Effects of pH and KCl on the conformations of creatine kinase from rabbit muscle. Infrared, circular dichroic and fluorescence studies

Detection of local polarity and conformational changes at the active site of rabbit muscle creatine kinase with a new arginine-specific fluorescent probe

A new polarity-sensitive fluorescent probe, 3-(4-chloro-6-p-glyoxal-phenoxy-1,3,5-triazinylamino)-7-(dimethylamino)-2-methylphenazine (CGTDP), is synthesized for selective labeling of active-site arginine residues. The probe comprises a neutral red moiety as a polarity-sensitive fluorophore and a phenylglyoxal unit as an arginine-specific labeling group. The probe exhibits a sensitive response of shift of fluorescence maximum emission wavelength to solvent polarity only instead of pH or temperature, which leads to the use of the probe in detecting the local polarity and conformational changes of the active site of rabbit muscle creatine kinase (CK) denatured by pH or temperature. The polarity of the active site domain has been first found to correspond to a dielectric constant of about 44, and the conformational change of the active site directly revealed by CGTDP occurs far before that of CK as a whole disclosed by the intrinsic tryptophan fluorescence during acid or thermal denaturation. The present strategy may provide a useful method to detect the local polarity and conformational changes of the active sites of many enzymes that employ arginine residues as anion recognition sites under different denaturation conditions.

Metal-Binding Properties of a Dicysteine-Containing Motif in Protein Tyrosine Kinases

Studying the structural consequences of the direct binding of arsenite, cadmium, cobalt, nickel, and lead to a number of protein tyrosine kinases led to the discovery of the metal-binding properties of a dicysteine-containing motif in the C-terminal (CT) lobe of the kinases. Of all the synthesized peptides derived from different domains of c-Src and Csk, only peptides based on a dicysteine-containing motif located in the CT lobe of the kinase domain-CPESLHDLMCQC and CPESLHDLMC in c-Src, and CPPAVYDVMKNC in Csk-exhibited significant conformational changes in the presence of all metals, as shown by circular dichroism (CD) analyses. Furthermore, CD analysis of natural enzymes c-Src, Csk, Fyn, c-Abl, Lck, EGFR, and c-Src domains containing the CT lobe in the presence of metals showed a significant concentration-dependent conformational change. ICP-MS, (113)Cd NMR, (33)S NMR, and/or molecular modeling studies of CPESLHDLMC and CPPAVYDVMKNC confirmed the binding between the free sulfhydryl groups of the cysteine residues and Cd(II) or As(III). UV-titration studies suggested a high-affinity interaction between Cd(II) and As(III) and the peptides (K(d) values in the range of 0.6-18.3 nM).

pH-controlled reversible assembly of peptide-functionalized gold nanoparticles

The assembly/disassembly process of carboxylated peptide-functionalized gold nanoparticles (peptide-GNPs) was studied within the pH interval of 2.5 to 10. The assembly process was not well controlled at pH 2.5, leading to the formation of 3D structures of GNPs, whereas at pH 4 we observed controlled assembly with the formation of only a network of 1D chains. In the pH range of 2.5 to 4, the assembly proceeded with the formation of a combination of two extremes (i.e., having both 1D and 2D nanostructures). The assembly process was reversed on changing the pH of the medium to 10. The assembly/disassembly process was monitored using UV-vis spectroscopy and finally confirmed by TEM analysis. This assembly resulted from the intermolecular H-bonding between two carboxylic acid groups of peptides bound to the two adjacent GNPs and were confirmed by FTIR spectroscopy.

Effect of pH on the Adsorption and Activity of Creatine Phosphokinase

The combination of in situ ellipsometry with atomic force microscopy in the liquid for the study of adsorption of creatine phosphokinase (CPK) onto silicon wafers was shown for the first time. The thickness, adsorbed amount, and topographic information of the adsorbed CPK layers were obtained under different pH conditions. The thickness values of adsorbed CPK layer determined by both techniques were in excellent agreement. At pH 4, CPK monomers present in solution adsorb, forming a very thin (approximately 0.8 nm) layer, indicating CPK unfolding. Upon increasing the pH to 6.8, the adsorbed layer is composed of a mixture of CPK dimers (native structure) and intermediates, increasing the film thickness (approximately 2.4 nm). At pH 9, CPK dimers form monolayers with the highest thickness (approximately 4.0 nm). The nature of interactions between CPK and Si wafers associated with the hydration force seems to control the degree of CPK unfolding upon adsorbing. The enzymatic activity of free CPK and of adsorbed CPK at pH 4, pH 6.8, and pH 9 was measured as a function of pH. In comparison to free CPK in solution, adsorbed CPK presented a shift of the optimal pH from 6.8 toward alkaline pH.

Conformational change in the C-terminal domain is responsible for the initiation of creatine kinase thermal aggregation

Protein conformational changes may be associated with particular properties such as its function, transportation, assembly, tendency to aggregate, and potential cytotoxicity. In this research, the conformational change that is responsible for the fast destabilization and aggregation of rabbit muscle creatine kinase (EC 2.7.3.2) induced by heat was studied by intrinsic fluorescence and infrared spectroscopy. A pretransitional change of the tryptophan microenvironments was found from the intrinsic fluorescence spectra. A further analysis of the infrared spectra using quantitative second-derivative and two-dimensional correlation analysis indicated that the changes of the beta-sheet structures in the C-terminal domain and the loops occurred before the formation of intermolecular cross-beta-sheet structures and the unfolding of alpha-helices. These results suggested that the pretransitional conformational changes in the active site and the C-terminal domain might result in the modification of the domain-domain interactions and the formation of an inactive dimeric form that was prone to aggregate. Our results highlighted the fact that some minor conformational changes, which were usually negligible or undetectable by normal methods, might play a crucial role in protein stability and aggregation. Our results also suggested that the changes in domain-domain interactions, but not the dissociation of the dimer, might play a crucial role in the thermal denaturation and aggregation of this dimeric two-domain protein.

Unfolding of rabbit muscle creatine kinase induced by acid. A study using electrospray ionization mass spectrometry, isothermal titration calorimetry, and fluorescence spectroscopy

Electrospray ionization mass spectrometry, isothermal titration calorimetry (ITC), fluorescence spectroscopy, and glutaraldehyde cross-linking SDS-PAGE have been used to study the unfolding of rabbit muscle creatine kinase (MM-CK) induced by acid. The mass spectrometric experiments show that MM-CK is unfolded gradually when titrated with acid. MM-CK is a dimer (the native state) at pH 7.0 and becomes an equilibrium mixture of the dimer and a partially folded monomer (the intermediate) between pH 6.7 and 5.0. The dimeric protein becomes an equilibrium mixture of the intermediate and an unfolded monomer (the unfolded state) between pH 5.0 and 3.0 and is almost fully unfolded at pH 3.0 reached. The results from a "phase diagram" method of fluorescence show that the conformational transition between the native state and the intermediate of MM-CK occurs in the pH range of 7.0-5.2, and the transition between the intermediate and the unfolded state of the protein occurs between pH 5.2 and 3.0. The intrinsic molar enthalpy changes for formation of the unfolded state of MM-CK induced by acid at 15.0, 25.0, 30.0, and 37.0 degrees C have been determined by ITC. A large positive molar heat capacity change of the unfolding, 8.78 kcal mol-1 K-1, at all temperatures examined indicates that hydrophobic interaction is the dominant driving force stabilizing the native structure of MM-CK. Combining the results from these four methods, we conclude that the acid-induced unfolding of MM-CK follows a "three-state" model and that the intermediate state of the protein is a partially folded monomer.

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

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

Acid-induced unfolding of flounder hemoglobin: evidence for a molten globular state with enhanced pro-oxidative activity

The acid-induced unfolding of flounder oxyhemoglobin was investigated and the effect on pro-oxidative activity assessed. Hemoglobin exhibited multistep unfolding transitions as pH was lowered, with the major transition between pH 3.5 and 4 5. The protein was maximally acid-unfolded (but not fully unfolded) at approximately pH 2.5, and further titration with HCl led to a partially refolded protein due to a stabilizing effect of Cl(-) anions. At low pH, the protein retained a sizable amount of secondary structure and had increased ANS binding, suggesting a molten globular form at low pH. Dramatic changes in the heme environment occurred concurrently with the changes in protein conformation. These changes resulted in an enhancement in the pro-oxidative activity of the protein. The results show that an increase in flounder hemoglobin pro-oxidation was correlated with the extent of its unfolding, and they provide useful insight into what may occur with hemoglobin in processes where highly acidic conditions are employed.

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

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

alpha-B- and alpha-A-crystallin prevent irreversible acidification-induced protein denaturation

alpha-Crystallin (alpha), a major structural protein of the mammalian lens, is a large, physically heterogeneous macromolecule with an average molecular weight of approximately 800 kDa and is composed of two 20-kDa polypeptides designated as alphaA and alphaB. A line of evidence strongly suggests that alphaB may have an essential nonlenticular function. Here it is demonstrated that alphaB can bind partially denatured enzymes effectively at acidic pH and prevent their irreversible aggregation, but cannot prevent loss of enzyme activity. However, when the inactive luciferase bound to alphaB was treated with reticulocyte lysate (a rich source of molecular chaperones) and an ATP-generating system, more than 50% of the original luciferase activity could be recovered. Somewhat less activation was observed when alphaA-bound enzyme or the alpha-bound enzyme was renatured similarly. The overall results suggest that alpha acts as a chaperone to stabilize denaturing proteins at acidic pH so that at a later time they can be reactivated by other chaperones.

Mitochondrial Creatine Kinase Binding to Phospholipids Decreases Fluidity of Membranes and Promotes New Lipid-Induced β Structures As Monitored by Red Edge Excitation Shift, Laurdan Fluorescence, and FTIR

Structural modifications induced by the binding of mitochondrial creatine kinase (mtCK) to saturated and unsaturated phospholipids were monitored by using Laurdan, a membrane probe sensitive to the polarity of the environment. The abrupt change characteristic of a phase transition of lipids alone was attenuated by addition of mtCK. Generalized polarization spectra indicated that mtCK surface binding changed the phospholipid liquid-crystalline state to a more rigid state. Infrared spectra of lipids further strengthened these results: upon mtCK binding, the phospholipid methylene chains had a more rigid conformation than that observed without mtCK at the same temperature. After mtCK binding to vesicles of perdeuterated dimyristoylphosphatidylcholine and nondeuterated dimyristoylphosphatidylglycerol, no lateral phase separation was observed, suggesting that both lipids were rigidified. Moreover, mtCK bound to liposomes exhibited an uncommon red edge excitation shift of 19 nm, while that of the soluble enzyme was only 6 nm. These results indicated that the environment of some mtCK tryptophan residues was motionally restricted. Strong stabilization of the enzyme structure against heat denaturation was observed upon lipid binding. In addition, lipids promoted a new reversible protein-protein or protein-lipid interaction, as evidenced by infrared data showing a slight modification of the beta sheet over alpha helix ratio with formation of a new 1632-cm(-)(1) beta sheet instead of the soluble protein 1636-cm(-)(1) one. Such modifications, inducing a decrease in the fluidity of the mitochondrial membranes, may play a role in vesicle aggregation; they could be implicated in the appearance of contact sites between internal and external mitochondrial membranes.

Structural changes of mitochondrial creatine kinase upon binding of ADP, ATP, or Pi, observed by reaction-induced infrared difference spectra

Structural modifications of rabbit heart mitochondrial creatine kinase induced by the binding of its nucleotide substrates and Pi were investigated. Reaction-induced difference spectra (RIDS), resulting from the difference between infrared spectra recorded before and after the photorelease of a caged ligand, allow us to detect very small variations in protein structure. Our results indicated that the protein secondary structure remained relatively stable during nucleotide binding. Indeed, this binding to creatine kinase affected only a few amino acids, and caused small peptide backbone deformations and alterations of the carbonyl side chains of aspartate or glutamate, reflecting modifications within preexisting elements rather than a net change in secondary structure. Nonetheless, MgADP and MgATP RIDS were distinct, whereas the MgPi RIDS presented some similarities with the MgATP one. The difference between MgADP and MgATP RIDS could reflect a distinct configuration of the two metal-nucleotide complexes inducing a different positioning and/or a distinct binding mode to the creatine kinase active site. Comparison of the MgATP and MgPi RIDS suggests that Pi binding took place at the same binding site as the gamma-phosphoryl group of ATP. Thus, the difference between MgADP and MgATP RIDS would mainly be due to the effect of the gamma-P of ATP. The differences observed when comparing the RIDS resulting from the binding of nucleotides to octameric mitochondrial creatine kinase or dimeric cytosolic isoform could reflect the distinct oligomerization states and physicochemical or kinetic properties of the two isoenzymes.

The unusual dodecameric ferritin from Listeria innocua dissociates below pH 2.0

The stability of the dodecameric Listeria innocua ferritin at low pH values has been investigated by spectroscopic methods and size-exclusion chromatography. The dodecamer is extremely stable in comparison to the classic ferritin tetracosamer and preserves its quaternary assembly at pH 2.0, despite an altered tertiary structure. Below pH 2.0, dissociation into dimers occurs and is paralleled by the complete loss of tertiary structure and a significant decrease in secondary structure elements. Dissociation of dimers into monomers occurs only at pH 1.0. Addition of NaCl to the protein at pH 2.0 induces structural changes similar to those observed upon increasing the proton concentration, although dissociation proceeds only to the dimer stage. Addition of sulfate at pH values >/= 1.5 prevents the dissociation of the dodecamer. The role played by hydrophilic and hydrophobic interactions in determining the resistance to dissociation of L. innocua ferritin at low pH is discussed in the light of its three-dimensional structure.

Magnesium-adenosine diphosphate binding sites in wild-type creatine kinase and in mutants: role of aromatic residues probed by Raman and infrared spectroscopies

Two distinct methods were used to investigate the role of Trp residues during Mg-ADP binding to cytosolic creatine kinase (CK) from rabbit muscle: (1) Raman spectroscopy, which is very sensitive to the environment of aromatic side-chain residues, and (2) reaction-induced infrared difference spectroscopy (RIDS) and photolabile substrate (ADP[Et(PhNO(2))]), combined with site-directed mutagenesis on the four Trp residues of CK. Our Raman results indicated that the environment of Trp and of Tyr were not affected during Mg-ADP binding to CK. Analysis of RIDS of wild-type CK, inactive W227Y, and active W210,217,272Y mutants suggested that Trp227 was not involved in the stacking interactions. Results are consistent with Trp227 being essential to prevent water molecules from entering in the active site [as suggested by Gross, M., Furter-Graves, E. M., Wallimann, T., Eppenberger, H. M., and Furter, R. (1994) Protein Sci. 3, 1058-1068] and that another Trp could in addition help to steer the nucleotide in the binding site, although it is not essential for the activity of CK. Raman and infrared spectra indicated that Mg-ADP binding does not involve large secondary structure changes. Only 3-4 residues absorbing in the amide I region are directly implicated in the Mg-ADP binding (corresponding to secondary structure changes less than 1%), suggesting that movement of protein domains due to Mg-nucleotide binding do not promote large secondary structure changes.

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.

Macrophage migration inhibitory factor interactions with glutathione and S-hexylglutathione

Macrophage migration inhibitory factor (MIF) has been reported to interact with glutathione and S-hexylglutathione and to possess glutathione S-transferase activity. However, contrary to these reports, a recent NMR study concluded that MIF shows no affinity for glutathione. Re-examination of the glutathione-MIF interactions indicates that the reported increase in fluorescence upon addition of glutathione is because of pH-induced unfolding of the protein and not to any direct interactions. Circular dichroism shows that MIF remains folded from pH 4.5-7.5 but is 50% unfolded at pH 2.9 +/- 0.2. The reported increase in fluorescence can be achieved by acid titration. Under strongly buffered conditions, no fluorescence change is observed upon addition of glutathione. In contrast to the results with glutathione, MIF binds S-hexylglutathione with a Kd of 2.5 +/- 0.6 mM. Using NMR spectroscopy, a binding site which clusters around the N-terminal proline was identified. These data indicate that the binding site for S-hexylglutathione is the same as the catalytic site for the dopachrome tautomerase activity of MIF. Consequently, the binding of S-hexylglutathione as well as hexanethiol inhibits this catalytic activity.

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.

ADP-binding and ATP-binding sites in native and proteinase-K-digested creatine kinase, probed by reaction-induced difference infrared spectroscopy

Conformational changes induced by nucleotide binding to native creatine kinase (CK) from rabbit muscle and to proteinase-K-digested (nicked) CK, were investigated by infrared spectroscopy. Photochemical release of ATP from ATP[Et(PhNO2)] in the presence of creatine and native CK produced reaction-induced difference infrared spectra (RIDS) of CK related to structural changes of the enzyme that paralleled the reversible phosphoryl transfer from ATP to creatine. Similarly the photochemical release of ADP from ADP[Et(PhNO2)] in the presence of phosphocreatine and native CK allowed us to follow the backward reaction and its corresponding RIDS. Infrared spectra of native CK indicated that carboxylate groups of Asp or Glu, and some carbonyl groups of the peptide backbone are involved in the enzymatic reaction. Native and proteinase nicked CK have similar Stokes' radii, tryptophan fluorescence, fluorescence fraction accessible to iodide, and far-ultraviolet CD spectra, indicating that native and modified enzymes have the same quaternary structures. However, infrared data showed that the binding site of the gamma-phosphate group of the nucleotide was affected in nicked CK compared with that of the native CK. Furthermore, the infrared absorptions associated with ionized carboxylate groups of Asp or Glu amino acid residues were different in nicked CK and in native CK.

Acid-induced disassembly of glutamate dehydrogenase from the hyperthermophilic archaeon Pyrococcus furiosus occurs below pH 2.0

The stability of the hexameric glutamate dehydrogenase from the hyperthermophilic archaeon Pyrococcus furiosus at low pH values has been studied by activity assay, spectroscopic methods, size-exclusion chromatography and ultracentrifugation analysis. The enzyme is exceptionally stable and at pH 2.0 its hexameric assembly is preserved despite the changes observed in its tertiary structure. Below pH 1.7 dissociation into monomers starts and is accompanied by a progressive loss of tertiary interactions. Dissociation intermediate(s) were not detectable. At pH 2.0 the addition of NaCl causes the same structural changes observed upon further addition of protons. The monomeric state of the enzyme at pH 1.0 shows a significant content of native secondary structure and can be unfolded by guanidinium chloride. The role of electrostatic interactions in the high stability of the enzyme structure at low pH values is discussed.

Conformational changes of arginine kinase induced by photochemical release of nucleotides from caged nucleotides--an infrared difference-spectroscopy investigation

The conformations of arginine kinase (AK) in AK x Mg x ADP, AK x Mg x ATP, AK x Mg x ADP x NO3-, AK x Mg x ADP x Arg and AK x Mg x ADP x NO3- x Arg complexes were investigated by measuring their reaction-induced infrared difference spectra (RIDS). The photochemical release of ATP from ATP[Et(PhNO2)] and of ADP from ADP[Et(PhNO2)] produced distinct RIDS of AK complexes, suggesting that binding of ADP and ATP promoted different structural alterations of the enzyme active-site. Small infrared changes in the amide-I region were observed, indicating that about 5-10 amino acid residues were involved in the nucleotide-binding site. These infrared changes were due to the structural alteration of the peptide backbone caused by the nucleotide-binding and to the coupling effects between the nucleotide-binding site and the other substrate (Arg or NO3-)-binding site. ATP binding to AK (as well as ADP-binding to AK in the presence of NO3-) induced protonation of a carboxylate group of Asp or Glu, as evidenced by the appearance of the 1733-cm(-1) band, which was not observed with the AK x Mg x ADP, AK x Mg x ADP x Arg and AK x Mg x ADP x NO3- x Arg complexes. The RIDS of the AK x Mg x ADP x NO3- x Arg complex showed new infrared bands at 1622 cm(-1) (negative) and at 1613 cm(-1) (positive), which were not seen in the RIDS of other complexes (without NO3- or/and Arg). In the transition-state-analog complex of AK, no protonation of the carboxylate residue (Asp or Glu) was observed, and the binding site of NO3- or the gamma-phosphate group of nucleotide was altered.

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.