Creatine kinase is modified by 2-chloromercuri-4-nitrophenol at the active site thiols with complete inactivation

Chemical modification studies on arginine kinase: Essential cysteine and arginine residues at the active site

Chemical modification was used to elucidate the essential amino acids in the catalytic activity of arginine kinase (AK) from Migratoria manilensis. Among six cysteine (Cys) residues only one Cys residue was determined to be essential in the active site by Tsou's method. Furthermore, the AK modified by DTNB can be fully reactivated by dithiothreitol (DTT) in a monophasic kinetic course. At the same time, this reactivation can be slowed down in the presence of ATP, suggesting that the essential Cys is located near the ATP binding site. The ionizing groups at the AK active site were studied and the standard dissociation enthalpy (DeltaH degrees ) was 12.38kcal/mol, showing that the dissociation group may be the guanidino of arginine (Arg). Using the specific chemical modifier phenylglyoxal (PG) demonstrated that only one Arg, located near the ATP binding site, is essential for the activity of AK.

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

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

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.

Conformational change and inactivation of arginine kinase from shrimp Feneropenaeus chinensis in oxidized dithiothreitol solutions

The effect of oxidized dithiothreitol (DTT) on the conformation and function of arginine kinase from shrimp Feneropenaeus chinensis was investigated with the methods of intrinsic fluorescence, ANS fluorescence, size exclusion chromatography (SEC), sodium dodecyl sulfate - polyacrylamide gel electrophoresis (SDS-PAGE), and activity assay. The excess molecular oxidized dithiothreitol could result in a loss of activity and conformational change of arginine kinase. The oxidized arginine kinase was characterized by monitoring the changes of fluorescence emission wavelength (excitation wavelength: 295 nm) and the intensity of 1-anilino-8-naphthalenesulfonate (ANS) binding (excitation wavelength: 380 nm) to the protein. The results of fluorescence spectra showed that the presence of oxidized DTT could result in a marked change in the enzyme tertiary structure. The conformational changes of native and oxidized arginine kinase are induced by the presence of the full set of transition state analog (TSA) components. The results of size exclusion chromatography and SDS-PAGE indicated that no disulfide bond was formed among the protein molecules in the oxidized-DTT solution.

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

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

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

The reduction of 5,5'-dithiobis-(2-nitrobenzoic acid)-modified creatine by dithiothreitol has been studied using the kinetic theory of the substrate reaction during modification of enzyme activity as previously described by C.L. Tsou (Adv. Enzymol. Rel. Areas Mol. Biol. 61 (1988) 381-436). The results show that the modified creatine kinase can be fully reactivated by an excess concentration of dithiothreitol in a monophasic kinetic course. The presence of ATP or the transition-state analogue markedly slows the apparent reactivation rate constant, while creatine shows no effect. The substrates creatine-ADP-Mg2+ can induce conformational changes of the modified enzyme but adding NO-3 cannot induce further changes that occur with the native enzyme. The reactive cysteines' location and role in the catalysis of creatine kinase are discussed. It is suggested that the cysteine may be located in the hinge area of the two domains of creatine kinase. The reactive cysteine of creatine kinase may play an important role not in the binding to the transition-state analogue but in the conformational changes caused by the transition-state analogue.

Transition state structure of arginine kinase: implications for catalysis of bimolecular reactions

Arginine kinase belongs to the family of enzymes, including creatine kinase, that catalyze the buffering of ATP in cells with fluctuating energy requirements and that has been a paradigm for classical enzymological studies. The 1.86-A resolution structure of its transition-state analog complex, reported here, reveals its active site and offers direct evidence for the importance of precise substrate alignment in the catalysis of bimolecular reactions, in contrast to the unimolecular reactions studied previously. In the transition-state analog complex studied here, a nitrate mimics the planar gamma-phosphoryl during associative in-line transfer between ATP and arginine. The active site is unperturbed, and the reactants are not constrained covalently as in a bisubstrate complex, so it is possible to measure how precisely they are pre-aligned by the enzyme. Alignment is exquisite. Entropic effects may contribute to catalysis, but the lone-pair orbitals are also aligned close enough to their optimal trajectories for orbital steering to be a factor during nucleophilic attack. The structure suggests that polarization, strain toward the transition state, and acid-base catalysis also contribute, but, in contrast to unimolecular enzyme reactions, their role appears to be secondary to substrate alignment in this bimolecular reaction.

Phosphocreatine-dependent glutamate uptake by synaptic vesicles. A comparison with atp-dependent glutamate uptake

ATP-dependent uptake of glutamate into synaptic vesicles has been well documented. Stimulation of glutamate uptake into synaptic vesicles by other high-energy phosphates has not been described. In this paper, we examine the stimulation of phosphocreatine (PCr)-induced glutamate uptake and determine whether this stimulation is secondary to conversion of PCr to ATP. We found the following. 1) PCr stimulates glutamate uptake into synaptic vesicles in the absence of added ATP. 2) At a glutamate concentration of 50 microM, no concentration of added ATP could produce the degree of stimulation seen in the presence of PCr. 3) 0.5 mM iodoacetamide completely inhibits synaptic vesicle creatine kinase activity but does not inhibit PCr-stimulated glutamate uptake. 4) PCr-dependent glutamate uptake, unlike ATP-dependent uptake, is not magnesium- or chloride-dependent. 5) 0.5 mM N-ethylmaleimide, a selective H+-ATPase inhibitor, completely inhibits ATP-dependent glutamate uptake but only slightly inhibits PCr-dependent glutamate uptake. 6) PCr-dependent glutamate uptake is sensitive to valinomycin, a K+/H+ translocator, whereas the ATP-dependent uptake is not. Therefore, it appears that in addition to the well-known ATP-dependent glutamate uptake system, there is a previously unreported PCr-dependent glutamate uptake system in synaptic vesicles. The total glutamate uptake by synaptic vesicles is likely the sum of both ATP- and PCr-dependent glutamate uptake.

Creatine kinase: the characteristics of the enzyme regenerated from the thio-methylated creatine kinase reflect a differentiation in function between the two reactive thiols

The activity of S-thiomethyl-modified creatine kinase is due to regeneration of the free thiol (Hou and Vollmer, Biochim Biophys Acta (1994) 1205, 83-88). Characteristics of enzyme regenerated from the S-thiomethyl-modified creatine kinase are reported in the present study. The intrinsic fluorescence of the regenerated enzyme is similar to that of the native enzyme in the presence or absence of the dead-end complex. Regenerated CK (rCK) with full activity has only one reactive thiol. The rate constant of the rCK-reactive thiol reacting with DTNB is close to that of the slow phase of the reactive thiols of the native enzyme. If the IAM-modified rCK is treated with the same method as that for obtaining the rCK, the thiol-methylated reactive thiol of the rCK is reduced to a free SH and a regenerated enzyme, RCK, is produced with about 10% of the rCK activity. Therefore, the different roles of the two reactive thiols of creatine kinase may stem from the characteristics of the rCK, which suggests that only one of the two reactive thiols is related to the activity of the enzyme and the slower phase thiol (the first SH) in the modification reaction with DTNB is directly related to the enzymatic activity, while the faster phase thiol (the second SH) assists the first SH. This compensatory mechanism is proposed in the present study to interpret the dispute on the reactive SH role in the enzymatic catalysis.

Kinetics of inactivation of aminoacylase by 2-chloromercuri-4-nitrophenol: a new type of complexing inhibitor

The kinetic theory of the substrate reaction during modification of enzyme activity previously described [Tsou (1988) Adv. Enzymol. 61, 381-436] has been applied to a study of the inactivation kinetics of aminoacylase by 2-chloromercuri-4-nitrophenol (MNP). The results indicate that the mechanism of reaction between MNP and aminoacylase is a special type of irreversible inhibition. The main features of this type of inhibitor are as follows: (i) the reaction kinetics of inhibitor with enzyme is a single exponential process; (ii) inhibition shows a noncompetitive, complexing behavior; (iii) the first inhibitor-enzyme complex, EI, still has some enzyme activity, and hence the plot of [P]infinity versus the reciprocal of inhibitor concentration gives a straight line with a positive intercept at the ordinate. On the basis of the kinetic equation of substrate reaction in the presence of the inhibitor, a plotting method has been developed for determining the inhibition kinetic constants. As an example, all reaction kinetic constants of aminoacylase with 2-chloromercuri-4-nitrophenol have been determined. The results of the present study suggest that the essential thiol group at the active site of aminoacylase may have a significant effect on the catalytic step but is not involved in substrate binding.

The activity of S-thiomethyl modified creatine kinase is due to the regeneration of free thiol at the active site

Creatine kinase modified by S-methyl methanethiosulfonate and devoid of reactive thiol group has been reported to retain about 18-40% of the activity of the native enzyme. It has now been found that during the reaction catalyzed by the modified enzyme the rate increases with time and if the reaction is allowed to continue sufficiently long, the enzyme eventually recovers full activity. The presence of substrates is not required for the reactivation as suitable dilution after removal of MMTS in excess leads to complete reactivation of the MMTS modified enzyme with the simultaneous regeneration of reactive thiol per each dimeric molecule as shown by determinations with DTNB and IAN. The addition of MMTS during the course of reactivation again inactivates the reactivated enzyme. The activity recovery is therefore due to the regeneration of reactive thiol and it appears that the active-site thiols are essential for the activity of rabbit muscle creatine kinase.

Is the subunit the minimal function unit of creatine kinase?

The dimeric rabbit muscle isozyme of creatine kinase (MM) is modified by iodoacetamide to produce the inactive dimer (M'M') and then hybridized with native dimeric brain isozyme (BB). The hybrid enzyme (M'B), as isolated by PAGE, has the same Km for both ATP and creatine but half the specific activity of the brain isozyme (BB). Likewise, the hybrid of the modified brain with the native muscle isozyme (MB') has half the activity of the native muscle enzyme. The M'B, MB' and MB hybrid dimers all have essentially the same electrophoretic properties, and their intrinsic fluorescence and CD spectra in the far-ultraviolet region are very similar to those of the homodimers MM and BB. Similar results were obtained for the hybrid (M"B) containing the muscle enzyme subunit modified at both the thiol group with iodoacetamide and the Trp residue with dimethyl(2-hydroxy-5-nitrobenzyl)sulfonium bromide and the native brain enzyme submit. The above results suggest strongly the independent catalytic function of the subunit of creatine kinase.