Molecular characterization and kinetic properties of a novel two-domain taurocyamine kinase from the lung flukeParagonimus westermani

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.

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.

In-Depth Proteomic Analysis of the Porcine Retina by Use of a four Step Differential Extraction Bottom up LC MS Platform

The eye of the house swine (Sus scrofa domestica Linnaeus, 1758) represents a promising model for the study of human eye diseases encircling neurodegenerative retina disorders that go along with proteomic changes. To provide an in-depth view into the "normal" (untreated & healthy) porcine retina proteome as an important reference, a proteomic strategy has been developed encircling stepwise/differential extraction, LC MS and peptide de novo sequencing. Accordingly, pooled porcine retina homogenates were processed by stepwise DDM, CHAPS, ASB14 and ACN/TFA extraction. Retinal proteins were fractionated by 1D-SDS PAGE and further analyzed by LC ESI MS following database and de novo sequencing related protein identification and functional analyses. In summary, >2000 retinal proteins (FDR < 1 %) could be identified by use of the highly reproducible and selective extraction procedure. Moreover, an identification surplus of 36 % comparing initial one step extraction to the four step method could be documented. Despite most proteins were identified in the DDM and CHAPS fraction, all extraction steps contributed exclusive proteins with nucleus proteins enriched in the final ACN/TFA fraction. Additionally, for the first time new non-annotated de novo peptides could be documented for the porcine retina. The generated porcine retina proteome reference map contributes importantly to the understanding of the pig eye proteome and the developed workflow has strong translational potential considering retina studies of various species.

Arginine kinase in Toxocara canis: Exon-intron organization, functional analysis of site-directed mutants and evaluation of putative enzyme inhibitors

OBJECTIVES

To determine exon/intron organization of the Toxocara canis (T. canis) AK (TCAK) and to test green and black tea and several other chemicals against the activity of recombinant TCAK in the guanidino-specific region by site-directed mutants.

METHODS

Amplification of genomic DNA fragments containing introns was carried out by PCRs. The open-reading frame (1200 bp) of TCAK (wild type) was cloned into the BamH1/SalI site of pMAL-c2X. The maltose-binding protein-TCAK fusion protein was expressed in Escherichia coli TB1 cells. The purity of the expressed enzyme was verified by SDS-PAGE. Mutations were introduced into the guanidino-specific region and other areas of pMAL/TCAK by PCR. Enzyme activity was measured with an NADH-linked assay at 25 °C for the forward reaction (phosphagen synthesis).

RESULTS

Arginine kinase in T. canis has a seven-exon/six-intron gene structure. The lengths of the introns ranged from 542 bp to 2 500 bp. All introns begin with gt and end with ag. Furthermore, we measured the enzyme activity of site-directed mutants of the recombinant TCAK. The K_m value of the mutant (Alanine to Serine) decreased indicating a higher affinity for substrate arginine than the wild-type. The K_m value of the mutant (Serine to Glycine) increased to 0.19 mM. The K_m value (0.19 mM) of the double mutant (Alanine-Serine to Serine-Glycine) was slightly greater than in the wild-type (0.12 mM). In addition, several other chemicals were tested; including plant extract Azadiracta indica (A. indica), an aminoglycoside antibiotic (aminosidine), a citrus flavonoid glycoside (rutin) and a commercially available catechin mixture against TCAK. Green and black tea (1:10 dilution) produced 15% and 25% inhibition of TCAK, respectively. The extract of A. indica produced 5% inhibition of TCAK. Moreover, green and black tea produced a non-competitive type of inhibition and A. indica produced a mixed-type of inhibition on TCAK.

CONCLUSIONS

Arginine kinase in T. canis has a seven-exon/six-intron gene structure. However, further studies are needed to identify a specific compound within the extract causing the inhibitory effect and also to determine the molecular mechanisms behind inhibition of arginine kinase in T. canis.

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.

Characterization of the arginine kinase isoforms in Caenorhabditis elegans

Phosphagen kinases (PKs) are well-studied enzymes involved in energy homeostasis in a wide range of animal, protozoan, and even some bacterial species. Recent genome efforts have allowed comparative work on the PKs to extend beyond the biochemistry of individual proteins to the comparative cellular physiology and examining of the role of all PK family members in an organism. The sequencing of the Caenorhabditis elegans genome and availability of sophisticated genetic tools within that system affords the opportunity to conduct a detailed physiological analysis of the PKs from a well known invertebrate for comparison with the extensive work conducted on vertebrate systems. As a first step in this effort we have carried out a detailed molecular genetic and biochemical characterization of the PKs in C. elegans. Our results reveal that C. elegans has five PK genes encoding arginine kinases that range in catalytic efficiency (kcat/KM(Arg)) from (3.1±0.6)×10(4) to (9±4)×10(5) M(-1) s(-1). This range is generally within the range seen for arginine kinases from a variety of species. Our molecular genetic and phylogenetic analysis reveals that the gene family has undergone extensive intron loss and gain within the suborder Rhabditina. In addition, within C. elegans we find evidence of gene duplication and loss. The analysis described here for the C. elegans AKs represents one of the most complete biochemical and molecular genetic analysis of a PK family within a genetically tractable invertebrate system and opens up the possibility of conducting detailed physiological comparisons with vertebrate systems using the sophisticated tools available with this model invertebrate system.

The substrate-free and -bound crystal structures of the duplicated taurocyamine kinase from the human parasite Schistosoma mansoni

The taurocyamine kinase from the blood fluke Schistosoma mansoni (SmTK) belongs to the phosphagen kinase (PK) family and catalyzes the reversible Mg(2+)-dependent transfer of a phosphoryl group between ATP and taurocyamine. SmTK is derived from gene duplication, as are all known trematode TKs. Our crystallographic study of SmTK reveals the first atomic structure of both a TK and a PK with a bilobal structure. The two unliganded lobes present a canonical open conformation and interact via their respective C- and N-terminal domains at a helix-mediated interface. This spatial arrangement differs from that observed in true dimeric PKs, in which both N-terminal domains make contact. Our structures of SmTK complexed with taurocyamine or l-arginine compounds explain the mechanism by which an arginine residue of the phosphagen specificity loop is crucial for substrate specificity. An SmTK crystal was soaked with the dead end transition state analog (TSA) components taurocyamine-NO3 (2-)-MgADP. One SmTK monomer was observed with two bound TSAs and an asymmetric conformation, with the first lobe semiclosed and the second closed. However, isothermal titration calorimetry and enzyme kinetics experiments showed that the two lobes function independently. A small angle x-ray scattering model of SmTK-TSA in solution with two closed active sites was generated.

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.

Paragonimiasis

Paragonimiasis is a zoonotic disease caused by lung flukes of the genus Paragonimus. Humans usually become infected by eating freshwater crabs or crayfish containing encysted metacercariae of these worms. However, an alternative route of infection exists: ingestion of raw meat from a mammalian paratenic host. Adult worms normally occur in pairs in cysts in the lungs from which they void their eggs via air passages. The pulmonary form is typical in cases of human infection due to P. westermani, P. heterotremus, and a few other species (Table 5.1). Worms may occupy other sites in the body, notably the brain, but lung flukes have made their presence felt in almost every organ. Ectopic paragonimiasis is particularly common when infection is due to members of the P. skrjabini complex (Table 5.1). Human paragonimiasis occurs primarily in the tropics and subtropics of Asia, Africa, and the Americas, with different species being responsible in different areas (Table 5.1).

Molecular cloning and characterization of taurocyamine kinase from Clonorchis sinensis: a candidate chemotherapeutic target

Background

Adult Clonorchis sinensis lives in the bile duct and causes endemic clonorchiasis in East Asian countries. Phosphagen kinases (PK) constitute a highly conserved family of enzymes, which play a role in ATP buffering in cells, and are potential targets for chemotherapeutic agents, since variants of PK are found only in invertebrate animals, including helminthic parasites. This work is conducted to characterize a PK from C. sinensis and to address further investigation for future drug development.

Methology/Principal findings

A cDNA clone encoding a putative polypeptide of 717 amino acids was retrieved from a C. sinensis transcriptome. This polypeptide was homologous to taurocyamine kinase (TK) of the invertebrate animals and consisted of two contiguous domains. C. sinensis TK (CsTK) gene was reported and found consist of 13 exons intercalated with 12 introns. This suggested an evolutionary pathway originating from an arginine kinase gene group, and distinguished annelid TK from the general CK phylogenetic group. CsTK was found not to have a homologous counterpart in sequences analysis of its mammalian hosts from public databases. Individual domains of CsTK, as well as the whole two-domain enzyme, showed enzymatic activity and specificity toward taurocyamine substrate. Of the CsTK residues, R58, I60 and Y84 of domain 1, and H60, I63 and Y87 of domain 2 were found to participate in binding taurocyamine. CsTK expression was distributed in locomotive and reproductive organs of adult C. sinensis. Developmentally, CsTK was stably expressed in both the adult and metacercariae stages. Recombinant CsTK protein was found to have low sensitivity and specificity toward C. sinensis and platyhelminth-infected human sera on ELISA.

Conclusion

CsTK is a promising anti-C. sinensis drug target since the enzyme is found only in the C. sinensis and has a substrate specificity for taurocyamine, which is different from its mammalian counterpart, creatine.

The food-borne clonorchiasis imposes public health problems on inhabitants in endemic areas. Praziquantel has been employed as an efficacious anthelminthic in large-scale campaigns as well as for individual treatment of Clonorchis sinensis human infections. Although praziquantel continues to have good efficacy, new drug development for this parasite has been recognized as a crucial issue to be investigated intensively. Clonorchis sinensis adults generate energy through glycolysis, actively utilizing exogenous glucose, and produce a large amount of eggs each day. Taurocyamine kinase (CsTK) is distributed abundantly in the locomotive and reproductive organs, and is an important enzyme in energy generation and homeostasis in adult C. sinensis. Enzymes of the glycolytic pathway are also expressed abundantly in these organs and in tegument, implying these organs play central roles which are essential for survival and reproduction of C. sinensis. The TK enzymes, including CsTK, are found only among invertebrate organisms and have substrate specificity for taurocyamine, which are significantly different from phosphagen kinases of vertebrate animals. With these molecular biological, enzymatic, and evolutionary characteristics, we propose here that CsTK could be a target for development of chemotherapeutic agents against C. sinensis and be a biomolecular model for other human-infecting trematodes.

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.

Characterization of a putative oomycete taurocyamine kinase: Implications for the evolution of the phosphagen kinase family

Phosphagen kinases (PKs) are known to be distributed throughout the animal kingdom, but have recently been discovered in some protozoan and bacterial species. Within animal species, these enzymes play a critical role in energy homeostasis by catalyzing the reversible transfer of a high-energy phosphoryl group from Mg⋅ATP to an acceptor molecule containing a guanidinium group. In this work, a putative PK gene was identified in the oomycete Phytophthora sojae that was predicted, based on sequence homology, to encode a multimeric hypotaurocyamine kinase. The recombinant P. sojae enzyme was purified and shown to catalyze taurocyamine phosphorylation efficiently (kcat/KM (taurocyamine) = 2 × 10(5) M(-1) s(-1)) and glycocyamine phosphorylation only weakly (kcat/KM (glycocyamine) = 2 × 10(2) M(-1) s(-1)), but lacked any observable kinase activity with the more ubiquitous guanidinium substrates, creatine or arginine. Additionally, the enzyme was observed to be dimeric but lacked cooperativity between the subunits in forming a transition state analog complex. These results suggest that protozoan PKs may exhibit more diversity in substrate specificity than was previously thought.

Gene structure of the two-domain taurocyamine kinase from Paragonimus westermani: evidence for a distinct lineage of trematode phosphagen kinases

Taurocyamine kinase (TK) is an enzyme that catalyzes the reversible transfer of a phosphate between ATP and taurocyamine. Annelid TKs were suggested to have evolved from a CK ancestor. However, TKs from the lung fluke Paragonimus westermani comprised another lineage. Construction of phylogenetic tree and comparison of exon/intron organization showed that P. westermani TK and other trematode TKs evolved from a molluscan arginine kinase (AK) gene. Exon shuffling probably caused the changes in amino acid sequence thereby changing the affinity from AK to TK. The present study provides new insights on the evolution of phosphagen kinases found in trematodes.

Phosphagen kinase in Schistosoma japonicum: characterization of its enzymatic properties and determination of its gene structure

Phosphagen kinases (PKs) play a major role in the regulation of energy metabolism in animals. Creatine kinase (CK) is the sole PK in vertebrates, whereas several PKs are present in invertebrates. Here, we report the enzymatic properties and gene structure of PK in the trematode Schistosoma japonicum (Sj). SjPK has a unique contiguous dimeric structure comprising domain 1 (D1) and domain 2 (D2). The three states of the recombinant SjPK (D1, D2, and D1D2) show a specific activity for the substrate taurocyamine. The comparison of the two domains of SjPK revealed that D1 had a high turnover rate (kcat=52.91) and D2 exhibited a high affinity for taurocyamine (Km(Tauro) =0.53±0.06). The full-length protein exhibited higher affinity for taurocyamine (Km(Tauro) =0.47±0.03) than the truncated domains (D1=1.30±0.10, D2=0.53±0.06). D1D2 also exhibited higher catalytic efficiency (kcat/Km(Tauro) =82.98) than D1 (40.70) and D2 (29.04). These results demonstrated that both domains of SjTKD1D2 interacted efficiently and remained functional. The three-dimensional structure of SjPKD1 was constructed by the homology modeling based on the transition state analog complex state of Limulus AK. This protein model of SjPKD1 suggests that the overall structure is almost conserve between SjPKD1 and Limulus AK except for the flexible loops, that is, particularly guanidino-specificity (GS) region, which is associated with the recognition of the corresponding guanidino substrate. The constructed NJ tree and the comparison of exon/intron organization suggest that SjTK has evolved from an arginine kinase (AK) gene. SjTK has potential as a novel antihelminthic drug target as it is absent in mammals and its strong activity may imply a significant role for this protein in the energy metabolism of the parasite.

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.

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.

Phosphagen kinases of parasites: unexplored chemotherapeutic targets

Due to the possible emergence of resistance and safety concerns on certain treatments, development of new drugs against parasites is essential for the effective control and subsequent eradication of parasitic infections. Several drug targets have been identified which are either genes or proteins essential for the parasite survival and distinct from the hosts. These include the phosphagen kinases (PKs) which are enzymes that play a key role in maintenance of homeostasis in cells exhibiting high or variable rates of energy turnover by catalizing the reversible transfer of a phosphate between ATP and naturally occurring guanidine compounds. PKs have been identified in a number of important human and animal parasites and were also shown to be significant in survival and adaptation to stress conditions. The potential of parasite PKs as novel chemotherapeutic targets remains to be explored.