Reversible dissociation and unfolding of dimeric creatine kinase isoenzyme MM in guanidine hydrochloride and urea

Metformin-Induced Invertase Unfolding: Enzyme Kinetics and Activity Regulation

The effects of metformin on invertase activity and its inhibition on sucrose digestion were studied. The rapid unfolding kinetics of invertases, followed a two-state model with an inactive intermediate formation. The dynamic interaction between metformin and invertase caused the secondary structure of the enzyme to become less β-sheet, more α-helix, and random coiling oriented, which weakened the binding force between enzyme and its substrate. Metformin acted as a chaotrope and disrupted the hydrogen bonds of water, which facilitated the unfolding of invertase. However, some sugar alcohols, which promoted the H-bond formation of water, could repair the secondary structure of metformin-denatured invertase and therefore regulate the enzyme activity. This research enriches our understanding of the mechanism of enzyme unfolding induced by guanidine compounds. Moreover, because metformin and sugar substitutes are of concern to diabetes, this research also provides useful information for understanding the activity of the digestive enzyme that coexists with metformin and sugar alcohols.

Spectroscopic probing of the refolding of an unfolded protein through the formation of mixed-micelles

We report the unfolding of the globular protein, Bovine Serum Albumin (BSA) induced by anionic surfactant sodium dodecyl sulfate (SDS) and subsequently monitored the refolding of this denatured BSA using triblock copolymers F127 and P123 through the formation of mixed micelles. Our study exclusively represents the reversibility of this unfolding-refolding process using pluronic triblock copolymers F127/P123 as refolding agents. We confirm the recovery of its native state from its denatured state estimating the α-helical structure of the denatured protein from the CD data which support our steady state fluorescence spectra monitoring the fluorescence of the intrinsic Trp molecules present in BSA. Time resolved study also corroborates the stepwise recovery of the denatured BSA as well as the reversibility of the processes. Isothermal Titration Calorimetry (ITC) data explain the negligible interactions between the triblock copolymers and the native state of BSA. The high binding constant of SDS and triblock copolymers probably play the crucial role in the stepwise recovery of the unfolded BSA followed by reversibility of the refolding processes through the formation of the mixed micelles. The mechanism of mixed-micelle formation has been substantiated by the fact that the Guanidine Hydrochloride denatured BSA does not react with F127/P123 whereby no recovery of the protein was observed.

Investigating the folding pathway and substrate induced conformational changes in B. malayi Guanylate kinase

Guanylate kinase is one of the key enzymes in nucleotide biosynthesis. The study highlights the structural and functional properties of Brugia malayi Guanylate kinase (BmGK) in the presence of chemical denaturants. An inactive, partially unfolded, dimeric intermediate was observed at 1-2M urea while GdnCl unfolding showed monomer molten globule like intermediate at 0.8-1.0M. The results also illustrate the protective role of substrates in maintaining the integrity of the enzyme. The thermo stability of protein was found to be significantly enhanced in the presence of the substrates. Furthermore, binding of the substrates, GMP and ATP to BmGK changed its GdnCl induced unfolding pattern. Docking and molecular dynamic simulation performed for native BmGK, BmGK bound to GMP and GMP+ATP showed change in the fluctuation in the region between 130 and 150 residues. Arg134 lost its interaction with GMP and Arg145 interaction shifted to ATP after 40ns simulation upon binding of ATP to BmGK-GMP complex. We, thus, propose the importance of specific rearrangements contributed by binding of substrates which participate in the overall stability of the protein. The work here emphasizes on detailed biophysical characterization of BmGK along with the significant role of substrates in modulating the structural and functional properties of BmGK.

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.

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.

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.

Interfacial behavior of cytoplasmic and mitochondrial creatine kinase oligomeric states

Adsorption to the air/water interface of isoenzymes of creatine kinase was investigated using surface pressure-area isotherms and Brewster angle microscopy (BAM) observations. Octameric mitochondrial creatine kinase (mtCK) exhibits a significant affinity for the air/water interface. Whatever the mode of formation of the interfacial film, i.e., injection of the protein in the subphase or spreading onto the buffer surface, the final arrangement and conformation adopted by mtCK molecules lead to a similar result. In contrast, the dimeric isoenzymes mtCK and cytosolic MMCK do not induce any surface pressure variation. However, when the subphase contains 0.3M NaCl, both isoenzymes adsorb to the interface. When treated with 0.8 or 3M GdnHCl, muscle creatine kinase (MMCK) becomes surface active and occupies a greater surface than mtCK. This result contrasts with previous observations, often derived from monomeric proteins, that their surface activity is increased upon unfolding. It underlines the possible influence exerted by the protein oligomeric state on its interfacial activity. At a subphase pH of 8.8, which corresponds to the pI of octameric mtCK, the profiles of the isotherms obtained with dimeric and octameric states and the resistance to compression of the protein monolayers are significantly affected when compared to those recorded at pH 7.4. These data suggest that the octamer is more hydrophobic than the dimer and may contribute to explaining why octamers bind to the inner mitochondrial membrane while dimers do not.

Calorimetric and spectroscopic investigations of the thermal denaturation of wild type nitrite reductase

Nitrite reductase (NiR) is a multicopper protein, with a trimeric structure containing two types of copper site: type 1 is present in each subunit whereas type 2 is localized at the subunits interface. The paper reports on the thermal behaviour of wild type NiR from Alcaligenes faecalis S-6. The temperature-induced changes of the copper centres are characterized by optical spectroscopy and electron paramagnetic resonance spectroscopy, and by establishing the thermal stability by differential scanning calorimetry. The calorimetric profile of the enzyme shows a single endothermic peak with maximum heat absorption at T(m) approximately 100 degrees C, revealing an exceptional thermal stability. The thermal transition is irreversible and the scan rate dependence of the calorimetric trace indicates that the denaturation of NiR is kinetically controlled. The divergence of the activation energy values determined by different methods is used as a criterion for the inapplicability of the one-step irreversible model. The best fit of the DSC profiles is obtained when the classical Lumry-Eyring model, N<-->U-->F, is considered. The simulation results indicate that the irreversible step prevails on the reversible one. Moreover, it is found that the conformational changes within the type-1 copper environments precede the denaturation of the whole protein. No evidence of protein dissociation within the temperature range investigated was observed.

The importance of being dimeric

Why are there so many dimeric proteins and enzymes? While for heterodimers a functional explanation seems quite reasonable, the case of homodimers is more puzzling. The number of homodimers found in all living organisms is rapidly increasing. A thorough inspection of the structural data from the available literature and stability (measured from denaturation-renaturation experiments) allows one to suggest that homodimers can be divided into three main types according to their mass and the presence of a (relatively) stable monomeric intermediate in the folding-unfolding pathway. Among other explanations, we propose that an essential advantage for a protein being dimeric may be the proper and rapid assembly in the cellular milieu.

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.

Random-coil behavior and the dimensions of chemically unfolded proteins

Spectroscopic studies have identified a number of proteins that appear to retain significant residual structure under even strongly denaturing conditions. Intrinsic viscosity, hydrodynamic radii, and small-angle x-ray scattering studies, in contrast, indicate that the dimensions of most chemically denatured proteins scale with polypeptide length by means of the power-law relationship expected for random-coil behavior. Here we further explore this discrepancy by expanding the length range of characterized denatured-state radii of gyration (R(G)) and by reexamining proteins that reportedly do not fit the expected dimensional scaling. We find that only 2 of 28 crosslink-free, prosthetic-group-free, chemically denatured polypeptides deviate significantly from a power-law relationship with polymer length. The R(G) of the remaining 26 polypeptides, which range from 16 to 549 residues, are well fitted (r(2) = 0.988) by a power-law relationship with a best-fit exponent, 0.598 +/- 0.028, coinciding closely with the 0.588 predicted for an excluded volume random coil. Therefore, it appears that the mean dimensions of the large majority of chemically denatured proteins are effectively indistinguishable from the mean dimensions of a random-coil ensemble.

Predissociated dimers and molten globule monomers in the equilibrium unfolding of yeast glutathione reductase

The equilibrium unfolding of dimeric yeast glutathione reductase (GR) by guanidine hydrochloride (GdnHCl) was investigated. Unfolding was monitored by a variety of techniques, including intrinsic fluorescence emission, anisotropy and iodide quenching measurements, far-ultraviolet circular dichroism and thiol reactivity measurements. At 1 M GdnHCl, one thiol group of GR became accessible to modification with 5,5'-dithiobis-(2-nitrobenzoic) acid (DTNB), whereas no changes could be detected in the spectroscopic properties (fluorescence, circular dichroism) of the protein. Between 2 and 3 M GdnHCl, two partially folded intermediate states possessing flexible tertiary structures (revealed by fluorescence data) but compact secondary structures (as indicated by circular dichroism measurements) were identified. The quaternary structure of GR in the presence of GdnHCl was also investigated by size-exclusion liquid chromatography. These results indicated the presence of an expanded predissociated dimer at 2.5 M GdnHCl and partially folded monomers at 3 M GdnHCl. Taken together, these results suggest the existence of two molten-globule-like intermediate species (one dimeric and one monomeric) in the unfolding of GR. The results are discussed in terms of the mechanism of GR folding and dimerization.

Inactivation and conformational changes of creatine kinase at low concentrations of hexafluoroisopropanol solutions

Using the methods of far-ultraviolet circular dichroism (CD) spectra, fluorescence spectra, and enzyme activity assays, the inactivation and conformational changes of creatine kinase (CK) induced by 1,1,1,3,3,3-hexafluoro-2-propanol (hexafluoroisopropanol (HFIP)) of different concentrations were investigated. To avoid the aggregation of CK that occurs with high HFIP, concentrations of 0%-5% HFIP were used in this study. The CD spectra showed that HFIP concentrations above 2.5% strongly induced the formation of secondary structures of CK. No marked conformational changes were observed at low concentrations of HFIP (0%-2.5%). After incubation with 0.2% HFIP for 10 min, CK lost most of its activity. The kinetic theory of the substrate reaction during irreversible inhibition of enzyme activity described previously by Tsou was applied to study the kinetics of CK inactivation during denaturation by HFIP. The inactivation rate constants for the free enzyme and the substrate-enzyme complex were determined by Tsou's method. The results suggested that low concentrations of HFIP had a high potential to induce helices of protein and that the active site of the enzyme was situated in a limited and flexible region of the enzyme molecule that was more susceptible to the denaturant than was the protein as a whole.

Conformational Dynamics of the GdmHCl-Induced Molten Globule State of Creatine Kinase Monitored by Hydrogen Exchange and Mass Spectrometry

Our understanding of the mechanism of protein folding can be improved by the characterization of folding intermediate states. Intrinsic tryptophan fluorescence measurements of equilibrium GdmHCl-induced unfolding of MM-CK allow for the construction of a "phase diagram", which shows the presence of five different conformational states, including three partially folded intermediates. However, only three states are detected by using pulsed-labeled H-D exchange analyzed by electrospray ionization mass spectrometry. One of them is the native state, and the two other species are present in proportions strongly dependent on the GdmHCl concentration and denaturation time. The low-mass peak is due to a largely exchange-incompetent state, which has gained only approximately 10 deuteriums more than the native protein. This population of MM-CK molecules has undergone a small conformational change induced by low GdmHCl concentrations. However, this limited change is in itself not sufficient to inactivate the enzyme or is easily reversible. The high-mass peak corresponds to a population of MM-CK that is fully deuterated. The comparison of fluorescence, activity, and H-D exchange measurements shows that the maximally populated intermediate at 0.8 M GdmHCl has the characteristics of a molten globule. It has no activity; it has 55% of its native alpha-helices and a maximum fluorescence emission wavelength of approximately 341 nm, and it binds ANS strongly. However, no protection against exchange is detected under the conditions used in this work. This paradox, the presence of significant residual secondary and tertiary structures detected by optical probes and the total deuteration of its amide protons detected by H-D exchange and mass spectrometry, could be explained by a highly dynamic MM-CK molten globule.

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.

Consequences of a six residual deletion from the N-terminal of rabbit muscle creatine kinase

A mutant of dimeric rabbit muscle creatine kinase (CK), in which six residues (residues 2-7) at the N-terminal were removed by the PCR method, was studied to assess the role of these residues in dimer cohesion and to determine the structural stability of the protein. The specific activity of the mutant was 70.39% of that of the wild-type CK, and the affinity for Mg-ATP and CK substrates was slightly reduced compared with the wild-type protein. The structural stability of the mutant was investigated by a comparative equilibrium urea denaturation study and a thermal denaturation study. The data acquired by intrinsic fluorescence and far-UV circular dichroism (CD) during urea unfolding indicated that, the secondary and tertiary structures of the mutant were more stable than those of wild-type CK. Furthermore, results of 8-anilino-1-naphthalene-sulfonic acid (ANS) fluorescence demonstrated that the hydrophobic surface of the mutant CKND(6) was more stable during urea titration. Data from size exclusion chromatography (SEC) experiments indicated that deletion of the six N-terminal residues resulted in a relatively loose molecular structure, but the dissociation of the mutant CKND(6) occurred later during the unfolding process than for wild-type CK. Consistent with this result, the differential scanning calorimetry (DSC) profiles demonstrated that the thermal stability of the enzyme was increased by removal of the six N-terminal residues. We conclude that a more stable quaternary structure was obtained by deletion of the six residues from the N-terminal of wild-type CK.

Nativelike intermediate on the unfolding pathway of pig kidney fructose-1,6-bisphosphatase

The unfolding and dissociation of the tetrameric enzyme fructose-1,6-bisphosphatase from pig kidney by guanidine hydrochloride have been investigated at equilibrium by monitoring enzyme activity, ANS binding, intrinsic (tyrosine) protein fluorescence, exposure of thiol groups, fluorescence of extrinsic probes (AEDANS, MIANS), and size-exclusion chromatography. The unfolding is a multistate process involving as the first intermediate a catalytically inactive tetramer. The evidence that indicates the existence of this intermediate is as follows: (1) the loss of enzymatic activity and the concomitant increase of ANS binding, at low concentrations of Gdn.HCl (midpoint at 0.75 M), are both protein concentration independent, and (2) the enzyme remains in a tetrameric state at 0.9 M Gdn.HCl as shown by size-exclusion chromatography. At slightly higher Gdn.HCl concentrations the inactive tetramer dissociates to a compact dimer which is prone to aggregate. Further evidence for dissociation of tetramers to dimers and of dimers to monomers comes from the concentration dependence of AEDANS-labeled enzyme anisotropy data. Above 2.3 M Gdn.HCl the change of AEDANS anisotropy is concentration independent, indicative of monomer unfolding, which also is detected by a red shift of MIANS-labeled enzyme emission. At Gdn.HCl concentrations higher than 3.0 M, the protein elutes from the size-exclusion column as a single peak, with a retention volume smaller than that of the native protein, corresponding to the completely unfolded monomer. In the presence of its cofactor Mg(2+), the denaturated enzyme could be successfully reconstituted into the active enzyme with a yield of approximately 70-90%. Refolding kinetic data indicate that rapid refolding and reassociation of the monomers into a nativelike tetramer and reactivation of the tetramer are sequential events, the latter involving slow and small conformational rearrangements in the refolded enzyme.

Equilibrium unfolding of dimeric human prostatic acid phosphatase involves an inactive monomeric intermediate

Guanidine hydrochloride (GdnHCl)-induced unfolding of human prostatic acid phosphatase (hPAP), a homodimer of 50 kDa subunit molecular weight, was investigated with activity measurements, size exclusion HPLC, tryptophan fluorescence, 1-anilinonaphtalene-8-sulfonate (ANS) binding and reactivity with 2-(4'-maleimidoanilino)naphthalene-6-sulfonate (MIANS). Equilibrium analysis was performed to shed light on the role of dimerization in the folding and stability of the catalytically active oligomeric protein. Unfolding was reversible, as verified by activity measurements and tryptophan fluorescence. The noncoincidence of the unfolding curves obtained by different techniques suggests the occurrence of a multiphasic process. The reaction of hPAP inactivation is accompanied by dissociation of the dimer into two monomers. The midpoint of this transition is at 0.65 M GdnHCl with 4.24+/-0.12 kcalmol(-1) free energy change. Binding of ANS to the inactive phosphatase monomer, especially remarkable in the region from 0.8 to 1.25M GdnHCl, suggests that the hydrophobic probe indicates exposition of the intersubunit hydrophobic surface and a loosening of the monomer's tertiary structure. Strong fluorescence of thiol group derivatives, the products of their reaction with MIANS, appears in a limited range of GdnHCl concentrations (1.2-1.6M). This shows that in the relaxed structure of the intermediate, the reagent is allowed to penetrate into the hydrophobic environment of the partially hidden thiol groups. The equilibrium unfolding reaction of hPAP, as monitored by tryptophan fluorescence, does not depend on the protein concentration and displays a single transition curve with a midpoint at 1.7 M GdnHCl and value of DeltaG(unf)(H(2)O)=3.38+/-0.08 kcalmol(-1) per monomer, a result implying that this transition is related to the conformational change of the earlier dissociated and already inactive subunit of the protein.

Generation of an active monomer of rabbit muscle creatine kinase by site-directed mutagenesis: the effect of quaternary structure on catalysis and stability

Cytosolic creatine kinase exists in native form as a dimer; however, the reasons for this quaternary structure are unclear, given that there is no evidence of active site communication and more primitive guanidino kinases are monomers. Three fully conserved residues found in one-half of the dimer interface of the rabbit muscle creatine kinase (rmCK) were selectively changed to alanine by site-directed mutagenesis. Four mutants were prepared, overexpressed, and purified: R147A, R151A, D209A, and R147A/R151A. Both the R147A and R147A/R151A were confirmed by size-exclusion chromatography and analytical ultracentrifugation to be monomers, whereas R151A was dimeric and D209A appeared to be an equilibrium mixture of dimers and monomers. Kinetic analysis showed that the monomeric mutants, R147A and R147A/R151A, showed substantial enzymatic activity. Substrate binding affinity by R147A/R151A was reduced approximately 10-fold, although k(cat) was 60% of the wild-type enzyme. Unlike the R147A/R151A, the kinetic data for the R147A mutant could not be fit to a random-order rapid-equilibrium mechanism characteristic of the wild-type, but could only be fit to an ordered mechanism with creatine binding first. Substrate binding affinities were also significantly lower for the R147A mutant, but k(cat) was 11% that of the native enzyme. Fluorescence measurements using 1-anilinonaphthalene-8-sufonate showed that increased amounts of hydrophobic surface area are exposed in all of the mutants, with the monomeric mutants having the greatest amounts of unfolding. Thermal inactivation profiles demonstrated that protein stability is significantly decreased in the monomeric mutants compared to wild-type. Denaturation experiments measuring lambda(max) of the intrinsic fluorescence as a function of guanidine hydrochloride concentration helped confirm the quaternary structures and indicated that the general unfolding pathway of all the mutants are similar to that of the wild-type. Collectively, the data show that dimerization is not a prerequisite for activity, but there is loss of structure and stability upon formation of a CK monomer.

Studies on the stability of creatine kinase isozymes

Research on the stabilizing properties of creatine kinase isozymes CK-BB, CK-MB, and CK-MM showed that minor alteration of their sequence and structure influenced their stability significantly. An analysis of the stability of the isozymes in storage after freeze drying indicates that creatine kinase isozymes are all in monomer form because of the loss of subunit interactions. Freeze-drying leads to the oxidization of CK-BB and rearrangement of CK-MB. There are also differences in the unfolding of the isozymes in urea. CK-BB and CK-MB are unfolded in lower urea concentrations than CK-MM. Differences in the thermal unfolding were also examined by differential scanning calorimetry. This paper discusses the potential biological significance of these results.

Effects of lactic acid and NaCl on creatine kinase from rabbit muscle

The lactic acid induced unfolding and the salt-induced folding of creatine kinase (CK) were studied by enzyme activity, fluorescence emission spectra, circular dichroism spectra, and native polyacrylamide gel electrophoresis. The results showed that the kinetics of CK inactivation was a monophase process. Lactic acid caused inactivation and unfolding of CK with no aggregation during CK denaturation. The unfolding of the whole molecule and the inactivation of CK in solutions of different concentration of lactic acid were compared. Much lower lactic acid concentration values were required to bring about inactivation than were required to produce significant conformational changes of the enzyme molecule. At higher concentrations of lactic acid (more than 0.2 mM) the CK dimers were partially dissociated, as proved by native polyacrylamide gel electrophoresis. NaCl induced the molten globule state with a compact structure after CK was denatured with 0.8 mM lactic acid, and the increasing of anions led to a tight side-chain. The above results suggest that the effect of lactic acid differed from that of other denaturants such as guanidine hydrochloride, HCI, or urea during CK folding, and the molten globule state indicates that intermediates exist during CK folding.

Effects of aspartate on rabbit muscle creatine kinase and the salt induced molten globule state

The aspartate (Asp)-induced unfolding and the salt-induced folding of creatine kinase (CK) have been studied by measuring enzyme activity, fluorescence emission spectra, circular dichroism (CD) spectra, native polyacrylamide gel electrophoresis and ultraviolet difference spectra. The results showed that Asp caused inactivation and unfolding of CK, with no aggregation during CK denaturation. The kinetics of CK unfolding followed a one phase process. At higher concentrations of Asp (>2.5mM), the CK dimers were partially dissociated. Inactivation occurred before noticeable conformational change during CK denaturation. Asp denatured CK was mostly reactivated and refolded by dilution. KCl induced the molten globule state with compact structure after CK was denatured with 10mM Asp. These results suggest that the effect of Asp differed from that of other denaturants such as guanidine, HCl or urea during CK unfolding. Asp is a reversible protein denaturant and the molten globule state indicates that intermediates exist during CK folding.

Role of C-terminal sequences in the folding of muscle creatine kinase

Proteinase K selectively nicks the native homodimeric muscle creatine kinase (MM-CK) into two 37.1 kDa N-terminal (K1) and two 5.8 kDa C-terminal (K2) fragments that remain firmly associated in a native-like, although inactive, heterotetrameric structure. This truncated protein has been named (K1K2)(2). To analyze the role of the C-terminal peptide in the protein structure acquisition, we studied in vitro refolding of the guanidinium chloride-denatured (K1K2)(2). Although they never reassociate with K2, in selected conditions the K1 fragments refold slowly to a dimeric state as shown by size exclusion chromatography data. This K1 dimer exhibits a fluorescence emission lambda max of 335 nm, a high degree of tyrosine exposure, strongly binds ANS but not MgADP, a CK substrate, and according to these structural characteristics, could be a dimeric molten globule species. We propose a folding model that takes into account the existence of a new transient intermediate state in the MM-CK refolding process. Besides two monomeric premolten and molten globule kinetic intermediates and the active final dimeric form, an inactive dimer, with partly compacted monomers, must ephemerally exist. Our results strongly suggest that the C-terminal end of the protein accelerates folding and plays a critical role for monomer final packing into a native-like conformation. The data also indicate that MM-CK catalytic efficiency is only acquired after dimerization.

Unraveling multistate unfolding of rabbit muscle creatine kinase

GdmCl-induced unfolding of rabbit muscle creatine kinase, CK, has been studied by a variety of physico-chemical methods including near and far UV CD, SEC, intrinsic fluorescence (intensity, anisotropy and lifetime) as well as intensity and lifetime of bound ANS fluorescence. The formation of several stable unfolding intermediates, some of which were not observed previously, has been established. This was further confirmed by representation of fluorescence data in terms of "phase diagram", i.e. I(lambda1) versus I(lambda2) dependence, where I(lambda1) and I(lambda2) are fluorescence intensity values measured on wavelengths lambda(1) and lambda(2) under the different experimental conditions for a protein undergoing structural transformations. The unfolding behavior of CK was shown to be strongly affected by association of partially folded intermediates. A model of CK unfolding, which takes into account both structural perturbations and association of partially folded intermediates has been elaborated.

Subunit composition and conformational stability of the oligomeric form of the avian reovirus cell-attachment protein sigmaC

Previous work has shown that the avian reovirus cell-attachment sigma C (sigmaC) protein is a multimer. In the first part of this study the oligomerization state of intracellularly synthesized sigmaC was analysed by different approaches, including SDS-PAGE, chemical cross-linking, sedimentation and gel filtration analysis. All these approaches indicated that protein sigmaC in its native state is a homotrimer. In the second part of the present work we investigated the effect of different factors and reagents on oligomer stability, in order to elucidate the nature of the forces that maintain the conformational stability of the homotrimer. Our results, based on the stabilizing effect conferred by reducing agents, demonstrate that the sigmaC subunits are not covalently bound via disulfide linkages. They further suggest that the formation of an intrachain disulfide bond between the two cysteine residues of the sigmaC polypeptide has a negative effect on oligomer stability. The susceptibility of the trimer to pH, temperature, ionic strength, chemical denaturants and detergents indicates that hydrophobic interactions contribute much more to oligomer stability than do ionic interactions and hydrogen bonding. Finally, our results also reveal that mammalian and avian reovirus cell attachment proteins follow different subunit dissociation pathways.

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 rabbit muscle creatine kinase denatured in 2,2,2-trifluoroethanol solutions

The unfolding and refolding of creatine kinase (ATP:creatine N-phosphotransferase (CK), EC 2.7.3.2) during denaturation and reactivation by trifluoroethanol (TFE) have been studied. Significant aggregation was observed when CK was denatured at TFE concentrations between 10% and 40% (v/v). 50% TFE (v/v) was used to study the denaturation and unfolding of CK. The activity loss of CK was a very quick process, as was the marked conformational changes during denaturation followed by fluorescence emission spectra and far-ultraviolet CD spectra. DTNB modification and size exclusion chromatography were used to find that CK dissociated and was in its monomer state after denaturation with 50% TFE. Reactivation and refolding were observed after 80-fold dilution of the denatured CK into 0.05 M Tris-HCl buffer, pH 8.0. The denatured CK recovered about 38% activity following a two phase course (k(1)=4.82+/-0.41x10(-3) s(-1), k(2)=0.60+/-0.01x10(-3) s(-1)). Intrinsic fluorescence maximum intensity changes showed that the refolding process also followed biphasic kinetics (k(1)=4.34+/-0.27x10(-3) s(-1), k(2)=0.76+/-0.02x10(-3) s(-1)) after dilution into the proper solutions. The far-ultraviolet CD spectra ellipticity changes at 222 nm during the refolding process also showed a two phase course (k(1)=4.50+/-0.07x10(-3) s(-1), k(2)=1.13+/-0.05x10(-3) s(-1)). Our results suggest that TFE can be used as a reversible denaturant like urea and GuHCl. The 50% TFE induced CK denaturation state, which was referred to as the 'TFE state', and the partially refolded CK are compared with the molten globule state. The aggregation caused by TFE during denaturation is also discussed in this paper.

Identification of equilibrium and kinetic intermediates involved in folding of urea-denatured creatine kinase

The unfolding transition and kinetic refolding of dimeric creatine kinase after urea denaturation were monitored by intrinsic fluorescence and far ultraviolet circular dichroism. An equilibrium intermediate and a kinetic folding intermediate were identified and characterized. The fluorescence intensity of the equilibrium intermediate is close to that of the unfolded state, whereas its ellipticity at 222 nm is about 50% of the native state. The transition curves measured by these two methods are therefore non-coincident. The kinetic folding intermediate, formed during the burst phase of refolding under native-like conditions, possesses 75% of the native secondary structure, but is mostly lacking in native tertiary structure. In moderate concentrations of urea, only the initial, rapid change in fluorescence intensity or negative ellipticity is observed, and the final state values do not reach the equivalent unfolding values. The unfolding and refolding transition curves measured under identical conditions are non-coincident within the transition from intermediate to fully unfolded state. It is observed by SDS-PAGE that disulfide bond-linked dimeric or oligomeric intermediates are formed in moderate urea concentrations, especially in the refolding reaction. These rapidly formed, soluble intermediates represent an off-pathway event that leads to the hysteresis in the refolding transition curves.

The two slow refolding processes of creatine kinase are catalyzed by cyclophilin

A burst phase occurs in the refolding kinetics of guanidine-denatured creatine kinase due to formation of an intermediate within the mixing dead time, with further refolding to the native state after the burst phase along a path following biphasic kinetics. In the presence of cyclophilin, the refolding rates of the two slow processes are accelerated and the values are proportional to the cyclophilin concentration. The activity of cyclophilin in accelerating the slow refolding processes of creatine kinase is totally inhibited by cyclosporin A, indicating that the cis-trans isomerization of the peptidyl-prolyl bonds is involved in the two slow refolding processes.

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.

Effect of Mg2+ during reactivation and refolding of guanidine hydrochloride-denatured creatine kinase

Creatine kinase (ATP: creatine N-phosphotransferase, EC 2.7.3.2) was completely denatured using 3 M guanidine hydrochloride for 2 h as in previous studies [Yao et al. (1982), Sci. Sin. 25B, 1296-1302; Yao et al. (1984), Biochemistry 23, 2740-2744; Yao et al. (1982), Sci. Sin. 25B, 1186-1193]. Under suitable conditions, about 60-70% of the activity can be recovered in the presence of different Mg2+ concentrations. Both the reactivation and the refolding processes follow two-phase courses after dilution in the proper solutions. A comparison of the rate constants for the refolding of unfolded creatine kinase with those for the recovery of its catalytic activity at various Mg2+ concentrations shows that these are not synchronized. The reactivity of guanidine hydrochloride-denatured creatine kinase can be inhibited by Mg2+; however, the rates of reactivation are independent of the Mg2+ concentration. In addition, Mg2+ affects the fluorescence intensity, but the rate constants of refolding are independent of Mg2+ concentration. Although the reactivation of GdHCl-denatured creatine kinase is complete about 3 h after dilution with reactivation solutions, the conformational changes during refolding occur in a much slower reaction. Mg2+ can induce complex changes in the relative fluorescence intensity during refolding over a broad range of concentrations.

Structural defects underlying protein dysfunction in human glucose-6-phosphate dehydrogenase A(-) deficiency

The enzyme variant glucose-6-phosphate dehydrogenase (G6PD) A(-), which gives rise to human glucose-6-phosphate dehydrogenase deficiency, is a protein of markedly reduced structural stability. This variant differs from the normal enzyme, G6PD B, in two amino acid substitutions. A further nondeficient variant, G6PD A, bears only one of these two mutations and is structurally stable. In this study, the synergistic structural defect in recombinant G6PD A(-) was reflected by reduced unfolding enthalpy due to loss of beta-sheet and alpha-helix interactions where both mutations are found. This was accompanied by changes in inner spatial distances between residues in the coenzyme domain and the partial disruption of tertiary structure with no significant loss of secondary structure. However, the secondary structure of G6PD A(-) was qualitatively affected by an increase in beta-sheets substituting beta-turns related to the lower unfolding enthalpy. The structural changes observed did not affect the active site of the mutant proteins, since its spatial position was unmodified. The final result is a loss of folding determinants leading to a protein with decreased intracellular stability. This is suggested as the cause of the enzyme deficiency in the red blood cell, which is unable to perform de novo protein synthesis.

Reactivation and refolding of a partially folded creatine kinase modified by 5,5'-dithio-bis(2-nitrobenzoic acid)

Creatine kinase with its thiol groups modified by 5, 5'-dithio-bis(2-nitrobenzoic acid) has been shown to be partially folded in a monomeric state using fluorescence, circular dichroism, proteolysis, and size exclusion chromatography studies. In the presence of DTT, the partially folded modified creatine kinase can be reactivated and refolded following a biphasic course, suggesting the existence of a monomeric intermediate during the refolding of CK. The results provide evidence for our previously suggested model of the refolding pathway of urea-denatured creatine kinase.

Thermodynamic analysis of unfolding and dissociation in lactose repressor protein

Lactose repressor protein, regulator of lac enzyme expression in Escherichia coli, maintains its structure and function at extremely low protein concentrations (<10(-)12 M). To examine the unfolding and dissociation of this tetrameric protein, structural transitions in the presence of varying concentrations of urea were monitored by fluorescence and circular dichroism spectroscopy, analytical ultracentrifugation, and functional activities. The spectroscopic data demonstrated a single cooperative transition with no evidence of folded dimeric or monomeric species of this protein. These spectroscopic transitions were reversible provided a long incubation step was employed in the refolding reaction at approximately 3 M urea. The refolded repressor protein possessed the same functional and structural properties as wild-type repressor protein. The absence of concentration dependence expected for tetramer dissociation to unfolded monomer (M4 <--> 4U) in the spectral transitions indicates that the disruption of the monomer-monomer interface and monomer unfolding are a concerted reaction (M4 <--> U4) that may occur prior to the dissociation of the dimer-dimer interface. Thus, we propose that the unfolded monomers remain associated at the C-terminus by the 4-helical coiled-coil structure that forms the dimer-dimer interface and that this intermediate is the end point detected in the spectral transitions. Efforts to confirm the existence of this species by ultracentrifugation were inhibited by the aggregation of this intermediate. Based upon these observations, the wild-type fluorescence and CD data were fit to a model, M4 <--> U4, which resulted in an overall DeltaG degrees for unfolding of 40 kcal/mol. Using a mutant protein, K84L, in which the monomer-monomer interface is stabilized, sedimentation equilibrium results demonstrated that the dimer-dimer interface of lac repressor could persist at higher levels of urea than the monomer-monomer interface. The tetramer-dimer transition monitored using this mutant repressor yields a DeltaG degrees of 20.4 kcal/mol. Using this free energy value for the dissociation process of U4 <--> 4U, an overall free energy change of approximately 60 kcal/mol was calculated for dissociation of all interfaces and unfolding of the tetrameric lac repressor, reflecting the exceptional stability of this protein.

Urea-induced unfolding and conformational stability of 3-isopropylmalate dehydrogenase from the Thermophile thermus thermophilus and its mesophilic counterpart from Escherichia coli

To reveal the basis of the thermal stability of 3-isopropylmalate dehydrogenase (IPMDH) from an extreme thermophile, Thermus thermophilus, urea-induced unfolding of the enzyme and of its mesophilic counterpart from Escherichia coli has been studied. The urea-induced equilibrium unfolding of T. thermophilus and E. coli IPMDHs at 27 degreesC was monitored by measuring the changes in far-UV CD, intrinsic fluorescence, anilinonaphthalenesulfonic acid (ANS) binding, and catalytic activity in the presence of nonionic detergent Tween 20. For both enzymes, the spectral methods revealed a biphasic unfolding transition. The first transition was protein concentration-independent, whereas the second was protein concentration-dependent for both enzymes. The observation suggested a three-state unfolding mechanism with a dimeric intermediate. However, the intermediates of the E. coli and the T. thermophilus IPMDHs seemed to be different from each other. The intermediate of the E. coli IPMDH lost its secondary and tertiary structure more than that of the thermophilic enzyme. E. coli IPMDH lost enzymatic activity through the transition from the native to the intermediate state, though the intermediate of the T. thermophilus enzyme was still active. The unfolding process of E. coli IPMDH can be explained by a sequential unfolding of individual folding domains, while there is only a small structural perturbation in the intermediate of T. thermophilus IPMDH. The higher thermal stability of T. thermophilus IPMDH can be attributed to the increase in the extent of interaction inside the first domain which unfolded prior to the unfolding of the whole molecular structure in E. coli IPMDH.

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

The kinetics of refolding of guanidinium chloride-denatured rabbit MM-creatine kinase was investigated. Recovery of enzymatic activity is biphasic, depending on the temperature but not on the protein or DTT concentration. Only 45% of the original, active dimeric form is recovered even after several hours of refolding. The reactivation yield is limited by the accumulation of a highly stable but nonproductive monomeric species. The ratio of "correct" to "incorrect" forms depends on the duration of exposure to the denaturant, which may be consistent with the existence of a heterogeneous population of unfolded states with regard to proline isomerization. The first fast reaction observed during renaturation results in the appearance of collapsed monomeric states, displaying features of a pre-molten globule state. These burst species are rapidly transformed into more structured monomers resembling a molten globule state possessing a partially folded C-terminal domain. A proportion of these latter transient intermediates (45%) associates into an active dimer, while the remainder (55%) is trapped by reshuffling in a monomeric dead-end product. Our results strongly indicate that (i) the dimeric state is a prerequisite for the expression of catalytic activity, (ii) the kinetic intermediates of refolding are very similar to those observed during equilibrium unfolding, and (iii) refolding of creatine kinase in these conditions is limited by the accumulation of inactive misfolded nondimerizable monomer.

Unfolding and refolding of dimeric creatine kinase equilibrium and kinetic studies

Equilibrium and kinetic studies of the guanidine hydrochloride induced unfolding-refolding of dimeric cytoplasmic creatine kinase have been monitored by intrinsic fluorescence, far ultraviolet circular dichroism, and 1-anilinonaphthalene-8-sulfonate binding. The GuHCl induced equilibrium-unfolding curve shows two transitions, indicating the presence of at least one stable equilibrium intermediate in GuHCl solutions of moderate concentrations. This intermediate is an inactive monomer with all of the thiol groups exposed. The thermodynamic parameters obtained by analysis using a three-state model indicate that this intermediate is similar in energy to the fully unfolded state. There is a burst phase in the refolding kinetics due to formation of an intermediate within the dead time of mixing (15 ms) in the stopped-flow apparatus. Further refolding to the native state after the burst phase follows biphasic kinetics. The properties of the burst phase and equilibrium intermediates were studied and compared. The results indicate that these intermediates are similar in some respects, but different in others. Both are characterized by pronounced secondary structure, compact globularity, exposed hydrophobic surface area, and the absence of rigid side-chain packing, resembling the "molten globule" state. However, the burst phase intermediate shows more secondary structure, more exposed hydrophobic surface area, and more flexible side-chain packing than the equilibrium intermediate. Following the burst phase, there is a fast phase corresponding to folding of the monomer to a compact conformation. This is followed by rapid assembly to form the dimer. Neither of the equilibrium unfolding transitions are protein concentration dependent. The refolding kinetics are also not concentration dependent. This suggests that association of the subunits is not rate limiting for refolding, and that under equilibrium conditions, dissociation occurs in the region between the two unfolding transitions. Based upon the above results, schemes of unfolding and refolding of creatine kinase are proposed.

Guanidine-induced denaturation of beta-glycosidase from Sulfolobus solfataricus expressed in Escherichia coli

Guanidine-induced denaturation of Sulfolobus solfataricus beta-glycosidase expressed in Escherichia coli, Sbetagly, was investigated at pH 6.5 and 25 degreesC by means of circular dichroism and fluorescence measurements. The process proved reversible when the protein concentration was lower than 0.01 mg mL-1. Moreover, the transition curves determined by fluorescence did not coincide with those determined by circular dichroism, and the GuHCl concentration corresponding at half-completion of the transition increased on raising the protein concentration in the range 0.001-0.1 mg mL-1. Gel filtration chromatography experiments showed that, in the range 2-4 M GuHCl, there was an equilibrium among tetrameric, dimeric, and monomeric species. These findings, unequivocally, indicated that the guanidine-induced denaturation of Sbetagly was not a two-state transition with concomitant unfolding and dissociation of the four subunits. A mechanism involving a dimeric intermediate species was proposed and was able to fit the experimental fluorescence intensity transition profiles, allowing the estimation of the total denaturation Gibbs energy change at 25 degreesC and pH 6.5. This figure, when normalized for the number of residues, showed that, at room temperature, Sbetagly has a stability similar to that of mesophilic proteins.

Role of quaternary structure in muscle creatine kinase stability: tryptophan 210 is important for dimer cohesion

A mutant of the dimeric rabbit muscle creatine kinase (MM-CK) in which tryptophan 210 was replaced has been studied to assess the role of this residue in dimer cohesion and the importance of the dimeric state for the native enzyme stability. Wild-type protein equilibrium unfolding induced by guanidine hydrochloride occurs through intermediate states with formation of a molten globule and a premolten globule. Unlike the wild-type enzyme, the mutant inactivates at lower denaturant concentration and the loss of enzymatic activity is accompanied by the dissociation of the dimer into two apparently compact monomers. However, the Stokes radius of the monomer increases with denaturant concentration as determined by size exclusion chromatography, indicating that, upon monomerization, the protein structure is destabilized. Binding of 8-anilinonaphthalene-1-sulfonate shows that the dissociated monomer exposes hydrophobic patches at its surface, suggesting that it could be a molten globule. At higher denaturant concentrations, both wild-type and mutant follow similar denaturation pathways with formation of a premolten globule around 1.5-M guanidine, indicating that tryptophan 210 does not contribute to a large extent to the monomer conformational stability, which may be ensured in the dimeric state through quaternary interactions.

High salt concentrations induce dissociation of dimeric rabbit muscle creatine kinase. Physico-chemical characterization of the monomeric species

Incubation of dimeric MM-creatine kinase (MM-CK) with high NaCl or LiCl concentrations results in dissociation of the subunits and complete enzyme inactivation. In NaCl, this process, which depends on protein concentration, may be described according to a two-state model where the dimer can be reversibly converted into compact folded monomers (D <--> 2M). At LiCl concentrations higher than 2-2.5 M, MM-CK is recovered in two monomeric states: an inactive compact species (M) and a more expanded form (EF), which represents 15-20% of the population. Thus, in LiCl, a three-state model (D <--> 2M --> 2EF) more adequately accounts for our experimental results. The monomeric species (M) obtained in NaCl and LiCl exhibits some properties of the molten globule state described in guanidine hydrochloride. Indeed, this form is compact and devoid of any enzymatic activity; it maintains a high degree of secondary structure and binds 8-anilino-1-naphthalenesulfonate. The formation of this intermediate induces the exposure of a second tryptophan (among the four present) which is located at the monomer-monomer interface in the native structure. In LiCl, the monomeric species (M) is irreversibly converted into a less compact form (EF) which seems to have lost a large part of its secondary structure.

Denaturation by guanidinium chloride of dimeric MM-creatine kinase and its proteinase K-nicked form: evidence for a multiple-step process

Cytosolic MM-creatine kinase is a homodimeric protein. Each monomer can be cleaved by proteinase K at an exposed surface loop, into two fragments K1 and K2, which remain associated. The nicked protein is thus a heterotetrameric protein, named (K1K2)2, made up of two heterodimers K1K2 linked together by their K1 subunit. In non-denaturing conditions, the cleaved protein does not present any measurable difference compared with uncleaved MM-creatine kinase, except for the loss of enzymatic activity. Comparative equilibrium denaturation of the two oligomeric proteins by guanidinium chloride indicates a multistep process with formation of either compact monomer or compact K1K2 dimer, a molten globule and a pre-molten globule state. In the case of the nicked-enzyme, the molten globule is composed of the two peptides K1 and K2, whereas in the pre-molten globule the interactions between K1 and K2 are too weak to maintain their cohesion. At low guanidinium chloride concentration, the proteinase K-nicked protein exhibits a higher accessibility of one of its tryptophan accompanied by a small decrease in its molar ellipticity suggesting a secondary structure loosening of the K1 peptide. Our results suggest that K1 and K2 are not strictly autonomous unfolding units and thus cannot be considered as independent domains.

Protease digestion studies of an equilibrium intermediate in the unfolding of creatine kinase

Protease digestion experiments have been used to characterize the structure of an equilibrium intermediate in the unfolding of creatine kinase (CK) by low concentrations (0.625 M) of guanidine hydrochloride (GdnHCl). Eighteen of the major products of digestion by trypsin, chymotrypsin and endoproteinase Glu-C have been identified by microsequencing after separation by SDS/PAGE and electroblotting on poly(vinylidene difluoride) membranes. The C-terminal portion (Gly215 to Lys380) was much more resistant to digestion than the N-terminal portion (Pro1 to Gly133), although the area most sensitive to proteolysis was in the middle of the CK sequence (Arg134 to Arg214). These experiments are consistent with the two-domain model for the CK monomer. The structure of the intermediate is proposed to consist of a folded C-terminal domain and a partly folded N-terminal domain separated by an unfolded central linker. Protease susceptibility is clustered within two N-terminal regions and one central region. These regions are evidently exposed as a result of the partial unfolding and/or separation of the N-terminal domain. Further evidence for the structure of this intermediate comes from gel filtration studies. Treatment of CK with 0.625 M GdnHCl resulted in slow aggregation at 37 degrees C, but not at 12 degrees C, a phenomenon previously reported for phosphoglycerate kinase. The aggregation did not occur at higher GdnHCl concentrations and was unaffected by a reducing agent. It is proposed that aggregation is a consequence of non-specific interactions between hydrophobic regions, possibly domain/domain interfaces, which become exposed in the intermediate.

Denaturation of MM-creatine kinase by sodium dodecyl sulfate

The denaturation of dimeric cytoplasmic MM-creatine kinase by sodium dodecyl sulfate (SDS) has been investigated using activity measurements, far-ultraviolet circular dichroism, SEC-HPLC, electric birefringence, intrinsic probes (cysteine and tryptophan residues), and an extrinsic fluorescent probe (ANS). Our results show that inactivation is the first detectable event; the inactivation curve midpoint is located around 0.9 mM SDS. The second event is dissociation and it occurs in parallel to tertiary and secondary perturbations, as demonstrated by the coincidence (near 1.3 mM) of the midpoints of the transition curves monitoring dissociation and structural changes. At high total SDS concentration (concentration higher than 2.5 mM), the monomer had bound 170 mol of SDS per mol of protein. In these conditions, electric birefringence experiments suggest that the SDS-CK complex may be described as a prolate ellipsoid with an axial ratio of 1.27 (14 nm x 11 nm). These results are compatible with recent models of SDS-protein complexes: the "protein decorated micelle structure" or the "necklace structure".