A Conserved Negatively Charged Cluster in the Active Site of Creatine Kinase Is Critical for Enzymatic Activity

In Vitro Selection of Short DNA Aptamers that Can Inhibit or Alleviate Cocaine and MK-801 Inhibition of Muscle-Type Nicotinic Acetylcholine Receptors

Ligands of high specificity and selectivity have been selected for biological molecules of interest including nicotinic acetylcholine receptor (nAChR) using combinatorial libraries of nucleic acids. The nAChR belongs to a group of structurally related proteins that regulate signal transmission between ~ 10¹² cells of the mammalian nervous system. It is inhibited by both therapeutic agents and abused drugs, including cocaine. A mechanism-based approach to alleviating noncompetitive inhibition of the mucle-type nAChR, including Torpedo, resulted in the selection of very short DNA aptamers only seven nucleotides long. By transient kinetic measurements, these DNA aptamers, which displaced cocaine from its binding site on the muscle-type nAChR, were classified into two groups based on their effects on the nAChR: Class I aptamers inhibit agonist-induced current in the muscle-type nAChR and Class II molecules alleviate inhibition by MK-801 [(+)-dizocilpine] without affecting the receptor function. The most potent Class I DNA aptamer, which inhibits the muscle-type nAChR, has an apparent dissociation constant (K_Iapt) of 5 μM, while the most efficient Class II DNA aptamer, which alleviates MK-801-induced inhibition, has an apparent dissociation constant (K_Apt) of 1.8 μM. An innovative aspect of the work is the identification of very short DNA aptamers with these properties that makes them attractive for therapeutic and diagnostic applications.

Tyrosine Phosphorylation of Mitochondrial Creatine Kinase 1 Enhances a Druggable Tumor Energy Shuttle Pathway

How mitochondrial metabolism is altered by oncogenic tyrosine kinases to promote tumor growth is incompletely understood. Here, we show that oncogenic HER2 tyrosine kinase signaling induces phosphorylation of mitochondrial creatine kinase 1 (MtCK1) on tyrosine 153 (Y153) in an ABL-dependent manner in breast cancer cells. Y153 phosphorylation, which is commonly upregulated in HER2+ breast cancers, stabilizes MtCK1 to increase the phosphocreatine energy shuttle and promote proliferation. Inhibition of the phosphocreatine energy shuttle by MtCK1 knockdown or with the creatine analog cyclocreatine decreases proliferation of trastuzumab-sensitive and -resistant HER2+ cell lines in culture and in xenografts. Finally, we show that cyclocreatine in combination with the HER2 kinase inhibitor lapatinib reduces the growth of a trastuzumab-resistant HER2+ patient-derived xenograft. These findings suggest that activation of the phosphocreatine energy shuttle by MtCK1 Y153 phosphorylation creates a druggable metabolic vulnerability in cancer.

The isomeric structure of pentacoordinate chiral spirophosphoranes in solution by the combined use of NMR experiments and GIAO DFT calculations of NMR parameters

The interplay of NMR experiments and DFT calculations of NMR parameters is a reliable method for determining the relative configurations of pentacoordinate chiral spirophosphoranes bearing two six- or five-membered rings at the phosphorus atom in solution. The major product of the Betti based derivatives corresponds to the isomers with both substituents at chiral carbons being opposite to the P-H proton. The next populated product corresponds to the isomer with different chiralities at carbons. The least populated isomer is one with both substituents being at the same side of the heterocycle as the P-H bond.

Submembranous recruitment of creatine kinase B supports formation of dynamic actin-based protrusions of macrophages and relies on its C-terminal flexible loop

Subcellular partitioning of creatine kinase contributes to the formation of patterns in intracellular ATP distribution and the fuelling of cellular processes with a high and sudden energy demand. We have previously shown that brain-type creatine kinase (CK-B) accumulates at the phagocytic cup in macrophages where it is involved in the compartmentalized generation of ATP for actin remodeling. Here, we report that CK-B catalytic activity also helps in the formation of protrusive ruffle structures which are actin-dependent and abundant on the surface of both unstimulated and LPS-activated macrophages. Recruitment of CK-B to these structures occurred transiently and inhibition of the enzyme's catalytic activity with cyclocreatine led to a general smoothening of surface morphology as visualized by scanning electron microscopy. Comparison of the dynamics of distribution of YFP-tagged CK-mutants and isoforms by live imaging revealed that amino acid residues in the C-terminal segment (aa positions 323-330) that forms one of the protein's two mobile loops are involved in partitioning over inner regions of the cytosol and nearby sites where membrane protrusions occur during induction of phagocytic cup formation. Although wt CK-B, muscle-type CK (CK-M), and a catalytically dead CK-B-E232Q mutant with intact loop region were normally recruited from the cytosolic pool, no dynamic transition to the phagocytic cup area was seen for the CK-homologue arginine kinase and a CK-B-D326A mutant protein. Bioinformatics analysis helped us to predict that conformational flexibility of the C-terminal loop, independent of conformational changes induced by substrate binding or catalytic activity, is likely involved in exposing the enzyme for binding at or near the sites of membrane protrusion formation.

Reconstitution of CKMT1 expression fails to rescue cells from mitochondrial membrane potential dissipation: implications for controlling RNAi experiments

RNA interference (RNAi) is an essential method in molecular biology to reduce the expression of target genes and thereby determine their function. Since this tool is known to also have unspecific effects, control experiments are needed, chiefly among them the exclusion of off-target effects and the reconstitution of the genes' expression for the rescue of the cellular RNAi effects. We show here that the knock-down of the mitochondrial creatine kinase-1 (CKMT1) by RNA interference causes the dissipation of the mitochondrial membrane potential ΔΨm. This was accomplished with 11 different RNAi constructs designed to target 7 distinct exons as well as exon/intron junctions making unspecific off-target effects unlikely. However, all our attempts failed to rescue human cells from ΔΨm dissipation by the expression of CKMT1 alleles not targeted by RNAi. This included the transient and stable expression of the murine CKMT1 homologue, the expression of human codon usage-modified alleles, the transfection of a novel splice-isoform of CKMT1, and even the introduction of a human genomic clone for CKMT1 with codon usage changes. Our results indicate that while off-target effects of RNA interference can easily be addressed, the rescue of the knock-down phenotype is not necessarily achievable.

Regulation of sodium-calcium exchanger activity by creatine kinase

It has been shown that in rat heart NCX1 exists in a macromolecular -complex including PKA, PKA-anchoring protein, PKC, and phosphatases PP1 and PP2A. In addition, several lines of evidence suggest that the interactions of the exchanger with other molecules are closely associated with its function in regulation of [Ca(2+)](i). NCX contains a large intracellular loop (NCXIL) that is responsible for regulating NCX activity. We used the yeast two-hybrid method to screen a human heart cDNA library and found that the C-terminal region of sarcomeric mitochondrial creatine kinase (sMiCK) interacted with NCX1IL. Among the four creatine kinase (CK) isozymes, both sMiCK and the muscle-type cytosolic creatine kinase (CKM) co-immunoprecipitated with NCX1. Both sMiCK and CKM were able to produce a recovery in the decreased NCX1 activity that was lost under energy-compromised conditions. This regulation is mediated through a putative PKC phosphorylation site of sMiCK and CKM. The catalytic activity of sMiCK and CKM is not required for their regulation of NCX1 activity. Our results suggest a novel mechanism for the regulation of NCX1 activity and a novel role for CK.

Reexamination of magnetic isotope and field effects on adenosine triphosphate production by creatine kinase

The influence of isotopically enriched magnesium on the creatine kinase catalyzed phosphorylation of adenosine diphosphate is examined in two independent series of experiments where adenosine triphosphate (ATP) concentrations were determined by a luciferase-linked luminescence end-point assay or a real-time spectrophotometric assay. No increase was observed between the rates of ATP production with natural Mg, (24)Mg, and (25)Mg, nor was any significant magnetic field effect observed in magnetic fields from 3 to 1,000 mT. Our results are in conflict with those reported by Buchachenko et al. [J Am Chem Soc 130:12868-12869 (2008)], and they challenge these authors' general claims that a large (two- to threefold) magnetic isotope effect is "universally observable" for ATP-producing enzymes [Her Russ Acad Sci 80:22-28 (2010)] and that "enzymatic phosphorylation is an ion-radical, electron-spin-selective process" [Proc Natl Acad Sci USA 101:10793-10796 (2005)].

The creatine kinase system and pleiotropic effects of creatine

The pleiotropic effects of creatine (Cr) are based mostly on the functions of the enzyme creatine kinase (CK) and its high-energy product phosphocreatine (PCr). Multidisciplinary studies have established molecular, cellular, organ and somatic functions of the CK/PCr system, in particular for cells and tissues with high and intermittent energy fluctuations. These studies include tissue-specific expression and subcellular localization of CK isoforms, high-resolution molecular structures and structure–function relationships, transgenic CK abrogation and reverse genetic approaches. Three energy-related physiological principles emerge, namely that the CK/PCr systems functions as (a) an immediately available temporal energy buffer, (b) a spatial energy buffer or intracellular energy transport system (the CK/PCr energy shuttle or circuit) and (c) a metabolic regulator. The CK/PCr energy shuttle connects sites of ATP production (glycolysis and mitochondrial oxidative phosphorylation) with subcellular sites of ATP utilization (ATPases). Thus, diffusion limitations of ADP and ATP are overcome by PCr/Cr shuttling, as most clearly seen in polar cells such as spermatozoa, retina photoreceptor cells and sensory hair bundles of the inner ear. The CK/PCr system relies on the close exchange of substrates and products between CK isoforms and ATP-generating or -consuming processes. Mitochondrial CK in the mitochondrial outer compartment, for example, is tightly coupled to ATP export via adenine nucleotide transporter or carrier (ANT) and thus ATP-synthesis and respiratory chain activity, releasing PCr into the cytosol. This coupling also reduces formation of reactive oxygen species (ROS) and inhibits mitochondrial permeability transition, an early event in apoptosis. Cr itself may also act as a direct and/or indirect anti-oxidant, while PCr can interact with and protect cellular membranes. Collectively, these factors may well explain the beneficial effects of Cr supplementation. The stimulating effects of Cr for muscle and bone growth and maintenance, and especially in neuroprotection, are now recognized and the first clinical studies are underway. Novel socio-economically relevant applications of Cr supplementation are emerging, e.g. for senior people, intensive care units and dialysis patients, who are notoriously Cr-depleted. Also, Cr will likely be beneficial for the healthy development of premature infants, who after separation from the placenta depend on external Cr. Cr supplementation of pregnant and lactating women, as well as of babies and infants are likely to be of benefit for child development. Last but not least, Cr harbours a global ecological potential as an additive for animal feed, replacing meat- and fish meal for animal (poultry and swine) and fish aqua farming. This may help to alleviate human starvation and at the same time prevent over-fishing of oceans.

Activity and function of rabbit muscle-specific creatine kinase at low temperature by mutation at gly268 to asn268

Carp muscle-specific creatine kinase M1 isoenzyme (M1-CK) seems to have evolved to adapt to synchronized changes in body temperature and intracellular pH. When gly(268) in rabbit muscle-specific creatine kinase was substituted with asn(268) as found in carp M1-CK, the rabbit muscle-specific CK G286N mutant specific activity at pH 8.0 and 10°C was more than 2-fold higher than that in the wild-type rabbit enzyme. Kinetic studies showed that K(m) values of the rabbit CK G268N mutant were similar to those of the wild-type rabbit enzyme, yet circular dichroism spectra showed that the overall secondary structures of the mutant enzyme, at pH 8.0 and 5°C, were almost identical to the carp M1-CK enzyme. The X-ray diffraction pattern of the mutant enzyme crystal revealed that amino acid residues involved in substrate binding are closer to one another than in the rabbit enzyme, and the cysteine283 active site of the mutant enzyme points away from the ADP binding site. At pH 7.4-8.0 and 35-10°C, with a smaller substrate, dADP, specific activities of the mutant enzyme were consistently higher than the wild-type rabbit enzyme and more similar to the carp M1-CK enzyme. Thus, the smaller active site of the RM-CK G268N mutant may be one of the reasons for its improved activity at low temperature.

Regulation of sodium-calcium exchanger activity by creatine kinase under energy-compromised conditions

Na(+)/Ca(2+) exchanger (NCX) is one of the major mechanisms for removing Ca(2+) from the cytosol especially in cardiac myocytes and neurons, where their physiological activities are triggered by an influx of Ca(2+). NCX contains a large intracellular loop (NCXIL) that is responsible for regulating NCX activity. Recent evidence has shown that proteins, including kinases and phosphatases, associate with NCX1IL to form a NCX1 macromolecular complex. To search for the molecules that interact with NCX1IL and regulate NCX1 activity, we used the yeast two-hybrid method to screen a human heart cDNA library and found that the C-terminal region of sarcomeric mitochondrial creatine kinase (sMiCK) interacted with NCX1IL. Moreover, both sMiCK and the muscle-type creatine kinase (CKM) coimmunoprecipitated with NCX1 using lysates of cardiacmyocytes and HEK293T cells that transiently expressed NCX1 and various creatine kinases. Both sMiCK and CKM were able to produce a recovery in the decreased NCX1 activity that was lost under energy-compromised conditions. This regulation is mediated through a putative PKC phosphorylation site of sMiCK and CKM. The autophosphorylation and the catalytic activity of sMiCK and CKM are not required for their regulation of NCX1 activity. Our results suggest a novel mechanism for the regulation of NCX1 activity.

Minimal RNA aptamer sequences that can inhibit or alleviate noncompetitive inhibition of the muscle-type nicotinic acetylcholine receptor

Combinatorially synthesized nucleotide polymers have been used during the last decade to find ligands that bind to specific sites on biological molecules, including membrane-bound proteins such as the nicotinic acetylcholine receptors (nAChRs). The neurotransmitter receptors belong to a group of four structurally related proteins that regulate signal transmission between ~10(11) neurons of the mammalian nervous system. The nAChRs are inhibited by compounds such as the anticonvulsant MK-801 [(+)-dizocilpine] and abused drugs such as cocaine. Based on predictions arising from the mechanism of receptor inhibition by MK-801 and cocaine, we developed two classes of RNA aptamers: class I members, which inhibit the nAChR, and class II members, which alleviate inhibition of the receptor by MK-801 and cocaine. The systematic evolution of ligands by the exponential enrichment (SELEX) method was used to obtain these compounds. Here, we report that we have truncated RNA aptamers in each class to determine the minimal nucleic acid sequence that retains the characteristic function for which the aptamer was originally selected. We demonstrate that a truncated class I aptamer containing a sequence of seven nucleotides inhibits the nAChR and that a truncated class II aptamer containing a sequence of only four nucleotides can alleviate MK-801 inhibition.

Proteomic identification of dopamine-conjugated proteins from isolated rat brain mitochondria and SH-SY5Y cells

Dopamine oxidation has been previously demonstrated to cause dysfunction in mitochondrial respiration and membrane permeability, possibly related to covalent modification of critical proteins by the reactive dopamine quinone. However, specific mitochondrial protein targets have not been identified. In this study, we utilized proteomic techniques to identify proteins directly conjugated with (14)C-dopamine from isolated rat brain mitochondria exposed to radiolabeled dopamine quinone (150 microM) and differentiated SH-SY5Y cells treated with (14)C-dopamine (150 microM). We observed a subset of rat brain mitochondrial proteins that were covalently modified by (14)C-dopamine, including chaperonin, ubiquinol-cytochrome c reductase core protein 1, glucose regulated protein 75/mitochondrial HSP70/mortalin, mitofilin, and mitochondrial creatine kinase. We also found the Parkinson's disease associated proteins ubiquitin carboxy-terminal hydrolase L1 and DJ-1 to be covalently modified by dopamine in both brain mitochondrial preparations and SH-SY5Y cells. The susceptibility of the identified proteins to covalent modification by dopamine may carry implications for their role in the vulnerability of dopaminergic neurons in Parkinson's disease pathogenesis.

Structure-function relationships in Escherichia coli adenylate cyclase

Class I adenylate cyclases are found in gamma- and delta-proteobacteria. They play central roles in processes such as catabolite repression in Escherichia coli or development of full virulence in pathogens such as Yersinia enterocolitica and Vibrio vulnificus. The catalytic domain (residues 2-446) of the adenylate cyclase of E. coli was overexpressed and purified. It displayed a V(max) of 665 nmol of cAMP x mg(-1) x min(-1) and a K(m) of 270 microM. Titration of the metal cofactor Mg(2+) against the substrate ATP showed a requirement for free metal ions in addition to the MgATP complex, suggesting a two-metal-ion mechanism as is known for class II and class III adenylate cyclases. Twelve residues which are essential for catalysis were identified by mutagenesis of a total of 20 polar residues conserved in all class I adenylate cyclases. Five essential residues (Ser(103), Ser(113), Asp(114), Asp(116) and Trp(118)) were part of a region which is found in all members of the large DNA polymerase beta-like nucleotidyltransferase superfamily. Alignment of the E. coli adenylate cyclase with the crystal structure of a distant member of the superfamily, archaeal tRNA CCA-adding enzyme, suggested that Asp(114) and Asp(116) are the metal-cofactor-ion-binding residues. The S103A mutant had a 17-fold higher K(m) than wild-type, demonstrating its important role in substrate binding. In comparison with the tRNA CCA-adding enzyme, Ser(103) of the E. coli adenylate cyclase apparently binds the gamma-phosphate group of ATP. Consistent with this function, the S103A mutation caused a marked reduction of discrimination between ATP- and ADP- or AMP-derived inhibitors.

Changing the substrate specificity of creatine kinase from creatine to glycocyamine: evidence for a highly evolved active site

Eight variants of creatine kinase were created to switch the substrate specificity from creatine to glycocyamine using a rational design approach. Changes to creatine kinase involved altering several residues on the flexible loops that fold over the bound substrates including a chimeric replacement of the guanidino specificity loop from glycocyamine kinase into creatine kinase. A maximal 2,000-fold change in substrate specificity was obtained as measured by a ratio of enzymatic efficiency (k(cat)/K(M).K(d)) for creatine vs. glycocyamine. In all cases, a change in specificity was accompanied by a large drop in enzymatic efficiency. This data, combined with evidence from other studies, indicate that substrate specificity in the phosphagen kinase family is obtained by precise alignment of substrates in the active site to maximize k(cat)/K(M).K(d) as opposed to selective molecular recognition of one guanidino substrate over another. A model for the evolution of the dimeric forms of phosphagen kinases is proposed in which these enzymes radiated from a common ancestor that may have possessed a level of catalytic promiscuity. As mutational events occurred leading to greater degrees of substrate specificity, the dimeric phosphagen kinases became evolutionary separated such that the substrate specificity could not be interchanged by a small number of mutations.

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.

Possible involvement of glutamic and/or aspartic acid residue(s) and requirement of mitochondrial integrity for the protective effect of creatine against inhibition of cardiac mitochondrial respiration by methylglyoxal

We had previously shown that creatine exerted a protective effect against inhibition of cardiac mitochondrial respiration by methylglyoxal (SinhaRoy S, Biswas S, Ray M, Ray S. Biochem J 372: 661-669,2003). In the present study, we have investigated the mechanism of this protective effect by specific amino acid modifying reagent and by several compounds, which are structurally related to creatine. The results show that the compounds, which contain guanidine group such as arginine and guanidinopropionic acid, exert a protective effect, which is quantitatively similar to creatine. This result suggests the presence of carboxylic acid(s) such as glutamic and/or aspartic acid(s) in the creatine-binding site, which has been further supported by experiments with N-ethyl-5-phenyl isoxazolium-3'-sulfonate a reagent known to modify these amino acids. Both polarographic and spectrophotometric assays were performed with NADH as respiratory substrate by using a) submitochondrial particles by sonication, b) freeze-thawed mitochondria and c) mitochondria permeabilized by alamethicin treatment. The results of these studies as compared to that of intact mitochondria indicate that structural integrity of mitochondria is essential for the protective effect of creatine.

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.

Isoleucine 69 and valine 325 form a specificity pocket in human muscle creatine kinase

Creatine kinase (CK) catalyzes the reversible phosphorylation of creatine by ATP. From a structural perspective, the enzyme utilizes two flexible loop regions to sequester and position the substrates for catalysis. There has been debate over the specific roles of the flexible loops in substrate specificity and catalysis in CK and other related phosphagen kinases. In CK, two hydrophobic loop residues, I69 and V325, make contacts with the N-methyl group of creatine. In this study, we report the alteration of the substrate specificity of CK through the mutagenesis of V325. The V325 to glutamate mutation results in a more than 100-fold preference for glycocyamine, while mutation of V325 to alanine results in a slight preference of the enzyme for cyclocreatine (1-carboxymethyl-2-iminoimidazolidine). This study enhances our understanding of how the active sites of phosphagen kinases have evolved to recognize their respective substrates and catalyze their reactions.

Consequences of a six residual deletion from the N-terminal of rabbit muscle creatine kinase

A mutant of dimeric rabbit muscle creatine kinase (CK), in which six residues (residues 2-7) at the N-terminal were removed by the PCR method, was studied to assess the role of these residues in dimer cohesion and to determine the structural stability of the protein. The specific activity of the mutant was 70.39% of that of the wild-type CK, and the affinity for Mg-ATP and CK substrates was slightly reduced compared with the wild-type protein. The structural stability of the mutant was investigated by a comparative equilibrium urea denaturation study and a thermal denaturation study. The data acquired by intrinsic fluorescence and far-UV circular dichroism (CD) during urea unfolding indicated that, the secondary and tertiary structures of the mutant were more stable than those of wild-type CK. Furthermore, results of 8-anilino-1-naphthalene-sulfonic acid (ANS) fluorescence demonstrated that the hydrophobic surface of the mutant CKND(6) was more stable during urea titration. Data from size exclusion chromatography (SEC) experiments indicated that deletion of the six N-terminal residues resulted in a relatively loose molecular structure, but the dissociation of the mutant CKND(6) occurred later during the unfolding process than for wild-type CK. Consistent with this result, the differential scanning calorimetry (DSC) profiles demonstrated that the thermal stability of the enzyme was increased by removal of the six N-terminal residues. We conclude that a more stable quaternary structure was obtained by deletion of the six residues from the N-terminal of wild-type CK.

The putative catalytic bases have, at most, an accessory role in the mechanism of arginine kinase

Arginine kinase is a member of the phosphagen kinase family that includes creatine kinase and likely shares a common reaction mechanism in catalyzing the buffering of cellular ATP energy levels. Abstraction of a proton from the substrate guanidinium by a catalytic base has long been thought to be an early mechanistic step. The structure of arginine kinase as a transition state analog complex (Zhou, G., Somasundaram, T., Blanc, E., Parthasarathy, G., Ellington, W. R., and Chapman, M. S. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 8449-8454) showed that Glu-225 and Glu-314 were the only potential catalytic residues contacting the phosphorylated nitrogen. In the present study, these residues were changed to Asp, Gln, and Val or Ala in several single and multisite mutant enzymes. These mutations had little impact on the substrate binding constants. The effect upon activity varied with reductions in kcat between 3000-fold and less than 2-fold. The retention of significant activity in some mutants contrasts with published studies of homologues and suggests that acid-base catalysis by these residues may enhance the rate but is not absolutely essential. Crystal structures of mutant enzymes E314D at 1.9 A and E225Q at 2.8 A resolution showed that the precise alignment of substrates is subtly distorted. Thus, pre-ordering of substrates might be just as important as acid-base chemistry, electrostatics, or other potential effects in the modest impact of these residues upon catalysis.

The 2.1 A structure of Torpedo californica creatine kinase complexed with the ADP-Mg(2+)-NO(3)(-)-creatine transition-state analogue complex

Creatine kinase (CK) catalyzes the reversible conversion of creatine and ATP to phosphocreatine and ADP, thereby helping maintain energy homeostasis in the cell. Here we report the first X-ray structure of CK bound to a transition-state analogue complex (CK-TSAC). Cocrystallization of the enzyme from Torpedo californica (TcCK) with ADP-Mg(2+), nitrate, and creatine yielded a homodimer, one monomer of which was liganded to a TSAC complex while the second monomer was bound to ADP-Mg(2+) alone. The structures of both monomers were determined to 2.1 A resolution. The creatine is located with the guanidino nitrogen cis to the methyl group positioned to perform in-line attack at the gamma-phosphate of ATP-Mg(2+), while the ADP-Mg(2+) is in a conformation similar to that found in the TSAC-bound structure of the homologue arginine kinase (AK). Three ligands to Mg(2+) are contributed by ADP and nitrate and three by ordered water molecules. The most striking difference between the substrate-bound and TSAC-bound structures is the movement of two loops, comprising residues 60-70 and residues 323-332. In the TSAC-bound structure, both loops move into the active site, resulting in the positioning of two hydrophobic residues (one from each loop), Ile69 and Val325, near the methyl group of creatine. This apparently provides a specificity pocket for optimal creatine binding as this interaction is missing in the AK structure. In addition, the active site of the transition-state analogue complex is completely occluded from solvent, unlike the ADP-Mg(2+)-bound monomer and the unliganded structures reported previously.

Determination of the affinity of each component of a composite quaternary transition-state analogue complex of creatine kinase

Recombinant rabbit muscle creatine kinase (CK) was titrated with MgADP in 50 mM Bicine and 5 mM Mg(OAc)2, pH 8.3, at 30.0 degrees C by following a decrease in the protein's intrinsic fluorescence. In the presence of 50 mM NaOAc, but in the absence of added creatine or nitrate, MgADP has an apparent K(d) of 135 +/- 7 microM, and the total change in fluorescence on saturation (Delta%F) is 15.3 +/- 0.3%. Acetate was used as the anion in this experiment because it does not promote the formation of a CK.MgADP.anion.creatine transition-state analogue complex (TSAC) [Millner-White and Watts (1971) Biochem. J. 122, 727-740]. In the presence of 80 mM creatine, but no nitrate, the apparent K(d) for MgADP remains essentially unchanged at 132 +/- 10 microM, while Delta%F decreases slightly to 13.2 +/- 0.3%. In the presence of 10 mM nitrate, but no creatine, the apparent K(d) is once again essentially unchanged at 143 +/- 23 microM, but the Delta%F is markedly reduced to 4.2 +/- 0.2%. The presence of both 10 mM nitrate and 80 mM creatine during titration reduces the apparent K(d) for MgADP 10-fold to 13.7 +/- 0.7 microM, and Delta%F increases to 20.6 +/- 0.3%, strongly suggesting that the simultaneous presence of saturating levels of creatine and nitrate increases the affinity of CK for MgADP and promotes the formation of the enzyme*MgADP*nitrate*creatine TSAC. When the fluorescence of CK was titrated with MgADP in the presence of 80 mM creatine and fixed saturating concentrations of various anions, apparent K(d) values for MgADP of 132 +/- 10 microM, 25.2 +/- 1.3 microM, 18.8 +/- 0.9 microM, 13.7 +/- 0.7 microM, and 6.4 +/- 0.7 microM were observed as the anion was changed from acetate to formate to chloride to nitrate to nitrite, respectively. This is the same trend reported by Millner-White and Watts for the effectiveness of various monovalent anions in forming the CK.MgADP.anion.creatine TSAC. On titration of CK with MgADP in the presence of 80 mM creatine and various fixed concentrations of NaNO3, the apparent K(d) for MgADP decreases with increasing fixed concentrations of nitrate. A plot of the apparent K(d) for MgADP vs [NO3-] suggests a K(d) for nitrate from the TSAC of 0.39 +/- 0.07 mM. Similarly, titration with MgADP in the presence of 10 mM NaNO3 and various fixed concentrations of creatine gives a value of 0.9 +/- 0.4 mM for the dissociation of creatine from the TSAC. The data were used to calculate K(TDAC), the dissociation constant of the quaternary TSAC into its individual components, of 3 x 10(-10) M3. To our knowledge this is the first reported dissociation constant for a ternary or quaternary TSAC.

Reduced creatine kinase activity in transgenic amyotrophic lateral sclerosis mice

Creatine (Cr), the substrate of the creatine kinase (CK) isoenzymes, was shown to be neuroprotective in several models of neurodegeneration, including amyotrophic lateral sclerosis (ALS). In order to investigate the mechanism of this beneficial effect, we determined CK activities and mitochondrial respiration rates in tissues from G93A transgenic mice, which overexpress a mutant form of human superoxide dismutase associated with familial ALS (FALS). While respiration rates of mitochondria from G93A transgenic or wild-type control mice isolated from spinal cord showed no difference, a significant and dramatic loss of CK activity could be detected in these tissues. In homogenates from spinal cord of G93A transgenic mice, CK activity decreased to 49% and in mitochondrial fractions to 67% compared to CK activities in wild-type control mice. Feeding the G93A transgenic mice with 2% Cr, the same tissues showed no statistically significant increase of CK activity compared to regular fed G93A transgenic mice. Experiments with isolated mitochondria, however, showed that Cr and adenosine triphosphate (ATP) protected mitochondrial CK activity against peroxynitrite-induced inactivation, which may play a role in tissue damage in neurodegeneration. Our data provide evidence for oxidative damage to the CK system in ALS, which may contribute to impaired energy metabolism and neurodegeneration.

An unusually low pK(a) for Cys282 in the active site of human muscle creatine kinase

All phosphagen kinases contain a conserved cysteine residue which has been shown by crystallographic studies, on both creatine kinase and arginine kinase, to be located in the active site. There are conflicting reports as to whether this cysteine is essential for catalysis. In this study we have used site-directed mutagenesis to replace Cys282 of human muscle creatine kinase with serine and methionine. In addition, we have replaced Cys282, conserved across all creatine kinases, with alanine. No activity was found with the C282M mutant. The C282S mutant showed significant, albeit greatly reduced, activity in both the forward (creatine phosphorylation) and reverse (MgADP phosphorylation) reactions. The K(m) for creatine was increased approximately 10-fold, but the K(m) for phosphocreatine was relatively unaffected. The V and V/K pH-profiles for the wild-type enzyme were similar to those reported for rabbit muscle creatine kinase, the most widely studied creatine kinase isozyme. However, the V/K(creatine) profile for the C282S mutant was missing a pK of 5.4. This suggests that Cys282 exists as the thiolate anion, and is necessary for the optimal binding of creatine. The low pK of Cys282 was also determined spectrophotometrically and found to be 5.6 +/- 0.1. The S284A mutant was found to have reduced catalytic activity, as well as a 15-fold increase in K(m) for creatine. The pK(a) of Cys282 in this mutant was found to be 6.7 +/- 0.1, indicating that H-bonding to Ser284 is an important, but not the sole, factor contributing to the unusually low pK(a) of Cys282.