Creatine kinase: a role for arginine-95 in creatine binding and active site organization

Phosphagen kinases from five groups of eukaryotic protists (Choanomonada, Alveolate, Stramenopiles, Haptophyta, and Cryptophyta): Diverse enzyme activities and phylogenetic relationship with metazoan enzymes

Among 28 groups of eukaryotes, apart from Metazoa, phosphagen kinase (PKs) is distributed in only a few protist groups, including the Choanomonada with the closest affinity to metazoans. To clarify the origin of metazoan PKs, we performed a database search and focused on 11 sequences of PK homologs from five groups of protists: the Choanomonada, Alveolata, Haptophyta, Stramenopiles, and Cryptophyta. The recombinant enzymes were prepared to determine their substrate specificity. Emiliania (Haptophyta), Anophryoides, Pseudocohnilembus, Vitrella and Chromera (Alveolata), and Monosiga (Choanomonada) all contained a gene for arginine kinase (AK). In contrast, Aphanomyces, Albugo and Ectocarpus (Stramenopiles), and Guillardia (Cryptophyta) possessed a gene for taurocyamine kinase (TK). The Guillardia TK enzyme exhibited rather strong substrate inhibition toward taurocyamine, which was analyzed using the most likely kinetic model. This was the first report of substrate inhibition in a TK. Together with the research results from other groups, the AK, TK, or creatine kinase (CK) activities have been observed sporadically in at least six groups of protists. However, it is not clear the three enzyme activities were emerged early in the evolution and divergence of protist groups, or some of enzyme activities were introduced to the protists by horizontal gene transfer. In addition, we found that seven protist enzymes examined in this study possess a myristoylation signaling sequence at the N-terminus. The amino-acid sequence around the guanidine-specificity region and the key residue at 89^th position of the protist AK and CK were homologous to those of the metazoan enzymes, but those for protist TKs were different indicating that the latter evolved independently.

Diversity of phosphagen kinases in annelids: The first sequence report for a putative opheline kinase

Opheline kinase (OK) is one of the phosphagen kinases (PKs) restricted to annelids, but the amino acid sequence has not been determined yet. The OK enzyme was isolated in 1966 from the polychaete Ophelia neglecta (Opheliidae) and shown to have somewhat broader activities for the various substrates opheline, lombricine and taurocyamine. To determine the OK sequence, we analyzed the RNA sequencing data for Ophelina sp. and Thoracophelia sp., belonging to Opheliidae. Four PK sequences, namely, taurocyamine kinase (TK), creatine kinase (CK), mitochondrial CK (MiCK) and putative OK, were identified in both species, and the recombinant Ophelina enzymes were expressed in E. coli and purified. Since the substrate opheline was not commercially available, we used the partial activity toward taurocyamine to infer the enzyme specificity. The putative Ophelina OK showed lower activity to taurocyamine with a V_max/K_m nearly identical to a previously published value for an OK from a related species Ophelia neglecta. Under the same conditions, the true Ophelina TK showed much higher activity. Thus, the putative Ophelina enzyme was determined to be OK. The amino acid sequence alignment indicated that Ophelina and Thoracophelia OKs have five amino acid deletions in the GS region, like those of LKs and AKs, and the guanidino substrate specific residue was Lys, the same as LKs. In the phylogenetic tree constructed from annelid PK amino acid sequences, the OK sequences formed a distinct cluster, and it was placed near the TK and lombricine kinase (LK) clusters. This is the first report of the amino acid sequence for the OK enzyme.

Structure and functional analysis reveal an important regulated role of arginine kinase in Patinopecten yessoensis under low pH stress

Arginine kinase (AK), an important member of the phosphokinase family, is involved in temporal and spatial adenosine triphosphate (ATP) buffering systems. AK plays an important role in physiological function and metabolic regulations, in particular tissues with high and fluctuating energy demands. In present study, four AK genes were firstly identified from Yesso scallop (Patinopecten yessoensis) genome, respectively named PyAK1-4. PyAKs have highly conserved structures with a six-exon/five-exon structure, except for PyAK3. PyAK3 contains an unusual two-domain structure and a "bridge intron" between the two domains, which may originate from gene duplication and subsequent fusion. Phylogenetic analysis showed that all PyAKs belonged to an AK supercluster together with other AK proteins from Mollusca, Platyhelminthes, Arthropoda, and Nematode. A transcriptome database demonstrated that PyAK3 and PyAK4 were the main functional executors with high expression level during larval development and in adult tissues, while PyAK1 and PyAK2 were expressed at a low level. Furthermore, both PyAK2 and PyAK3 showed notably high expression in the male gonad, and PyAK4 was broadly expressed in almost all tissues with the highest level in striated muscle, indicating a tissue-specific expression pattern of PyAKs. In addition, quantitative real-time PCR results demonstrated that the expression of PyAK2, PyAK3 and PyAK4 were significantly upregulated in response to pH stress, especially in an extremely acidifying condition (pH 6.5), revealing the possible involvement of PyAKs in energetic homeostasis during environmental changes. Collectively, a comprehensive analysis of PyAKs was conducted in P. yessoensis. The diversity of PyAKs and their specific expression patterns promote a better understanding of energy metabolism in the growth, development and environmental response of P. yessoensis.

Arginine Kinases from the Precious Corals Corallium rubrum and Paracorallium japonicum: Presence of Two Distinct Arginine Kinase Gene Lineages in Cnidarians

The cDNA sequence of arginine kinase (AK) from the precious coral Corallium rubrum was assembled from transcriptome sequence data, and the deduced amino acid sequence of 364 residues was shown to conserve the structural features characteristic of AK. Based on the amino acid sequence, the DNA coding C. rubrum AK was synthesized by overlap extension PCR to prepare the recombinant enzyme. The following kinetic parameters were determined for the C. rubrum enzyme: K _a^Arg (0.10 mM), K _ia^Arg (0.79 mM), K _a^ATP (0.23 mM), K _ia^ATP (2.16 mM), and k _cat (74.3 s^-1). These are comparable with the kinetic parameters of other AKs. However, phylogenetic analysis suggested that the C. rubrum AK sequence has a distinct origin from that of other known cnidarian AKs with unusual two-domain structure. Using oligomers designed from the sequence of C. rubrum AK, the coding region of genomic DNA of another coral Paracorallium japonicum AK was successfully amplified. Although the nucleotide sequences differed between the two AKs at 14 positions in the coding region, all involved synonymous substitutions, giving the identical amino acid sequence. The P. japonicum AK gene contained one intron at a unique position compared with other cnidarian AK genes. Together with the observations from phylogenetic analysis, the comparison of exon/intron organization supports the idea that two distinct AK gene lineages are present in cnidarians. The difference in the nucleotide sequence between the coding regions of C. rubrum and P. japonicum AKs was 1.28%, which is twice that (0.54%) of mitochondrial DNA, is consistent with the general observation that the mitochondrial genome evolves slower than the nuclear one in cnidarians.

Biochemical and structural characterization of a novel arginine kinase from the spider Polybetes pythagoricus

Energy buffering systems are key for homeostasis during variations in energy supply. Spiders are the most important predators for insects and therefore key in terrestrial ecosystems. From biomedical interest, spiders are important for their venoms and as a source of potent allergens, such as arginine kinase (AK, EC 2.7.3.3). AK is an enzyme crucial for energy metabolism, keeping the pool of phosphagens in invertebrates, and also an allergen for humans. In this work, we studied AK from the Argentininan spider Polybetes pythagoricus (PpAK), from its complementary DNA to the crystal structure. The PpAK cDNA from muscle was cloned, and it is comprised of 1068 nucleotides that encode a 384-amino acids protein, similar to other invertebrate AKs. The apparent Michaelis-Menten kinetic constant (K_m) was 1.7 mM with a k_cat of 75 s⁻¹. Two crystal structures are presented, the apoPvAK and PpAK bound to arginine, both in the open conformation with the active site lid (residues 310–320) completely disordered. The guanidino group binding site in the apo structure appears to be organized to accept the arginine substrate. Finally, these results contribute to knowledge of mechanistic details of the function of arginine kinase.

Insights into the Phosphoryl Transfer Mechanism of Human Ubiquitous Mitochondrial Creatine Kinase

Human ubiquitous mitochondrial creatine kinase (uMtCK) is responsible for the regulation of cellular energy metabolism. To investigate the phosphoryl-transfer mechanism catalyzed by human uMtCK, in this work, molecular dynamic simulations of uMtCK∙ATP-Mg2+∙creatine complex and quantum mechanism calculations were performed to make clear the puzzle. The theoretical studies hereof revealed that human uMtCK utilizes a two-step dissociative mechanism, in which the E227 residue of uMtCK acts as the catalytic base to accept the creatine guanidinium proton. This catalytic role of E227 was further confirmed by our assay on the phosphatase activity. Moreover, the roles of active site residues in phosphoryl transfer reaction were also identified by site directed mutagenesis. This study reveals the structural basis of biochemical activity of uMtCK and gets insights into its phosphoryl transfer mechanism.

Arginine kinase from Myzostoma cirriferum, a basal member of annelids

We assembled a phosphagen kinase gene from the Expressed Sequence Tags database of Myzostoma cirriferum, a basal member of annelids. The assembled gene sequence was synthesized using an overlap extension polymerase chain reaction method and was expressed in Escherichia coli. The recombinant enzyme (355 residues) exhibited monomeric behavior on a gel filtration column and showed strong activity only for l-arginine. Thus, the enzyme was identified as arginine kinase (AK). The two-substrate kinetic parameters were obtained and compared with other AKs. Phylogenetic analysis of amino acid sequences of phosphagen kinases indicated that the Myzostoma AK gene lineage differed from that of the polychaete Sabellastarte spectabilis AK, which is a dimer of creatine kinase (CK) origin. It is likely that the Myzostoma AK gene lineage was lost at an early stage of annelid evolution and that Sabellastarte AK evolved secondarily from the CK gene. This work contributes to our understanding of the evolution of phosphagen kinases of annelids with marked diversity.

Cooperativity and evolution of Tetrahymena two-domain arginine kinase

Tetrahymena pyriformis contains two arginine kinases, a 40-kDa enzyme (AK1) with a myristoylation signal sequence at the N-terminus and a two-domain 80-kDa enzyme (AK2). The former is localized mainly in cilia and the latter is in the cytoplasm. AK1 was successfully synthesized using an insect cell-free protein synthesis system and subjected to peptide mass fingerprinting (PMF) analysis. The masses corresponding to unmodified N-terminal tryptic peptide or N-terminal myristoylated peptide were not observed, suggesting that N-terminal peptides were not ionized in this analysis. We performed PMF analyses for two other phosphagen kinases (PKs) with myristoylation signals, an AK from Nematostella vectensis and a PK from Ectocarpus siliculosus. In both cases, the myristoylated, N-terminal peptides were clearly identified. The differences between the experimental and theoretical masses were within 0.0165-0.0583 Da, supporting the accuracy of the identification. Domains 1 and 2 of Tetrahymena two-domain AK2 were expressed separately in Escherichia coli and the extent of cooperativity was estimated on the basis of their kinetic constants. The results suggested that each of the domains functions independently, namely no cooperativity is displayed between the two domains. This is in sharp contrast to the two-domain AK from Anthopleura.

Arginine kinases from the marine feather star Tropiometra afra macrodiscus: The first finding of a prenylation signal sequence in metazoan phosphagen kinases

Two arginine kinase cDNAs (AK1 and AK2) were isolated from the marine feather star Tropiometra afra macrodiscus, and the gene structure (exon/intron organization) of AK1 was determined. The cDNA-derived amino acid sequences and the exon/intron organization of the Tropiometra AK1 gene were homologous to those of a human creatine kinase (CK) as well as the AK of the sea cucumber Stichopus. Phylogenetic analysis also supports the close relationship between human CKs and echinoderm AKs, indicating that the latter AKs evolved from an ancestral CK gene. We observed that the Tropiometra AK1 gene has a novel C-terminal extension (approximately 50 amino acid residues) encoded by a unique exon. Moreover, a typical prenylation signal sequence (CSLL) was found at the C-terminal end of this extension, suggesting that AK1 is anchored to a membrane. AK2 had no such C-terminal extension. This is the first finding of a prenylation signal in metazoan phosphagen kinases. Recombinant Tropiometra AK1 and AK2 enzymes were successfully expressed in Escherichia coli, and their kinetic constants were determined. Both enzymes showed activity comparable to that of typical invertebrate AKs.

Arginine kinase in the cladoceran Daphnia magna: cDNA sequencing and expression is associated with resistance to toxic Microcystis

Nutrient loading derived from anthropogenic activities into lakes have increased the frequency, severity and duration of toxic cyanobacterial blooms around the world. Although herbivorous zooplankton are generally considered to be unable to control toxic cyanobacteria, populations of some zooplankton, including Daphnia, have been shown to locally adapt to toxic cyanobacteria and suppress cyanobacterial bloom formation. However, little is known about the physiology of zooplankton behind this phenomenon. One possible explanation is that some zooplankton may induce more tolerance by elevating energy production, thereby adding more energy allocation to detoxification expenditure. It is assumed that arginine kinase (AK) serves as a core in temporal and spatial adenosine triphosphate (ATP) buffering in cells with high fluctuating energy requirements. To test this hypothesis, we studied the energetic response of a single Daphnia magna clone exposed to a toxic strain of Microcystis aeruginosa, PCC7806. Arginine kinase of D. magna (Dm-AK) was successfully cloned. An ATP-gua PtransN domain which was described as a guanidine substrate specificity domain and an ATP-gua Ptrans domain which was responsible for binding ATP were both identified in the Dm-AK. Phylogenetic analysis of AKs in a range of arthropod taxa suggested that Dm-AK was as dissimilar to other crustaceans as it was to insects. Dm-AK transcript level and ATP content in the presence of M. aeruginosa were significantly lower than those in the control diet containing only the nutritious chlorophyte, Scenedesmus obliquus, whereas the two parameters in the neonates whose mothers had been previously exposed to M. aeruginosa were significantly higher than those of mothers fed with pure S. obliquus. These findings suggest that Dm-AK might play an essential role in the coupling of energy production and utilization and the tolerance of D. magna to toxic cyanobacteria.

Phosphagen kinase in Schistosoma japonicum: II. Determination of amino acid residues essential for substrate catalysis using site-directed mutagenesis

Phosphagen kinases (PKs) play major roles in the regulation of energy metabolism in animals. Creatine kinase (CK) is the sole PK in vertebrates, whereas several PKs are present in invertebrates. We previously identified a contiguous dimer taurocyamine kinase (TK) from the trematode Schistosoma japonicum (Sj), a causative agent of schistosomiasis. SjTK contiguous dimer is comprised of domain 1 (D1) and domain 2 (D2). In this study, we used SjTK contiguous dimer (SjTKD1D2) or truncated single-domain constructs (SjTKD1 or SjTKD2) and employed site-directed mutagenesis to investigate the enzymatic properties of TK mutants. Mutation in SjTKD1 or SjTKD2 (D1E222G or D2E225G) caused complete loss of activity for the substrate taurocyamine. Likewise, a double mutant (D1E222GD2E225G) in the contiguous dimer (D1D2) exhibited complete loss of activity for the substrate taurocyamine. However, catalytic activity in the contiguous dimer remained in both of D1 inactive mutant (D1D2D1E222G) and D2 inactive mutant (D1D2D2E225G), suggesting that efficient catalysis of SjTKD1D2 is dependent on the activity of D1 and D2. The catalytic efficiency of the mixture of both single domains (WTD1+WTD2) showed same enzymatic properties (Km(Tauro)=0.68;Vmax/Km(Tauro)=137.04) to WTD1D2 (Km(Tauro)=0.47; Vmax/Km(Tauro)=144.30). This result suggests that the contiguous dimeric structure is not essential for the catalytic efficiencies of both domains of SjTK. Vmax/Km(Tauro) of the mixture of wild-type and inactivated domains (78.02 in WTD1+D2E225G and 128.24 in D1E222G+WTD2) were higher than the corresponding mutants (47.25 in D1D2D1E222G and 46.77 in D1D2D2E225G). To identify amino acid residues that are critical for taurocyamine binding, we performed alanine scanning mutagenesis at positions 57-63 on the guanidino specificity (GS) region of the SjTKD1, which is considered to be involved in guanidino-substrate recognition. R63A and R63Y mutants lost activity for taurocyamine, suggesting that these residues are associated with taurocyamine binding. In addition, we investigated the role of Tyr84 in D1 and found an association with substrate alignment. The Y84 residue was replaced with R, H, K, I, A, and G. Although the activities of each mutant were decreased (Vmax=2.36-67.50μmolPi/min/mgprotein), Y84 mutants possess binding affinity for taurocyamine (Km(Tauro)=3.19-10.04mM). The D1Y84R, D1Y84H, D1Y84K, and D1Y84A mutants exhibited low activity for taurocyamine, whereas the D1Y84I and D1Y84G mutants exhibited slightly decreased activity compared with the other Y84 mutants. The D1Y84K mutant lost substrate synergy between taurocyamine and ATP, suggesting that this mutation moves the position of the GS loop, similar to that of lombricine kinase (LK), and interferes with taurocyamine binding. This is the first comprehensive investigation of essential amino acid residues for substrate catalysis in trematode TK.

Two arginine kinases of Tetrahymena pyriformis: characterization and localization

Two cDNAs, one coding a typical 40-kDa arginine kinase (AK1) and the other coding a two-domain 80-kDa enzyme (AK2), were isolated from ciliate Tetrahymena pyriformis, and their recombinant enzymes were successfully expressed in Escherichia coli. Both enzymes had an activity comparable to those of typical invertebrate AKs. Interestingly, the amino acid sequence of T. pyriformis AK1, but not AK2, had a distinct myristoylation signal sequence at the N-terminus, suggesting that 40-kDa AK1 targets the membrane. Moreover, Western blot analysis showed that the AK1 is mainly localized in the ciliary fraction. Based on these results, we discuss the phosphoarginine shuttle, which enables a continuous energy flow to dynein for ciliary movement in T. pyriformis, and the role of AK1 in this model.

The role of Y84 on domain 1 and Y87 on domain 2 of Paragonimus westermani taurocyamine kinase: Insights on the substrate binding mechanism of a trematode phosphagen kinase

The two-domain taurocyamine kinase (TK) from Paragonimus westermani was suggested to have a unique substrate binding mechanism. We performed site-directed mutagenesis on each domain of this TK and compared the kinetic parameters Km(Tc) and Vmax with that of the wild-type to determine putative amino acids involved in substrate recognition and binding. Replacement of Y84 on domain 1 and Y87 on domain 2 with R resulted in the loss of activity for the substrate taurocyamine. Y84E mutant has a dramatic decrease in affinity and activity for taurocyamine while Y87E has completely lost catalytic activity. Substituting H and I on the said positions also resulted in significant changes in activity. Mutation of the residues A59 on the GS region of domain 1 also caused significant decrease in affinity and activity while mutation on the equivalent position on domain 2 resulted in complete loss of activity.

T273 plays an important role in the activity and structural stability of arginine kinase

Arginine kinase (AK) is a key enzyme for cellular energy metabolism, catalyzing the reversible phosphoryl transfer from phosphoarginine to ADP in invertebrates. The amino acid residue C271 is involved in keeping AK's activity and constraining the orientation of the substrate arginine. However, the roles of the C271 interaction amino acid residues in AK's substrate synergism, activity and structural stability are still unclear. The crystal structure of AK implied that the amino acid residue T273 interacted with the residue C271 and might play vital roles in keeping AK's activity, substrate synergism and structural stability. The mutations T273G and T273A led to significantly loss of activity, obviously decreased of substrate synergism and structural stability. Furthermore, spectroscopic experiments indicated that mutations T273G and T273A impaired the structure of AK and led them to a partially unfolded state. The inability to fold to the functional state made the mutations prone to aggregate under environmental stresses. Moreover, the mutations T273S and T273D almost had no effects on AK's activity and structural stability. This study herein indicated that the residue T273 played key roles in AK's activity, substrate synergism and structural stability.

A novel taurocyamine kinase found in the protist Phytophthora infestans

Phosphagen kinase (PK), which is typically in the form of creatine kinase (CK; EC 2.7.3.2) in vertebrates or arginine kinase (AK; EC 2.7.3.3) in invertebrates, plays a key role in ATP buffering systems of tissues and nerves that display high and variable rates of ATP turnover. The enzyme is also found with intermittent occurrence as AK in unicellular organisms, protist and bacteria species, suggesting an ancient origin of AK. Through a database search, we identified two novel PK genes, coding 40- and 80-kDa (contiguous dimer) enzymes in the protist Phytophthora infestans. Both enzymes showed strong activity for taurocyamine and, in addition, we detected taurocyamine in cell extracts of P. infestans. Thus, the enzyme was identified to be taurocyamine kinase (TK; EC 2.7.3.4). This was the first phosphagen kinase, other than AK, to be found in unicellular organisms. Their position on the phylogenetic tree indicates that P. infestans TKs evolved uniquely at an early stage of evolution. Occurrence of TK in protists suggests that PK enzymes show flexible substrate specificity.

Cold-adapted features of arginine kinase from the deep-sea clam Calyptogena kaikoi

The heterodont clam Calyptogena kaikoi, which inhabits depths exceeding 3,500 m where low ambient temperatures prevail, has an unusual two-domain arginine kinase (AK) with molecular mass of 80 kDa, twice that of typical AKs. The purpose of this work is to investigate the nature of the adaptations of this AK for functioning at low temperatures. Recombinant C. kaikoi AK constructs were expressed, and their two-substrate kinetic constants (k(cat), K(a), and K(ia)) were determined at 10°C and 25°C, respectively. When measured at 25°C, the K(ia) values were tenfold larger than those for corresponding K(a) values, while at 10°C, the K(ia) values decreased remarkably, but the K (a) values were almost unchanged. The Calyptogena two-domain enzyme has threefold higher catalytic efficiency, calculated by k (cat)/(K(a)(ARG)·K(ia)(ATP) ), at 10°C, than that at 25°C, reflecting adaptation for function at reduced ambient temperatures. The activation energy (E(a)) and thermodynamic parameters were determined for Calyptogena two-domain enzyme and compared with those of two-domain enzymes from mesophilic Corbicula and Anthopleura. The value for E(a) of Calyptogena enzyme were about half of those for mesophilic enzymes, and a larger decrease in entropy was observed in Calyptogena AK reaction. Although large decrease in entropy increases the ΔG(o‡) value and consequently lowers the k(cat) value, this is compensated with its lower E(a) value thereby minimizing the reduction in its k(cat) value. These thermodynamic properties, together with the kinetic ones, are also present in the separated domain 2 of the Calyptogena two-domain enzyme.

The arginine kinase in Zhikong scallop Chlamys farreri is involved in immunomodulation

Arginine kinase (AK) catalyzes the reversible phosphorylation of l-arginine to form phosphoarginine, and plays a critical role in energy metabolism in invertebrates. In the present study, a scallop AK gene was identified from Chlamys farreri with an open reading frame (ORF) of 1101bp encoding for a protein of 366 amino acids (designed as CfAK). An ATP-gua PtransN domain which was described as a guanidine substrate specificity domain (GS domain) and an ATP-gua Ptrans domian which was responsible for binding ATP, were both identified in CfAK. The mRNA transcripts of CfAK were detectable in haemocytes, hepatopancreas, adductor muscle, mantle, gill, kidney and gonad, with the highest expression level in the muscle and the lowest level in the hemocytes. The expression level of CfAK mRNA increased from fertilized eggs to eyebot, and reached the highest in the trochophore stage. The relative expression level of CfAK mRNA in muscle was up-regulated significantly after LPS (0.5mg/mL) stimulation, and reached the peak at 6h (5.2-fold, P<0.05). The activity of inducible nitric oxide synthase (iNOS) in the supernatant of muscle homogenate increased significantly from 3.2U/mg at 0 h to 9.7 U/mg at 12h after LPS stimulation, while the concentration of nitric oxide (NO) in the supernatant of muscle homogenate began to increase at 3h (21.55 μmol/L), and reached the top concentration at 24h (42.27 μmol/L), then recovered to the normal level after 48 h. The recombinant protein of CfAK (rCfAK) expressed in Escherichia coli displayed Arginine kinase activity, and its apparent K(m) was 0.82 ± 0.11 and 1.24 ± 0.13 mM for L-arginine and ATP-Na, respectively. The results indicated that the CfAK was involved in energy production and utilization during the whole life process, and might refer to the immunomodulation process via altering the NO concentration and iNOS activity in scallop Chlamys farreri.

A diverse array of creatine kinase and arginine kinase isoform genes is present in the starlet sea anemone Nematostella vectensis, a cnidarian model system for studying developmental evolution

Phosphagen (guanidino) kinases (PK) constitute a family of homologous phosphotransferases catalyzing the reversible transfer of the high-energy phosphoryl group of ATP to naturally occurring guanidine compounds. Prior work has shown that PKs can be phylogenetically separated into two distinct groups- an arginine kinase (AK) subfamily and a creatine kinase (CK) subfamily. The latter includes three CK isoforms- cytoplasmic CK (CyCK), mitochondrial CK (MiCK) and three-domain flagellar CK (fCK). In the present study we identified six unique PK genes from the draft genome sequence of the starlet sea anemone Nematostella vectensis, a well-known model organism for understanding metazoan developmental evolution. Using reverse transcription polymerase chain reaction (RTPCR) methods, full length cDNAs were amplified for all of these PKs. These cDNAs were cloned and expressed in Escherichia coli as 6x His-tagged fusion proteins. The six PKs were identified as the three typical CK isoforms (CyCK, MiCK and fCK), two unusual AKs (a two-domain AK (2DAK) and a three-domain AK (3DAK)) and a PK which phosphorylated arginine. The latter enzyme had a very low AK activity (its apparent V(max) value being less than 0.2% that of 3DAK), lacks several key residues necessary for AK enzyme activity, and was tentatively designated as AK1. As far as we know, this constitutes the first report of an AK with the three fused AK domains. The Bayesian tree suggested that the third domain of 3DAK likely evolved from the gene for domain 2 of typical two-domain AK found widely in cnidarians. Construction of phylogenetic trees and comparison of exon-intron organizations of their respective genes indicated that the N. vectensis three-domain fCK and 3DAK evolved independently, and both enzymes are likely to be targeted to cell membranes since they have a myristoylation signal at their respective N-termini. These results complement prior work on other basal invertebrates showing that multiple CK and AK isoform genes were present at the dawn of the radiation of metazoans. The presence of isoform diversity in an organism lacking in structural complexity reflects an early imperative for targeting of PKs to particular cellular contexts such as muscle fibers, neurons, ciliated/flagellated epithelia and spermatozoa.

Identification of amino acid residues responsible for taurocyamine binding in mitochondrial taurocyamine kinase from Arenicola brasiliensis

In order to investigate the residues associated with binding of the substrate taurocyamine in Arenicola mitochondrial taurocyamine kinase (TK), we performed Ala-scanning of the amino acid sequence HTKTV at positions 67-71 on the GS loop, and determined apparent K(m) and V(max) (appK(m) and appV(max), respectively) of the mutant forms for the substrates taurocyamine and glycocyamine. The appK(m) values for taurocyamine of the K69A, T70A and V71A mutants were significantly increased as compared with wild-type, suggesting that these residues are associated with taurocyamine binding. Of special interest is a property of V71A mutant: its catalytic efficiency for glycocyamine was twice that for taurocyamine, indicating that the V71A mutant acts like a glycocyamine kinase, rather than a TK. The role of the amino acid residue K95 of Arenicola MiTK was also examined. K95 was replaced with R, H, Y, I, A and E. K95R, K95H and K95I have a 3-fold higher affinity for taurocyamine, and activity was largely lost in K95E. On the other hand, the K95Y mutant showed a rather unique feature; namely, an increase in substrate concentration caused a decrease in initial velocity of the reaction (substrate inhibition). This is the first report on the key amino acid residues responsible for taurocyamine binding in mitochondrial TK.

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.

Mutation of the conserved Asp122 in the linker impedes creatine kinase reactivation and refolding

Creatine kinase (CK), a key enzyme in maintaining the intracellular energetic homeostasis, contains two domains connected by a long linker. In this research,we found that the mutations of the conserved Asp122 in the linker slightly affected CK activity, structure and stability. The hydrogen bonding and the ion pair contributed 2-5 kJ/mol to the conformational stability of CK. Interestingly, the ability of CK reactivation from the denatured state was completely removed by the mutations. These results suggested that the electrostatic interactions were crucial to the action of the linker in CK reactivation.

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.

A novel arginine kinase from the shrimp Neocaridina denticulata: the fourth arginine kinase gene lineage

Arginine kinases (AK) evolved independently at least three times: first at an early stage of phosphagen kinase evolution (typical AK), second from the cytoplasmic creatine kinase (CK) gene later in metazoan evolution (Stichopus AK) and third from the mitochondrial CK gene in the course of annelid radiation (Sabellastarte AK). Here, we present a possible fourth AK lineage. We amplified cDNA encoding three AKs (AK1, AK2 and AK3) from the shrimp Neocaridina denticulata, and determined their amino acid sequences (355-356 residues). Each cDNA sequence was cloned in a pET plasmid and the corresponding recombinant kinase was expressed in E. coli. The AKs showed monomeric nature similar to typical AKs on an analytical gel filtration column. While the amino acid sequence of AK2 corresponded to that of typical AK, containing the conserved key residues established in Limulus AK for the substrate binding site, those of AK1 and AK3 lacked some of these key residues, indicating a similar evolution to Stichopus and Sabellastarte AKs. Moreover, phylogenetic analysis of phosphagen kinases indicated that Neocaridina AK1 and AK3 diverged at the deepest branching point close to the root of the tree and formed a distinct cluster outside the typical AK cluster, which included Neocaridina AK2. Kinetic constants of Neocaridina AKs were similar to those of other AKs. However, activation energy (E(a)) for the transition state of AK1 and AK3 was about 1.5-fold larger than that of AK2. The DeltaH(o++) values for AK1 and AK3 were also about 1.5-fold larger than that for AK2, but all three DeltaG(o++) values were very similar (71-72 kJ/mol); this results in similar reaction velocities for the three AK reactions due to a lower decrease in entropy in the activated complexes of AK1 and AK3 reactions compared to that of AK2. These findings suggest that Neocaridina AK1 and AK3 are unique from the known three AK gene lineages and therefore comprises a fourth AK lineage.

Two-domain arginine kinase from the deep-sea clam Calyptogena kaikoi--evidence of two active domains

The cDNA and deduced amino acid sequences for arginine kinase (AK) from the deep-sea clam Calyptogena kaikoi have been determined revealing an unusual two-domain (2D) structure with molecular mass of 80 kDa, twice that of normal AK. The amino acid sequences of both domains contain most of the residues thought to be required for substrate binding found in the horseshoe crab Limulus polyphemus AK, a well studied system for which several X-ray crystal structures exist. However, two highly conserved residues, D62 and R193, that form a salt bridge thereby stabilizing the substrate-bound structure have been replaced by G and N in domain 1, and G and P in domain 2, respectively. The present effort probes whether both domains of Calyptogena AK are catalytically competent. Recombinant constructs of the wild-type enzyme of both single domains, and of selected mutants of the Calyptogena AK have been expressed as fusion proteins with the maltose-binding protein. The wild-type two-domain enzyme (2D[WT]) had high AK activity (k(cat)=23 s(- 1), average value of the two domains), and the single domain 2 (D2[WT]) showed 1.5-times higher activity (k(cat)=38 s(- 1)) than the wild-type 2D[WT]. Interestingly, the single domain 1 (D1[WT]) showed only a very low activity (k(cat) approximately 0.016 s(- 1)). Introduction of a Y68A mutation in both domains virtually abolished catalytic activity. On the other hand, significant residual activity was observed (k(cat)=2.8 s(- 1)), when the Y68A mutation was introduced only into domain 2 of the two-domain enzyme. A similar mutation in domain 1 of the two-domain enzyme reduced activity to a much lower extent (k(cat)=11.1 s(- 1)). Although the domains of this "contiguous" dimeric AK each have catalytic capabilities, the presence of domain 2 strongly influences the stability and activity of domain 1.

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

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

A novel arginine kinase with substrate specificity towards D-arginine

We determined the cDNA-derived amino acid sequences of two arginine kinases (AK1, AK2) from the annelid Sabellastarte indica, cloned the cDNAs into pMAL plasmid and expressed them in E. coli. The phylogenetic analyses suggested that Sabellastarte AKs have evolved from a CK-related gene, not from the usual AK gene. The recombinant Sabellastarte AK1 showed a broad specificity towards various guanidine compounds, while the Sabellastarte AK2 mainly showed stronger activity for both D- and L-arginine, a very unique substrate specificity not seen before in usual AKs. We isolated guanidino compounds from the body wall musculature of Sabellastarte, and found that the major compound is D-arginine with a concentration of 4.85 +/- 0.51 mmol/kg. From these results, we suggest strongly that in Sabellastarte, D-arginine is the major phosphagen substrate and that the AK2 with substrate specificity towards D-arginine, catalyzes the phosphorylation of D-arginine.

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.

Impact of intra-subunit domain-domain interactions on creatine kinase activity and stability

Creatine kinase (CK) is a key enzyme in vertebrate excitable tissues. In this research, five conserved residues located on the intra-subunit domain-domain interface were mutated to explore their role in the activity and structural stability of CK. The mutations of Val72 and Gly73 decreased both the activity and stability of CK. The mutations of Cys74 and Val75, which had no significant effect on CK activity and structure, gradually decreased the stability and reactivation of CK. Our results suggested that the mutations might modify the correct positioning of the loop contributing to domain-domain interactions, and result in decreased stability against denaturation.

Hypotaurocyamine kinase evolved from a gene for arginine kinase

Hypotaurocyamine kinase (HTK) is a member of the highly conserved family of phosphagen kinases that includes creatine kinase (CK) and arginine kinase (AK). HTK is found only in sipunculid worms, and it shows activities for both the substrates hypotaurocyamine and taurocyamine. Determining how HTK evolved in sipunculids is particularly insightful because all sipunculid-allied animals have AK and only some sipunculids have HTK. We determined the cDNA sequence of HTK from the sipunculid worm Siphonosoma cumanense for the first time, cloned it in pMAL plasmid and expressed it in E. coli as a fusion protein with maltose-binding protein. The cDNAderived amino acid sequence of Siphonosoma HTK showed high amino acid identity with molluscan AKs. Nevertheless, the recombinant enzyme of Siphonosoma HTK showed no activity for the substrate arginine, but showed activity for taurocyamine. Comparison of the amino acid sequences of HTK and AK indicated that the amino acid residues necessary for the binding of the substrate arginine in AK have been completely lost in Siphonosoma HTK sequence. The phylogenetic analysis indicated that the HTK amino acid sequence was placed just outside the molluscan AK cluster, which formed a sister group with the arthropod and nematode AKs. These results suggest that Siphonosoma HTK evolved from a gene for molluscan AK. Moreover, to confirm this assertion, we determined by PCR that the gene for Siphonosoma HTK has a 5-exon/4-intron structure, which is homologous with that of the molluscan AK genes. Further, the positions of splice junctions were conserved exactly between the two genes. Thus, we conclude that Siphonosoma HTK has evolved from a primordial gene for molluscan AK.

Transition state stabilization by six arginines clustered in the active site of creatine kinase

Six fully conserved arginine residues (R129, R131, R235, R291, R319, and R340) closely grouped in the nucleotide binding site of rabbit muscle creatine kinase (rmCK) were mutated; four to alanine and all six to lysine. Kinetic analyses in the direction of phosphocreatine formation showed that all four alanine mutants led to substantial losses of activity with three (R129A, R131A, and R235A) having no detectable activity. All six lysine mutants retained variable degrees of reduced enzymatic activity. Static quenching of intrinsic tryptophan fluorescence was used to measure the binding constants for MgADP and MgATP. Nucleotide binding was at most only modestly affected by mutation of the arginine residues. Thus, the cluster of arginines seem to be primarily responsible for transition state stabilization which is further supported by the observation that none of the inactive mutants demonstrated the ability to form a transition analogue complex of MgADP.nitrate.creatine as determined by fluorescence quenching assays. As a whole, the results suggest that the most important role these residues play is to properly align the substrates for stabilization of the phosphoryl transfer reaction.

Phosphagen kinase of the giant tubeworm Riftia pachyptila. Cloning and expression of cytoplasmic and mitochondrial isoforms of taurocyamine kinase

The giant tubeworm Riftia pachyptila lives at deep-sea hydrothermal vents along the East Pacific Rise and the Galapagos Rift. The large size and high growth rate of R. pachyptila is supported by an endosymbiotic relationship with a chemosynthetic bacterium. Elucidation of the regulation of energy metabolism of the giant tubeworm remains an interesting problem. The purpose of this study is to determine the cDNA sequence of phosphagen kinase, one of the most important enzymes in energy metabolism, and to characterize its function. Two phosphagen kinase cDNA sequences amplified from the cDNA library of R. pachyptila showed high derived amino acid sequence identity (74%) with those of cytoplasmic taurocyamine kinase (TK) and mitochondrial TK from an annelid Arenicola brasiliensis. The cytoplasmic form of the Riftia recombinant enzyme showed stronger activity for the substrates taurocyamine and also considerable activity for lombricine (21% that of taurocyamine). The mitochondrial form, which was structurally similar to mitochondrial creatine kinase, showed stronger activity for taurocyamine, and a broader activity for various guanidine compounds: glycocyamine (35% that of taurocyamine), lombricine (31%) and arginine (3%). Both forms showed no activity for creatine. The difference in substrate specificities between the cytoplasmic and mitochondrial forms might be attributable to the large difference in the amino acid sequence of the GS region and/or several key amino acid residues for establishing guanidine substrate specificity. Based on these results, we conclude that Riftia contains at least two forms of TK as phosphagen kinase. We also report the kinetic parameters, Km and kcat, of Arenicola and Riftia TKs for the first time. The Km values for taurocyamine of Arenicola and Riftia TKs ranged from 0.9 to 4.0 mM and appear to be comparable to those of other annelid-specific enzymes, lombricine kinase and glycocyamine kinase, but are significantly lower than those of Neanthes cytoplasmic and mitochondrial creatine kinases. Comparison of kcat/Km value in various annelid phosphagen kinases indicates that Arenicola mitochondrial TK has the highest catalytic efficiency (16.2 s-1 mM-1). In Arenicola TKs, the mitochondrial form has seven-fold higher efficiency than the cytoplasmic form.

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.

Amino acid residues 62 and 193 play the key role in regulating the synergism of substrate binding in oyster arginine kinase

The purpose of this study is to clarify the amino acid residues responsible for the synergism in substrate binding of arginine kinase (AK), a key enzyme in invertebrate energy metabolism. AKs contain a pair of highly conserved amino acids (D62 and R193) that form an ion pair, and replacement of these residues can cause a pronounced loss of activity. Interestingly, in the oyster Crassostrea AK, these residues are replaced by an N and a K, respectively. Despite this replacement, the enzyme retains high activity and moderate synergism in substrate binding (Kd/Km=2.3). We replaced the N62 by G or D and the K193 by G or R in Crassostrea AK, and also constructed the double mutants of N62G/K193G and N62D/K193R. All of the mutants retained 50-90% of the wild-type activity. In N62G and N62D mutants, the Kmarg for arginine binding was comparable to that of wild-type enzyme, but the Kdarg was increased 2-5-fold, resulting in a strong synergism (Kd/Km=4.9-11.3). On the other hand, in K193G and K193R mutants, the Kmarg was increased 4-fold, and synergism was lost almost completely (Kd/Km=1.0-1.4). The N62G/K193G double mutant showed similar characteristics to the K193G and K193R mutants. Another double mutant, N62D/K193R, similar to the amino acid pair in the wild-type enzyme, had characteristics similar to those of the wild-type enzyme. These results indicate that the amino acid residues 62 and 193 play the key role in mediating the synergism in substrate binding of oyster arginine kinase.

Role of amino-acid residue 95 in substrate specificity of phosphagen kinases

The purpose of this study is to elucidate the mechanisms of guanidine substrate specificity in phosphagen kinases, including creatine kinase (CK), glycocyamine kinase (GK), lombricine kinase (LK), taurocyamine kinase (TK) and arginine kinase (AK). Among these enzymes, LK is unique in that it shows considerable enzyme activity for taurocyamine in addition to its original target substrate, lombricine. We earlier proposed several candidate amino acids associated with guanidine substrate recognition. Here, we focus on amino-acid residue 95, which is strictly conserved in phosphagen kinases: Arg in CK, Ile in GK, Lys in LK and Tyr in AK. This residue is not directly associated with substrate binding in CK and AK crystal structures, but it is located close to the binding site of the guanidine substrate. We replaced amino acid 95 Lys in LK isolated from earthworm Eisenia foetida with two amino acids, Arg or Tyr, expressed the modified enzymes in Escherichia coli as a fusion protein with maltose-binding protein, and determined the kinetic parameters. The K95R mutant enzyme showed a stronger affinity for both lombricine (Km=0.74 mM and kcat/Km=19.34 s(-1) mM(-1)) and taurocyamine (Km=2.67 and kcat/Km=2.81), compared with those of the wild-type enzyme (Km=5.33 and kcat/Km=3.37 for lombricine, and Km=15.31 and kcat/ Km=0.48for taurocyamine). Enzyme activity of the other mutant, K95Y, was dramatically altered. The affinity for taurocyamine (Km=1.93 and kcat/Km=6.41) was enhanced remarkably and that for lombricine (Km=14.2 and kcat/Km=0.72) was largely decreased, indicating that this mutant functions as a taurocyamine kinase. This mutant also had a lower but significant enzyme activity for the substrate arginine (Km=33.28 and kcat/Km=0.01). These results suggest that Eisenia LK is an inherently flexible enzyme and that substrate specificity is strongly controlled by the amino-acid residue at position 95.

Asparagine 285 plays a key role in transition state stabilization in rabbit muscle creatine kinase

To explore the possibility that asparagine 285 plays a key role in transition state stabilization in phosphagen kinase catalysis, the N285Q, N285D, and N285A site-directed mutants of recombinant rabbit muscle creatine kinase (rmCK) were prepared and characterized. Kinetic analysis of phosphocreatine formation showed that the catalytic efficiency of each N285 mutant was reduced by approximately four orders of magnitude, with the major cause of activity loss being a reduction in k(cat) in comparison to the recombinant native CK. The data for N285Q still fit a random-order, rapid-equilibrium mechanism, with either MgATP or creatine binding first with affinities very nearly equal to those for native CK. However, the affinity for the binding of the second substrate is reduced approximately 10-fold, suggesting that addition of a single methylene group at position 285 disrupts the symphony of substrate binding. The data for the N285A mutant only fit an ordered binding mechanism, with MgATP binding first. Isosteric replacement to form the N285D mutant has almost no effect on the K(M) values for either creatine or MgATP, thus the decrease in activity is due almost entirely to a 5000-fold reduction in k(cat). Using the quenching of the intrinsic CK tryptophan fluorescence by added MgADP (Borders et al. 2002), it was found that, unlike native CK, none of the mutants have the ability to form a quaternary TSAC. We use these data to propose that asparagine 285 indeed plays a key role in transition state stabilization in the reaction catalyzed by creatine kinase and other phosphagen kinases.

Generation of an active monomer of rabbit muscle creatine kinase by site-directed mutagenesis: the effect of quaternary structure on catalysis and stability

Cytosolic creatine kinase exists in native form as a dimer; however, the reasons for this quaternary structure are unclear, given that there is no evidence of active site communication and more primitive guanidino kinases are monomers. Three fully conserved residues found in one-half of the dimer interface of the rabbit muscle creatine kinase (rmCK) were selectively changed to alanine by site-directed mutagenesis. Four mutants were prepared, overexpressed, and purified: R147A, R151A, D209A, and R147A/R151A. Both the R147A and R147A/R151A were confirmed by size-exclusion chromatography and analytical ultracentrifugation to be monomers, whereas R151A was dimeric and D209A appeared to be an equilibrium mixture of dimers and monomers. Kinetic analysis showed that the monomeric mutants, R147A and R147A/R151A, showed substantial enzymatic activity. Substrate binding affinity by R147A/R151A was reduced approximately 10-fold, although k(cat) was 60% of the wild-type enzyme. Unlike the R147A/R151A, the kinetic data for the R147A mutant could not be fit to a random-order rapid-equilibrium mechanism characteristic of the wild-type, but could only be fit to an ordered mechanism with creatine binding first. Substrate binding affinities were also significantly lower for the R147A mutant, but k(cat) was 11% that of the native enzyme. Fluorescence measurements using 1-anilinonaphthalene-8-sufonate showed that increased amounts of hydrophobic surface area are exposed in all of the mutants, with the monomeric mutants having the greatest amounts of unfolding. Thermal inactivation profiles demonstrated that protein stability is significantly decreased in the monomeric mutants compared to wild-type. Denaturation experiments measuring lambda(max) of the intrinsic fluorescence as a function of guanidine hydrochloride concentration helped confirm the quaternary structures and indicated that the general unfolding pathway of all the mutants are similar to that of the wild-type. Collectively, the data show that dimerization is not a prerequisite for activity, but there is loss of structure and stability upon formation of a CK monomer.