Isoenzyme-specific thermostability of human cytosolic creatine kinase

Dissociative mechanism for irreversible thermal denaturation of oligomeric proteins

Protein stability is a fundamental characteristic essential for understanding conformational transformations of the proteins in the cell. When using protein preparations in biotechnology and biomedicine, the problem of protein stability is of great importance. The kinetics of denaturation of oligomeric proteins may have characteristic properties determined by the quaternary structure. The kinetic schemes of denaturation can include the multiple stages of conformational transitions in the protein oligomer and stages of reversible dissociation of the oligomer. In this case, the shape of the kinetic curve of denaturation or the shape of the melting curve registered by differential scanning calorimetry can vary with varying the protein concentration. The experimental data illustrating dissociative mechanism for irreversible thermal denaturation of oligomeric proteins have been summarized in the present review. The use of test systems based on thermal aggregation of oligomeric proteins for screening of agents possessing anti-aggregation activity is discussed.

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.

Purification and stability of octameric mitochondrial creatine kinase isoform from herring (Clupea harengus) organ of vision

Creatine kinases (CKs) constitute a large family of isoenzymes that are involved in intracellular energy homeostasis. In cells with high and fluctuating energy requirements ATP level is maintained via phosphocreatine hydrolysis catalyzed by creatine kinase. In contrast to invertebrates and higher vertebrates, in poikilothermic vertebrates the adaptations for the regulation of energy metabolism by changes in the oligomeric state of CK isoforms are not well known. The present study aimed at identification of herring eye CK isoforms and focuses on factors affecting the CK-octamer stability. In addition to the CK octamer, three different dimeric isoforms of CK were detected by cellulose acetate native electrophoresis. Destabilization of octamer was studied in the presence of TSAC substrates and about 50% of octamers dissociated into dimers within 24h. Moreover, we found that the increase of temperature from 4 °C to 30 °C caused rapid inactivation of dimers in TSAC-treated samples but did not affect octameric structures. In a thermostability assay we demonstrated that octamers retain their activity even at 50 °C. Our results indicate that destabilization of the octameric structure can lead to loss of enzyme activity at higher temperatures (above 30 °C). Furthermore, our results based on N-terminal sequence analysis suggest that probably the mitochondrial s-type CK, rather than u-type, is predominantly expressed in herring eye. In conclusion the existence of four various CK isoforms in one organ may reflect complex regulation of energy metabolism in the phototransduction process in teleost fishes.

Effects of the Fc-III tag on activity and stability of green fluorescent protein and human muscle creatine kinase

The Fc-III tag is a newly developed fusion tag that can be applied to protein purification and detection. In the present work, we use the Fc-III-tagged green fluorescent protein (GFP) and human muscle creatine kinase (CK) as model systems to investigate effects of the Fc-III tag on activities and stabilities of the expressed multicysteine-containing proteins. Our results show the Fc-III tag has no adverse effects on the fluorescence of GFP and reduces the occurrence of GFP misfolding due to incorrect Cys oxidation compared with the His-tagged protein. The activity and stability of the Fc-III-tagged CK is slightly lower than that of the tag-free CK, but is higher than that of the His-tagged CK as determined by the ratio of the oxidized versus reduced CK. A major portion of His-tagged CK is in its oxidized form, while that of the Fc-III-tagged CK is in its reduced form. A folding model of CK with different tags was proposed, which may provide insights into the effect of the Fc-III tag on the conformations of disulfide-bridged proteins.

Inactivation of recombinant human brain-type creatine kinase during denaturation by guanidine hydrochloride in a macromolecular crowding system

In this study, we quantitatively examined the effects of the macromolecular crowding agents, polyethylene glycol 2000 (PEG 2000) and dextran 70, on guanidine hydrochloride (GdnHCl)-induced denaturation of recombinant human brain-type creatine kinase (rHBCK). Our results showed that both PEG 2000 and dextran 70 had a protective effect on the inactivation of rHBCK induced by 0.5 M GdnHCl at 25 °C. The presence of 200 g/L PEG 2000 resulted in the retention of 35.33 % of rHBCK activity after 4 h of inactivation, while no rHBCK activity was observed after denaturation in the absence of macromolecular crowding agents. The presence of PEG 2000 and dextran 70 at a concentration of 100 g/L could decelerate the k (2) value of the slow track to 21 and 33 %, respectively, in comparison to values obtained in the absence of crowding agents. Interestingly, inactivation of rHBCK in the presence of 200 g/L PEG 2000 followed first-order monophasic kinetics, with an apparent rate constant of 8 × 10(-5) s(-1). The intrinsic fluorescence results showed that PEG 2000 was better than dextran 70 at stabilizing rHBCK conformation. In addition, the results of the phase diagram indicate that more intermediates may be captured when rHBCK is denatured in a macromolecular crowding system. Mixed crowding agents did not produce better results than single crowding agents, but the protective effects of PEG 2000 on the inactivation and unfolding of rHBCK tended to increase as the ratio of PEG 2000 increased in the mixed crowding agent solution. Though it is not clear which crowding agents more accurately simulated the intracellular environment, this study could lead to a better understanding of protein unfolding in the intracellular environment.

Effect of SNPs on creatine kinase structure and function: identifying potential molecular mechanisms for possible creatine kinase deficiency diseases

Single-nucleotide polymorphisms (SNPs) are common genetic material changes that often occur naturally. SNPs can cause amino acid replacements that may lead to severe diseases, such as the well-known sickle-cell anemia. We constructed eight SNP mutants of human brain-type creatine kinase (CKB) based on bioinformatics predictions. The biochemical and biophysical characteristics of these SNP mutants were determined and compared to those of the wild-type creatine kinase to explore the potential molecular mechanisms of possible creatine kinase SNP-induced diseases. While the reactivation of six SNP mutants after heat shock dropped more than 45%, only three of them showed notable increases in ANS fluorescence intensity and decreases in catalytic efficiency. Among them, H26Y and P36T bind substrates as well as the wild-type form does, but the melting temperatures (T_m) dropped below body temperature, while the T59I mutant exhibited decreased catalytic activity that was most likely due to the much reduced binding affinity of this mutant for substrates. These findings indicate that SNPs such as H26Y, P36T and T59I have the potential to induce genetic diseases by different mechanisms.

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.

Metallothioneins protect cytosolic creatine kinases against stress induced by nitrogen-based oxidants

The formation of intracellular nitrogen-based oxidants has important physiological and pathological consequences. CK (creatine kinase), which plays a key role in intracellular energy metabolism, is a main target of low concentrations of oxidative and nitrative stresses. In the present study, the interaction between cytosolic CKs [MM-CK (muscle-type CK) and BB-CK (brain-type CK)] and MTs [metallothioneins; hMT2A (human MT-IIA) and hMT3 (human MT-III)] were characterized by both in vitro and intact-cell assays. MTs could successfully protect the cytosolic CKs against inactivation induced by low concentrations of PN (peroxynitrite) and NO both in vitro and in hMT2A-overexpressing H9c2 cells and hMT3-knockdown U-87 MG cells. Under high PN concentrations, CK formed granule-like structures, and MTs were well co-localized in these aggregated granules. Further analysis indicated that the number of cells containing the CK aggregates negatively correlated with the expression levels of MTs. In vitro experiments indicated that MTs could effectively protect CKs against aggregation during refolding, suggesting that MT might function as a chaperone to assist CK re-activation. The findings of the present study provide direct evidence of the connection between the two well-characterized intracellular systems: the precisely balanced energy homoeostasis by CKs and the oxidative-stress response system using MTs.

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.

Hydrogen peroxide targets the cysteine at the active site and irreversibly inactivates creatine kinase

In our study, we showed that at a relatively low concentration, H(2)O(2) can irreversibly inactivate the human brain type of creatine kinase (HBCK) and that HBCK is inactivated in an H(2)O(2) concentration-dependent manner. HBCK is completely inactivated when incubated with 2mM H(2)O(2) for 1h (pH 8.0, 25°C). Inactivation of HBCK is a two-stage process with a fast stage (k(1)=0.050 ± 0.002 min(-1)) and a slow (k(2)=0.022 ± 0.003 min(-1)) stage. HBCK inactivation by H(2)O(2) was affected by pH and therefore we determined the pH profile of HBCK inactivation by H(2)O(2). H(2)O(2)-induced inactivation could not be recovered by reducing agents such as dl-dithiothreitol, N-acetyl-L-cysteine, and l-glutathione reduced. When HBCK was treated with DTNB, an enzyme substrate that reacts specifically with active site cysteines, the enzyme became resistant to H(2)O(2). HBCK binding to Mg(2+)ATP and creatine can also prevent H(2)O(2) inactivation. Intrinsic and 1-anilinonaphthalene-8-sulfonate-binding fluorescence data showed no tertiary structure changes after H(2)O(2) treatment. The thiol group content of H(2)O(2)-treated HBCK was reduced by 13% (approximately 1 thiol group per HBCK dimer, theoretically). For further insight, we performed a simulation of HBCK and H(2)O(2) docking that suggested the CYS283 residue could interact with H(2)O(2). Considering these results and the asymmetrical structure of HBCK, we propose that H(2)O(2) specifically targets the active site cysteine of HBCK to inactivate HBCK, but that substrate-bound HBCK is resistant to H(2)O(2). Our findings suggest the existence of a previously unknown negative form of regulation of HBCK via reactive oxygen species.

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.