Reactivation kinetics of 5,5'-dithiobis-(2-nitrobenzoic acid)-modified creatine kinase reactivated by dithiothreitol

The role of Cys271 in conformational changes of arginine kinase

Arginine kinase (AK), a crucial enzyme in energy metabolism, buffers cellular ATP levels by catalyzing the reversible phosphoryl transfer between ATP and arginine. To better understand the role of Cys271 in conformational changes of AK from greasyback shrimp (Metapenaeus ensis), we replaced the residue with serine and alanine. A detailed comparison of the catalytic activity and conformation was made between wild-type AK and the mutants by means of activity analysis, ultraviolet (UV) difference, fluorescence spectrum and size exclusion chromatography (SEC). The results indicated that the catalytic activity of the two mutants was gone. The substrates, arginine-ADP-Mg(2+) could induce conformational changes, and additional NO(3)(-) could induce further changes in both the native enzyme and the variants. We speculated that Cys271 might be located in the hinge region between the two domains of AK and cause enzyme conformational changes upon addition of substrate.

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.

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.

Exploration of the polar microenvironment around the reactive cysteine in rabbit muscle creatine kinase

The polar microenvironment around the reactive Cys283 of rabbit muscle creatine kinase was explored using kinetic analysis of substrates reaction in the presence of modifiers. In the present study, three specific sulphydryl reagents, 5,5'-dithiobis(2-nitrobenzoic acid), 6,6'-dithiodinicotinic acid and 2,2'-dithiodipyridine, were applied as modifiers to react with Cys283 of creatine kinase. The inactivation kinetics of creatine kinase by the modifiers was analyzed. The microscopic rate constants for reactions of the modifiers with free enzyme and enzyme-substrate complexes were also determined. The results suggested that the inactivation rate of creatine kinase by 5,5'-dithiobis(2-nitrobenzoic acid) was the fastest, followed by 6,6'-dithiodinicotinic acid and then 2,2'-dithiodipyridine. Interestingly, 5,5'-dithiobis(2-nitrobenzoic acid) and 6,6'-dithiodinicotinic acid functioned as non-complexing modifiers, while 2,2'-dithiodipyridine did a complexing modifier. The results here indicated that the electrophilic group was predominant around Cys283, and that the presence of substrates seemed to have different effects on the inactivation reactions of creatine kinase by the three modifiers. Furthermore, the findings in this study may provide a novel explanation for the low pKa value of Cys283.

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.

The kinetic study of arginine kinase from the sea cucumber Stichopus japonicus with 5,5'-dithiobis-(2-nitrobenzoic acid)

The Stichopus japonicus arginine kinase (AK) is a significant dimeric enzyme. Its modification and inactivation course with 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) and the reactivation course of DTNB-modified AK by dithiothreitol were investigated on the basis of the kinetic theory of the substrate reaction during the modification of enzyme activity. The results show that the modification is a biphasic course while the inactivation is monophasic, with one essential reactive cysteine per subunit. The Cys274 (numbering from the Stichopus sequence) is exposed to DTNB and is near the ATP binding site. The modified AK can be reactivated by an excess concentration of dithiothreitol in a monophasic kinetic course. The presence of ATP or the transition-state analog markedly slows the apparent reactivation rate constant. The analog components, arginine-ADP-Mg2+ can induce conformational changes of the modified enzyme, but adding NO3- cannot induce further changes that occur with the native enzyme. The reactive cysteines' location and its role in the catalysis of AK are discussed. The results suggest that the cysteine may be located in the hinge area of the two domains of AK. The reactive cysteine of AK, which was proposed to be Cys274, may play an important role not in the binding of the transition-state analog but in the conformational changes caused by the transition-state analog.

Implications of the role of reactive cystein in arginine kinase: reactivation kinetics of 5,5′-dithiobis-(2-nitrobenzoic acid)-modified arginine kinase reactivated by dithiothreitol

The reduction of 5,5'-dithiobis-(2-nitrobenzoic acid)-modified arginine kinase by dithiothreitol has been investigated using the kinetic theory of the substrate reaction during modification of enzyme activity. The results show that the modified arginine kinase can be fully reactivated by an excess concentration of dithiothreitol in a monophasic kinetic course. The presence of ATP or the transition-state analog markedly slows the apparent reactivation rate constant, while arginine shows no effect. The results of ultraviolet (UV) difference and intrinsic fluorescence spectra indicate that the substrate arginine-ADP-Mg2+ can induce conformational changes of the modified enzyme but adding NO3- cannot induce further changes that occur with the native enzyme. The reactive cysteines' location and role in the catalysis of arginine kinase are discussed. It is suggested that the cysteine may be located in the hinge region of the two domains of arginine kinase. The reactive cysteine of arginine kinase may play an important role not in the binding to the transition-state analog but in the conformational changes caused by the transition-state analog.

Effect of thiohydroxyl compounds on tyrosinase: inactivation and reactivation study

An unusual thioether bridge (Cys-His) has been detected at the active site of mushroom tyrosinase, and the effects of thiohydroxyl compounds such as dithiothreitol (DTT) and beta-mercaptoethanol (beta-ME) on Cu2+ at the active site have been elucidated. Treatment with DTT and beta-ME on mushroom tyrosinase completely inactivated 3,4-dihydroxyphenylalanine oxidase activity in a dose-dependent manner. Sequential kinetic studies revealed that DTT and beta-ME caused different mixed-type inhibition mechanisms: the slope-parabolic competitive inhibition (Ki = 0.143 mM) by DTT and slope-hyperbolic noncompetitive inhibition (Ki = 0.0128 mM) by beta-ME, respectively. Kinetic Scatchard analysis consistently showed that mushroom tyrosinase had multiple binding sites for DTT and beta-ME with different affinities. Reactivation study of inactivated enzyme by addition of Cu2+ confirmed that DTT and beta-ME directly bound with Cu2+ at the active site. Our results may provide useful information regarding interactions of tyrosinase inhibitor for designing an effective whitening agent targeted to the tyrosinase active site.

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.

Conformational changes and inactivation of rabbit muscle creatine kinase in dimethyl sulfoxide solutions

The effects of dimethyl sulfoxide (DMSO) on creatine kinase (CK) conformation and enzymatic activity were studied by measuring activity changes, aggregation, and fluorescence spectra. The results showed that at low concentrations (< 65% v/v), DMSO had little effect on CK activity and structure. However, higher concentrations of DMSO led to CK inactivation, partial unfolding, and exposure of hydrophobic surfaces and thiol groups. DMSO caused aggregation during CK denaturation. A 75% DMSO concentration induced the most significant aggregation of CK. The CK inactivation and unfolding kinetics were single phase. The unfolding of CK was an irreversible process in the DMSO solutions. The results suggest that to a certain extent, an enzyme can maintain catalytic activity and conformation in water-organic mixture environments. Higher concentrations of DMSO affected the enzyme structure but not its active site. Inactivation occurred along with noticeable conformational change during CK denaturation. The inactivation and unfolding of CK in DMSO solutions differed from other denaturants such as guanidine, urea, and sodium dodecyl sulfate. The exposure of hydrophobic surfaces was a primary reason for the protein aggregation.

Inhibitive effect of zinc ion on fatty acid synthase from chicken liver

Fatty acid synthase (FAS; acyl-CoA:malonyl-CoA C-acyltransferase [decarboxylating, oxoacyl- and enoyl-reducing and thioester-hydrolyzing], EC 2.3.1.85) is an important enzyme participating in energy metabolism in vivo which is related to adiposis and cancer [Cancer Lett. 167 (1) (2001) 99; Nat. Med. 8 (4) (2002) 335]. Tests of fast- and slow-binding inhibitions showed that fatty acid synthase of chicken liver is rapidly and irreversibly inactivated by low Zn(2+) concentrations. Electrophoresis and FPLC results showed that FAS cross-links occurred in the presence of high Zn(2+) concentrations (>4 microM) which may be another reason that FAS lost its activity. The modification velocity of FAS by DTNB decreased with increasing Zn(2+) concentration, which confirmed that Zn(2+) interacted with SH groups. Substrate protective experiments and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) showed that all three substrates tested had some protective effects on FAS in the presence of Zn(2+), and malonyl-CoA was the most effective of the three substrates. In the presence of malonyl-CoA, the activity loss of FAS decreased sharply and almost no cross-link was observed in SDS-PAGE. This suggests that the phosphopantetheine SH group is the critical group in the cross-link and inhibition of FAS in the presence of Zn(2+).

Effects of acrylamide on creatine kinase from rabbit muscle

The mechanism of inhibition of creatine kinase (CK) by acrylamide (Acr) has been examined (in vitro). Within the concentration range of 0 to 1 M, Acr markedly inhibited CK and depleted the protein thiols. Both inactivation and thiol depletion were time- and Acr concentration-dependent. Addition of dithiothreitol (DTT) did not reactivate CK inactivated by Acr. However, CK with 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) pre-blocked thiols can be reactivated by DTT after incubation with Acr. The transition-state analogue also had a significant protective effect on CK against Acr inhibition. We conclude that thiol alkylation is a critical event in inactivation of CK by Acr. Furthermore, Acr binding to CK changed its surface charge, which may be the same effect for the toxicity of Acr towards other proteins.

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.

Reactivation kinetics of guanidine hydrochloride-denatured creatine kinase measured using the substrate reaction

Guanidine hydrochloride-denatured creatine kinase (CK) can very quickly form a dimer with reactivity when the denaturant is diluted into the reaction system in the presence of DTT or EDTA. Tsou's method and its applied equation [Tsou (1988), Adv. Enzymol. Rel. Areas Mol. Biol. 61, 381-436; Yang and Zhou (1998), Biochim. Biophys. Acta 1388, 190-198] were used to measure the kinetic reactivation rate constants and the reactivation degree for reassociated CK dimers. Partial reactivation (about 50% at best) occurred following a monophasic course during the substrate reaction when compared with previous time interval measurements. The reactivation degree increased with increasing DTT (0.1-5 mM) and EDTA (0.1-1 mM) concentrations. The apparent forward rate constants do not change with concentration, showing that the reactivation is a reversible first-order reaction, but not of complex formation type. However, the apparent forward rate constants do change with EDTA concentration, showing that the reactivation with EDTA is a reversible first-order reaction as well as of complex formation type. Excess DTT concentrations have an inhibitory effect, indicating that the excessive EDTA acts as a metal chealate not only for free Mg2+, but also for MgATP during the enzyme catalysis. This study shows that additional information about the reactivation of CK can be obtained from examining the substrate reaction. The possible refolding pathway of CK is discussed.

Folding character of cytochrome c studied by o-nitrobenzyl modification of methionine 65 and subsequent ultraviolet light irradiation

The protein folding character of cyt c was studied with the use of a photocleavable o-nitrobenzyl derivative of Met65 (NBz-Met65). For the NBz-Met65 cyt c, the Soret absorption band slightly blue shifted compared with the unlabeled cyt c, the 695 nm absorption band related to the Met80 sulfur ligation to the heme iron disappeared, and its resonance Raman spectrum was characteristic of a six-coordinate low-spin species, all characters demonstrating coordination of a non-native ligand, probably a histidine, instead of Met80 to the heme iron. The far-UV circular dichroism (CD) spectrum of cyt c was altered, and the transition midpoint concentration value of guanidine hydrochloride (GdnHCl) for unfolding the protein decreased by 0.9 M by the modification, which showed perturbation of the structure and decrease in protein stability, respectively. With irradiation of 308 nm laser pulses on the NBz-Met65 cyt c, the Soret absorption band slightly red shifted, the 695 nm absorption band appeared, and the CD spectrum shifted toward that of the native protein, which demonstrated recovery of the methionine heme coordination and the native protein structure, due to reconversion of NBz-Met65 to unlabeled methionine. A fast phase was detected as a change in Soret absorbance with a rate constant of 21 000 +/- 4000 s(-)(1) during refolding of cyt c initiated by irradiation of a 308 nm pulse on the NBz-Met65 cyt c in the presence of 2 M GdnHCl. The observed rate constant corresponded well with that reported by the tryptophan fluorescence study [Shastry, M. C. R. S., and Roder, H. (1998) Nat. Struct. Biol. 5, 385-392]. The intermediate decayed with a rate constant of 90 +/- 15, followed by another phase with a rate constant of 13 +/- 3 s(-)(1), and was not seen in the absence of GdnHCl.

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.

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.