-antichymotrypsin, were hydrolyzed effectively by enzyme, but with unexceptional rates. Our results indicate that the natural substrate(s) and function(s) of cathepsin G still remain to be discovered.

Requirement of anionic groups for the mitogenicity of a fungal mitogen, vesiculogen

Active site(s) of a B-cell mitogen, vesiculogen, was investigated by means of chemical and enzymic modifications. Vesiculogen is a non-dialyzable fraction of a hot water extract from a fungus, Peziza vesiculosa (Yadomae et al, Microbiol. Immunol. 23, 997 (1979], and was composed of protein (approximately 60%), carbohydrate (approximately 30%), and a small amount of amino sugar, uronic acid, phosphate, and lipid. The mitogenicity was not affected by periodate oxidation, N-acylation, defatting treatment, destruction of the three dimensional structure, protease digestion, nuclease digestion, N-bromosuccinimide treatment, and beta-mercaptoethanol treatment. However, the mitogenicity was decreased by a modification of carboxyl groups, such as 1-ethyl-3(3-dimethylamino propyl)-carbodiimide or dicyclohexylcarbodiimide treatment. Vesiculogen contains a large amount of acidic amino acids. These results suggest that the mitogenicity of vesiculogen is due to the presence of anionic groups, such as aspartic acid and glutamic acid, and that the mitogenic substance is a novel, heat stable polyanionic B-cell activator obtained from Ascomycotina.

Age-dependent changes of protein structure. The properties of young and old rabbit aldolase are restored after reversible denaturation

The significantly increased helical content is observed in muscle aldolase molecule of old rabbits. The unfolding and refolding of protein conformation followed by circular dichroism, fluorescence and enzyme activity showed the recovery of initial conformation after the denaturation. The protein folds into the form that existed prior to denaturation--"young" into "young" and "old" into "old"--the conformational differences between them being restored. This suggests that the primary structure modifications prior to the folding of the native protein conformation are the origin of the age-dependent differences of aldolase structure and function.

Electrostatic influence of local cysteine environments on disulfide exchange kinetics

The ionic strength dependence of the bimolecular rate constant for reaction of the negative disulfide 5,5'-dithiobis (2-nitrobenzoic acid) with cysteines in fragments of naturally occurring proteins was determined by stopped-flow spectroscopy. The Debye-Hückel relationship was applied to determine the effective charge at the cysteine and thereby determine the extent to which nearby neighbors in the primary sequence influence the kinetics. Corrections for the secondary salt effect on cysteine pKs were determined by direct spectrometric pH titration of sulfhydryl groups or by observation of the ionic strength dependence of kinetics of cysteine reaction with the neutral disulfide 2,2'-dithiodipyridine. Quantitative expressions was verified by model studies with N-acetyl-cystein. At ionic strengths equal to or greater than 20 mM, the net charge at the polypeptide cysteine site is the sum of the single negative charge of the thiolate anion and the charges of the amino acids immediately preceding and following the cysteine in the primary sequence. At lower ionic strengths, more distant residues influence kinetics. At pH 7.0, 23 degree C, and an ionic strength of 20 mM, rate constants for reaction of the negative disulfide with a cysteine having two positive neighbors, one positive and one neutral neighbor, or two neutral neighbors are 132000, 3350, and 367 s-1 M-1, respectively. This corresponds to a contribution to the activation energy of 0.65- 1.1 kcal/mol per ion pair involved in collision between the cysteine and disulfide regions. The results permit the estimation that cysteine local environments may provide a means of achieving a 10(6)-fold range in rate constants in disulfide exchange reactions in random-coil proteins. This range may prove useful in developing strategies for directing disulfide pairing in synthetic proteins.

The influence of effectors on the refolding (reactivation) of immobilized trypsin

The refolding of immobilized trypsin in the presence of various effectors of its enzymatic activity has been studied. Trypsin covalently bound to Sephadex G-200 was made to unfold in a concentrated solution of urea; at the same time its S-S bonds were split with the help of dithiothreitol. The preparation was then separated from the splitting agents, and one of the effectors of the enzymatic activity of trypsin (boric acid, benzamidine, pancreatic or soybean inhibitors of trypsin, N-benzoyl-L-arginine ethyl ester, and N-tosyl-L-arginine methyl ester) was added and the reactivation of immobilized enzyme was studied in the absence of catalysts of thiol-disulphide exchange. The following effects were found. 1. The reactivation of immobilized trypsin in the presence of specific substrates or protein inhibitors proceeds with the same yield (2-5%) as in their absence. 2. In the presence of benzamidine or boric acid (competitive inhibitors) the reactivation yields of the immobilized trypsin increased 5-fold and 12-fold respectively, and became equal to 15% and 40%. Comparison of these results with the statistical probability of formation of the six native S-S bonds from twelve SH groups (approximately 0.01%) shows that if trypsin is made to refold in an immobilized state in the presence of a 'good' effector, the yield of the reactivation of the enzyme can be increased several thousand times. 3. Similar effects were observed for trypsin immobilized on Sepharose 4B. A model is suggested in terms of which the influence of various effectors on trypsin refolding is explained as being a result of their ability to bind with an intermediate folded forms of protein followed by a shift of the equilibrium towards 'regular' conformers.

Preparation of a two-disulfide bonded enzymically active derivative from hen egg lysozyme

A method has been developed for preparation of an enzymically active two-disulfide bonded derivative from hen egg lysozyme. Lysozyme (0.15 mM) is incubated with 2 mM dithiothreitol at pH 7.8, 23 degrees for 40 min. The products are reacted with [1-14C]iodoacetic acid and then purified by gel filtration and ion-exchange chromatography. An enzymically active derivative containing 4 mol of [1-14C] carboxymethyl groups and no free sulfhydryl groups is obtained in approximately 18% yield. Examinations of hydrodynamic volume, tryptophan fluorescence, CD and tryptic peptides containing [1-14C] carboxymethyl cysteine indicate that this derivative contains two presumably native disulfide bonds and two open disulfide bonds between Cys 6 and Cys 127 and between Cys 76 and Cys 94. The rest of the species in the incubation mixture are intact lysozyme. Thus, the species containing two presumably native disulfide bonds and four free sulfhydryl groups at Cys 6, Cys 76, Cys 94 and Cys 127 appears to be only the intermediate accumulating during reduction of lysozyme with dithiothreitol.

The refolding of denatured rabbit muscle creatine kinase

1. The refolding of rabbit muscle creatine kinase (ATP:creatine N-phosphotransferase, EC 2.7.3.2) which had been denatured in 3 M guanidine hydrochloride was monitored by studying the regain of enzyme activity. Full activity could be regained provided that the residual denaturant concentration was less than or equal to 0.1 M. 2. The refolded product was shown by a number of criteria (CD, kinetic parameters and polyacrylamide gel electrophoresis) to be identical with the native enzyme. 3. The rate of regain of enzyme activity was studied as a function of protein concentration. It was found that 70% of the activity was regained in a rapid, first-order process. The remaining activity was regained more slowly. In the rapid phase the number of reactive thiol groups per subunit declined from four to two; the further decline to one per subunit occurred more slowly. 4. It was found that the presence of the reducing agent dithiothreitol was not necessary for the regain of full activity, provided that the chelating agent EDTA was present. 5. The subunit structure of the enzyme during refolding was studied using dimethylsuberimidate as a cross-linking agent. From these experiments, a pathway for the refolding process could be proposed.

Protein folding

This review describes recent advances in studies on the stabilities of the three-dimensional structures of proteins and on the processes leading to the formation of these structures. The term ‘protein folding’ will be used here to denote the process of the conversion of an open polypeptide chain into the unique three-dimensional conformation of the native protein. Experimental and theoretical aspects of protein folding have been reviewed by anfinsen & Scheraga (1975). In the present article, we emphasize advances made since the writing of that review, together with a brief summary of the background of recent studies.