Renaturation of glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides after denaturation in 4 M guanidine hydrochloride: kinetics of aggregation and reactivation

Trehalose and 6-aminohexanoic acid stabilize and renature glucose-6-phosphate dehydrogenase inactivated by glycation and by guanidinium hydrochloride

A number of naturally occurring small organic molecules, primarily involved in maintaining osmotic pressure in the cell, display chaperone-like activity, stabilizing the native conformation of proteins and protecting them from various kinds of stress. Most of them are sugars, polyols, amino acids or methylamines. In addition to their intrinsic protein-stabilizing activity, these small organic stress molecules regulate the activity of some molecular chaperones, and may stabilize the folded state of proteins involved in unfolding or in misfolding diseases, such as Alzheimer's and Parkinson's diseases, or alpha1-antitrypsin deficiency and cystic fibrosis, respectively. Similar to molecular chaperones, most of these compounds have no substrate specificity, but some specifically stabilize certain proteins, e.g., 6-aminohexanoic acid (AHA) stabilizes apolipoprotein A. In the present work, the capacity of 6-aminohexanoic acid to stabilize non-specifically other proteins is demonstrated. Both trehalose and AHA significantly protect glucose-6-phosphate dehydrogenase (G6PD) against glycation-induced inactivation, and renatured enzyme already inactivated by glycation and by guanidinium hydrochloride (GuHCl). To the best of our knowledge, there are no data on the effect of these compounds on protein glycation. The correlation between the recovery of enzyme activity and structural changes indicated by fluorescence spectroscopy and Western blotting contribute to better understanding of the protein stabilization mechanism.

Reversible Equilibrium Unfolding of Triosephosphate Isomerase from Trypanosoma cruzi in Guanidinium Hydrochloride Involves Stable Dimeric and Monomeric Intermediates

The reversible guanidinium hydrochloride-induced unfolding of Trypanosoma cruzi triosephosphate isomerase (TcTIM) was characterized under equilibrium conditions. The catalytic activity was followed as a native homodimeric functional probe. Circular dichroism, intrinsic fluorescence, and size-exclusion chromatography were used as secondary, tertiary, and quaternary structural probes, respectively. The change in ANS fluorescence intensity with increasing denaturant concentrations was also determined. The results show that two stable intermediates exist in the transition from the homodimeric native enzyme to the unfolded monomers: one (N(2*)) is a slightly more expanded, non-native, and active dimer, and the other is a partially expanded monomer (M) that binds ANS. Spectroscopic and activity data were used to reach a thermodynamic characterization. The results indicate that the Gibbs free energies for the partial reactions are 4.5 (N(2) <==> N(2*)), 65.8 (N(2*) <==> 2M), and 17.8 kJ/mol (M <==> U). It appears that TcTIM monomers are more stable than those found for other TIM species (except yeast TIM), where monomer stability is only marginal. These results are compared with those for the guanidinium hydrochloride-induced denaturation of TIM from different species, where despite the functional and three-dimensional similarities, a remarkable heterogeneity exists in the unfolding pathways.

Aggregation and folding of recombinant human creatine kinase

The processes of aggregation and refolding of recombinant human creatine kinase (rHCK) were studied. Most of the rHCK expressed in E. coli was present in the insoluble traction and it could be solubilized in 6 M urea solution. Unfolding of rHCK in 6 M urea showed biphasic kinetic courses (kappa1 = 6.5 x 10(-3) s(-1); kappa2 = 0.54 x 10(-3) s(-1)) as observed by maximum fluorescence wavelength change. During refolding of the rHCK dissolved in urea, significant aggregation was noticed following first-order kinetics. Aggregation rate constants were influenced by the concentration of NaCl, which increased the difference in transition-free energy (deltadeltaG), showing that stabilization of folding intermediates by NaCl could efficiently reduce the formation of insoluble aggregates. Formations of aggregate were also reduced by adjusting temperature, pH, and concentration of rHCK. Refolding of rHCK under the optimized condition which prevented the aggregation also showed multi-kinetic phases (kappa1 = 3.0 x 10(-3) s(-1); kappa2 = 0.64 x 10(-3) s(-1)). Under optimized conditions applied in this study, rHCK could correctly refold retrieving the high specific enzymatic activity.

Effects of macromolecular crowding on the refolding of glucose- 6-phosphate dehydrogenase and protein disulfide isomerase

The effects of polysaccharide, polyethylene glycol, and protein-crowding agents on the refolding of glucose-6-phosphate dehydrogenase (G6PDH) and protein disulfide isomerase have been examined. By increasing concentration during refolding, the reactivation yields of the two proteins decrease with the formation of soluble aggregates. In the presence of high concentrations of crowding agents the reactivation yields remain constant but with decreased refolding rates. The refolding of G6PDH changes from monophasic to biphasic first-order reactions in the presence of crowding agents, and the amplitude of the new slow phase increases with increasing concentrations of crowding agents. The molecular chaperone GroEL reverses the refolding kinetics of G6PDH from biphase back to monophase and accelerates the refolding process. Our results display the complexity and diversity of the effects of macromolecular crowding on both the thermodynamics and kinetics of protein folding.

Unfolding and inactivation of fatty acid synthase from chicken liver during urea denaturation

The inactivation and conformational changes of the multifunctional fatty acid synthase (acyl-CoA:malonyl-CoA C-acyltransferase (decarboxylating, oxoacyl- and enoyl-reducing and thioester-hydrolyzing), EC 2.3.1.85) from chicken liver have been studied in urea solution. The results show that complete inactivation of the fatty acid synthase occurs before obvious conformational changes with regard to the overall, beta-ketoacyl reduction and acetoacetyl-CoA reduction reactions. Significant conformational changes indicated by the changes of the intrinsic fluorescence emission and the circular dichroism spectra occurred at higher urea concentrations. The kinetic rate constants for the two phase inactivation and unfolding reactions were measured and semilogarithmic plots of the activity versus time gave curves which could be resolved into two straight lines, indicating that both the inactivation and unfolding processes consisted of fast and slow phases as a first-order reaction. The results from Lineweaver-Burk plots indicated that urea is a competitive inhibitor for acetyl-CoA and malonyl-CoA, with K(m) increasing with increasing urea concentrations. However, urea is a noncompetitive inhibitor for NADPH, the substrate of the overall reaction and beta-ketoacyl reduction reaction, and acetylacetate, the substrate of the beta-ketoacyl reduction reaction. Activation by low concentrations of urea was observed although this activation was only temporarily induced in an early stage of inactivation. The aggregation phenomenon of the fatty acid synthase in a certain concentration range of urea (3-4 M) was also observed during unfolding. This result shows that this multifunctional enzyme unfolds with competition with misfolding in the folding pathway. Comparison of inactivation and conformational changes of the enzyme as well as aggregation imply that unfolding intermediates may exist during urea denaturation. The possible unfolding pathway of fatty acid synthase is also discussed in this paper.

Role of proline, glycerol, and heparin as protein folding aids during refolding of rabbit muscle creatine kinase

Aggregation of 3 M guanidine hydrochloride denatured creatine kinase (ATP: creatine N-phosphotransferase, EC 2.7.3.2) occurs after dilution into the refolding solution. Proline, glycerol and heparin sodium act as folding aids which can effectively inhibit aggregation of creatine kinase during refolding. Proline at 1 M concentration, glycerol at 10% concentration and heparin at 25 mg/ml not only completely prevented creatine kinase aggregation but also enabled the creatine kinase to return to its native state as well as to recover most of its native activity. The reactivity after the aggregation was completely blocked by the presence of each folding aid reached 65-80% of the native activity. Results of turbidity, activity, intrinsic fluorescence and 1-anilinonaphthalene-8-sulfonate binding fluorescence measurements suggested that the effect of heparin differs from that of proline and glycerol in its artificial chaperone-like behavior. Heparin may bind with creatine kinase both in the native state and during the refolding course. The results showed that this heparin-creatine kinase complex favorably restored the creatine kinase reactivity.

Commitment to folded and aggregated states occurs late in interleukin-1 beta folding

A point mutation, lysine 97 to isoleucine, in the all-beta cytokine interleukin-1 beta (IL-1 beta) exhibits an increased propensity to form inclusion bodies in vivo and aggregates in vitro. In an effort to better understand the aggregation reaction and determine when intervention may allow rescue of protein from aggregation during renaturation, we developed a novel application of mass spectrometry using isotopic labeling to determine the step(s) at which K97I commits to either the native or aggregated state. Interestingly, despite the early formation of a folding intermediate ensemble at an observed rate lambda(2) of 4.0 s(-1), K97I commits to folding at a significantly slower rate lambda(CF) of 0.021 s(-1). This rate of commitment to folding is in excellent agreement with the observed rate of K97I native state formation (lambda(1) = 0.018 s(-1)). K97I also commits slowly to aggregation at an observed rate lambda(CA) of 0.023 s(-1). Earlier folding species and aggregates present prior to these commitment steps are likely to be in a reversible equilibrium between monomeric folding intermediates and higher-order oligomers. Kinetic and equilibrium experimental measurements of folding and aggregation processes are consistent with a nucleation-dependent model of aggregation.

Aggregation events occur prior to stable intermediate formation during refolding of interleukin 1beta

A point mutation, lysine 97 --> isoleucine (K97I), in a surface loop in the beta-sheet protein interleukin 1beta (IL-1beta), exhibits increased levels of inclusion body (IB) formation relative to the wild-type protein (WT) when expressed in Escherichia coli. Despite the common observation that less stable proteins are often found in IBs, K97I is more stable than WT. We examined the folding pathway of the mutant and wild-type proteins at pH 6.5 and 25 degrees C with manual-mixing and stopped-flow optical spectroscopy to determine whether changes in the properties of transiently populated species in vitro correlate with the observation of increased aggregation in vivo. The refolding reactions of the WT and K97I proteins are both described by three exponential processes. Two exponential processes characterize fast events (0.1-1.0 s) in folding while the third exponential process correlates with a slow (70 s) single pathway to and from the native state. The K97I replacement affects the earlier steps in the refolding pathway. Aggregation, absent in the WT refolding reaction, occurs in K97I above a critical protein concentration of 18 microM. This observation is consistent with an initial nucleation step mediating protein aggregation. Stopped-flow kinetic studies of the K97I aggregation process demonstrate that K97I aggregates most rapidly during the earliest refolding times, when unfolded protein conformers remain highly populated and the concentration of folding intermediates is low. Folding and aggregation studies together support a model in which the formation of stable folding intermediates afford protection against further K97I aggregation.

Expression of bovine mitochondrial elongation factor Ts in Escherichia coli and characterization of the heterologous complex formed with prokaryotic elongation factor Tu

When bovine mitochondrial elongation factor Ts (EF-Ts(mt)) is expressed in Escherichia coli, it forms a tightly associated complex with E. coli EF-Tu (EF-Tu(Eco) x Ts(mt)). This complex is active in poly(U)-directed polymerization and this activity is inhibited by kirromycin. The EF-Tu(Eco) x Ts(mt) complex does not bind guanine nucleotides detectably and is not dissociated to a significant extent by either GDP or GTP. A portion of the EF-Tu(Eco) x Ts(mt) complex can be dissociated by aa-tRNA in the presence of GTP. The heterologous complex cannot be dissociated completely in the presence of either the 8 M urea or 8 M guanidine hydrochloride, suggesting that EF-Ts(mt) has an unusually tight interaction with E. coli EF-Tu. The EF-Tu(Eco) x Ts(mt) complex can be dissociated by denaturation using 2 M guanidine thiocyanate. Free EF-Ts(mt) can then be purified and renatured. The refolded EF-Ts(mt) is active in stimulating the activity of expressed mitochondrial EF-Tu (EF-Tu(mt)) in poly(U)-directed polymerization. Almost all the EF-Ts(mt) molecules appear to refold into a conformation which can interact with EF-Tu(mt). Protease mapping of EF-Ts(mt) indicates that the first 54 residues fold into an independent domain. Analysis of deletion derivatives of EF-Ts(mt) indicates that extensive regions of this factor are required for its tight interaction with EF-Tu.

Denaturation and reactivation of dimeric human glutathione reductase--an assay for folding inhibitors

Human glutathione reductase (GR; which catalyzes the reaction NADPH + GSSG + H+ --> 2 GSH + NADP+) is an obligatory FAD-containing homodimer of known geometry. Native human GR, a potential target of antimalarial and cytostatic agents, cannot be dissociated by dilution or by means of subunit-interface mimetics, similarly to well-studied viral dimeric proteins. However, ab initio folding and/or dimerization of human GR can be inhibited by point mutations or by peptides corresponding to subunit-interface areas, for example synthetic peptide P11, which represents the intersubunit-contact helix H11. The structure of this peptide, which might assist inhibitor design, was solved by high-resolution NMR spectroscopy. Residues 440-453, were found to be alpha helical in the isolated peptide. To quantitate the efficacy of inhibitors such as P11, we developed the following unfolding/reactivation assay. The effects of various guanidine hydrochloride (Gdn/HCl) concentrations were studied by analytical ultracentrifugation. It was shown that human GR denatured by greater than 3 M Gdn/HCl is monomeric and free of FAD. Circular-dichroism experiments at 223 nm indicated a half-life of approximately 20 s at 20 degrees C for the unfolding process. To optimize the reactivation yield, four parameters [protein concentration (x) in the range 0.3-10 microg/ml, cofactor supplementation, temperature (y: 0-32 degrees C), and time (0-72 h)] were varied systematically, and a reactivation score z was given to each constellation of parameters. This type of analysis might be useful to optimize refolding and activation yields for other proteins. For human GR, the highest recovery was found not to occur at one of the corners of the x,y plane, but close to its center. Consequently, the optimal assay conditions for folding and dimerization inhibitors are as follows. The enzyme (at 300 microg/ml) is denatured by 5 M guanidine hydrochloride/5 mM dithiothreitol, then reactivated by dilution to 1 microg/ml at pH 6.9 and 20 degrees C. In the absence of inhibitors, this procedure leads to 70% of the control activity within 8 h. Peptides representing the upper subunit interface (for instance residues 436-478) of human GR were found to inhibit refolding with EC50% values in the micromolar range, whereas fragments from other regions of the protein had no influence on this process. For peptide P11, the EC50% value was 20 microM. In conclusion, hGR, enzyme with a tight intersubunit contact area of 21 nm2, appears to be suitable for studying protein folding, dimerization, and prosthetic-group complexation in the absence and presence of compounds that inhibit these processes. There is a shortage, at least for oligomeric enzymes of eukaryotes, of published systematic studies on protein (re)activation.

The folding intermediate of reversibly denatured human prostatic acid phosphatase

Human prostatic acid phosphatase (hPAP) [EC 3.1.3.2], a homodimer of ca. 50 kDa subunit molecular weight, shows reversible denaturation in 6 M urea at pH 2.5. Rapid dilution of the denatured enzyme allowed partial renaturation of hPAP, as measured by enzyme activity, to a level which depended on the composition of the dilution solution employed and time of the reaction. The renaturation reaction of hPAP was examined using spectral analysis (circular dichroism and fluorometry), fast size-exclusion chromatography and proteolysis by trypsin. The observed results are in agreement with the concentration-dependent kinetics of hPAP reactivation, assuming that the reconstitution of the active enzyme requires the association of subunits in dimeric form. Moreover, it suggests formation of an inactive intermediate during refolding of the denatured PAP. A mechanism of renaturation of the active enzyme from denatured PAP is proposed.

Activation of chicken liver dihydrofolate reductase in concentrated urea solutions

The activation and inactivation of dihydrofolate reductase from chicken liver during denaturation in a wide concentration range of urea are compared with changes in intrinsic fluorescence. At 2 M urea the enzyme is activated 3.6-fold and is stable up to 12 h in the activated form. At 4 M urea, the enzyme activity increases about 5-fold initially but the activated enzyme loses activity rapidly to a level well below that of the native enzyme. The activated enzyme is stabilized in presence of either DHF or NADPH. The Kd and Km of the enzyme for the substrates at various urea concentrations were determined and compared. In the presence of 3 M urea, the values of Kd for DHF and NADPH increase 4-fold and 10-fold, respectively, whereas the corresponding Km values increase 25-fold and 3-fold. A large increase in Vmax is mainly responsible for the activation. The inactivation and unfolding in urea are both biphasic processes. For the fast phase, the rate constant of inactivation is 10-fold greater than that of unfolding in 4 M urea. The effect of (NH4)2SO4 on the activation and unfolding of the enzyme was also studied. The results suggest that the active site of the enzyme is more easily perturbed by denaturants; and the activated enzyme appears to have a more open and flexible conformation at the active site, which is favorable for the full expression of the catalytic power of the enzyme. A scheme for the sequential activation and inactivation of DHFR accompanying its unfolding by increasing concentrations of urea is proposed.