Crystallization and preliminary X-ray analysis of two different forms of mitochondrial creatine kinase from chicken cardiac muscle

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.

Direct evidence for the control of mitochondrial respiration by mitochondrial creatine kinase in oxidative muscle cells in situ

The efficiency of stimulation of mitochondrial respiration in permeabilized muscle cells by ADP produced at different intracellular sites, e.g. cytosolic or mitochondrial intermembrane space, was evaluated in wild-type and creatine kinase (CK)-deficient mice. To activate respiration by endogenous production of ADP in permeabilized cells, ATP was added either alone or together with creatine. In cardiac fibers, while ATP alone activated respiration to half of the maximal rate, creatine plus ATP increased the respiratory rate up to its maximum. To find out whether the stimulation by creatine is a consequence of extramitochondrial [ADP] increase, or whether it directly correlates with ADP generation by mitochondrial CK in the mitochondrial intermembrane space, an exogenous ADP-trap system was added to rephosphorylate all cytosolic ADP. Under these conditions, creatine plus ATP still increased the respiration rate by 2.5 times, compared with ATP alone, for the same extramitochondrial [ADP] of 14 microM. Moreover, this stimulatory effect of creatine, observed in wild-type cardiac fibers disappeared in mitochondrial CK deficient, but not in cytosolic CK-deficient muscle. It is concluded that respiration rates can be dissociated from cytosolic [ADP], and ADP generated by mitochondrial CK is an important regulator of oxidative phosphorylation.

Structural changes of creatine kinase upon substrate binding

Small-angle x-ray scattering was used to investigate structural changes upon binding of individual substrates or a transition state analog complex (TSAC; Mg-ADP, creatine, and KNO3) to creatine kinase (CK) isoenzymes (dimeric muscle-type (M)-CK and octameric mitochondrial (Mi)-CK) and monomeric arginine kinase (AK). Considerable changes in the shape and the size of the molecules occurred upon binding of Mg-nucleotide or TSAC. The radius of gyration of Mi-CK was reduced from 55.6 A (free enzyme) to 48.9 A (enzyme plus Mg-ATP) and to 48.2 A (enzyme plus TSAC). M-CK showed similar changes from 28.0 A (free enzyme) to 25.6 A (enzyme plus Mg-ATP) and to 25.5 A (enzyme plus TSAC). Creatine alone did not lead to significant changes in the radii of gyration, nor did free ATP or ADP. AK also showed a change of the radius of gyration from 21.5 A (free enzyme) to 19.7 A (enzyme plus Mg-ATP), whereas with arginine alone only a minor change could be observed. The primary change in structure as seen with monomeric AK seems to be a Mg-nucleotide-induced domain movement relative to each other, whereas the effect of substrate may be of local order only. In CK, however, additional movements have to be involved.

The active site histidines of creatine kinase. A critical role of His 61 situated on a flexible loop

A histidine residue with a pKa of 7 has been inferred to act as a general acid-base catalyst for the reaction of creatine kinase (CK), catalyzing the reversible phosphorylation of creatine by ATP. The chicken sarcomeric muscle mitochondrial isoenzyme Mib-CK contains several histidine residues that are conserved throughout the family of creatine kinases. By X-ray crystal structure analysis, three of them (His 61, His 92, and His 186) were recently shown to be located close to the active site of the enzyme. These residues were exchanged against alanine or aspartate by in vitro mutagenesis, and the six mutant proteins were expressed in E. coli and purified. Structural integrity of the mutant proteins was checked by small-angle X-ray scattering. Kinetic analysis showed the mutant His 61 Asp to be completely inactive in the direction of ATP consumption while exhibiting a residual activity of 1.7% of the wild-type (wt) activity in the reverse direction. The respective His to Ala mutant of residue 61 showed approximately 1% wt activity in the forward and 10% wt activity in the reverse reaction. All other mutants showed near wt activities. Changes in the kinetic parameters K(m) or Vmax, as well as a significant loss of synergism in substrate binding, could be observed with all active mutants. These effects were most pronounced for the binding of creatine and phosphocreatine, whereas ATP or ADP binding were less severely affected. Based on our results, we assume that His 92 and His 186 are involved in the binding of creatine and ATP in the active site, whereas His 61 is of importance for the catalytic reaction but does not serve as an acid-base catalyst in the transphosphorylation of creatine and ATP. In addition, our data support the idea that the flexible loop bearing His 61 is able to move towards the active site and to participate in catalysis.

Expression, purification from inclusion bodies, and crystal characterization of a transition state analog complex of arginine kinase: a model for studying phosphagen kinases

Phosphagen kinases catalyze the reversible transfer of a phosphoryl group between guanidino phosphate compounds and ADP, thereby regenerating ATP during bursts of cellular activity. Large quantities of highly pure arginine kinase (EC 2.7.3.3), the phosphagen kinase present in arthropods, have been isolated from E. coli, into which the cDNA for the horseshoe crab enzyme had been cloned. Purification involves size exclusion and anion exchange chromatographies applied in the denatured and refolded states. The recombinant enzyme has been crystallized as a transition state analog complex. Near complete native diffraction data have been collected to 1.86 A resolution. Substitution of a recombinant source for a natural one, improvement in the purification, and data collection at cryo temperatures have all yielded significant improvements in diffraction.

Structure of mitochondrial creatine kinase

Creatine kinase (CK, EC 2.7.3.2), an enzyme important for energy metabolism in cells of high and fluctuating energy requirements, catalyses the reversible transfer of a phosphoryl goup from phosphocreatine to ADP. We have solved the structure of the octameric mitochondrial isoform, Mib-CK, which is located in the intermembrane compartment and along the cristae membranes. Mib-CK consumes ATP produced in the mitochondria for the production of phosphocreatine, which is then exported into the cytosol for fast regeneration of ATP by the cytosolic CK isoforms. The octamer has 422 point-group symmetry, and appears as a cube of side length 93 angstrom with a channel 20 angstrom wide extending along the four-fold axis. Positively charged amino acids at the four-fold faces of the octamer possibly interact with negatively charged mitochondrial membranes. Each monomer consists of a small alpha-helical domain and a large domain containing an eight-stranded antiparallel beta-sheet flanked by seven alpha-helices. The conserved residues of the CK family form a compact cluster that covers the active site between the domains.

Expression of active octameric chicken cardiac mitochondrial creatine kinase in Escherichia coli

Sarcomeric mitochondrial creatine kinase (Mib-CK) of chicken was expressed in Escherichia coli as a soluble enzyme by using an inducible phage-T7 promoter. Up to one third of the protein in E. coli extracts consisted of soluble recombinant Mib-CK in an enzymically active form. Approx. 20 mg of nearly-homogenous Mib-CK was isolated in a two-step isolation procedure starting with 1 litre of isopropyl beta-D-thiogalactopyranoside-induced E. coli culture, whereas previous attempts to express other CK genes in E. coli have resulted in 20-fold lower yields and inclusion-body formation. Selection of the Mib-CK expression plasmid on media containing kanamycin rather than ampicillin extended the time period of maximal Mib-CK expression. Recombinant Mib-CK displayed an identical N-terminal amino acid sequence, identical Km for phosphocreatine and Vmax. values, the same electrophoretic behaviour and the same immunological cross-reactivity as the native enzyme isolated from chicken heart mitochondria. The recombinant Mib-CK had the same molecular mass as native chicken Mib-CK in m.s. analysis, indicating that post-translational modification of the enzyme in chicken tissue does not occur. As judged by gel-permeation chromatography and electron microscopy, recombinant enzyme formed predominantly octameric oligomers with the same overall structure as the chicken heart enzyme. Furthermore, the enzymes isolated from both sources formed protein crystals of space group P42(1)2, when grown in the absence of ATP, with one Mi-CK octamer per asymmetric unit. The indistinguishable X-ray-diffraction patterns indicate identical structures for the native and recombinant proteins.

Intracellular compartmentation, structure and function of creatine kinase isoenzymes in tissues with high and fluctuating energy demands: the 'phosphocreatine circuit' for cellular energy homeostasis

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Adult rat cardiomyocytes cultured in creatine-deficient medium display large mitochondria with paracrystalline inclusions, enriched for creatine kinase

In adult regenerating cardiomyocytes in culture, in contrast to fetal cells, mitochondrial creatine kinase (Mi-CK) was expressed. In the same cell, two populations of mitochondria, differing in shape, in distribution within the cell and in content of Mi-CK, could be distinguished. Immunofluorescence studies using antibodies against Mi-CK revealed a characteristic staining pattern for the two types of mitochondria: giant, mostly cylindrically shaped, and, as shown by confocal laser light microscopy, randomly distributed mitochondria exhibited a strong signal for Mi-CK, whereas small, "normal" mitochondria, localized in rows between myofibrils, gave a much weaker signal. Transmission EM of the giant mitochondria demonstrated paracrystalline inclusions located between cristae membranes. Immunogold labeling with anti-Mi-CK antibodies revealed a specific decoration of these inclusions for Mi-CK. Addition of 20 mM creatine, the substrate of Mi-CK, to the essentially creatine-free culture medium caused the disappearance of the giant cylindrically shaped mitochondria as well as of the paracrystalline inclusions, accompanied by an increase of the intracellular level of total creatine. Replacement of creatine in the medium by the creatine analogue and competitor beta-guanidinopropionic acid caused the reappearance of the enlarged mitochondria. It is believed that the accumulation of Mi-CK within the paracrystalline inclusions, similar to those observed in certain myopathies, represents a compensatory effect of the cardiomyocytes to cope with a metabolic stress situation caused by low intracellular total creatine levels.