Is the subunit the minimal function unit of creatine kinase?

Mitochondrial Proteolipid Complexes of Creatine Kinase

Isoforms of creatine kinase (CK) generate and use phosphocreatine, a concentrated and highly diffusible cellular "high energy" intermediate, for the main purpose of energy buffering and transfer in order to maintain cellular energy homeostasis. The mitochondrial CK isoform (mtCK) localizes to the mitochondrial intermembrane and cristae space, where it assembles into peripherally membrane-bound, large cuboidal homooctamers. These are part of proteolipid complexes wherein mtCK directly interacts with cardiolipin and other anionic phospholipids, as well as with the VDAC channel in the outer membrane. This leads to a stabilization and cross-linking of inner and outer mitochondrial membrane, forming so-called contact sites. Also the adenine nucleotide translocator of the inner membrane can be recruited into these proteolipid complexes, probably mediated by cardiolipin. The complexes have functions mainly in energy transfer to the cytosol and stimulation of oxidative phosphorylation, but also in restraining formation of reactive oxygen species and apoptosis. In vitro evidence indicates a putative role of mtCK in mitochondrial phospholipid distribution, and most recently a role in thermogenesis has been proposed. This review summarizes the essential structural and functional data of these mtCK complexes and describes in more detail the more recent advances in phospholipid interaction, thermogenesis, cancer and evolution of mtCK.

Structural basis for the mechanism and substrate specificity of glycocyamine kinase, a phosphagen kinase family member

Glycocyamine kinase (GK), a member of the phosphagen kinase family, catalyzes the Mg(2+)-dependent reversible phosphoryl group transfer of the N-phosphoryl group of phosphoglycocyamine to ADP to yield glycocyamine and ATP. This reaction helps to maintain the energy homeostasis of the cell in some multicelullar organisms that encounter high and variable energy turnover. GK from the marine worm Namalycastis sp. is heterodimeric, with two homologous polypeptide chains, alpha and beta, derived from a common pre-mRNA by mutually exclusive N-terminal alternative exons. The N-terminal exon of GKbeta encodes a peptide that is different in sequence and is 16 amino acids longer than that encoded by the N-terminal exon of GKalpha. The crystal structures of recombinant GKalphabeta and GKbetabeta from Namalycastis sp. were determined at 2.6 and 2.4 A resolution, respectively. In addition, the structure of the GKbetabeta was determined at 2.3 A resolution in complex with a transition state analogue, Mg(2+)-ADP-NO(3)(-)-glycocyamine. Consistent with the sequence homology, the GK subunits adopt the same overall fold as that of other phosphagen kinases of known structure (the homodimeric creatine kinase (CK) and the monomeric arginine kinase (AK)). As with CK, the GK N-termini mediate the dimer interface. In both heterodimeric and homodimeric GK forms, the conformations of the two N-termini are asymmetric, and the asymmetry is different than that reported previously for the homodimeric CKs from several organisms. The entire polypeptide chains of GKalphabeta are structurally defined, and the longer N-terminus of the beta subunit is anchored at the dimer interface. In GKbetabeta the 24 N-terminal residues of one subunit and 11 N-terminal residues of the second subunit are disordered. This observation is consistent with a proposal that the GKalphabeta amino acids involved in the interface formation were optimized once a heterodimer emerged as the physiological form of the enzyme. As a consequence, the homodimer interface (either solely alpha or solely beta chains) has been corrupted. In the unbound state, GK exhibits an open conformation analogous to that observed with ligand-free CK or AK. Upon binding the transition state analogue, both subunits of GK undergo the same closure motion that clasps the transition state analogue, in contrast to the transition state analogue complexes of CK, where the corresponding transition state analogue occupies only one subunit, which undergoes domain closure. The active site environments of the GK, CK, and AK at the bound states reveal the structural determinants of substrate specificity. Despite the equivalent binding in both active sites of the GK dimer, the conformational asymmetry of the N-termini is retained. Thus, the coupling between the structural asymmetry and negative cooperativity previously proposed for CK is not supported in the case of GK.

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

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

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

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

Two fused proteins combining Stichopus japonicus arginine kinase and rabbit muscle creatine kinase

Two fused proteins of dimeric arginine kinase (AK) from sea cucumber and dimeric creatine kinase (CK) from rabbit muscle, named AK-CK and CK-AK, were obtained through the expression of fused AK and CK genes. Both AK-CK and CK-AK had about 50% AK activity and about 2-fold K(m) values for arginine of native AK, as well as about 50% CK activity and about 2-fold K(m) values for creatine of native CK. This indicated that both AK and CK moieties are fully active in the two fused proteins. The structures of AK, CK, AK-CK, and CK-AK were compared by collecting data of far-UV circular dichroism, intrinsic fluorescence, 1-anilinonaphthalene-8-sulfonate binding fluorescence, and size-exclusion chromatography. The results indicated that dimeric AK and CK differed in the maximum emission wavelength, the exposure extent of hydrophobic surfaces, and molecular size, though they have a close evolutionary relationship. The structure and thermodynamic stability of AK, CK, AK-CK, and CK-AK were compared by guanidine hydrochloride (GdnHCl) titration. Dimeric AK was more dependent on the cooperation of two subunits than CK according to the analysis of residual AK or CK activity with GdnHCl concentration increase. Additionally, AK and CK had different denaturation curves induced by GdnHCl, but almost the same thermodynamic stability. The two fused proteins, AK-CK and CK-AK, had similar secondary structure, tertiary structure, molecular size, structure, and thermodynamic stability, which indicated that the expression order of AK and CK genes might have little effect on the characteristics of the fused proteins and might further verify the close relationship of dimeric AK and CK.

Relating structure to mechanism in creatine kinase

Found in all vertebrates, creatine kinase catalyzes the reversible reaction of creatine and ATP forming phosphocreatine and ADP. Phosphocreatine may be viewed as a reservoir of "high-energy phosphate" which is able to supply ATP, the primary energy source in bioenergetics, on demand. Consequently, creatine kinase plays a significant role in energy homeostasis of cells with intermittently high energy requirements. The enzyme is of clinical importance and its levels are routinely used as an indicator of myocardial and skeletal muscle disorders and for the diagnosis of acute myocardial infarction. First identified in 1928, the enzyme has undergone intensive investigation for over 75 years. There are four major isozymes, two cytosolic and two mitochondrial, which form dimers and octamers, respectively. Depending on the pH, the enzyme operates by a random or an ordered bimolecular mechanism, with the equilibrium lying towards phosphocreatine production. Evidence suggests that conversion of creatine to phosphocreatine occurs via the in-line transfer of a phosphoryl group from ATP. A recent X-ray structure of creatine kinase bound to a transition state analog complex confirmed many of the predictions based on kinetic, spectroscopic, and mutagenesis studies. This review summarizes and correlates the more significant mechanistic and structural studies on creatine kinase.

The tryptophane residues of dimeric arginine kinase: roles of Trp-208 and Trp-218 in active site and conformation stability

Roles of the two tryptophane residues of dimeric arginine kinase (AK) were individually investigated by site-directed mutagenesis. Both residues were fully conserved in the phosphogen kinase family and the mutant proteins were analyzed by enzyme kinetics, fluorescence spectroscopy, fluorescence quenching experiments, thermal stability and conformational stability. Our studies revealed that Trp-218 was located at the active site of AK and was the major fluorescence contributor (96.9%). Single replacement of this residue by alanine led to almost complete inactivation of the enzyme. In addition, a decrease in the melting temperature in differential scanning calorimetry (DSC) profiles and the equilibrium studies in guanidine hydrochloride (GdnHCl) denaturation after mutagenesis also suggested that Trp-218 takes part in stabilizing the conformational structure of AK. Although another tryptophane, Trp-208 was not located at the active sites, it may take part in maintaining the correct dimer conformation for catalysis. Replacement of this tryptophane by alanine decreased the activity to 70.3% and made it susceptible to heat and denaturants, such as GdnHCl. In addition, Trp-208 also seemed to play an important role in correct protein folding.

Evidence for proximal cysteine and lysine residues at or near the ative site of arginine kinase of Stichopus japonicus

Inactivation of arginine kinase (AK) of Stichopus japonicus by o-phthalaldehyde (OPTA) was investigated. The modified enzyme showed an absorption peak at 337 nm and a fluorescent emission peak at 410 nm, which are characteristic of an isoindole derivative formed by OPTA binding to a thiol and an amine group in proximity within the enzyme. Loss of enzymatic activity was concomitant with an increase in fluorescence intensity at 410 nm. Stoichiometry studies by Tsou's method showed that among the cysteine residues available for OPTA modification in the enzyme, only one was essential for the enzyme activity. This cysteine residue is located in a highly hydrophobic environment, presumably near ATP and ADP binding region. This conclusion was verified by 5,5 -dithiobis(2-nitrobenzoic acid) modification. In addition, these results were supported by means of electrophoresis and ultraviolet, fluorescence, circular dichroism spectroscopy and fast performance liquid chromatography. Sequence comparison suggested that this essential cysteine residue maybe the conservative Cys274.

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.

Inactivation of rabbit muscle creatine kinase by reversible formation of an internal disulfide bond induced by the fungal toxin gliotoxin

The biological activity of gliotoxin is dependent on the presence of a strained disulfide bond that can react with accessible cysteine residues on proteins. Rabbit muscle creatine kinase contains 4 cysteines per 42-kDa subunit and is active in solution as a dimer. Only Cys-282 has been identified as essential for activity. Modification of this residue results in loss of activity of the enzyme. Treatment of creatine kinase with gliotoxin resulted in a time-dependent loss of activity abrogated in the presence of reducing agents. Activity was restored when the inactivated enzyme was treated with reducing agents. Inactivation of creatine kinase by gliotoxin was accompanied by the formation of a 37-kDa form of the enzyme. This oxidized form of creatine kinase was rapidly reconverted to the 42-kDa species by the addition of reducing agents concomitant with restoration of activity. A 1:1 mixture of the oxidized and reduced monomer forms of creatine kinase as shown on polyacrylamide gel electrophoresis was equivalent to the activity of the fully reduced form of the enzyme consistent with only one reduced monomer of the dimer necessary for complete activity. Conversion of the second monomeric species of the dimer to the oxidized form by gliotoxin correlated with loss of activity. Our data are consistent with gliotoxin inducing the formation of an internal disulfide bond in creatine kinase by initially binding and possibly activating a cysteine residue on the protein, followed by reaction with a second neighboring thiol. The recently published crystal structure of creatine kinase suggests the disulfide is formed between Cys-282 and Cys-73.

Why is creatine kinase a dimer? Evidence for cooperativity between the two subunits

The dimeric chicken brain type isoenzyme of creatine kinase (BB-CK) was mutated by a C283S amino acid exchange in the catalytic site to produce a basically inactive dimer (B*B*-CK). The mutated enzyme showed a residual activity of about 4% compared to the wild-type, whereas substrate binding parameters were not altered. The inactivated dimer was hybridized with native dimeric muscle enzyme (MM-CK) to produce a partially inactivated MB*-CK heterodimeric hybrid and also to a his-tagged BB-CK (hBhB-CK) resulting in a partially inactive hBB*-CK homodimer. The generated hybrids were purified by chromatography. The V(max) and substrate binding parameters K(m) and K(d) were determined for both directions of the CK reaction and compared to the parameters of the wild-type enzymes (MM-, BB-, hBhB-, MB-CK). In the direction of ATP synthesis (reverse reaction), the MB*- and hBB*-CK hybrids showed a decrease of V(max) to 34% and 32%, respectively, compared to the unmodified wild-type isoform. The inactivation of a single subunit in MB*-CK led to an increase in the K(d) value resulting in an significant substrate synergism, not seen with the MB-CK wild-type enzyme. In the direction of phosphocreatine synthesis (forward reaction), the modified hybrids showed a decrease of V(max) to 50% of the wild-type enzymes and no significant alterations of the K(m) and K(d) parameters. These results strongly suggest an enzymatic cooperativity of the two subunits in the reverse reaction but independent catalytic function in the forward reaction.

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.

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.

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.

Nucleotide binding sites in wild-type creatine kinase and in W227Y mutant probed by photochemical release of nucleotides and infrared difference spectroscopy

Structural changes induced by nucleotide binding to the wild-type rabbit muscle creatine kinase (CK) and to its W227Y mutant were compared and probed by reaction-induced difference spectroscopy (RIDS). The reaction was induced by the photorelease of nucleotide from the caged nucleotides ADP[Et(PhNO2)] or ATP[Et(PhNO2)], producing the RIDS of CK. The concomitant addition of a saturated concentration of nucleotide and caged nucleotide modified the RIDS of CK, permitting structural changes caused by nucleotide binding in the wild-type creatine kinase to be identified. The W227Y mutant was inactive and its nucleotide binding site was partially impaired as shown by the disappearance or decrease of several nucleotide-sensitive bands in the RIDS of W227Y mutant. The magnitude of the decrease was not the same for each band, suggesting that distinct groups of W227Y mutant were affected differently during nucleotide binding. More precisely, the binding sites for gamma-phosphate and beta-phosphate of the nucleotide were not accessible in W227Y mutant as shown by the absence of the phosphate-sensitive 1666-1667-cm(-1) and 1625-cm(-1) bands in the RIDS of W227Y mutant. However the binding site of other parts of the nucleotide was partially accessible, since the 1638-1639-cm(-1) phosphate-insensitive band did not completely vanish in the RIDS of W227Y mutant. The RIDS of W227Y mutant with ADP[Et(PhNO2)] and creatine lacked the 1613-cm(-1) and 1581-cm(-1) bands, associated with vibrational modes of creatine, suggesting that coupling between the binding sites of the nucleotide and of creatine was altered in W227Y mutant. These results are in accordance with the earlier suggestions that residue W227 in CK is essential for preventing water molecules from penetrating into the active site and for orienting nucleotide in the binding site, by forming stacking interactions between its indole group and purine of the nucleotide and its indole group.

Minimal functional unit of lactate dehydrogenase

The tetrameric heart isozyme of lactate dehydrogenase (H4) is modified by p-chloromercuribenzoate (PCMB) to produce the inactive tetramer (H4) and then hybridized with native tetrameric muscle isozyme (M4). The hybrid mixture (M4, H'M3, H2'M2, H3'M, and H4') was isolated by polyacrylamide gel electrophoresis (PAGE) and then stained for enzyme activity and with Coomassie brilliant blue. Only three bands were found on the gels in either case. The hybrid enzymes (H'M3 and H2'M2) as isolated by PAGE have half the specific activity of the native muscle enzyme. The electrophoresis properties of H'M3 are very similar to those of HM3, while the electrophoresis properties of H2'M2 are very similar to those of H2M2. The above results strongly suggest that the tetramer having enzymatic activity contains at least two native subunits, and the di-subunit in the tetrameric enzyme is the minimal functional unit.

Changes of creatine kinase secondary structure induced by the release of nucleotides from caged compounds. An infrared difference-spectroscopy study

Light-induced release of ADP and ATP from their respective caged nucleotides produced small distinct difference infrared spectra of creatine kinase (CK), indicating that ADP and ATP binding to CK promoted different structural alteration. The positive band at 1638-1640 cm-1 and the negative band at about 1650-1652 cm-1 on the reaction-induced infrared difference spectra in the amide I region were insensitive to the deuteration effects. They were assigned to the peptide backbone of the ADP/ATP-binding site. In addition Pi or ATP binding produced another positive band at 1657-1659 cm-1 corresponding to the C = O (amide I band) associated with the gamma-phosphate of ATP. This site was also affected when ADP was added, indicating coupling interactions between both sites. No additional structural changes were observed when creatine and ADP were added, suggesting that the creatine-binding site was uncoupled from the ADP-binding site. The infrared difference spectra of a transition-state-analog complex formed by the addition of ADP, creatine and NO3- (a planar-phosphate-mimicking group) lacked the 1657-1659-cm-1 band indicating that the binding site of gamma-phosphate within CK, was not affected. Infrared changes in the 1560-1590-cm-1 region suggested that carboxylate groups of Asp or Glu were involved in the binding of Pi, ADP and ATP.

Dissociation, unfolding and inactivation of creatine kinase in urea solutions

It was previously reported that, during unfolding of creatine kinase in guanidinium chloride or urea solutions, inactivation occurred before noticeable conformational change could be detected, suggesting that the conformation at the active site is more easily perturbed and, hence, more flexible than the molecule as a whole [Tsou (1986) Trends Biochem. Sci 11, 427-429]. In the present paper, the ureagradient electrophoresis and the isoenzyme hybrid of creatine kinase has been studied. The results show that at low urea concentrations, creatine kinase is still in the dimeric state or only slightly dissociated. The dissociation and inactivation of creatine kinase during denaturation by urea are also compared. It was found that the enzyme was nearly inactivated in low urea concentrations before noticeable dissociation was detected. It therefore appears that in low urea concentrations, inactivation of creatine kinase is not due to the dissociation of the active dimer. The present result supports the hypothesis of the conformational flexibility of the active site in this enzyme.

An equilibrium study of the dependence of secondary and tertiary structure of creatine kinase on subunit association

Several physical properties of dimeric creatine kinase in increasing concentrations of guanidine hydrochloride (GDN/HCl) were evaluated and correlated with degree of subunit dissociation, determined by isozyme competitive hybridization. Three distinct stages were observed that correlated with phases before, during and after dissociation. In 0.2 M GDN/HCl, before significant dimer dissociation, creatinine kinase has 75% of its original activity, and exhibits only small decreases in circular dichroism, intrinsic fluorescence and emission maximum. The spectral characteristics of creatine kinase (CK) derivatized with 5-[(((2-iodoacetyl)amino)ethyl)amino]naphthalene-1- sulfonate (AEDANS-CK) at the cysteine near the active site are suggestive of a slightly more non-polar environment. A decrease in steady-state anisotropy (0.140 to 0.132) was characterized by time-resolved methods. The slow component of the time-resolved anisotropy decay law, which reflects global protein rotation, is decreased only from 36.6 to 33.4 ns. The faster component decreases from 1.95 to 0.74 ns which suggests the active-site domain is more sensitive to conformation perturbation than the protein as a whole. Overall these observations suggest the subunits within the dimeric state are rather stable in dilute denaturant, but undergo a minor contraction in conformation. The region of the active site, as reported by the extrinsic fluorophore, is less polar but apparently more flexible in dilute denaturant. Between 0.5 M and 1 M GDN/HCl, most of the dimers dissociate, 63% of helical content is lost and inactivation is complete. The intrinsic fluorescence shifts 8 nm to the red and increases by 35%, indicating exposure of tryptophans to solvent and release of quenching, perhaps between residues on separate subunits. Over the same denaturant range, the spectral characteristics and lifetime of AEDANS-CK suggests less exposure of the active site to solvent. Time-resolved anisotropy measurements show that the sharp decrease in steady-state anisotropy to 0.086 is due to a decrease in macromolecular rotation to 22 ns. This may represent the rotational correlation time of a relatively intact subunit, and suggests limited subunit unfolding accompanying dissociation. Dissociation is complete in 1.5 M GDN/HCl. The subunits still retain 20% helical content in 2 M denaturant and not until 5 M GDN/HCl is all helical structure eliminated. Above 2 M GDN/HCl, AEDANS-CK exhibits sharp decreases in steady-state anisotropy, fluorescence lifetime and the long-lived component in the time-resolved anisotropy decay law. These results reveal a catastrophic loss of tertiary structure by the subunits and may define the physical properties of the random coil.

Heterogeneous flexibilities of the active site domains of homodimeric creatine kinase: effect of substrate

(1) A single subunit and both subunits of creatine kinase from rabbit muscle was derivatized at the active site with the thiol-specific reagent 2-(4'-(iodoacetamido)anilino)-naphthalene-6-sulfonic acid. (2) The highly biphasic kinetics of the labelling reaction were characterized from measurements of activity, steady-state fluorescence and anisotropy. Derivatization of one thiol and both thiols resulted in 48 and 100% inhibition, respectively. The dead-end complex (DEC), consisting of creatine, MgADP and protein, inhibited the rate, but not the extent, of derivatization and resulted in a 2-fold increase in fluorescence. (3) The fluorescence of singlylabelled (1AANS/CK) and doublylabelled (2AANS/CK) protein exhibited three discrete lifetime components or a two-term Lorentzian distribution. The decay laws for both preparations were not remarkably different, except that, unlike 1AANS/CK, the longer decay component of 2AANS/CK was distributed, which narrowed in the presence of the DEC. (4) The steady-state anisotropies of 1AANS/CK and 2AANS/CK at 25 degrees C were 0.305 and 0.240, respectively. It was concluded that the fast reacting site was immobile and the slow reacting site was flexible. Kinetics of labelling and anisotropy emission spectra indicated that the DEC immobilized the flexible site. (5) The anisotropy decay of 1AANS/CK with and without the DEC was described by a rotational correlation time of about 50 ns, characteristic of the molecular rotation of the CK dimer. At least two terms were required to fit the data for 2AANS/CK, indicating additional segmental motion which was eliminated upon formation of the DEC. (6) Energy transfer from tryptophans to AANS indicated movement of approx. 3 A accompanying formation of the DEC.