Arginine kinase from Nautilus pompilius, a living fossil. Site-directed mutagenesis studies on the role of amino acid residues in the Guanidino specificity region

A comprehensive review of arginine kinase proteins: What we need to know?

The enzyme arginine kinase (AK), EC 2.7.3.3, catalyzes the reversible phosphorylation of arginine with adenosine triphosphate, forming phosphoarginine, which acts as an energy reservoir due to its high-energy phosphate content that can be rapidly transferred to ADP for ATP renewal. It has been proposed that AK should be associated with some ATP biosynthesis mechanisms, such as glycolysis and oxidative phosphorylation. Arginine kinase is an analogue of creatine kinase found in vertebrates. A literature survey has recovered the physicochemical and structural characteristics of AK. This enzyme is widely distributed in invertebrates such as protozoa, bacteria, porifera, cnidaria, mollusca, and arthropods. Arginine kinase may be involved in the response to abiotic and biotic stresses, being up regulated in several organisms and controlling energy homeostasis during environmental changes. Additionally, phosphoarginine plays a role in providing energy for the transport of protozoa, the beating of cilia, and flagellar movement, processes that demand continuous energy. Arginine kinase is also associated with allergies to shellfish and arthropods, such as shrimp, oysters, and cockroaches. Phenolic compounds such as resveratrol, which decrease AK activity by 50 % in Trypanosoma cruzi, inhibit the growth of the epimastigote and trypomastigote forms, making them a significant target for the development of medications for Chagas Disease treatment.

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.

Insight into Structural Aspects of Histidine 284 of Daphnia magna Arginine Kinase

Arginine kinase (AK), a bioenergy-related enzyme, is distributed widely in invertebrates. The role of highly conserved histidines in AKs is still unascertained. In this study, the highly conserved histidine 284 (H284) in AK of Daphnia magna (DmAK) was replaced with alanine to elucidate the role of H284. We examined the alteration of catalytic activity and structural changes of H284A in DmAK. The catalytic activity of H284A was reduced dramatically compared to that in wild type (WT). Thus the crystal structure of H284A displayed several structural changes, including the alteration of D324, a hydrogen-bonding network around H284, and the disruption of π-stacking between the imidazole group of the H284 residue and the adenine ring of ATP. These findings suggest that such alterations might affect a conformational change of the specific loop consisting of G310-V322 at the antiparallel β-sheet region. Thus, we speculated that the H284 residue might play an important role in the conformational change of the specific loop when ATP binds to the substrate-binding site of DmAK.

Natural Products Containing 'Rare' Organophosphorus Functional Groups

Phosphorous-containing molecules are essential constituents of all living cells. While the phosphate functional group is very common in small molecule natural products, nucleic acids, and as chemical modification in protein and peptides, phosphorous can form P⁻N (phosphoramidate), P⁻S (phosphorothioate), and P⁻C (e.g., phosphonate and phosphinate) linkages. While rare, these moieties play critical roles in many processes and in all forms of life. In this review we thoroughly categorize P⁻N, P⁻S, and P⁻C natural organophosphorus compounds. Information on biological source, biological activity, and biosynthesis is included, if known. This review also summarizes the role of phosphorylation on unusual amino acids in proteins (N- and S-phosphorylation) and reviews the natural phosphorothioate (P⁻S) and phosphoramidate (P⁻N) modifications of DNA and nucleotides with an emphasis on their role in the metabolism of the cell. We challenge the commonly held notion that nonphosphate organophosphorus functional groups are an oddity of biochemistry, with no central role in the metabolism of the cell. We postulate that the extent of utilization of some phosphorus groups by life, especially those containing P⁻N bonds, is likely severely underestimated and has been largely overlooked, mainly due to the technological limitations in their detection and analysis.

Characterization and in vitro expression of arginine kinase gene in the invasive western flower thrips, Frankliniella occidentalis

Arginine kinase (AK) plays a critical role in insect energy metabolism and has been proposed to be a potential insecticide target for commercial exploitation. In this study, the full length cDNA encoding a typical group 1 insect AK (FoAK) was isolated from the western flower thrips (WFT), Frankliniella occidentalis (Pergande). Sequence analysis showed that FoAK contains an open reading frame of 1068 nucleotides, which encods a protein of 355 amino acid residues including the signature sequence pattern of ATP-guanidino kinases. Genomic structure analysis showed that the coding region of FoAK contains five exons connected by four introns. RT-qPCR analysis revealed that the mRNA expression of FoAK was developmentally regulated with the lowest level in prepupal stage. Enzymatic activity analysis of the recombinant enzymes expressed in Escherichia coli showed that FoAK is highly stereo specific for L-arginine versus D-arginine and the apparent Michaelis constant for L-arginine (Km^Arg) is comparable to that of AKs from a variety of species. This research should enable further investigation of the function as well as in vitro screening for inhibitors of FoAK.

Positive selection adaptation of two-domain arginine kinase (AK) from cold seep Vesicomyidae clams

Arginine kinase (AK) is an important member of Phosphagen kinases which engage in energy metabolism process, and AKs from cold seep clams may develop an effective mechanism to adapt a special habitat (e.g. low temperature). Three Vesicomyidae clams and seven Veneridae clams (belong to the same Order Veneroida) were chosen to analyze the evolution of two-domain AKs. In the present study, ten two-domain AKs were identified and Neighbor-joining tree showed that AKs were divided into two groups. Branch-site model indicated that two-domain AKs were subjected to strong positive selection (ω_2a = 17.5058). 16 positively selective sites were detected and five of them showed posterior probabilities of 0.95 or more. Comparative analysis found that domain 2 might be suffered from more evolutionary selection pressure than domain 1, as most positively sites were located at domain 2. Residue Pro (positively selective site) (587P in ApAK) in domain 2 from all Vesicomyidae AKs might participate in change of the synergism and in the function of its cold-adapted characteristics. In conclusion, our studies provide evidence of positive Darwinian selection in the two-domain AKs family of Vesicomyidae clams, and may contribute to a better understanding of its adaptation mechanisms to cold seep habitats.

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.

Characterization of Arginine Kinase in the Barnacle Amphibalanus Amphitrite and Its Role in the Larval Settlement

Energy metabolism is a key process in larval settlement of barnacles, but the underlying molecular mechanisms remain ambiguous. Arginine kinase (AK) mainly participates in energy metabolism in invertebrates. So far, its roles in barnacles have not been studied. In the present study, we raised an antibody against AK from Amphibalanus amphitrite Darwin to characterize the roles of AK in the larval settlement process. Among the developmental stages, AK was highly expressed during the cypris stage. Along with the aging process in cyprids, the level of AK decreased. The immunostaining results showed that AK was localized to muscular tissues in cyprids, including antennules, antennular muscles, and thoracic limbs. The larval settlement rate decreased and larval movement was inhibited in response to treatments with high concentrations of AK inhibitors (rutin and quercetin). These results demonstrated that AK was involved in the larval settlement of A. amphitrite through mediating energy supply in muscle tissues. Moreover, further analysis indicated that both the p38 MAPK and NO/cGMP pathways positively mediated the expression of AK in cyprids.

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.

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.

Purification, cloning, and immunological characterization of arginine kinase, a novel allergen of Octopus fangsiao

Arginine kinase (AK) is an important enzyme participating in energy metabolism in invertebrates, but, to date, there have been no reports that AK from octopus is an allergen. In this study, octopus AK was purified, and its molecular biological, immunological, and physicochemical characterizations were analyzed. The results showed that octopus AK was purified and confirmed by mass spectrometry for the first time, and its molecular mass was 38 kDa. The full-length gene sequence of octopus AK encompassed 1209 bp and was predicted to encode a protein with 348 amino acid residues. The homology of octopus AK and crustacean AK was about 54%, but the similarity between their three-dimensional structures was high. Octopus AK could react with mouse anti-shrimp AK and rabbit anti-crab AK polyclonal antibody singly. Octopus AK could also react with specific IgE of the sera from octopus-allergic patients effectively, whereas crab AK could inhibit the reaction between them. Finally, the IgE-binding activity of octopus AK could be reduced in the processes of thermal or acid-alkali treatment. In summary, AK was identified as a novel allergen in octopus, which had a sensitizing ability similar to that of crustacean AK. This is significant in allergy diagnosis and the treatment of octopus-allergic disorders.

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.

DNA barcoding in Nautilus pompilius (Mollusca : Cephalopoda): evolutionary divergence of an ancient species in modern times

DNA barcoding studies to elucidate the evolutionary and dispersal history of the current populations of Nautilus pompilius allow us to develop a greater understanding of their biology, their movement and the systematic relationships between different groups. Phylogenetic analyses were conducted on Australian N. pompilius, and COI sequences were generated for 98 discrete accessions. Sequences from samples collected across the distribution were sourced from GenBank and included in the analyses. Maximum likelihood revealed three distinct clades for N. pompilius: (1) populations sourced from west Australia, Indonesia and the Philippines; (2) populations collected from east Australia and Papua New Guinea; (3) western Pacific accessions from Vanuatu, American Samoa and Fiji, supporting previous findings on the evolutionary divergence of N. pompilius. A minimum spanning tree revealed 49 discrete haplotypes for the 128 accessions, from a total of 16 discrete sampling locations. Population similarity reflects oceanic topographic features, with divergence between populations across the N. pompilius range mirroring geographical separation. This illustrates the success of DNA barcoding as a tool to identify geographic origin, and looks to the future role of such technology in population genetics and evolutionary biology.

Molecular and catalytic properties of an arginine kinase from the nematode Ascaris suum

We amplified the cDNA coding for arginine kinase (AK) from the parasitic nematode Ascaris suum, cloned it in pMAL plasmid and expressed the enzyme as a fusion protein with the maltose-binding protein. The whole cDNA was 1260 bp, encoding 400 amino acids, and the recombinant protein had a molecular mass of 45,341 Da. Ascaris suum recombinant AK showed significant activity and strong affinity ( K(m)(Arg) = 0.126 mM) for the substrate L-arginine. It also exhibited high catalytic efficiency ( k(ca)/K(m)(Arg) = 352) comparable with AKs from other organisms. Sequence analysis revealed high amino acid sequence identity between A. suum AK and other nematode AKs, all of which cluster in a phylogenetic tree. However, comparison of gene structures showed that A. suum AK gene intron/exon organization is quite distinct from that of other nematode AKs. Phosphagen kinases (PKs) from certain parasites have been shown to be potential novel drug targets or tools for detection of infection. The characterization of A. suum AK will be useful in the development of strategies for control not only of A. suum but also of related species infecting humans.

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.

The role of Cys271 in conformational changes of arginine kinase

Arginine kinase (AK), a crucial enzyme in energy metabolism, buffers cellular ATP levels by catalyzing the reversible phosphoryl transfer between ATP and arginine. To better understand the role of Cys271 in conformational changes of AK from greasyback shrimp (Metapenaeus ensis), we replaced the residue with serine and alanine. A detailed comparison of the catalytic activity and conformation was made between wild-type AK and the mutants by means of activity analysis, ultraviolet (UV) difference, fluorescence spectrum and size exclusion chromatography (SEC). The results indicated that the catalytic activity of the two mutants was gone. The substrates, arginine-ADP-Mg(2+) could induce conformational changes, and additional NO(3)(-) could induce further changes in both the native enzyme and the variants. We speculated that Cys271 might be located in the hinge region between the two domains of AK and cause enzyme conformational changes upon addition of substrate.

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.

cDNA identification, comparison and phylogenetic aspects of lombricine kinase from two oligochaete species

Creatine kinase and arginine kinase are the typical representatives of an eight-member phosphagen kinase family, which play important roles in the cellular energy metabolism of animals. The phylum Annelida underwent a series of evolutionary processes that resulted in rapid divergence and radiation of these enzymes, producing the greatest diversity of the phosphagen kinases within this phylum. Lombricine kinase (EC 2.7.3.5) is one of such enzymes and sequence information is rather limited compared to other phosphagen kinases. This study presents data on the cDNA sequences of lombricine kinase from two oligochaete species, the California blackworm (Lumbriculus variegatus) and the sludge worm (Tubifex tubifex). The deduced amino acid sequences are analyzed and compared with other selected phosphagen kinases, including two additional lombricine kinase sequences extracted from DNA databases and provide further insights in the evolution and position of these enzymes within the phosphagen kinase family. The data confirms the presence of a deleted region within the flexible loop (the GS region) of all six examined lombricine kinases. A phylogenetic analysis of these six lombricine kinases clearly positions the enzymes together in a small subcluster within the larger creatine kinase (EC 2.7.3.2) clade.

Cloning, expression, characterization and phylogenetic analysis of arginine kinase from greasyback shrimp (Metapenaeus ensis)

Arginine kinase (AK) plays an important role in cellular energy metabolism in invertebrate. The encoding AK gene from Shrimp Metapenaeus ensis (M. ensis) was cloned in prokaryotic expression plasmid pET-28a, and it was then expressed in Escherichia coil in dissoluble form. The recombinant protein was purified by following three chromatography steps in turn: CM-Cellulose cation-exchange, Sephacryl S-100HR gel filtrate and DEAE-Sepharose anion-exchange. The purified AK's apparent K(m) was 2.33+/-0.1 and 1.59+/-0.2 mM for ATP and l-arginine, respectively, while its optimum pH and temperature was 8.5 and 30 degrees C in the process of forward reaction, respectively. Phylogenetic analysis of cDNA-derived amino acid sequences for the AKs indicated a close affinity of M. ensis and another shrimp (Litopenaeus vannamei).

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.

Arginine kinase: differentiation of gene expression and protein activity in the red imported fire ant, Solenopsis invicta

Arginine kinase (AK), a primary enzyme in cell metabolism and adenosine 5'-triphosphate (ATP)-consuming processes, plays an important role in cellular energy metabolism and maintaining constant ATP levels in invertebrate cells. In order to identify genes that are differentially expressed between larvae and adults, queens and workers, and female alates (winged) and queens (wingless), AK cDNA was obtained from the red imported fire ant. The cDNA sequence of the gene has open reading frames of 1065 nucleotides, encoding a protein of 355 amino acid residues that includes the substrate recognition region, the signature sequence pattern of ATP:guanidino kinases, and an "actinin-type" actin binding domain. Northern blot analysis and protein activity analysis demonstrated that the expression of the AK gene and its protein activity were developmentally, caste specifically, and tissue specifically regulated in red imported fire ants with a descending order of worker> alate (winged adult) female> alate (winged adult) male> larvae> worker pupae approximately alate pupae. These results suggest a different demand for energy-consumption and production in the different castes of the red imported fire ant, which may be linked to their different missions and physiological activities in the colonies. The highest level of the AK gene expression and activity was identified in head tissue of both female alates and workers and thorax tissue of workers, followed by thorax tissue of female alates and abdomen tissue of male alates, suggesting the main tissues or cells in these body parts, such as brain, neurons and muscles, which have been identified as the major tissues and/or cells that display high and variable rates of energy turnover in other organisms, play a key role in energy production and its utilization in the fire ant. In contrast, in the male alate, the highest AK expression and activity were found in the abdomen, suggesting that here energy demand may relate to sperm formation and reproduction.

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.

Cytoplasmic and mitochondrial creatine kinases from the skeletal muscle of sperm whale (Physeter macrocephalus). Molecular cloning and enzyme characterization

We have amplified two cDNAs, coding for creatine kinases (CKs), from the skeletal muscle of sperm whale Physeter macrocephalus by PCR, and cloned these cDNAs into pMAL plasmid. These are the first CK cDNA and deduced amino acid sequences from cetaceans to be reported. One of the two amino acid sequences is a cytoplasmic, muscle-type isoform (MCK), while the other was identified as a sarcomeric, mitochondrial isoform (sMiCK) that included a mitochondrial targeting peptide. The amino acid sequences of sperm whale MCK and sMiCK showed 94-96% sequence identity with corresponding isoforms of mammalian CKs, and all of the key residues necessary for CK function were conserved. The phylogenetic analyses of vertebrate CKs with three independent methods (neighbor-joining, maximum-likelihood and Bayes) supported the clustering of sperm whale MCK with Bos and Sus MCKs, in agreement with the contemporary view that these groups are closely related. Sperm whale MCK and sMiCK were expressed in Escherichia coli as a fusion protein with maltose-binding protein, and the kinetic constants (K (m), K (d) and k (cat)) were determined for the forward reaction. Comparison of kinetic constants with those of human and mouse CKs indicated that sperm whale MCK has a comparable affinity for creatine (K (m) (Cr) = 9.38 mM) to that of human MCK, and the sMiCK has two times higher affinity for creatine than the human enzyme. Both the MCK and sMiCK of sperm whale display a synergistic substrate binding (K (d) /K (m) = 3.1-7.8) like those of other mammalian CKs.

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.

Arginine kinase from the beetle Cissites cephalotes (Olivier). Molecular cloning, phylogenetic analysis and enzymatic properties

Here, we report the PCR amplification and cloning of a cDNA for arginine kinase (AK) from the beetle Cissites cephalotes (Olivier). The cDNA is 1210bp and has an open reading frame of 1125bp and 5' and 3'-untranslated regions of 30 and 55bp, respectively. The open reading frame encodes a 374 amino acid protein with most of the residues considered necessary for AK function: five residues predicted to interact with the substrate arginine (S77, Y82, E239, C285 and E328), and five residues predicted to interact with the substrate ADP (R138, R140, R243, R294 and R323). A phylogenetic tree of arthropod AKs indicated clearly that insect AKs can be separated into typical AKs from various insect species (group 1) and putative AK sequences deduced from genomic sequences (group 2). Cissites AK clustered in group 2 and provides the first evidence that a group-2 gene is indeed expressed in insects. Moreover, we expressed Cissites AK protein in Escherichia coli as a fusion with maltose-binding protein, and kinetic constants (K(m), K(d), V(max) and k(cat)) were determined for the forward reaction. Comparison of kinetic constants with those of AKs from other sources (insects, mollusks and echinoderms) indicated that insect AKs from Cissites and Periplaneta have two very unique features, the lowest k(cat) (and k(cat)/K(m)(arg)) among AKs, and a lack of synergistic substrate binding (K(d)/K(m) approximately 1).

Metabolic responses and arginine kinase expression under hypoxic stress of the kuruma prawn Marsupenaeus japonicus

In response to hypoxia at PO(2) 1.3-1.7 mg/L for 6 h, the kuruma prawn Marsupenaeus (Penaeus) japonicus showed a dramatic decrease in phosphoarginine storage in muscle, with normal levels restored during 4-h post-hypoxic recovery. Large stores of muscle glycogen only decreased between 4 and 6 h during hypoxia, but greatly diminished during recovery. Muscle ATP levels and energy charge decreased only slightly under hypoxia. Lactate levels increased slightly during hypoxia and promptly returned to control levels during recovery. These data indicate that phosphoarginine works in muscle as an ATP buffer during hypoxia and glycogen is utilized as an energy source during recovery. Under hypoxia, up- and down-regulated proteins were identified after 2D electrophoresis and partial sequences were obtained after protease digestion. Fructose bisphosphate aldolase was down-regulated during hypoxia, suggesting the suppression of glycolysis under hypoxia. Several partial sequences from three protein spots up-regulated under hypoxia were all assigned to arginine kinase, suggesting the existence of several isoforms of arginine kinase in the muscle of M. japonicus. This arginine kinase up-regulation under hypoxia may indicate a provision for oxygen re-supply after anaerobiosis. This is consistent with the prompt replenishment of phosphoarginine stores during recovery from hypoxia.

The roles of C-terminal loop residues of dimeric arginine kinase from sea cucumber Stichopus japonicus in catalysis, specificity and structure

Arginine kinase (AK) catalyzes the reversible phosphorylation of arginine by MgATP to form a high-energy compound phosphoarginine (Parg) and MgADP in forward reaction in invertebrates. To detect the different catalytical mechanisms among Stichopus-AK (dimer) and Limulus-AK (monomer) and Torpedo creatine kinase (dimeric CK) and to reveal the structural role of the C-terminal domain loop (C-loop) of dimeric AK, six single-site mutants, E314D, E314Q, E314V, F315A, F315H and F315Y were constructed as well as two multi-site variants, S312R/F315H/V319E (formed by substituting the C-loop of monomeric AK for that of dimeric AK, termed the AAloop) and S312G/E314V/F315D/E317A/S318A/G321S (formed by substituting the C-loop of dimeric CK for that of dimeric AK, termed the ACloop). The AK activity of the three mutants at Glu(314) decreased significantly, from 60- to 500-fold. The ACloop showed only slight AK activity, unlike the same construction in Limulus-AK. In addition, all Phe(315) mutants including the AAloop which retained Glu(314) had modest AK activity (5-84% of the wild type). All the results above suggested that Glu(314) played a more significant role in catalysis in dimeric AK than in the monomer. In addition, ANS profiles indicated that the tolerance of the three Glu(314) mutants to denaturant decreased slightly compared with wild type AK. Though monomeric AK has a His residue at site 315, mutants F315H and the AAloop could not resist any perturbation of denaturant, and the mutants showed a Gibbs free energy of about 2.7kJ/mol lower than wild type AK. Therefore Phe(315) in dimeric AK has a different role from His(315) in monomeric AK. This might contribute to the stabilization of the native conformation, while His(315) in Limulus AK directly binded to the carboxylate of arginine. Taking all the results above together, we suggested a unique mechanism in dimeric AK, different from both monomeric AK and dimeric CK.

Evolution of the arginine kinase gene family

Arginine kinase (AK), catalyzing the reversible transfer of phosphate from MgATP to arginine yielding phosphoarginine and MgADP, is widely distributed throughout the invertebrates and is also present in certain protozoa. Typically, these proteins are found as monomers targeted to the cytoplasm, but true dimeric and contiguous dimeric AKs as well as mitochondrial AK activities have been observed. In the present study, we have obtained the sequences of the genes for AKs from two distantly related molluscs-the cephalopod Nautilus pompilius and the bivalve Crassostrea gigas. These new data were combined with available gene structure data (exon/intron organization) extracted from EST and genome sequencing project databases. These data, comprised of 23 sequences and gene structures from Protozoa, Cnidaria, Platyhelminthes, Mollusca, Arthropoda and Nematoda, provide great insight into the evolution and divergence of the AK family. Sequence and phylogenetic analyses clearly show that the AKs are homologous having arisen from some common ancestor. However, AK gene organization is highly divergent and variable. Molluscan AK genes typically have a highly conserved six-exon/five-intron organization, a structure that is very similar to that of the platyhelminth Schistosoma mansoni Arthropod and nematode AK genes have fewer introns, while the cnidarian and protozoan genes each display unique exon/intron organization when compared to the other AK genes. The non-conservative nature of the AK genes is in sharp contrast to the relatively high degree of conservation of intron positions seen in a homologous enzyme creatine kinase (CK). The present results also show that gene duplication and subsequent fusion events forming unusual two-domain AKs occurred independently at least four times as these contiguous dimers are present in Protozoa, Cnidaria, Platyhelminthes and Mollusca. Detailed analyses of the amino acid sequences indicate that two AKs (one each from Drosophila and Caenorhabditis) have what appear to be N-terminal mitochondrial targeting sequences, providing the first evidence for true mitochondrial AK genes. The AK gene family is ancient and the lineage has undergone considerable divergence as well as multiple duplication and fusion events.