Immunological and physical comparison of monomeric and dimeric phosphagen kinases: Some evolutionary implications

The antigenic and physical properties of several representative invertebrate phosphagen kinases have been examined in order to further characterize the relationship between taxonomic assignment, quaternary protein structure and evolution of this class of enzymes. Antibodies against dimeric arginine kinase from the sea cucumber cross-reacted with dimeric arginine kinase purified from sea urchin eggs, but failed to react with extracts from any species known to contain monomeric arginine kinase. However, strong immunoreactivity was observed when antibodies against purified dimeric arginine kinase were reacted with pure creatine kinase from the human muscle (CK-MM) and brain (CK-BB) as well as extracts from several species known to contain dimeric creatine kinase. Of particular interest with regard to evolution of the phosphagen kinases, we confirm the presence of creatine kinase activity in the very primitive sponge Tethya aurnatium and detect a reaction with antibodies against dimeric, but not monomeric, arginine kinase. This observation is consistent with recent studies of phosphagen kinase evolution. Substrate utilization was very specific with creatine kinase using only creatine. Arginine kinase catalyzed phosphorylation of arginine but enzymes from several species could also phosphorylate canavanine. No activities were detected with d-arginine. Isoelectric points, evaluated for several pure arginine kinases suggest that generally the monomeric forms are more acidic than the dimeric proteins. Heat inactivation of arginine kinase in several species indicated a wide range of stabilities, which did not appear to be correlated with quaternary structure, but rather distinguished by the organism's environment. On the other hand, homodimeric arginine kinase proteins from species inhabiting disparate environments are sufficiently homologous to form a catalytically active hybrid.     

Phosphagen kinase evolution. Expression in echinoderms

Arginine kinase and creatine kinase that catalyze the transfer of a phosphate group between ATP and arginine and creatine, respectively, play an important role in cellular energetics. In contrast to most animals which exhibit a single phosphagen kinase activity (creatine kinase in chordates and arginine kinase in protostomians), echinoderms exhibit both arginine kinase and creatine kinase activities, sometimes in the same tissue. In contrast to chordates in which creatine kinases are dimers (consisting of two subunits of 40 kDa) and protostomians in which arginine kinases are usually monomers (40 kDa), echinoids contain specific phosphagen kinases: a dimeric arginine kinase (consisting of two subunits of 42 kDa) in eggs and a monomeric creatine kinase (145 kDa) in sperm. We have examined echinoderms from the five existing classes (echinoids, asteroids, ophiuroids, holothurians and crinoids) for the expression of these specific phosphagen kinases in different tissues. Gel filtration was used to determine the molecular masses of the native enzymes. Antibodies specific for arginine kinase or for creatine kinase were used to characterize the subunit composition of arginine kinase and creatine kinase after SDS/PAGE and transfer. In all echinoderms analyzed, arginine kinase always occurred as an enzyme of about 81 kDa consisting of two subunits of 42 kDa and creatine kinase as a monomeric enzyme of 140-155 kDa. The occurrence in echinoderms of both phosphagen kinases with distinct specificities and specific molecular structures is discussed from both a developmental and evolutionary point of view.     

Origin and properties of cytoplasmic and mitochondrial isoforms of taurocyamine kinase

Taurocyamine kinase (TK) is a member of the highly conserved family of phosphagen kinases that includes creatine kinase (CK) and arginine kinase. TK is found only in certain marine annelids. In this study we used PCR to amplify two cDNAs coding for TKs from the polychaete Arenicola brasiliensis, cloned these cDNAs into the pMAL plasmid and expressed the TKs as fusion proteins with the maltose-binding protein. These are the first TK cDNA and deduced amino acid sequences to be reported. One of the two cDNA-derived amino acid sequences of TKs shows a high amino acid identity to lombricine kinase, another phosphagen kinase unique to annelids, and appears to be a cytoplasmic isoform. The other sequence appears to be a mitochondrial isoform; it has a long N-terminal extension that was judged to be a mitochondrial targeting peptide by several on-line programs and shows a higher similarity in amino acid sequence to mitochondrial creatine kinases from both vertebrates and invertebrates. The recombinant cytoplasmic TK showed activity for the substrates taurocyamine and lombricine (9% of that of taurocyamine). However, the mitochondrial TK showed activity for taurocyamine, lombricine (30% of that of taurocyamine) and glycocyamine (7% of that of taurocyamine). Neither TK catalyzed the phosphorylation of creatine. Comparison of the deduced amino acid sequences of mitochondrial CK and TK indicated that several key residues required for CK activity are lacking in the mitochondrial TK sequence. Homology models for both cytoplasmic and mitochondrial TK, constructed using CK templates, provided some insight into the structural correlation of differences in substrate specificity between the two TKs. A phylogenetic analysis using amino acid sequences from a broad spectrum of phosphagen kinases showed that annelid-specific phosphagen kinases (lombricine kinase, glycocyamine kinase and cytoplasmic and mitochondrial TKs) are grouped in one cluster, and form a sister-group with CK sequences from vertebrate and invertebrate groups. It appears that the annelid-specific phosphagen kinases, including cytoplasmic and mitochondrial TKs, evolved from a CK-like ancestor(s) early in the divergence of the protostome metazoans. Furthermore, our results suggest that the cytoplasmic and mitochondrial isoforms of TK evolved independently.     

Purification and characterization of human mitochondrial creatine kinase. A single enzyme form

Purification of human mitochondrial creatine kinase has been difficult and procedures that were highly successful in purifying canine enzyme failed for human mitochondrial creatine kinase. In the present study, we employed ultracentrifugation to remove the lipid, urea to prevent aggregation, followed by a final step of chromatofocusing which yielded a preparation of human mitochondrial creatine kinase with a specific enzyme activity of greater than 400 IU/mg. Biochemical and immunological characterization showed the preparation to be highly pure and free of even trace amounts of other creatine kinase isoenzymes. Antiserum specific for mitochondrial creatine kinase was developed which exhibited no cross-reactivity to cytosolic creatine kinase and mitochondrial creatine kinase did not cross-react with antiserum to the cytosolic forms. Marked differences were noted, both biochemically and immunologically, between mitochondrial creatine kinase and the cytosolic forms. Human mitochondrial creatine kinase was shown to have a molecular weight of around 82,000 and to be composed of two subunits of equal molecular weights around 41,000. Aggregates of mitochondrial creatine kinase were observed with molecular weights of around 200,000 in the absence of urea or if isolated from material after having undergone proteolysis. Isolation from fresh material or in the presence of urea inhibited aggregate formation for both canine and human mitochondrial creatine kinase. Despite claims of several investigators that mitochondrial creatine kinase exhibits two to three forms with varying molecular weights, our data indicate a single enzyme form made up of a subunit with a molecular weight of 41,000 and the high molecular weight aggregates appear to be induced artifacts. A radioimmunoassay was developed for human mitochondrial creatine kinase which, with appropriate modifications, should detect mitochondrial creatine kinase in human plasma.     

The structure of lombricine kinase: implications for phosphagen kinase conformational changes

Lombricine kinase is a member of the phosphagen kinase family and a homolog of creatine and arginine kinases, enzymes responsible for buffering cellular ATP levels. Structures of lombricine kinase from the marine worm Urechis caupo were determined by x-ray crystallography. One form was crystallized as a nucleotide complex, and the other was substrate-free. The two structures are similar to each other and more similar to the substrate-free forms of homologs than to the substrate-bound forms of the other phosphagen kinases. Active site specificity loop 309-317, which is disordered in substrate-free structures of homologs and is known from the NMR of arginine kinase to be inherently dynamic, is resolved in both lombricine kinase structures, providing an improved basis for understanding the loop dynamics. Phosphagen kinases undergo a segmented closing on substrate binding, but the lombricine kinase ADP complex is in the open form more typical of substrate-free homologs. Through a comparison with prior complexes of intermediate structure, a correlation was revealed between the overall enzyme conformation and the substrate interactions of His(178). Comparative modeling provides a rationale for the more relaxed specificity of these kinases, of which the natural substrates are among the largest of the phosphagen substrates.     

Functional expression of phosphagen kinase systems confers resistance to transient stresses in Saccharomyces cerevisiae by buffering the ATP pool

Phosphagen kinase systems provide different advantages to tissues with high and fluctuating energy demands, in particular an efficient energy buffering system. In this study we show for the first time functional expression of two phosphagen kinase systems in Saccharomyces cerevisiae, which does not normally contain such systems. First, to establish the creatine kinase system, in addition to overexpressing creatine kinase isoenzymes, we had to install the biosynthesis pathway of creatine by co-overexpression of L-arginine:glycine amidinotransferase and guanidinoacetate methyltransferase. Although we could achieve considerable creatine kinase activity, together with more than 3 mM intracellular creatine, this was not sufficient to confer an obvious advantage to the yeast under the specific stress conditions examined here. Second, using arginine kinase, we successfully installed an intracellular phosphagen pool of about 5 mM phosphoarginine. Such arginine kinase-expressing yeast showed improved resistance under two stress challenges that drain cellular energy, which were transient pH reduction and starvation. Although transient starvation led to 50% reduced intracellular ATP concentrations in wild-type yeast, arginine kinase overexpression stabilized the ATP pool at the pre-stress level. Thus, our results demonstrate that temporal energy buffering is an intrinsic property of phosphagen kinases that can be transferred to phylogenetically very distant organisms.     

Purification of mitochondrial creatine kinase. Biochemical and immunological characterization

Mitochondrial creatine kinase was purified from canine myocardium. The preparation exhibited a positively charged isoenzyme free of other creatine kinase isoenzymes and on sodium dodecyl sulfate gel exhibited a single protein band. Amino acid composition showed mitochondrial creatine kinase to be different from that of MM or BB creatine kinase and did not hybridize with the M or B subunits of the cytosolic forms. Antiserum was developed to mitochondrial creatine kinase which did not cross-react with cytosolic creatine kinases. Antiserum to cytosolic creatine kinase exhibited no reaction to mitochondrial creatine kinase. Utilizing the specific antiserum, a radioimmunoassay was developed for the specific detection of mitochondrial creatine kinase. Thus, mitochondrial creatine kinase was purified and shown to be comprised of a unique subunit which is biochemically and immunologically distinct from the cytosolic creatine kinases.     

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.     

Characterization of creatine kinase isoforms in herring (Clupea harengus) skeletal muscle

It is known that mitochondrial creatine kinase (MtCK) in mammals is always expressed in conjunction with one of the cytosolic forms of creatine kinase (CK), either muscle-type (MM-CK) or brain-type (BB-CK) in tissues of high, sudden energy demand. The two creatine kinase (CK) isoforms were detected in herring (Clupea harengus) skeletal muscle: cytosolic CK and mitochondrial CK (MtCK) that displayed the different electrophoretic mobility. These isoforms differ in molecular weight and some biochemical properties. Isolation and purification procedures allowed to obtain purified enzymes with specific activity of the 206 μmol/min/mg for cytosolic CK and 240 μmol/min/mg for MtCK. Native M_rs of the cytosolic CK and MtCK determined by gel permeation chromatography were 86.000 and 345.000, respectively. The results indicate that one of isoforms found in herring skeletal muscle is a cytosolic dimer and the other one, is a mitochondrial octamer. Octamerization of MtCK is not an advanced feature and also exists in fish. These values correspond well with published values for MtCKs and cytosolic CK isoforms from higher vertebrate classes and even from lower invertebrates.     

Induced fit in guanidino kinases--comparison of substrate-free and transition state analog structures of arginine kinase

Arginine kinase (AK) is a member of the guanidino kinase family that plays an important role in buffering ATP concentration in cells with high and fluctuating energy demands. The AK specifically catalyzes the reversible phosphoryl transfer between ATP and arginine. We have determined the crystal structure of AK from the horseshoe crab (Limulus polyphemus) in its open (substrate-free) form. The final model has been refined at 2.35 A with a final R of 22.3% (R(free) = 23.7%). The structure of the open form is compared to the previously determined structure of the transition state analog complex in the closed form. Classically, the protein would be considered two domain, but dynamic domain (DynDom) analysis shows that most of the differences between the two structures can be considered as the motion between four rigid groups of nonsequential residues. ATP binds near a cluster of positively charged residues of a fixed dynamic domain. The other three dynamic domains close the active site with separate hinge rotations relative to the fixed domain. Several residues of key importance for the induced motion are conserved within the phosphagen kinase family, including creatine kinase. Substantial conformational changes are induced in different parts of the enzyme as intimate interactions are formed with both substrates. Thus, although induced fit occurs in a number of phosphoryl transfer enzymes, the conformational changes in phosphagen kinases appear to be more complicated than in prior examples.     

Evolution of the diverse array of phosphagen systems present in annelids

Annelids as a group express a variety of phosphagen kinases including creatine kinase (CK), glyocyamine kinase (GK), lombricine kinase (LK), taurocyamine kinase (TK) and a unique arginine kinase (AK) restricted to annelids. In prior work, we have determined and compared the intron/exon organization of the annelid genes for cytoplasmic GK, LK, AK, and mitochondrial TK and LK (MiTK and MiLK, respectively), and found that these annelid genes, irrespective of cytoplasmic or mitochondrial, have the same 8-intron/9-exon organization strikingly similar to mitochondrial CK (MiCK) genes. These results support the view that the MiCK gene is basal and ancestral to the phosphagen kinases unique to annelids. To gain a greater understanding of the evolutionary processes leading to the diversity of annelid phosphagen kinases, we determined for the first time the intron/exon organization of a cytoplasmic CK gene from a polychaete as well as that of another polychaete MiCK gene. These gene structures, coupled with a phylogenetic analyses of annelid enzymes and assessment of the fidelity of substrate specificity of some these phosphagen kinases, provide insight into the pattern of radiation of the annelid enzymes. Annelid phosphagen kinases appeared to have diverged in the following order (earliest first): (1) cytoplasmic AK, LK and TK, (2) GK, and (3) mitochondrial MiLK and MiTK. Interestingly, phylogenetic analyses showed that the above phosphagen kinases appear to be basal to all CK isoforms (mitochondrial, cytoplasmic and flagellar CKs). This somewhat paradoxical placement of CKs most likely reflects a higher rate of evolution and radiation of the annelid-specific LK, TK and GK genes than the CK isoform genes.     

Variable intron/exon structure in the oligochaete lombricine kinase gene

Lombricine kinase is an annelid enzyme that belongs to the phosphagen kinase family of which creatine kinase and arginine kinase are the typical representatives. The enzymes play important roles in the cellular energy metabolism of animals. Biochemical, physiological and molecular information with respect to lombricine kinase is limited compared to other phosphagen kinases. This study presents data on the cDNA sequences of lombricine kinase from two smaller oligochaetes, Enchytraeus sp. and Stylaria sp. The deduced amino acid sequences are analyzed and compared with other selected phosphagen kinases. The intron/exon structure of the lombricine kinase gene was determined for these two species as well as two additional oligochaetes, Lumbriculus variegatus and Tubifex tubifex, and compared with available data for annelid phosphagen kinases. The data indicate the existence of a variable organization of the proposed 8-intron/9-exon gene structure. The results provide further insights in the evolution and position of these enzymes within the phosphagen kinase family.     

The role of creatine kinase and arginine kinase in muscle.

Arginine and creatine kinase activities in different muscles are compared with calculated maximum rates of ATP turnover. The magnitude of the kinase activities decreases in the following order: anaerobic muscles and vertebrate skeletal muscles greater than heart muscle greater than insect flight muscle. The maximum activity of phosphagen kinases (i.e. creatine kinase and arginine kinase), in the direction of phosphagen formation, is lower than the calculated maximum rate of ATP turnover in insect flight muscle or rat heart.     

Comparison of the two different phosphagen systems in the lugworm Arenicola marina

The different phosphagen systems in the lugworm Arenicola marina, the phosphotaurocyamine/taurocyamine kinase system of the body wall and the phosphocreatine/creatine kinase system of the spermatozoa, have been investigated to answer the question whether the change reflects different functional modes of these phosphagen systems. Enzyme analyses have shown that in contrast to the body wall taurocyamine kinase, creatine kinase of spermatozoa exists in at least two different forms which are compartmented in the mitochondria (creatine kinase I) and in the flagellum (creatine kinase II). Creatine kinase I is strongly attached to cell structures which require detergents and high phosphate concentrations for solubilization. The affinities of taurocyamine kinase and creatine kinase for all substrates are very similar except the extremely high K _m for creatine of both creatine kinase I and II. The level of creatine in spermatozoa is fivefold higher than taurocyamine in the body wall at similar phosphorylation potential (ATP/ADO_free) and ATP-buffer capacity (phosphagen/ATP), reflecting the higher equilibrium constants of the creatine kinase reaction compared to that of the taurocyamine kinase reaction (Ellington 1989). The high creatine concentration gives the phosphocreatine/creatine kinase system an advantage over the phosphotaurocyamine/taurocyamine kinase system for transport of energyrich phosphate at high phosphorylation potential by increasing the radial diffusion flux. The maximum diffusive flux of free ADP in spermatozoa is three orders of magnitude below the respiratory ATP production while the creatine flux would allow an unlimited energy transport over the long diffusion distance. In lugworm body wall, however, the low ATP turnover and the low diffusion distances between mitochondria and myosin-ATPases do not require a phosphagen shuttle.Abbreviations ADP _free cytoplasmic adenosine diphosphate - Ap ₅ A P₁, P₅-di(adenosine-5-) pentaphosphate - AK arginine kinase - CK creatine kinase (EC 2.7.3.2) - DTT dithiothreitol - GAPDH glyceraldehydephosphate dehydrogenase (EC 1.2.1.12) - HOADH 3-hydroxyacyl-CoA dehydrogenase (EC 1.1.1.35) - IEP isoelectric point - MIM mitochondria isolating medium - P _i-free cytoplasmic inorganic phosphate - (P)Arg (phospho)arginine - (P)Cr (phospho)creatine - (P)Tc (phospho)taurocyamine - SEM scanning electron microscopy - TK taurocyamine kinase - TEM transmission electron microscopy     

The role of Arg-96 in Danio rerio creatine kinase in substrate recognition and active center configuration

In creatine kinases (CKs), the amino acid residue-96 is a strictly conserved arginine. This residue is not directly associated with substrate binding, but it is located close to the binding site of the substrate creatine. On the other hand, the residue-96 is known to be involved in expression in the substrate specificity of various other phosphagen (guanidino) kinases, since each enzyme has a specific residue at this position: arginine kinase (Tyr), glycocyamine kinase (Ile), taurocyamine kinase (His) and lombricine kinase (Lys). To gain a greater understanding of the role of residue-96 in CKs, we replaced this residue in zebra fish Danio rerio cytoplasmic CK with other 19 amino acids, and expressed these constructs in Escherichia coli. All the twenty recombinant enzymes, including the wild-type, were obtained as soluble form, and their activities were determined in the forward direction. Compared with the activity of wild-type, the R96K mutant showed significant activity (8.3% to the wild-type), but 10 mutants (R96Y, A, S, E, H, T, F, C, V and N) showed a weak activity (0.056–1.0%). In the remaining mutants (R96Q, G, M, P, L, W, D and I), the activity was less than 0.05%. Our mutagenesis studies indicated that Arg-96 in Danio CK can be substituted for partially by Lys, but other replacements caused remarkable loss of activity. From careful inspection of the crystal structures (transition state analog complex (TSAC) and open state) of Torpedo cytoplasmic CK, we found that the side chain of R96 forms hydrogen bonds with A339 and D340 only in the TSAC structure. Based on the assumption that CKs consist of four dynamic domains (domains 1–3, and fixed domain), the above hydrogen bonds act to link putative domains 1 and 3 in TSAC structure. We suggest that residue-96 in CK and equivalent residues in other phosphagen kinases, which are structurally similar, have dual roles: (1) one involves in distinguishing guanidino substrates, and (2) the other plays a key role in organizing the hydrogen-bond network around residue-96 which offers an appropriate active center for the high catalytic turnover. The mode of development of the network appears to be unique each phosphagen kinase, reflecting evolution of each enzyme.     

Arginine kinase in Phytomonas, a trypanosomatid parasite of plants

Phytomonas are trypanosomatid plant parasites closely related to parasites that cause several human diseases. Little is known about the biology of these organisms including aspects of their metabolism. Arginine kinase (E.C. 2.7.3.3) is a phosphotransferase which catalyzes the interconversion between the phosphagen phosphoarginine and ATP. This enzyme is present in some invertebrates and is a homolog of another widely distributed phosphosphagen kinase, creatine kinase. In this work, a single canonical arginine kinase isoform was detected in Phytomonas Jma by enzymatic activity assays, PCR, and Western Blot. This arginine kinase is very similar to the canonical isoforms found in T. cruzi and T. brucei, presenting about 70% of amino acid sequence identity and a very similar molecular weight (40kDa). The Phytomonas phosphagen system seems to be very similar to T. cruzi, which has only one isoform, or T. brucei (three isoforms); establishing a difference with other trypanosomatids, such as Leishmania, which completely lacks phosphagen kinases, probably by the presence of the arginine-consuming enzyme, arginase. Finally, phylogenetic analysis suggests that Kinetoplastids' arginine kinase was acquired, during evolution, from the arthropod vectors by horizontal gene transfer.     

Evolution of the Cytoplasmic and Mitochondrial Phosphagen Kinases Unique to Annelid Groups

Creatine kinase (CK) is a member of a group of phosphoryl transfer enzymes called phosphagen kinases that play a key role in cellular energy transactions in animals. Three CK isoform gene families are known—cytoplasmic CK (CK), flagellar CK (fCK), and mitochondrial CK (MiCK). Each of the isoforms has a unique gene structure (intron/exon organization). A broad array of other phosphagen kinases is present in animals. Some of these enzymes are found only in annelids and closely related groups including glyocyamine kinase (GK), lombricine kinase (LK), taurocyamine kinase (TK), and a unique arginine kinase (AK) restricted to annelids. Phylogenetic analyses of these annelid phosphagen kinases indicate that they appear to have evolved from a CK-like ancestor. To gain a greater understanding of the relationship of the CK isoforms to the annelid enzymes, we have determined the intron/exon organization of the genes for the following phosphagen kinases: Eisenia LK, Sabellastarte AK, and Arenicola mitochondrial TK (MiTK). Analysis of genomic database for the polychaete Capitella sp. yielded two putative LK genes [cytoplasmic LK and mitochondrial LK (MiLK)]. The intron/exon organization of these genes was compared with available data for cytoplasmic and mitochondrial CKs, and an annelid GK. Surprisingly, these annelid genes, irrespective of whether they are cytoplasmic (LK, AK, and GK) or mitochondrial (MiTK and MiLK), had the same 8-intron/9-exon organization and were strikingly similar to MiCK genes sharing seven of eight splice junctions. These results support the view that the MiCK gene is basal and ancestral to the phosphagen kinases unique to annelids.     

Inverse metabolic engineering with phosphagen kinase systems improves the cellular energy state

Inverse metabolic engineering attempts to identify or construct desired phenotypes of applied interest to endow them on appropriate host organisms. A particular desirable phenotype is the ATP homeostasis exhibited by animal cells with high and variable ATP turnover through temporal and spatial energy buffering. This buffering is achieved by phosphagen kinase systems that consist of a specific kinase and its cognate phosphagen, which functions as a large pool of 'high-energy phosphates' that are used to replenish ATP during periods of high energetic demand. This review discusses recent advances and potentials of inverse metabolic engineering of cell types that do not normally contain such systems--bacteria, yeast, plants, and liver--with creatine or arginine kinase systems. Examples are discussed that illustrate how microbial metabolism can be tailored for large-scale industrial processes with imperfect mixing and how the liver can be protected from metabolic insults or stimulated for better regeneration.     

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.     

Mitochondrial creatine kinase in human health and disease

Mitochondrial creatine kinase (MtCK), together with cytosolic creatine kinase isoenzymes and the highly diffusible CK reaction product, phosphocreatine, provide a temporal and spatial energy buffer to maintain cellular energy homeostasis. Mitochondrial proteolipid complexes containing MtCK form microcompartments that are involved in channeling energy in form of phosphocreatine rather than ATP into the cytosol. Under situations of compromised cellular energy state, which are often linked to ischemia, oxidative stress and calcium overload, two characteristics of mitochondrial creatine kinase are particularly relevant: its exquisite susceptibility to oxidative modifications and the compensatory up-regulation of its gene expression, in some cases leading to accumulation of crystalline MtCK inclusion bodies in mitochondria that are the clinical hallmarks for mitochondrial cytopathies. Both of these events may either impair or reinforce, respectively, the functions of mitochondrial MtCK complexes in cellular energy supply and protection of mitochondria form the so-called permeability transition leading to apoptosis or necrosis.