Oxidative renaturation of lysozyme at high concentrations

Newly synthesized cloned gene proteins expressed in bacteria frequently accumulate in insoluble aggregates or inclusion bodies. Active protein can be recovered by solubilization of inclusion bodies followed by renaturation of the solubilized (unfolded) protein. The recovery of active protein is highly dependent on the renaturation conditions chosen. The renaturation process is generally conducted at low protein concentrations (0.01-0.2 mg/mL) to avoid aggregation. We have investigated the potential of successfully refolding reduced and denatured hen egg white lysozyme at high concentrations (1 and 5 mg/mL). By varying the composition of the renaturation media, optimum conditions which kinetically favor proper folding over inactivation were found. Solubilizing agents such as guanidinium chloride (GdmCl) and folding aids such as L-arginine present in low concentrations during refolding effectively enhanced renaturation yields by suppressing aggregation resulting in reactivation yields as high as 95%. Quantitatively the kinetic competition between lysozyme folding and aggregation can be described using first-order kinetics for the renaturation reaction and third-order kinetics for the overall aggregation pathway. The rate constants for both reactions have been found to be strongly dependent on denaturant and thiol concentration. This strategy supercedes the necessity to reactivate proteins at low concentrations using large renaturation volumes. The marked increase in volumetric productivity makes this a viable option for recovering biologically active protein efficiently and in high yield in vitro from proteins produced as inclusion bodies within microbial cells. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 54: 221-230, 1997.     

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.     

Purification and renaturation of recombinant human interleukin-2.

Recombinant human interleukin-2 (IL-2) expressed as Escherichia coli was isolated as insoluble aggregates of protein (inclusion bodies) after cell breakage. IL-2 and contaminants were dissolved in 6 M-guanidinium chloride/10 mM-dithiothreitol, pH 8.5, and further purified in reduced and denatured form by gel-permeation chromatography in the same solvent. Renaturation was effected by dilution and autoxidation; IL-2 of native specific activity was isolated at over 95% purity by reversed-phase h.p.l.c.; an additional peak of reduced protein was also observed. Most losses of native IL-2 occurred on refolding, probably because of an aggregation process; concentrations around 1 microgram/ml were necessary to achieve 30% recovery. It was essential to maintain the denatured protein in reduced form before renaturation and autoxidation, which was most efficient at pH 8.5 with 1.5 microM-CuSO4. A procedure based on these observations has been used to prepare IL-2 on the 50 micrograms scale.     

Renaturation and purification of rhGM-CSF with ion-exchange chromatography

The renaturation and purification of recombinant human granulocyte macrophage colony stimulation factor (rhGM-CSF) expressed in Escherichia coli with strong anion-exchange chromatography (SAX) were studied. The effects of pH values, ratios of concentrations of GSH/GSSG, and urea concentrations in the mobile phase on the renaturation and purification of rhGM-CSF with SAX were investigated, respectively. The results show that the above three factors have remarkable influences on the efficiency of renaturation and mass recovery of rhGM-CSF. The addition of GSH/GSSG in the mobile phase can improve the formation of correct disulfide bonds in rhGM-CSF so that its renaturation yield increases. In addition, to enhance the mass recovery of rhGM-CSF with SAX, the low concentration of urea was added in the mobile phase to prevent denatured protein aggregation. Under the optimal conditions, rhGM-CSF was renatured with simultaneous purification on SAX column within 30 min only by one step. After that its specific bioactivity, mass recovery, and purity reached 1.66 x 10(7) IU x mg, 58.8%, and 96.2%, respectively.     

Oxidative renaturation of hen egg-white lysozyme in polyethylene glycol-salt aqueous two-phase systems.

Aqueous two-phase systems have been widely used for the separation and concentration of proteins. In this work we investigated the possibility of using aqueous two-phase system for the renaturation of inclusion body proteins by studying the effect of polyethylene glycol (PEG)-salt systems on the oxidative renaturation of hen egg-white lysozyme (HEWL) with guanidinium chloride (GdmCl) present in the system. To accomplish phase separation at moderately low concentrations of polymer and salt, the total GdmCl concentration had to be kept low (<1 M). The unfolded protein exhibited very low solubility under these conditions. In an attempt to increase the solubility of the protein, temperatures of 40, 50, and 60 degrees C were investigated. The effect of PEG molecular weight was also addressed. Best renaturation yields were obtained when using PEG 3400 and working at 50 degrees C. However, the total protein concentration had to be kept at a low level of 0.2 mg/mL. Lowering the total GdmCl concentration in the system resulted in increased aggregation.     

Renaturation of Escherichia coli-derived recombinant human macrophage colony-stimulating factor.

Recombinant human macrophage colony-stimulating factor (rhM-CSF), a homodimeric, disulfide bonded protein, was expressed in Escherichia coli in the form of inclusion bodies. Reduced and denatured rhM-CSF monomers were refolded in the presence of a thiol mixture (reduced and oxidized glutathione) and a low concentration of denaturing agent (urea or guanidinium chloride). Refolding was monitored by nonreducing gel electrophoresis and recovery of bioactivity. The effects of denaturant type and concentration, protein concentration, concentration of thiol/disulfide reagents, temperature, and presence of impurities on the kinetics of rhM-CSF renaturation were investigated. Low denaturant concentrations (<0.5 M urea) and high protein concentrations (>0.4 mg/ml) in the refolding mixture resulted in increased formation of aggregates, although aggregation was never significant even when refolding was carried out at room temperature. Higher protein concentration resulted in higher rates but did not lead to increased yields, due to the formation of unwanted aggregates. Experiments conducted at room temperature resulted in slightly higher rates than those conducted at 4 degrees C. Although the initial renaturation rate for solubilized inclusion body protein without purification was higher than that of the reversed-phase purified reduced denatured rhM-CSF, the final renaturation yield was much higher for the purified material. A maximum refolding yield of 95% was obtained for the purified material at the following refolding conditions: 0.5 M urea, 50 mM Tris, 1.25 mM DTT, 2 mM GSH, 2 mM GSSG, 22 degrees C, pH 8, [protein] = 0.13 mg/ml.     

Refolding of thioredoxin reductase assisted by groEL and PDI

Thioredoxin reductase was unfolded in 2 M guanidine hydrochloride as revealed by fluorescence and CD spectroscopy. Spontaneous refolding of denatured species resulted in low recovery of 10% catalytic activity after 4 h incubation at 25 degrees C. Addition of groEL or protein disulfide isomerase to the renaturation buffer accelerated the rate of recovery of catalytic activity to a level of 35 and 15%, respectively. Fluorescence spectroscopy has been used to investigate the interaction of groEL and protein disulfide isomerase with denatured thioredoxin reductase tagged with a fluorescent probe. The fluorescence emitted by the denatured protein was quenched upon binding to either groEL or protein disulfide isomerase. It is suggested that encapsulation of the protein substrate by the chaperone plays an important role in the process of folding by facilitating the formation of correctly folded species.     

Different molten globule-like folding intermediates of hen egg white lysozyme induced by high pH and tertiary butanol

We have provided evidence that hen egg white lysozyme (HEWL) existed in alpha helical and beta structure dominated molten globule (MG) states at high pH and in the presence of tertiary butanol, respectively. Circular dichroism (CD), intrinsic fluorescence, ANS binding and acrylamide-induced fluorescence quenching techniques have been used to investigate alkali-induced unfolding of HEWL and the effect of tertiary butanol on the alkaline-induced state. At pH 12.75, HEWL existed as molten globule like intermediate. The observed MG-like intermediate was characterized by (i) retention of 77% of the native secondary structure, (ii) enhanced binding of ANS (approximately 5 times) compared to native and completely unfolded state, (iii) loss of the tertiary structure as indicated by the tertiary structural probes (near-UV, CD and Intrinsic fluorescence) and (iv) acrylamide quenching studies showed that MG state has compactness intermediate between native and completely unfolded states. Moreover, structural properties of the protein at isoelectric point (pI) and denatured states have also been described. We have also shown that in the presence of 45% tertiary butanol (t-butanol), HEWL at pH 7.0 and 11.0 (pI 11.0) existed in helical structure without much affecting tertiary structure. Interestingly, MG state of HEWL at pH 12.7 transformed into another MG state (MG2) at 20% t-butanol (v/v), in which secondary structure is mainly beta sheets. On further increasing the t-butanol concentration alpha helix was found to reform. We have proposed that formation of both alpha helical and beta sheet dominated intermediate may be possible in the folding pathway of alpha + beta protein.     

Multi-block poloxamer surfactants suppress aggregation of denatured proteins

On the basis of elastic light scattering, we have compared the capacity of the multi-block, surfactant copolymers Poloxamer 108 (P108), Poloxamer 188 (P188), and Tetronic 1107 (T1107), of average molecular weight 4700, 8400, and 15,000, respectively, with that of polyethylene glycol (PEG, molecular weight 8000) to suppress aggregation of heat-denatured hen egg white lysozyme (HEWL) and bovine serum albumin (BSA). We also compared the capacity of P188 to that of PEG to suppress aggregation of carboxypeptidase A denatured in the presence of trifluoroethanol and to facilitate recovery of catalytic activity. In contrast to the multi-block copolymers, PEG had no effect in inhibiting aggregation of HEWL or of carboxypeptidase A with the recovery of catalytic activity. At very high polymer:protein ratios (>or=10:1), PEG increased aggregation of heat-denatured HEWL and BSA, consistent with its known properties to promote macromolecular crowding and crystallization of proteins. At a polymer:protein ratio of 2:1, the tetra-block copolymer T1107 was the most effective of the three surfactant copolymers, completely suppressing aggregation of heat-denatured HEWL. At a T1107:BSA ratio of 10:1, the poloxamer suppressed aggregation of heat-denatured BSA by 50% compared to that observed in the absence of the polymer. We showed that the extent of suppression of aggregation of heat-denatured proteins by multi-block surfactant copolymers is dependent on the size of the protein and the copolymer:protein molar ratio. We also concluded that at least one of the tertiary nitrogens in the ethylene-1,2-diamine structural core of the T1107 copolymer is protonated, and that this electrostatic factor underlies its capacity to suppress aggregation of denatured proteins more effectively than nonionic, multi-block poloxamers. These results indicate that amphiphilic, surfactant, multi-block copolymers are efficient as additives to suppress aggregation and to facilitate refolding of denatured proteins in solution. Because of these properties, multi-block, surfactant copolymers are suitable for application to a variety of biotechnological and biomedical problems in which refolding of denatured or misfolded proteins and suppression of aggregation are important objectives.     

Technical refolding of proteins: Do we have freedom to operate?

Expression as inclusion bodies in Escherichia coli is a widely used method for the large-scale production of therapeutic proteins that do not require post-translational modifications. High expression yields and simple recovery steps of inclusion bodies from the host cells are attractive features industrially. However, the value of an inclusion body-based process is dominated by the solubilization and refolding technologies. Scale-invariant technologies that are economical and applicable for a wide range of proteins are requested by industry. The main challenge is to convert the denatured protein into its native conformation at high yields. Refolding competes with misfolding and aggregation. Thus, the yield of native monomer depends strongly on the initial protein concentrations in the refolding solution. Reasonable yields are attained at low concentrations (≤0.1 mg/mL). However, large buffer tanks and time-consuming concentration steps are required. We attempt to answer the question of the extent to which refolding of proteins is protected by patents. Low-molecular mass additives have been developed to improve refolding yields through the stabilization of the protein in solution and shielding hydrophobic patches. Progress has been made in the field of high-pressure renaturation and on-column refolding. Mixing times of the denatured protein in the refolding buffer have been reduced using newly developed devices and the introduction of specific mixers. Concepts of continuous refolding have been introduced to reduce tank sizes and increase yields. Some of the patents covering refolding of proteins will soon expire or have already expired. This gives more freedom to operate.     

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.     

ATP causes small heat shock proteins to release denatured protein.

Small heat-shock proteins (sHSPs) are a ubiquitous family of low molecular mass (15-30 kDa) stress proteins that have been found in all organisms. Under stress, sHSPs such as alpha-crystallin can act as chaperones binding partially denatured proteins and preventing further denaturation and aggregation. Recently, it has been proposed that the function of sHSPs is to stabilize stress-denatured protein and then act cooperatively with other HSPs to renature the partially denatured protein in an ATP-dependent manner. However, the process by which this occurs is obscure. As no significant phosphorylation of alpha-crystallin was observed during the renaturation, the role of ATP is not clear. It is now shown that ATP at normal physiological concentrations causes sHSPs to change their confirmation and release denatured protein, allowing other molecular chaperones such as HSP70 to renature the protein and renew its biological activity. In the absence of ATP, sHSPs such as alpha-crystallin are more efficient than HSP70 in preventing stress-induced protein aggregation. This work also indicates that in mammalian systems at normal cellular ATP concentrations, sHSPs are not effective chaperones.     

The renaturation of reduced chymotrypsinogen A in guanidine HCl. Refolding versus aggregation

The refolding and reoxidation of fully reduced and denatured chymotrypsinogen A have been studied in the presence of low concentrations of guanidine HCl or urea. Renaturation yields of 60 to 70% were observed when the reoxidation was facilitated by mixtures of reduced and oxidized glutathione. Refolding occurred within a narrow range of denaturant concentration (1.0 to 1.3 M guanidine HCl and 2 M urea) in which the native protein was shown to be stable, and the reduced protein was shown to regain the correct disulfide pairing. Renatured chymotrypsinogen is indistinguishable from the native zymogen in chromatographic behavior, potential chymotryptic activity, sedimentation coefficient, and spectral properties. The kinetics of renaturation were determined. Some of the protein species obtained at various times of renaturation were characterized as incorrectly oxidized molecules which could be renatured by thiol-catalyzed interchange of disulfide bonds.     

Protein refolding at high concentration using size-exclusion chromatography

A new method to improve refolding yields and to increase the concentration of refolded proteins in a single operation has been developed. The method uses size-exclusion chromatography matrices to perform buffer exchange, aggregate removal, and the folding reaction. The reduced diffusion of proteins in gel-filtration media has been shown to suppress the nonspecific interactions of partially folded molecules, thus reducing aggregation. Hen egg white lysozyme (HEWL) and bovine carbonic anhydrase (CAB) were successfully refolded from initial protein concentrations of up to 80 mg/mL using Sephacryl S-100 (HR). The aggregation reaction for lysozyme was reduced and was only detected at the highest protein concentration used. The average recovery of lysozyme was 63%, with an average specific activity of 104%. Carbonic anhydrase experiments also showed that aggregation was suppressed and the average protein recovery from the column was 56%, with a specific activity of 81%. This process enables refolding and the purification of active species to be achieved in a single step. (c) 1996 John Wiley & Sons, Inc.     

Kinetic model of lysozyme renaturation with the molecular chaperone GroEL

From the renaturation kinetics of denatured/reduced lysozyme assisted by the molecular chaperone GroEL, a simplified kinetic model was established based on the competition between protein folding and aggregation. In the presence of GroEL and ATP, the aggregate formation was a second order reaction. With 2 mM ATP, a renaturation yield of 90% at a high renaturation rate was obtained when the molar ratio of GroEL to lysozyme was 1:1.     

Topological considerations in protein structure. I. Disulfide linkage

The patterns of disulfide bridges in proteins were considered by using the concept of topological information content. It was proposed that the difference between topological informal ion content in the half-cystine residues of the native state and of the fully reduced form of a protein is related to the problems of the renaturation of the fully reduced and denatured protein. Specifically, when there is no difference between the topological information contents of the two states, the reduced protein is able to recover its native conformation. The concept reported in the present paper is consistent without exception with reported experimental results.     

Control of aggregation in protein refolding: a variety of surfactants promote renaturation of carbonic anhydrase II.

The denaturation and renaturation of carbonic anhydrase II (CAII) has been studied in several laboratories. Both thermodynamic and kinetic evidence support the existence of at least two intermediates between denatured and native protein. Previous studies have shown that on rapid dilution of a CAII solution from 5 M to 1 M guanidinium chloride, aggregation strongly competes with renaturation at higher protein concentrations, suggesting an upper limit for [CAII] of approximately 0.1%. Our experiments show 60% renaturation at 0.4% [CAII] and that aggregate formation is partially reversible. This yield can be substantially increased by several surfactant additives, including simple alkanols as well as micelle-forming surfactants. Effective surfactants (promoters) act by suppressing initial aggregate formation, not by dissolving aggregates. Promoters act on either the first folding intermediate (I1) or oligomers thereof. Eight of the 18 surfactants examined showed promoter activity, and no correlation was evident between promoter activity and chemical structure or surface tension lowering. These results indicate discrimination (molecular recognition) by I1 and/or its oligomers.     

重组人尿激酶原的体外变复性研究

重组人尿激酶原在大肠杆菌中过量表达时形成不溶物包涵体,需经体外变复性后才能获得生物活性。本文旨在提高包涵体中变性尿激酶原的复性效率,通过对pH,温度,变性剂种类及浓度,蛋白浓度,以及巯基氧化还原对的比率等的定性定量分析,研究重组人尿激酶原体外变复性的基本条件,并比较了添加某些非特异有效成分,脉冲稀释,梯度透析等方法对提高重组人尿激酶原体外变复性效率的作用。确定了重组人尿激酶原体外变复性的适宜方法,复性效率可达20%～30%。     

Renaturation studies of free and immobilized D-amino-acid oxidase

The renaturation of free and Sepharose-immobilized D-amino-acid oxidase (D-amino-acid:oxygen oxidoreductase (deaminating), EC 1.4.3.3), after its denaturation with 6 M guanidine hydrochloride, was investigated. No reactivation, or extremely limited reactivation (less than or equal to 4+), was obtained with the free enzyme, is spite of various attempts including the use of dialysis or buffers containing cofactors, different types of anions, surfactants and low concentrations of denaturing agents. The main obstacle to renaturation appeared to be the interaction among denatured or partially renatured monomers giving rise to inactive aggregates. In contrast, using the immobilized enzyme approach, substantial renaturation (up to 50%) of D-amino-acid oxidase was achieved. The denaturation-renaturation process was followed by monitoring the catalytic activity as well as the intrinsic protein fluorescence. An inverse correlation was found to exist between the degree of matrix activation by CNBr and the yield of enzyme reactivation. The anions of the lyotropic series markedly influenced the reactivation, showing an effectiveness opposite to their salting-out potential (thiocyanate congruent to iodide greater than chloride greater than phosphate congruent to sulphate congruent to citrate). Instead, the anions considerably increased the activity and stability of free and immobilized enzyme, according to their salting-out potential. Immobilized monomers of D-amino-acid oxidase, which in solution undergoes self-association, showed poor capacity to interact with the free enzyme: thus they appear unsuitable for analytical and preparative purposes.     

Renaturation of human proinsulin--a study on refolding and conversion to insulin

The production of human proinsulin in Escherichia coli usually leads to the formation of inclusion bodies. As a consequence, the recombinant protein must be isolated, refolded under suitable redox conditions, and enzymatically converted to the biologically active insulin. In this study we describe a detailed in vitro renaturation protocol for human proinsulin that includes native structure formation and the enzymatic conversion to mature insulin. We used a His(8)-Arg-proinsulin that was renatured from the completely reduced and denatured state in the presence of a cysteine/cystine redox couple. The refolding process was completed after 10-30 min and was shown to be strongly dependent on the redox potential and the pH value, but not on the temperature. Refolding yields of 60-70% could be obtained even at high concentrations of denaturant (3M guanidinium-HCl or 4M urea) and protein concentrations of 0.5mg/ml. By stepwise renaturation a concentration of about 6 mg/ml of native proinsulin was achieved. The refolded proinsulin was correctly disulfide-bonded and native and monomeric as shown by RP-HPLC, ELISA, circular dichroism, and analytical gel filtration. Treatment of the renatured proinsulin with trypsin and carboxypeptidase B yielded mature insulin.