Interaction of creatine kinase from monkey brain with substrate: analysis of kinetics and fluorescence polarization

Cold-adapted features of arginine kinase from the deep-sea clam Calyptogena kaikoi

The heterodont clam Calyptogena kaikoi, which inhabits depths exceeding 3,500 m where low ambient temperatures prevail, has an unusual two-domain arginine kinase (AK) with molecular mass of 80 kDa, twice that of typical AKs. The purpose of this work is to investigate the nature of the adaptations of this AK for functioning at low temperatures. Recombinant C. kaikoi AK constructs were expressed, and their two-substrate kinetic constants (k(cat), K(a), and K(ia)) were determined at 10°C and 25°C, respectively. When measured at 25°C, the K(ia) values were tenfold larger than those for corresponding K(a) values, while at 10°C, the K(ia) values decreased remarkably, but the K (a) values were almost unchanged. The Calyptogena two-domain enzyme has threefold higher catalytic efficiency, calculated by k (cat)/(K(a)(ARG)·K(ia)(ATP) ), at 10°C, than that at 25°C, reflecting adaptation for function at reduced ambient temperatures. The activation energy (E(a)) and thermodynamic parameters were determined for Calyptogena two-domain enzyme and compared with those of two-domain enzymes from mesophilic Corbicula and Anthopleura. The value for E(a) of Calyptogena enzyme were about half of those for mesophilic enzymes, and a larger decrease in entropy was observed in Calyptogena AK reaction. Although large decrease in entropy increases the ΔG(o‡) value and consequently lowers the k(cat) value, this is compensated with its lower E(a) value thereby minimizing the reduction in its k(cat) value. These thermodynamic properties, together with the kinetic ones, are also present in the separated domain 2 of the Calyptogena two-domain enzyme.

The role of Arg-96 in Danio rerio creatine kinase in substrate recognition and active center configuration

In creatine kinases (CKs), the amino acid residue-96 is a strictly conserved arginine. This residue is not directly associated with substrate binding, but it is located close to the binding site of the substrate creatine. On the other hand, the residue-96 is known to be involved in expression in the substrate specificity of various other phosphagen (guanidino) kinases, since each enzyme has a specific residue at this position: arginine kinase (Tyr), glycocyamine kinase (Ile), taurocyamine kinase (His) and lombricine kinase (Lys). To gain a greater understanding of the role of residue-96 in CKs, we replaced this residue in zebra fish Danio rerio cytoplasmic CK with other 19 amino acids, and expressed these constructs in Escherichia coli. All the twenty recombinant enzymes, including the wild-type, were obtained as soluble form, and their activities were determined in the forward direction. Compared with the activity of wild-type, the R96K mutant showed significant activity (8.3% to the wild-type), but 10 mutants (R96Y, A, S, E, H, T, F, C, V and N) showed a weak activity (0.056-1.0%). In the remaining mutants (R96Q, G, M, P, L, W, D and I), the activity was less than 0.05%. Our mutagenesis studies indicated that Arg-96 in Danio CK can be substituted for partially by Lys, but other replacements caused remarkable loss of activity. From careful inspection of the crystal structures (transition state analog complex (TSAC) and open state) of Torpedo cytoplasmic CK, we found that the side chain of R96 forms hydrogen bonds with A339 and D340 only in the TSAC structure. Based on the assumption that CKs consist of four dynamic domains (domains 1-3, and fixed domain), the above hydrogen bonds act to link putative domains 1 and 3 in TSAC structure. We suggest that residue-96 in CK and equivalent residues in other phosphagen kinases, which are structurally similar, have dual roles: (1) one involves in distinguishing guanidino substrates, and (2) the other plays a key role in organizing the hydrogen-bond network around residue-96 which offers an appropriate active center for the high catalytic turnover. The mode of development of the network appears to be unique each phosphagen kinase, reflecting evolution of each enzyme.

Evidence that amino-acid residues are responsible for substrate synergism of locust arginine kinase

A series of mutants were constructed to investigate the amino-acid residues responsible for the synergism in substrate binding of arginine kinase (AK). AK contains a pair of highly conserved amino acids (Y75 and P272) that form a hydrogen bond. In the locust (Locusta migratoria manilensis) AK, mutants in two highly conserved sites can cause pronounced loss of activity, conformational changes and distinct substrate synergism alteration. The Y75F and Y75D mutants showed strong synergism (Kd/Km=6.2-13.4), while in single mutants, P272G and P272R, and a double mutant, Y75F/P272G, the synergism was almost completely lost (Kd/Km=1.1-1.4). Another double mutant, Y75D/P272R, had characteristics similar to those of the wild-type enzyme. All these results suggest that the amino-acid residues 75 and 272 play an important role in regulating the synergism in substrate binding of AK. Fluorescence spectra showed that all mutants except Y75D/P272R displayed a red shift to different degrees. All the results provided direct evidence that there is a subtle relationship between the synergism in substrate binding and the conformational change.

Kinetic analysis of two purified forms of arginine kinase: absence of cooperativity in substrate binding of dimeric phosphagen kinase

Arginine kinase from sea urchin eggs and sea cucumber muscle are dimeric enzymes, unlike the more widely distributed monomeric enzyme found in other invertebrates. Both purified enzymes exhibited features characteristic of the monomeric arginine kinases including pH optima, formation of a catalytic dead-end complex (enzyme-MgADP-arginine) and stabilization of this complex by monovalent anions. A complete analysis of initial velocity data, in both directions for each substrate, indicated that substrate binding cooperativity was either minimal or non-existent. Unlike many other multi-subunit enzymes, the significance of the dimeric state of the phosphagen kinases remains unclear. These present results would suggest that (a) cooperativity, or so-called synergism in substrate binding is not a characteristic of the dimeric state of the protein and (b) the functional significance of the dimeric state is not related to the ability of some of these enzymes to undergo cooperativity in substrate binding. The significance of the dimeric state for the creatine kinases and arginine kinases remains to be established.

Immunological and physical comparison of monomeric and dimeric phosphagen kinases: Some evolutionary implications

The antigenic and physical properties of several representative invertebrate phosphagen kinases have been examined in order to further characterize the relationship between taxonomic assignment, quaternary protein structure and evolution of this class of enzymes. Antibodies against dimeric arginine kinase from the sea cucumber cross-reacted with dimeric arginine kinase purified from sea urchin eggs, but failed to react with extracts from any species known to contain monomeric arginine kinase. However, strong immunoreactivity was observed when antibodies against purified dimeric arginine kinase were reacted with pure creatine kinase from the human muscle (CK-MM) and brain (CK-BB) as well as extracts from several species known to contain dimeric creatine kinase. Of particular interest with regard to evolution of the phosphagen kinases, we confirm the presence of creatine kinase activity in the very primitive sponge Tethya aurnatium and detect a reaction with antibodies against dimeric, but not monomeric, arginine kinase. This observation is consistent with recent studies of phosphagen kinase evolution. Substrate utilization was very specific with creatine kinase using only creatine. Arginine kinase catalyzed phosphorylation of arginine but enzymes from several species could also phosphorylate canavanine. No activities were detected with d-arginine. Isoelectric points, evaluated for several pure arginine kinases suggest that generally the monomeric forms are more acidic than the dimeric proteins. Heat inactivation of arginine kinase in several species indicated a wide range of stabilities, which did not appear to be correlated with quaternary structure, but rather distinguished by the organism's environment. On the other hand, homodimeric arginine kinase proteins from species inhabiting disparate environments are sufficiently homologous to form a catalytically active hybrid.

Amino acid residues 62 and 193 play the key role in regulating the synergism of substrate binding in oyster arginine kinase

The purpose of this study is to clarify the amino acid residues responsible for the synergism in substrate binding of arginine kinase (AK), a key enzyme in invertebrate energy metabolism. AKs contain a pair of highly conserved amino acids (D62 and R193) that form an ion pair, and replacement of these residues can cause a pronounced loss of activity. Interestingly, in the oyster Crassostrea AK, these residues are replaced by an N and a K, respectively. Despite this replacement, the enzyme retains high activity and moderate synergism in substrate binding (Kd/Km=2.3). We replaced the N62 by G or D and the K193 by G or R in Crassostrea AK, and also constructed the double mutants of N62G/K193G and N62D/K193R. All of the mutants retained 50-90% of the wild-type activity. In N62G and N62D mutants, the Kmarg for arginine binding was comparable to that of wild-type enzyme, but the Kdarg was increased 2-5-fold, resulting in a strong synergism (Kd/Km=4.9-11.3). On the other hand, in K193G and K193R mutants, the Kmarg was increased 4-fold, and synergism was lost almost completely (Kd/Km=1.0-1.4). The N62G/K193G double mutant showed similar characteristics to the K193G and K193R mutants. Another double mutant, N62D/K193R, similar to the amino acid pair in the wild-type enzyme, had characteristics similar to those of the wild-type enzyme. These results indicate that the amino acid residues 62 and 193 play the key role in mediating the synergism in substrate binding of oyster arginine kinase.

The mechanism and modes of inhibition of arginine kinase from the cockroach (Periplaneta americana)

The kinetic mechanism and evaluation of several potential inhibitors of purified arginine kinase from the cockroach (Periplanta americana) were investigated. This monomeric phosphagen kinase is important in maintaining ATP levels during the rapid energy demands of muscle required for contraction and motility. Analysis reveals the following dissociation constants (mM) for the binary complex: E.Arg P-->E+Arg P, K=1.0; E.Arg-->E+Arg, K=0.45; E.MgATP-->E+MgATP, K=0.17; E.MgADP-->E+MgADP, K=0.12; and the ternary complex: Arg P.E.MgADP-->E.MgADP+Arg P, K=0.94; Arg.E.MgATP-->E.MgATP+Arg, K=0.49; MgATP.Enz.Arg-->E.Arg+MgATP, K=0.14; MgADP.E.Arg P-->E.Arg P+MgADP, K=0.09. For a particular substrate, the ratio of the dissociation constants for the binary to ternary complex is close to one, indicating little, if any, cooperativity in substrate binding for the rapid equilibrium, random addition mechanism. The time course of the arginine kinase reaction exhibits a pronounced curvature, which, as described for enzyme from other sources, is attributed to formation of an inhibitory catalytic dead-end complex, MgADP.E.Arg. The curvature is accentuated by the addition of monovalent anions, including borate, thiocyanate, and, most notably, nitrite and nitrate. This effect is attributed to stabilization of the dead-end complex through formation of a transition state analog. However, the substantial decrease in initial velocity (92%) caused by nitrate is due to an additional inhibitory effect, further characterized as non-competitive inhibition (Ki=8.0 mM) with the substrate L-arginine. On the other hand, borate inhibition of the initial velocity is only 30% with significant subsequent curvature, suggesting that this anion functions as an inhibitor mainly by formation of a transition state analog. However, some component of the borate inhibition appears to be mediated by an apparent partial competitive inhibition with L-arginine. D-arginine is not a substrate for arginine kinase from the cockroach, but is an effective competitive inhibitor with a Ki=0.31 mM. L-Canavanine is a weak substrate for arginine kinase (Km=6.7 mM) with a Vmax for the pure enzyme that is approximately one-third that of L-arginine. However, initial velocity experiments of substrate mixtures suggest that competition between L-canavanine and L-arginine may not be a simple summation effect and may involve a structural modification. Sensitivity of arginine kinase activity to D-arginine as well as nitrate and borate anions, coupled with the fact that L-arginine is an essential amino acid for the cockroach, suggest that arginine kinase could be a useful chemotherapeutic target for the control of cockroach proliferation.

Purification and characterization of arginine kinase from the American cockroach (Periplaneta americana)

The isolation and characterization of homogeneous arginine kinase from the cockroach is reported. The purification protocol produces 6.6 mg of pure enzyme from 6.8 g of whole cockroach. The purified enzyme cross-reacts with a heterologous antibody and monoclonal antibody against arginine kinase from the shrimp. Both antibody preparations also cross-react with extracts from several species known to contain monomeric arginine kinase, but fail to react with extracts from organisms containing dimeric arginine kinase. Cockroach arginine kinase has a molecular mass of approximately 43,000 determined from measurements by gel filtration and gel electrophoresis. Compared with other arginine kinases, the enzyme from the cockroach is relatively thermostable (50% activity retained at 50 degrees C for 10 min) and has a pH optima of 8.5 and 6.5-7.5, for the forward and reverse reactions, respectively. Treatment with 5,5'dithiobis[2-nitrobenzoic acid] indicates that arginine kinase has a single reactive sulfhydryl group and, interestingly, the reaction is biphasic. The Michaelis constants for the phosphagen substrates, arginine: 0.49 mM, phosphoarginine: 0.94 mM, and nucleotide substrates MgATP: 0.14 mM, MgADP: 0.09 mM, are in the range reported for other arginine kinases. A 1% solution of pure enzyme has an absorbance of 7.0 at 280 nm. Calculations based on circular dichroic spectra indicate that arginine kinase from the cockroach has 12% alpha-helical structure. The intrinsic protein fluorescence emission maximum at 340 nm suggests that tryptophan residues are below the surface of the protein and not exposed to solvent. Arginine kinase from the cockroach and shrimp are known to be deleterious immunogens towards humans. The availability of pure protein, its characterization and potential regulation of activity, will be useful in developing agents to control the cockroach population and its destructive role in agriculture and human health.

Application of aqueous two-phase partition to the production of homogeneous preparations of fluorescently labelled human serum albumin

The development of a traceable molecular probe was investigated for the monitoring of partition behaviour of biomolecules in aqueous two-phase systems. This work was based upon the selective labelling of the free thiol group of human serum albumin (i.e. Cys34) with the fluorophore N-(iodoacetylaminoethyl)-5-naphthylamine-1-sulphonic acid. The preparation of homogeneously labelled protein required purification operations. A succession of five processes was successfully applied, comprising two size-exclusion chromatographic operations by gel filtration and a series of three appropriately manipulated aqueous two-phase systems comprising PEG 1450 and phosphate salt. Aqueous two-phase partitioning is herein presented as an alternative to difficult separation and could be applied for 'fine' purifications.

Subunit conformation and dynamics in a heterodimeric protein: studies of the hybrid isozyme of creatine kinase

Several physical properties of creatine kinase (EC 2.7.3.2) isozymes MM (CK-MM, muscle-type) and BB (CK-BB, brain-type), both homodimers, and isozyme MB (CK-MB), a heterodimer, were compared to determine how formation of the hybrid modifies subunit conformation and dynamics. Circular dichroic spectra revealed additional alpha-helical content for the hybrid isozyme. Double-beam absorption difference spectra between CK-MB and a stoichiometric mixture of CK-MM and CK-BB revealed decreased exposure of intrinsic chromophores in the hybrid. The relative intensity of the intrinsic fluorescence of CK-MB was between the two homodimers, but was 16% closer to the less fluorescent CK-MM. Steady state anisotropy spectra and decay of the anisotropy of CK derivatized on a single subunit with the fluorescent sulfhydryl reagent 5-[2-(iodoacetyl)amino-ethyl]aminonaphthalene-1-sulfonate indicated that the derivatized sites are more flexible in the heterodimer. The slow component in the anisotropy decay suggests that hybridization results in a small increase in the packing density or contraction of overall conformation of the B-subunit. The KM for MgATP with singly derivatized CK-MB was the same as the KM for the native enzyme. However, derivatization of a single subunit caused the Vmax to decrease by greater than 50%, which indicates that subunit-subunit interactions may modulate the activity of CK. A model for assembly of CK-MB is proposed which includes subunit characteristics more similar to those found in the muscle-type homodimer than in the brain-type homodimer and increased flexibility of the active site domain of both subunits.

A physicochemical comparison of the isozymes of creatine kinase from rabbit brain and muscle

A comparison of specific structural features of creatine kinase from rabbit muscle and brain was undertaken to determine if the observed isozyme specific differences in catalytic cooperativity are related to conformational differences, particularly differences in packing density. The intrinsic fluorescence of the brain isozyme is 2-fold higher than the muscle isozyme. In the denatured state, both proteins display the characteristic red shift in emission maximum; however, the emission intensity of the brain isozyme increases only 5% upon denaturation compared to nearly 100% increase for the muscle protein. The fluorescence lifetimes are 2.65 ns (67%) and 0.48 ns for native muscle enzyme and 4.38 ns (65%) and 0.80 ns for brain enzyme. Upon denaturation, the lifetimes are 3.98 ns (77%) and 0.99 ns for muscle protein and 3.82 ns (79%) and 0.86 ns for brain protein. Stern-Volmer plots of quenching by acrylamide are essentially the same for both native isozymes indicating that the differences of the intrinsic fluorescence of the native proteins are not due to differences in solvent accessibility. The spectral and lifetime differences in the isozymes in the native state and changes accompanying denaturation are consistent with the occurrence of energy transfer in native muscle isozyme. The rotational correlation times of 5-[2-(iodoacetyl)aminoethyl]aminonaphthalene-1-sulfonate conjugated proteins, derivatized at the active site reactive thiol, are best described by two term decay laws. The slower rotations, 45.1 ns (75%) and 40.6 ns (71%) reflect overall macromolecular rotation for the muscle and brain isozymes, respectively. The faster motions, 2.4 ns for muscle isozyme and 0.4 ns for the brain isozyme, are attributed to the probe or probe associated segmental motions and indicate these motions are more restricted in the muscle protein. Reactivity of creatine kinase (2.5-10 microM) with the amino-specific reagent trinitrobenzene sulfonate (0.4-2 mM) was analyzed by pseudo-first-order and second order models, neither of which was adequate for the entire range of data. However, in every case, the rate constants were faster for brain creatine kinase but the extent of reaction was greater for muscle creatine kinase. The faster initial reactivity of the brain isozyme is consistent with greater accessibility for lysine derivatization.(ABSTRACT TRUNCATED AT 400 WORDS)

Resonance energy transfer between the active sites of creatine kinase from rabbit brain

Resonance energy transfer was measured between the active site domains of the brain isozyme of creatine kinase (CK-BB). The reactive thiol near the active sites, one on each subunit of the dimeric protein, was derivatized using 5-[2-[iodoacetyl)amino)ethyl]aminonaphthalene-1-sulfonic acid (AED), 2-[4'-iodoacetamidoanilino]naphthalene-6-sulfonic acid (AANS) and 5-iodoacetamidofluorescein (AF). Suitable donor/acceptor protein conjugated hybrids were prepared by controlled kinetics producing CK-BB-AED/AF and CK-BB-AANS/AF. Transfer efficiencies, measured from the quenching of the donor lifetime and steady-state sensitized acceptor emission, ranged from 0.10 to 0.17. From determination of the donor/acceptor overlap integrals, donor quantum yields and attempts to delimit the orientation factor using steady-state and phase-resolved anisotropy measurements, it was found that a suitable estimate of the range between the active sites was between 45 and 57 A. This range is similar to that reported previously for the muscle isozyme of creatine kinase (Grossman, S.H. (1989) Biochemistry 28, 4894-4902) but is a significantly greater distance than detected for the hybrid, myocardial specific isozyme (Grossman, S.H. (1983) Biochemistry 22, 5369-5375).

Monoclonal antibody studies suggest a catalytic site at the interface between domains in creatine kinase

We have located the epitopes recognized by four different monoclonal antibodies which bind to partially unfolded creatine kinase (CK) (ATP: creatine N-phosphotransferase, EC 2.7.3.2) but not to the native enzyme. The epitopes appear to be buried within the CK structure in its native, proteinase-resistant, state. When the epitopes are made accessible to antibody by mild denaturation, CK becomes enzymically-inactive and can be cleaved by proteinase V8 into two large fragments which retain the epitopes and may represent domains. Epitopes on each V8 fragment are associated with highly conserved sequences and are brought physically close to the active site of the enzyme during the later stages of CK refolding and reactivation. The results suggest a catalytic site formed at the interface between two domains which carry the epitopes on their interacting surfaces. Separation of loosely associated domains before or during immunization may account for the origin of antibodies against buried epitopes.

Resonance energy transfer between the active sites of rabbit muscle creatine kinase: analysis by steady-state and time-resolved fluorescence

Resonance energy transfer between the reactive thiols of rabbit muscle creatine kinase was evaluated. The reactive thiols are located at the active site, one occurring on each subunit of the dimeric protein that is known to be a constituent of the M-line structure of the myofibril. Transfer efficiency was evaluated from energy donor quenching of fluorescence by steady-state and phase-modulation lifetime measurements and determination of sensitized emission of the acceptor. Several sulfhydryl-specific donor fluorophores were used including 5-[[[(iodoacetyl)amino]ethyl]amino]naphthalene-1-sulfonic acid, 7-(dimethylamino)-3-(4-maleimidylphenyl)-4-methylcoumarin, and 2-[4-(iodoacetamido)anilino]naphthalene-6-sulfonic acid (IAANS). Energy transfer acceptors included 5-(iodoacetamido)fluorescein and the nonfluorescent dye [4-[[4-(dimethylamino)phenyl]azo]phenyl]iodoacetamide. In order to prepare the necessary homodimer labeled with both donor and acceptor, advantage was taken of the biphasic reaction between creatine kinase and IAANS. In some instances, donor/acceptor hybrids were prepared by denaturation/renaturation procedures, and possible deviations from expected hybridization stoichiometry were considered. Disproportionation of singly labeled dimers (to unlabeled and doubly labeled dimers) was not observed when the brain isozyme of creatine kinase was used to trap dissociated dye-conjugated or unlabeled muscle-type subunits of creatine kinase. From studies of five different donor/acceptor combinations, the efficiency of energy transfer was found to occur over a range of 5-14%, indicating that the reactive thiols are well separated. Overlap integrals and quantum yields were evaluated, and estimates of the range of orientation factor were obtained to determine a range for the distance between the active sites of creatine kinase. When the ranges are overlapped, a limited distance of 48.6-60.4 A is obtained.(ABSTRACT TRUNCATED AT 250 WORDS)

Monoclonal antibody studies of creatine kinase. Antibody-binding sites in the N-terminal region of creatine kinase and effects of antibody on enzyme refolding

(1) The binding sites of two monoclonal antibodies, CK-2A7 and CK-5H5, have been located to a 60-amino-acid sequence in the N-terminal region of creatine kinase (CK) by the use of chemical cleavage with formic acid (which cleaves proteins at Asp-Pro bonds) and cyanogen bromide (which cleaves at Met residues). (2) A simple method for preparing chemically-cleaved fragments of proteins for electrophoresis and Western blotting is described. (3) Binding studies with CK preparations from different animal species show that single amino acid changes at residues 39 or 82 prevent binding of CK-2A7 and CK-5H5 respectively. We suggest that Lys-39 and Glu-82 form parts of the binding sites on CK for the two monoclonal antibodies. The two sites lie in variable regions at each end of a highly-conserved sequence (residues 46 to 79) and are inaccessible to antibody in the native enzyme. (4) One of the antibodies, CK-2A7, inhibits the refolding of CK to native enzyme after denaturation by urea.

Interaction of brain-type creatine kinase with its transition state analog: kinetics of inhibition and conformational changes

The effects of components of the transition state analog (creatine, MgADP, planar anion) on the kinetics and conformation of creatine kinase isozyme BB from monkey brain was studied. From analysis of the reaction time course using the pH stat assay, it was shown that during accumulation of the reaction products (ADP and creatine phosphate), among several anions added, nitrate proved the most effective in inhibiting catalytic activity. Maximum inhibition (77%) was achieved with 50 mM nitrate. The Km for ATP was 0.48 mM and in the presence of 2.5 mM nitrate, 2.2 mM; for ATP in the presence of the dead-end complex, creatine and ADP, the apparent Km was 2.0 mM and the Ki was 0.16 mM; in the presence of the transition state analog, MgADP + NO3- + creatine, the Ki was estimated to be 0.04 mM. Ultraviolet difference spectra of creatine kinase revealed significant differences only in the presence of the complete mixture of the components of the transition state analog. Comparison of gel filtration elution profiles for creatine kinase in the absence and presence of the complete mixture of components of the transition state analog did not reveal any differences in elution volume. Addition of components of the transition state analog to creatine kinase resulted in only a marginal change in intrinsic fluorescence. The presence of the components of the transition state analog increased the rate of reactivity of the enzyme with trinitrobenzenesulfonic acid from k = 6.06 +/- 0.05 M-1 min-1 to 6.96 +/- 0.11 M-1 min-1.(ABSTRACT TRUNCATED AT 250 WORDS)

An analysis of the reassembly of denatured creatine kinase from monkey brain

Creatine kinase isolated from monkey brain was characterized with respect to denaturation/inactivation and renaturation/reactivation/reassociation in order to determine the mechanism of reassembly. Enzyme unfolded in 8 M urea exhibits several characteristics of denatured protein: complete loss of enzymatic activity, decrease in intrinsic fluorescence with a red shift in the emission maximum and loss of circular dichroism at 220 nm. The renatured protein reassembles to its apparently native condition as judged by these criteria, but small differences of uncertain origin persist. Dependence of activity and fluorescence on denaturant concentration indicate that inactivation is more sensitive to urea than is unfolding; spectral changes at the intermediate urea concentrations suggest formation of aggregated protein. Upon dilution, enzyme previously exposed to 8 M urea for 40 min regains 70-80% native activity, independent of protein concentration over the range of 0.56-160 nM. Reactivation kinetics, measured using the assay mixture with and without trypsin, are independent of protein concentration, and are adequately described by a single rate constant, 3.2 X 10(-3) s-1 and 4.2 X 10(-3) s-1, respectively. Reactivation is completed 20-30 min after initiation of renaturation. Fluorescence changes during refolding are at least biphasic, exhibiting a rapid increase, then a slow decrease completed at approximately 15-20 min after initiating refolding. Reassociation is measured by competitive hybridization between dimerizing B subunits and M subunits to form MB heterodimer. The time dependent decay in heterodimer formation during competitive dimerization shows that reassociation is completed between 60 and 90 min after initiation of reassembly. These results indicate that the brain isozyme of creatine kinase, like the muscle form, is composed of subunits which do not require association for expression of catalytic activity. Furthermore, a comparison of spectral data and susceptibility to trypsin inactivation between the muscle and brain isozymes supports previous suggestions that in the native state, the brain isozyme is a conformationally looser, more open protein.

Purification and characterization of creatine kinase isozymes from the nurse shark Ginglymostoma cirratum

Creatine kinase from nurse shark brain and muscle has been purified to apparent homogeneity. In contrast to creatine kinases from most other vertebrate species, the muscle isozyme and the brain isozyme from nurse shark migrate closely in electrophoresis and, unusually, the muscle isozyme is anodal to the brain isozyme. The isoelectric points are 5.3 and 6.2 for the muscle and brain isozymes, respectively. The purified brain preparation also contains a second active protein with pI 6.0. The amino acid content of the muscle isozyme is compared with other isozymes of creatine kinase using the Metzger Difference Index as an estimation of compositional relatedness. All comparisons show a high degree of compositional similarity including arginine kinase from lobster muscle. The muscle isozyme is marginally more resistant to temperature inactivation than the brain isozyme; the muscle protein does not exhibit unusual stability towards high concentrations of urea. Kinetic analysis of the muscle isozyme reveals Michaelis constants of 1.6 mM MgATP, 12 mM creatine, 1.2 mM MgADP and 50 mM creatine phosphate. Dissociation constants for the same substrate from the binary and ternary enzyme-substrate complex do not differ significantly, indicating limited cooperatively in substrate binding. Enzyme activity is inhibited by small planar anions, most severely by nitrate. Shark muscle creatine kinase hybridizes in vitro with rabbit muscle or monkey brain creatine kinase; shark brain isozyme hybridizes with monkey brain or rabbit brain creatine kinase. Shark muscle and shark brain isozymes, under a wide range of conditions, failed to produce a detectable hybrid.

Further characterization of the reassembly of creatine kinase and effect of substrate

Upon exposure to 8 M urea, creatine kinase from rabbit muscle exhibited a rapid increase in intrinsic fluorescence and a rapid decrease in fluorescence polarization. Polarization changes were complete after 5 min, while fluorescence changes continued for at least 15 min. Fluorescence polarization changes accompanying reassembly were complex, and appeared to involve a concentration dependent reaction. Enzyme sampled at intervals during denaturation exhibited refolding kinetics displaying two first-order rate constants, the first dependent and the second independent of the duration of exposure to urea. There was evidence for an additional renaturation step, occurring within the mixing phase of the denatured protein with solvent. Reactivation kinetics and yield of reactivated enzyme exhibited a dependency upon length of exposure to denaturant. The exposure of renaturing creatine kinase to trypsin was shown to prevent further reactivation, and provided use of a method to determine reactivation rates at discrete intervals after initiation of reassembly. The presence of 2 mM MgADP during reactivation enhanced the rate of reactivation immediately after initiation of reactivation. Reactivation was not accelerated if nucleotide substrate was added after reactivation was initiated nor did nucleotide substrate increase the overall reactivation yield. The presence of MgADP also enhanced the rate of refolding at an early stage as judged by changes in intrinsic fluorescence and resistance to tryptic hydrolysis. While in addition to MgADP, creatine phosphate accelerated resistance by refolding creatine kinase to trypsin, according to the other criteria measured, the phosphagen substrates did not promote reactivation or renaturation. The unfolding-refolding studies and role of substrate in reassembly were consistent with a mechanism involving at least two steps, possibly involving cis-trans isomerization of proline. These data also supported the suggestion that the formation of the nucleotide binding region is an early event in the refolding of creatine kinase in vitro.

Creatine kinase protein sequence encoded by a cDNA made from Torpedo californica electric organ mRNA

Creatine kinase (ATP creatine N-phosphotransferase, EC 2.7.3.2) is important in the maintenance of ATP levels in high energy-requiring tissues such as muscle and brain. A complete understanding of its function requires knowledge of its amino acid sequence. To obtain cDNA clones encoding creatine kinase sequences, a cDNA bank was constructed using mRNA from the electric organ of Torpedo californica and was screened by comparing differential colony hybridization of electric organ and liver-derived 32P-labeled cDNAs. Cloned DNAs have been isolated that can arrest the abundant synthesis of Mr 40,000-43,000 material seen after in vitro translation of electric organ mRNA. One of the clones, CK52g8, was sequenced by the dideoxy M13 method and was found to encode a Mr 42,941 protein, which is 68% homologous to a known partial sequence of rabbit muscle creatine kinase and which has a composition similar to creatine kinases from chicken and rabbit tissues. By contrast, no significant homology was found with the known sequences of kinases that use other substrates. RNA blot hybridization analysis indicated that CK52g8 is complementary to a 1600-base-pair mRNA. Primer extension analysis indicated that CK52g8 is only 5 nucleotides short of a full-length cDNA, implying that it encodes a complete protein sequence. The availability of this complete sequence should be useful in further studies of creatine kinase structure and function using techniques such as site-specific mutagenesis.