Evidence for kinetic intermediate states during the refolding of GdnHCl-denatured MM-creatine kinase. Characterization of a trapped monomeric species

Improvement Thermal Stability of D-Lactate Dehydrogenase by Hydrophobin-1 and in Silico Prediction of Protein-Protein Interactions

Hydrophobins are small surface-active proteins. They can connect to hydrophobic or hydrophilic regions and oligomerize in solution to form massive construction. In nature, these proteins are produced by filamentous fungi at different stages of growth. So far, researchers have used them in various fields of biotechnology. In this study, recombinant hydrophobin-1 (rHFB1, 7.5 kDa) was used to stabilize recombinant D-lactate dehydrogenase (rD-LDH, 35 kDa). rD-LDH is a sensitive enzyme deactivated and oxidized by external agents such as O₂ and lights. So, its stabilization with rHFB1 can be the best index to demonstrate the positive effect of rHFB1 on preserving and improving enzyme's activity. The unique ability of rHFB1 for interacting with hydrophobic regions of rD-LDH was predicted by protein-protein docking study with ClusPro and PIC servers and confirmed by fluorescence experiments, and Colorless Native-PAGE. Measurement of thermodynamic parameters allows for authenticating the role of rHFB1 as a thermal stabilizer in the protein-protein complex (rD-LDH@rHFB1). Interaction between rHFB1 and rD-LDH improved half-life of enzyme 2.25-fold at 40 °C. Investigation of the kinetic parameters proved that the presence of rHFB1 along with the rD-LDH enhancement strongly the affinity of the enzyme for pyruvate. Furthermore, an increase of Kcat/Km for complex displayed the effect of rHFB1 for improving the enzyme's catalytic efficiency.

A single residue substitution accounts for the significant difference in thermostability between two isoforms of human cytosolic creatine kinase

Creatine kinase (CK) helps maintain homeostasis of intracellular ATP level by catalyzing the reversible phosphotransfer between ATP and phosphocreatine. In humans, there are two cytosolic CK isoforms, the muscle-type (M) and the brain-type (B), which frequently function as homodimers (hMMCK and hBBCK). Interestingly, these isoenzymes exhibit significantly different thermostabilities, despite high similarity in amino acid sequences and tertiary structures. In order to investigate the mechanism of this phenomenon, in this work, we first used domain swapping and site-directed mutagenesis to search for the key residues responsible for the isoenzyme-specific thermostability. Strikingly, the difference in thermostability was found to principally arise from one single residue substitution at position 36 (Pro in hBBCK vs. Leu in hMMCK). We then engaged the molecular dynamics simulations to study the molecular mechanism. The calculations imply that the P36L substitution introduces additional local interactions around residue 36 and thus further stabilizes the dimer interface through a complex interaction network, which rationalizes the observation that hMMCK is more resistant to thermal inactivation than hBBCK. We finally confirmed this molecular explanation through thermal inactivation assays on Asp36 mutants that were proposed to devastate the local interactions and thus the dimer associations in both isoenzymes.

Folding studies on muscle type of creatine kinase from Pelodiscus sinensis

A folding study of creatine kinase from Pelodiscus sinensis has not yet been reported. To gain more insight into structural and folding mechanisms of P. sinensis CK (PSCK), denaturants such as SDS, guanidine HCl, and urea were applied in this study. We purified PSCK from the muscle of P. sinensis and conducted inhibition kinetics with structural unfolding studies under various conditions. The results revealed that PSCK was completely inactivated at 1.8 mM SDS, 1.05 M guanidine HCl, and 7.5 M urea. The kinetics via time-interval measurements showed that the inactivation by SDS, guanidine HCl, and urea were all first-order reactions with kinetic processes shifting from monophase to biphase at increasing concentrations. With respect to tertiary structural changes, PSCK was unfolded in different ways; SDS increased the hydrophobicity but retained the most tertiary structural conformation, while guanidine HCl and urea induced conspicuous changes in tertiary structures and initiated kinetic unfolding mechanisms. Our study provides information regarding PSCK and enhances our knowledge of the reptile-derived enzyme folding.

Dissimilarity in the folding of human cytosolic creatine kinase isoenzymes

Creatine kinase (CK, EC 2.7.3.2) plays a key role in the energy homeostasis of excitable cells. The cytosolic human CK isoenzymes exist as homodimers (HMCK and HBCK) or a heterodimer (MBCK) formed by the muscle CK subunit (M) and/or brain CK subunit (B) with highly conserved three-dimensional structures composed of a small N-terminal domain (NTD) and a large C-terminal domain (CTD). The isoforms of CK provide a novel system to investigate the sequence/structural determinants of multimeric/multidomain protein folding. In this research, the role of NTD and CTD as well as the domain interactions in CK folding was investigated by comparing the equilibrium and kinetic folding parameters of HMCK, HBCK, MBCK and two domain-swapped chimeric forms (BnMc and MnBc). Spectroscopic results indicated that the five proteins had distinct structural features depending on the domain organizations. MBCK BnMc had the smallest CD signals and the lowest stability against guanidine chloride-induced denaturation. During the biphasic kinetic refolding, three proteins (HMCK, BnMc and MnBc), which contained either the NTD or CTD of the M subunit and similar microenvironments of the Trp fluorophores, refolded about 10-fold faster than HBCK for both the fast and slow phase. The fast folding of these three proteins led to an accumulation of the aggregation-prone intermediate and slowed down the reactivation rate thereby during the kinetic refolding. Our results suggested that the intra- and inter-subunit domain interactions modified the behavior of kinetic refolding. The alternation of domain interactions based on isoenzymes also provides a valuable strategy to improve the properties of multidomain enzymes in biotechnology.

Dissecting the key residues crucial for the species-specific thermostability of muscle-type creatine kinase

Species-specific protein thermal stability is closely correlated to the living conditions of the organism, especially to its body temperature. In this research, human and zebrafish muscle-type creatine kinases (MMCKs) were taken as model proteins to investigate the molecular adaptation of proteins in poikilothermal and homoiothermal animals. Both the optimal temperature for catalysis and the thermal stability of human MMCK was much higher than those of zebrafish MMCK. Sequence alignment identified 9 amino acid variations conserved in either the teleost MMCKs or the mammal and electric ray MMCKs. Bidirectional mutations were performed to find the residues with beneficial mutations. The results showed that two residues close to the dimer interface of MMCK, the 46th and 146th residue, were crucial for species-specific thermal stability.

Sequential events in the irreversible thermal denaturation of human brain-type creatine kinase by spectroscopic methods

The non-cooperative or sequential events which occur during protein thermal denaturation are closely correlated with protein folding, stability, and physiological functions. In this research, the sequential events of human brain-type creatine kinase (hBBCK) thermal denaturation were studied by differential scanning calorimetry (DSC), CD, and intrinsic fluorescence spectroscopy. DSC experiments revealed that the thermal denaturation of hBBCK was calorimetrically irreversible. The existence of several endothermic peaks suggested that the denaturation involved stepwise conformational changes, which were further verified by the discrepancy in the transition curves obtained from various spectroscopic probes. During heating, the disruption of the active site structure occurred prior to the secondary and tertiary structural changes. The thermal unfolding and aggregation of hBBCK was found to occur through sequential events. This is quite different from that of muscle-type CK (MMCK). The results herein suggest that BBCK and MMCK undergo quite dissimilar thermal unfolding pathways, although they are highly conserved in the primary and tertiary structures. A minor difference in structure might endow the isoenzymes dissimilar local stabilities in structure, which further contribute to isoenzyme-specific thermal stabilities.

Mutation of the conserved Asp122 in the linker impedes creatine kinase reactivation and refolding

Creatine kinase (CK), a key enzyme in maintaining the intracellular energetic homeostasis, contains two domains connected by a long linker. In this research,we found that the mutations of the conserved Asp122 in the linker slightly affected CK activity, structure and stability. The hydrogen bonding and the ion pair contributed 2-5 kJ/mol to the conformational stability of CK. Interestingly, the ability of CK reactivation from the denatured state was completely removed by the mutations. These results suggested that the electrostatic interactions were crucial to the action of the linker in CK reactivation.

Intermediates in the refolding of urea-denatured dimeric arginine kinase from Stichopus japonicus

The refolding of urea-denatured dimeric AK was investigated by both equilibrium and kinetic measurements. Both studies indicated that the refolding of dimeric AK is a multiphasic process. The equilibrium studies, monitored by enzyme activity, intrinsic protein fluorescence, circular dichroism (CD), 1-anilinonaphtalene-8-sulfonate (ANS) binding, size-exclusion chromatography and glutaraldehyde cross-linking showed that there were at least two intermediates involved in this process: I(1) (existing in 1.8-1.4M urea) and I(2) (existing in 0.8-0.4M urea). I(1) was a monomeric intermediate and possessed characteristic similar to the globular folding intermediates described in the literature. I(2) was an active native-like intermediate. The kinetic studies suggested that the refolding of AK possessed a burst phase, fast phase and slow phase, which involved at least the burst phase intermediates (I(B)). Comparison of the properties of these intermediates suggested that I(B) in the kinetic process corresponded to I(1) in the equilibrium process. Based on these results, a scheme for refolding of urea-denatured AK was proposed.

Towards creatine kinase aggregation due to the cysteine modification at the flexible active site and refolding pathway

The dimeric native state of creatine kinase (CK) was aggregated at conspicuous levels during cysteine modification at the active site with using 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) under a high enzyme concentration. Measuring the ANS-binding fluorescence revealed that the hydrophobic surface of CK was increased by cysteine modification due to the flexible active site, and this resulted in insoluble aggregation, probably via non-specific hydrophobic interactions. To determine whether the aggregates can be refolded, 3M guanidine hydrochloride (GdnHCl) was used to dissolve the aggregates into the denatured form. Refolding of the solubilized enzyme sample was then conducted, accompanied by deprivation of DTNB from the CK in the presence of DTT. As a result, CK was reactivated by up to 40% with partial recovery of the tertiary (78%) and secondary structures (77%). To further elucidate its kinetic refolding pathway, both time interval measurements and a continuous substrate reaction were performed. The results showed that the refolding behavior was similar to the manner of normal CK folding with respect to the following two-phase kinetic courses. Additionally, the rate constants for the dimerization of the unfolded CK were dependent on the enzyme concentration and this was irrespective to the DTT concentrations, suggesting the rate-limiting steps of CK reassociation. The present study will expand our insight into the flexibility of the enzyme active site, which might act as a risk factor for inducing the unfavorable aggregation and partial refolding pathway of CK, as well as inducing an intermediate-like state recovery from aggregation.

Despite its high similarity with monomeric arginine kinase, muscle creatine kinase is only enzymatically active as a dimer

Although having highly similar primary to tertiary structures, the different guanidino kinases exhibit distinct quaternary structures: monomer, dimer or octamer. However, no evidence for communication between subunits has yet been provided, and reasons for these different levels of quaternary complexity that can be observed from invertebrate to mammalian guanidino kinases remain elusive. Muscle creatine kinase is a dimer and disruption of the interface between subunits has been shown to give rise to destabilized monomers with slight residual activity; this low activity could, however, be due to a fraction of protein molecules present as dimer. CK monomer/monomer interface involves electrostatic interactions and increasing salt concentrations unfold and inactivate this enzyme. NaCl and guanidine hydrochloride show a synergistic unfolding effect and, whatever the respective concentrations of these compounds, inactivation is associated with a dissociation of the dimer. Using an interface mutant (W210Y), protein concentration dependence of the NaCl-induced unfolding profile indicates that the active dimer is in equilibrium with an inactive monomeric state. Although highly similar to muscle CK, horse shoe crab (Limulus polyphemus) arginine kinase (AK) is enzymatically active as a monomer. Indeed, high ionic strengths that can monomerize and inactivate CK, have no effect on AK enzymatic activity or on its structure as judged from intrinsic fluorescence data. Our results indicate that expression of muscle creatine kinase catalytic activity is dependent on its dimeric state which is required for a proper stabilization of the monomers.

Effects of the single point genetic mutation D54G on muscle creatine kinase activity, structure and stability

Aberrant folding of important proteins caused by genetic mutations is closely correlated to many diseases. Due to the important physiological role in excitable cells, the activity and level of creatine kinase (CK) play a crucial role in maintaining body functions. Muscle CK deficiency disease was identified by an unusual CK activity decrease in an acute myocardial infarction patient caused by the single point mutation D54G. In this research, it was found that the D54G mutant had substantially decreased activity, substrate binding affinity and stability. Spectroscopic experiments indicated that the mutation impaired the structure of CK, which resulted in a partially unfolded state with more hydrophobic exposure and exposed Trp residues. The inability to fold to the functional compact state made the mutant be prone to aggregate upon microenvironmental stresses, and might gradually decrease the CK level of the patient.

Mixed Macromolecular Crowding Accelerates the Refolding of Rabbit Muscle Creatine Kinase: Implications for Protein Folding in Physiological Environments

The effects of four single macromolecular crowding agents, Ficoll 70, dextran 70, polyethylene glycol (PEG) 2000, and calf thymus DNA (CT DNA), and three mixed crowding agents containing both CT DNA and polysaccharide (or PEG 2000) on the refolding of guanidine hydrochloride-denatured rabbit muscle creatine kinase (MM-CK) have been examined by activity assay. When the total concentration of the mixed crowding agent is 100 g/l, in which the weight ratio of CT DNA to Ficoll 70 is 1:9, the refolding yield of MM-CK after refolding for 3 h under these conditions increases 23% compared with that in the presence of 10 g/l CT DNA, 18% compared with 100 g/l Ficoll 70, and 19% compared with that in the absence of crowding agents. A remarkable increase in the refolding yield of MM-CK by a mixed crowding agent containing CT DNA and dextran 70 (or PEG 2000) is also observed. Further folding kinetics analyses show that these three mixed crowding agents remarkably accelerate the refolding of MM-CK, compared with single crowding agents. Aggregation of MM-CK in the presence of any of the three mixed crowding agents is less serious than that in the presence of a single crowding agent at the same concentration but more serious than that in the absence of crowding agents. Both the refolding yield and the refolding rate of MM-CK in mixtures of these agents are increased relative to the individual agents by themselves, indicating that mixed macromolecular crowding agents are more favorable to MM-CK folding and can be used to reflect the physiological environment more accurately than single crowding agents.

Impact of intra-subunit domain-domain interactions on creatine kinase activity and stability

Creatine kinase (CK) is a key enzyme in vertebrate excitable tissues. In this research, five conserved residues located on the intra-subunit domain-domain interface were mutated to explore their role in the activity and structural stability of CK. The mutations of Val72 and Gly73 decreased both the activity and stability of CK. The mutations of Cys74 and Val75, which had no significant effect on CK activity and structure, gradually decreased the stability and reactivation of CK. Our results suggested that the mutations might modify the correct positioning of the loop contributing to domain-domain interactions, and result in decreased stability against denaturation.

Change in kinetic regime of protein aggregation with temperature increase. Thermal aggregation of rabbit muscle creatine kinase

Creatine kinase thermal aggregation kinetics has been studied in 30 mM Hepes-NaOH buffer, pH 8.0, at two temperatures: 50.6 and 60 degrees C. Aggregation kinetics was analyzed by measuring the growth of apparent absorption (A) at 400 nm. It was found that the limiting value of apparent absorption (A(lim)) is proportional to protein concentration at both temperatures. The first order rate constant (k(I)) does not depend on protein concentration in the range 0.05-0.2 mg/ml at temperature 50.6 degrees C, but at temperature 60 degrees C it increases with the growth of protein concentration in the range 0.1-0.4 mg/ml. Kinetic curves, shown in coordinates {A/A(lim); t}, in experiments at 50.6 degrees C fuse to a common curve, which coincides with the theoretical curve of creatine kinase denaturation calculated using the denaturation rate constant determined from differential scanning calorimetry. At temperature 60 degrees C, half-transformation time t(1/2) = ln2/k(I) decreases when protein concentration grows. We conclude that when temperature increased from 50.6 to 60 degrees C, change in the kinetic regime of thermal creatine kinase aggregation took place: at 50.6 degrees C aggregation rate is limited by the stage of protein molecule denaturation, but at 60 degrees C it is limited by the stage of protein aggregate growth, which proceeds as a reaction of pseudo-first order. Small heat shock protein Hsp 16.3 Mycobacterium tuberculosis suppresses the creatine kinase aggregation.

Multiple unfolding states of glutathione transferase from Physa acuta (Gastropada: Physidae)

The equilibrium unfolding of the major Physa acuta glutathione transferase isoenzyme (P. acuta GST(3)) has been performed using guanidinium chloride (GdmCl), urea, and acid denaturation to investigate the unfolding intermediates. Protein transitions were monitored by intrinsic fluorescence. The results indicate that unfolding of P. acuta GST(3) using GdmCl (0-3.0M) is a multistep process, i.e., three intermediates coexist in equilibrium. The first intermediate, a partially dissociated dimer, exists at low GdmCl concentration (approximately at 0.7M). At 1.2M GdmCl, a dimeric intermediate with a compact structure was observed. This intermediate undergoes dissociation into structural monomers at 1.75M of GdmCl. The monomeric intermediate started to be completely unfolding at higher GdmCl concentrations (>1.8M). Unfolding using urea (0-7.0M) and acid-induced structures as well as the fluorescence of 8-anilino-1-naphthalenesulfonate in the presence of different GdmCl concentrations confirmed that the unfolding is a multistep process. At concentrations of GdmCl or urea less than the midpoints or at the midpoint pH (pH 4.2-4.6), the unfolding transition is protein concentration independent and involved a change in the subunit tertiary structure yielding a partially active dimeric intermediate. The binding of glutathione to the enzyme active site stabilizes the native dimeric state.

Conformational change in the C-terminal domain is responsible for the initiation of creatine kinase thermal aggregation

Protein conformational changes may be associated with particular properties such as its function, transportation, assembly, tendency to aggregate, and potential cytotoxicity. In this research, the conformational change that is responsible for the fast destabilization and aggregation of rabbit muscle creatine kinase (EC 2.7.3.2) induced by heat was studied by intrinsic fluorescence and infrared spectroscopy. A pretransitional change of the tryptophan microenvironments was found from the intrinsic fluorescence spectra. A further analysis of the infrared spectra using quantitative second-derivative and two-dimensional correlation analysis indicated that the changes of the beta-sheet structures in the C-terminal domain and the loops occurred before the formation of intermolecular cross-beta-sheet structures and the unfolding of alpha-helices. These results suggested that the pretransitional conformational changes in the active site and the C-terminal domain might result in the modification of the domain-domain interactions and the formation of an inactive dimeric form that was prone to aggregate. Our results highlighted the fact that some minor conformational changes, which were usually negligible or undetectable by normal methods, might play a crucial role in protein stability and aggregation. Our results also suggested that the changes in domain-domain interactions, but not the dissociation of the dimer, might play a crucial role in the thermal denaturation and aggregation of this dimeric two-domain protein.

Catalysis of Creatine Kinase Refolding by Protein Disulfide Isomerase Involves Disulfide Cross-link and Dimer to Tetramer Switch

Protein disulfide isomerase (PDI) functions as an isomerase to catalyze thiol:disulfide exchange, as a chaperone to assist protein folding, and as a subunit of prolyl-4-hydroxylase and microsomal triglyceride transfer protein. At a lower concentration of 0.2 microm, PDI facilitated the aggregation of unfolded rabbit muscle creatine kinase (CK) and exhibited anti-chaperone activity, which was shown to be mainly due to the hydrophobic interactions between PDI and CK and was independent of the cross-linking of disulfide bonds. At concentrations above 1 microm, PDI acted as a protector against aggregation but an inhibitor of reactivation during CK refolding. The inhibition effect of PDI on CK reactivation was further characterized as due to the formation of PDI-CK complexes through intermolecular disulfide bonds, a process involving Cys-36 and Cys-295 of PDI. Two disulfide-linked complexes containing both PDI and CK were obtained, and the large, soluble aggregates around 400 kDa were composed of 1 molecule of tetrameric PDI and 2 molecules of inactive intermediate dimeric CK, whereas the smaller one, around 200 kDa, was formed by 1 dimeric PDI and 1 dimeric CK. To our knowledge this is the first study revealing that PDI could switch its conformation from dimer to tetramer in its functions as a foldase. According to the observations in this research and our previous study of the folding pathways of CK, a working model was proposed for the molecular mechanism of CK refolding catalyzed by PDI.

Denaturant sensitive regions in creatine kinase identified by hydrogen/deuterium exchange

The GdmHCl-induced unfolding of creatine kinase (CK) has been studied by hydrogen/deuterium (H/D) exchange combined with mass spectrometry. MM-CK unfolded for various periods in different denaturant concentrations was pulsed-labeled with deuterium to identify different conformational intermediate states. For all denaturation times or GdmHCl concentrations, we observed variable proportions of only two species. The low-mass envelope of isotope peaks corresponds to a species that has gained about 10 deuteriums more than native CK, and the high-mass envelope to a completely deuterated species. To localize precisely the unfolded regions in the states highly populated during denaturation, the protein was digested with two proteases (pepsin and type XIII protease) after H/D exchange and rapid quenching of the reaction. The two sets of fragments obtained were analyzed by liquid chromatography coupled to mass spectrometry to determine the deuterium level in each fragment. Bimodal distributions of deuterium were found for most peptides, indicating that these regions were either folded or unfolded. This behavior is consistent with cooperative, localized unfolding. However, we observed a monomodal distribution of deuterium in two regions (1-12 and 162-186). We conclude that the increment of mass observed in the low-mass species of the intact protein (+10 Da) has its origin in these two segments. These regions, which are very sensitive to low GdmHCl concentrations, are involved in the monomer-monomer interface of CK and their perturbation is likely to weaken the dimeric structure. At higher denaturant concentration, this would induce dissociation of the dimer.

Effects of arginine on rabbit muscle creatine kinase and salt-induced molten globule-like state

The arginine (Arg)-induced unfolding and the salt-induced folding of creatine kinase (CK) have been studied by measuring enzyme activity, fluorescence emission spectra, native polyacrylamide gel electrophoresis and size exclusion chromatography (SEC). The results showed that Arg caused inactivation and unfolding of CK, but there was no aggregation during CK denaturation. The kinetics of CK unfolding followed a one-phase process. At higher concentrations of Arg (>160 mM), the CK dimers were fully dissociated, the alkali characteristic of Arg mainly led to the dissociation of dimers, but not denaturation effect of Arg's guanidine groups on CK. The inactivation of CK occurred before noticeable conformational changes of the whole molecules. KCl induced monomeric and dimeric molten globule-like states of CK denatured by Arg. These results suggest that as a protein denaturant, the effect of Arg on CK differed from that of guanidine and alkali, its denaturation for protein contains the double effects, which acts not only as guanidine hydrochloride but also as alkali. The active sites of CK have more flexibility than the whole enzyme conformation. Monomeric and dimeric molten globule-like states of CK were formed by the salt inducing in 160 and 500 mM Arg H(2)O solutions, respectively. The molten globule-like states indicate that monomeric and dimeric intermediates exist during CK folding. Furthermore, these results also proved the orderly folding model of CK.

Unfolding of rabbit muscle creatine kinase induced by acid. A study using electrospray ionization mass spectrometry, isothermal titration calorimetry, and fluorescence spectroscopy

Electrospray ionization mass spectrometry, isothermal titration calorimetry (ITC), fluorescence spectroscopy, and glutaraldehyde cross-linking SDS-PAGE have been used to study the unfolding of rabbit muscle creatine kinase (MM-CK) induced by acid. The mass spectrometric experiments show that MM-CK is unfolded gradually when titrated with acid. MM-CK is a dimer (the native state) at pH 7.0 and becomes an equilibrium mixture of the dimer and a partially folded monomer (the intermediate) between pH 6.7 and 5.0. The dimeric protein becomes an equilibrium mixture of the intermediate and an unfolded monomer (the unfolded state) between pH 5.0 and 3.0 and is almost fully unfolded at pH 3.0 reached. The results from a "phase diagram" method of fluorescence show that the conformational transition between the native state and the intermediate of MM-CK occurs in the pH range of 7.0-5.2, and the transition between the intermediate and the unfolded state of the protein occurs between pH 5.2 and 3.0. The intrinsic molar enthalpy changes for formation of the unfolded state of MM-CK induced by acid at 15.0, 25.0, 30.0, and 37.0 degrees C have been determined by ITC. A large positive molar heat capacity change of the unfolding, 8.78 kcal mol-1 K-1, at all temperatures examined indicates that hydrophobic interaction is the dominant driving force stabilizing the native structure of MM-CK. Combining the results from these four methods, we conclude that the acid-induced unfolding of MM-CK follows a "three-state" model and that the intermediate state of the protein is a partially folded monomer.

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.

Folding pathway for partially folded rabbit muscle creatine kinase

Rabbit muscle creatine kinase (CK) was modified by 5,5'-dithio-bis(2-nitrobenzoic acid) accompanied by 3 M guanidine hydrochloride denaturation to produce a partially folded state with modified thiol groups. The partially folded CK was in a monomeric state detected by size exclusion chromatography, native-polyacrylamide gel electrophoresis, circular dichroism, and intrinsic fluorescence studies. After dithiothreitol (DTT) treatment, about 70% CK activity was regained with a two-phase kinetic course. Rate constants calculated for regaining of activity and refolding were compared with those for CK modified with various treatments to show that refolding and recovery of activity were synchronized. To further characterize the partially folded CK state and its folding pathway, the molecular chaperone GroEL was used to evaluate whether it can bind with partly folded CK during refolding, and 1-anilinonaphthalene-8-sulfonate was used to detect the hydrophobic surface of the monomeric state of CK. The monomeric state of CK did not bind with GroEL, although it had a larger area of hydrophobic surface relative to the native state. These results may provide different evidence for the structural requirement of GroEL recognition to the substrate protein compared with previously reported results that GroEL bound with substrate proteins mainly through hydrophobic surface. The present study provides data for a monomeric intermediate trapped by the modification of the SH groups during the refolding of CK. Schemes are given for explaining both the partial folding CK pathway and the refolding pathway.Key words: creatine kinase; partially folded state; reactivation; refolding; GroEL; intermediate.

Relationship between kinetic and equilibrium folding intermediates of creatine kinase

Creatine kinase (CK) is a dimeric enzyme important in ATP regeneration in cells where energy demands are high. The folding of CK under equilibrium and transient conditions has been studied in detail and is found to be complex. At equilibrium in 0.8 M GuHCl, 90% of CK molecules are in the form of a partially structured, monomeric intermediate. We exploit this property to measure kinetics of refolding and unfolding to and from this equilibrium intermediate (EI), using far-UV circular dichroism and intrinsic fluorescence as structural probes. We are thus able to compare the properties of EI and the kinetic intermediate formed during the burst phase in refolding. Native CK and EI unfold with rate constants in seconds and milliseconds, respectively. As is observed for refolding of fully-denatured CK, refolding from EI to the native state shows a burst phase followed by two exponential phases. The burst phase refolding intermediate is inferred to have more structure and greater stability than the equilibrium intermediate. When refolding from the fully-denatured state in 0.8 M GuHCl, the equilibrium intermediate is formed within the dead-time of mixing in the stopped-flow apparatus. The equilibrium intermediate may thus represent a kinetic intermediate formed early during folding.

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.

Reactivation and refolding of reassociated dimers of rabbit muscle creatine kinase

Creatine kinase (ATP:creatine N-phosphotransferase, EC 2.7.3.2) is a good model for studying dissociation and reassociation during unfolding and refolding. This study compares self-reassociated CK dimers and CK dimers that contain hybrid dimers under proper conditions. Creatine kinase forms a monomer when denatured in 6 M urea for 1 h which will very quickly form a dimer when the denaturant is diluted under suitable conditions. After modification by DTNB, CK was denatured in 6 M urea to form a modified CK monomer. Dimerization of this modified subunit of CK occurred upon dilution into a suitable buffer containing DTT. Therefore, three different types of reassociated CK dimers including a hybrid dimer can be made from two different CK monomers in the proper conditions. The CK monomers are from a urea-denatured monomer of DTNB-modified CK and from an unmodified urea dissociated monomer. Equal enzyme concentration ratios of these two monomers were mixed in the presence of urea, then diluted into the proper buffer to form the three types of reassociated CK dimers including the hybrid dimer. Reassociated CK dimers including all three different types recover about 75% activity following a two-phase course (k1 = 4.88 x 10(-3) s(-1), k2 = 0.68 x 10(-3) s(-1)). Intrinsic fluorescence spectra of the three different CK monomers which were dissociated in 6 M urea, dissociated in 6 M urea after DTNB modification, and a mixture of the first two dissociated enzymes were studied in the presence of the denaturant urea. The three monomers had different fluorescence intensities and emission maxima. The intrinsic fluorescence maximum intensity changes of the reassociated CK dimers were also studied. The refolding processes also follow biphasic kinetics (k1 = 3.28 x 10(-)3 s(-1), k2 = 0.11 x 10(-3) S(-1)) after dilution in the proper solutions. Tsou's method [Tsou (1988), Adv. Enzymol. Rel. Areas Mol. Biol. 61, 381-436] was also used to measure the kinetic reactivation rate constants for the different three types of reassociated CK dimers, with different kinetic reactivation rate constants observed for each type. CK dissociation and reassociation schemes are suggested based on the results.

Multiple unfolded states of glutathione transferase bbGSTP1-1 by guanidinium chloride

Inactivation, dissociation, and unfolding of the homodimeric glutathione transferase (bbGSTP1-1) from Bufo bufo embryos were investigated at equilibrium, using guanidinium chloride (GdmCl) as denaturant. Protein transitions were monitored by enzyme activity, intrinsic fluorescence, far UV circular dichroism, glutaraldehyde cross-linking, and gel-filtration chromatography. At low denaturant concentrations (less than 0.5 M), reversible inactivation of the enzyme occurs. At denaturant concentrations between 0.5 and 1.5 M the enzyme progressively dissociates into structured monomers. At higher denaturant concentrations the monomers unfold completely. Refolding studies indicate that a total reactivation occurs only by starting from the enzyme denatured at concentrations below 0.5 M. The enzyme denatured at GdmCl concentrations higher than 0.5 M only partially refolds. Globally our results indicate that unfolding of the amphibian bbGSTP1-1 is a multistep process, i.e., inactivation of the structured dimer, dissociation into partially structured monomers, followed by complete unfolding.