Cloning and molecular characterization of P . westermani

Paragonimiasis is a food-borne trematode infection that affects 22 million people in at least 20 countries (World Health Organization [WHO], 2002) with 293 million more at risk of infection (Keiser & Utzinger, 2007). In East, Southeast, and South Asia, pulmonary paragonimiasis is commonly caused by the trematode Paragonimus westermani (Blair et al., 2007). Humans get infected by this lung fluke by ingesting metacercariae present in raw freshwater crabs or by eating raw meat of paratenic hosts such as omnivorous mammals (Miyazaki & Habe, 1976). Human paragonimiasis has been reported to be re-emerging in previously endemic areas in Japan (Mukae et al., 2001; Nakano et al., 2001, and Kirino et al., 2009) and increasing in some regions of China (Lieu et al., 2008). New foci of transmission have also been reported in Lao PDR (Odermatt et al., 2009). Currently, either praziquantel or triclabendazole is effective for the treatment of pulmonary paragonimiasis. However, since these are the only viable drugs against this infection, there is a need to develop back up drugs while drug resistance is not yet emerging (Keiser & Utzinger, 2007). In addition, a more specific and sensitive detection tool is needed for diagnosing pulmonary paragonimiasis since a considerable number of cases are misdiagnosed as tuberculosis or vice versa due to similarity of some signs and symptoms (WHO, 2002). The rapid availability of parasite genomic sequences coupled with development of robust bioinformatics tools have resulted in the identification of numerous potential drug targets. These include the phosphagen kinases (PKs) that catalyze the reversible transfer of a phosphate between ATP and naturally occurring guanidino substrates. These enzymes play a key role in maintaining cellular energy homeostasis through temporal energy buffering that stabilizes the cellular ATP/ADP hydrolysis (Ellington, 2001). Studies on PKs of parasitic protozoans and nematodes have shown that PKs are important in energy metabolism and adaptation to stress conditions (Platzer et al, 1999; Alonso et al., 2001; Miranda et al., 2006; Pereira et al., 2003; Pereira et al., 2002). Enzyme activity of PK in Trypanosoma cruzi, the causative agent of Chagas disease, has been shown to be inhibited by various compounds such as catechin gallate (Paveto et al., 2004) and arginine analogs


Introduction
Paragonimiasis is a food-borne trematode infection that affects 22 million people in at least 20 countries (World Health Organization [WHO], 2002) with 293 million more at risk of infection (Keiser & Utzinger, 2007).In East, Southeast, and South Asia, pulmonary paragonimiasis is commonly caused by the trematode Paragonimus westermani (Blair et al., 2007).Humans get infected by this lung fluke by ingesting metacercariae present in raw freshwater crabs or by eating raw meat of paratenic hosts such as omnivorous mammals (Miyazaki & Habe, 1976).Human paragonimiasis has been reported to be re-emerging in previously endemic areas in Japan (Mukae et al., 2001;Nakano et al., 2001, andKirino et al., 2009) and increasing in some regions of China (Lieu et al., 2008).New foci of transmission have also been reported in Lao PDR (Odermatt et al., 2009).Currently, either praziquantel or triclabendazole is effective for the treatment of pulmonary paragonimiasis.However, since these are the only viable drugs against this infection, there is a need to develop back up drugs while drug resistance is not yet emerging (Keiser & Utzinger, 2007).In addition, a more specific and sensitive detection tool is needed for diagnosing pulmonary paragonimiasis since a considerable number of cases are misdiagnosed as tuberculosis or vice versa due to similarity of some signs and symptoms (WHO, 2002).The rapid availability of parasite genomic sequences coupled with development of robust bioinformatics tools have resulted in the identification of numerous potential drug targets.These include the phosphagen kinases (PKs) that catalyze the reversible transfer of a phosphate between ATP and naturally occurring guanidino substrates.These enzymes play a key role in maintaining cellular energy homeostasis through temporal energy buffering that stabilizes the cellular ATP/ADP hydrolysis (Ellington, 2001).Studies on PKs of parasitic protozoans and nematodes have shown that PKs are important in energy metabolism and adaptation to stress conditions (Platzer et al, 1999;Alonso et al., 2001;Miranda et al., 2006;Pereira et al., 2003;Pereira et al., 2002).Enzyme activity of PK in Trypanosoma cruzi, the causative agent of Chagas disease, has been shown to be inhibited by various compounds such as catechin gallate (Paveto et al., 2004) and arginine analogs (Pereira et al., 2003).In this chapter the cloning, expression and molecular characterization of a novel phosphagen kinase from the lung fluke P. westermani will be described.Mutation studies for the elucidation of substrate binding mechanism and phylogenetic analyses will also be presented.

Cloning and molecular characterization of P. westermani PK
At present, eight PKs and their corresponding phosphagens have been identified (Ellington, 2001).Creatine kinase (CK) is the sole PK in vertebrates.In addition to CK, the following PKs are found in various invertebrate species: arginine kinase (AK) (Uda et al. 2006), hypotaurocyamine kinase (HTK), glycocyamine kinase (GK), thalessemine kinase (ThK); opheline kinase (OK), lombricine kinase (LK), and taurocyamine kinase (TK) (Robin, 1974;Thoai, 1968;Morrison, 1973).Among the PKs, AK is the most widely distributed being present in deuterostomes, protostomes, basal metazoans, some protozoans (Uda et al. 2006) and in the prokaryote Desulfotalea psychrophila (Andrews et al., 2008).Besides T. cruzi, phosphagen kinases have also been identified in other important animal and human parasites.AKs were cloned from the nematodes Ascaris suum and Toxocara canis which can cause visceral larva migrans (VLM) in humans (Nagataki et al., 2008;Wickramasinghe et al., 2007) and from T. brucei which causes human sleeping sickness and Nagana in livestock (Pereira et al., 2002).The PK from the trematode Schistosoma mansoni was also recently described by Awama et al. (2008).To determine the cDNA sequence of the PK in P. westermani, total RNA was first isolated from samples collected from definitive hosts in Bogil Island, South Korea using the acid guanidinium thiocyanate-phenol-chloroform extraction method (Chomczynski & Sacchi, 1987).Messenger RNA (mRNA) was purified with a poly (A)+ isolation kit (Nippon Gene, Tokyo, Japan).First-strand cDNA was synthesized from 20 ng to 2 μg of mRNA using the Ready-To-Go You-Prime First-Strand Beads (Amersham Pharmacia Biotech, NJ, USA) which utilizes Moloney Murine Leukemia Virus (M-MuLV) as reverse transcriptase.One microliter of 10 pmol lock-docking oligo(dT) primer was used for the first-strand synthesis.The 5' half of P. westermani PK cDNA was first amplified by RT-PCR using the universal PK primers SmTcPKptnF1 and SmTcPKptnR1 and ExTaq DNA polymerase (Takara, Kyoto, Japan) as the amplifying enzyme.The PCR reaction components are listed in Table 1 while the amplification conditions are in Table 2.

Components
Concentration/25 µL reaction volume The amplified products were purified using GeneClean ® II Kit (QBIOgene, USA) and subcloned into the pGEM ® T-vector (Promega, USA) and transformed into E. coli JM109 cells (Takara, Japan).After transformation, the bacteria are plated on Luria-Bertani plate containing 5-bromo-4-chloro-3-indoyl-β-D-galactoside (X-gal), isopropyl β-D-1 galactopyranoside (IPTG), and ampicillin.Positive clones were obtained and cultured overnight in liquid 2 ml Luria Bertani (LB) medium with ampicillin.Plasmid DNA was extracted using the alkaline SDS method and nucleotide sequencing was done with an ABI PRISM 3100-Avant DNA sequencer using a Big Dye Terminators v3.1 Cycle Sequencing Kit (Applied Biosystems, CA, USA).From the obtained partial sequence, the specific primer PwKoreaPKF1 was designed and used, together with the lock-docking oligo (dT) primer, to amplify and determine the remaining sequence of the 5' half.A poly (G) + tail was added to the 3' end of the P. westermani cDNA pool with terminal deoxynucleotidyl transferase (Promega, WI, USA).The 3' half of the PK cDNA was then amplified using the oligo(dC) primer and a specific primer PwKoreaPKR3 designed from the sequence of the 5' half.The amplified products were purified, subcloned, and sequenced as described above.The sequences of the primers used for cDNA amplification are listed on Table 3.

Primer name
Sequence (5'-3') PwKoreaPKR3 TTTTTGTTGTGGAAGATCCC Table 3. Primers used for the amplification of P. westermani PK cDNA.Two-domain AKs were also reported for the sea anemone Anthopleura japonicus (Suzuki et al., 1997), the clams Pseudocardium sachalinensis (Suzuki et al., 1998), Corbicula japonica, Solen strictus (Suzuki et al., 2002), Ensis directus (Compaan & Ellington, 2003), and Calyptogena kaikoi (Uda et al., 2008).The PK found in S. mansoni also has a contiguous two-domain structure (Awama et al., 2008).These multiple domain AKs are products of gene duplication and subsequent fusion as suggested by the presence of a bridge intron separating the two domains of A. japonicus and P. sachalinensis AKs (Suzuki & Yamamoto, 2000).Members of the phosphagen kinase family share high sequence identity suggesting that these enzymes have evolved from a common ancestor (Suzuki et al., 1997).Phylogenetic analyses have shown that PKs can be divided into two distinct lineages, an AK lineage and a CK lineage (Uda et al., 2005).It is probable that the ancestral PK was monomeric and an early gene duplication event resulted in the formation of these lineages.Moreover, the genes coding for the ancestral AK and CK could have been present early in the evolution of metazoans since AKs and CKs are found in both deuterostomes and protostomes (Ellington & Suzuki, 2006).
The phylogenetic tree constructed based on the amino acid sequence of P. westermani PK and other phosphagen kinases using the Neighbor-joining method in MEGA version 4 (Tamura et al., 2007) shows the presence of two major clusters: a CK cluster and an AK cluster.The CK cluster includes the CKs from vertebrates and other PKs from annelids.The AK cluster, on the other hand, is divided into two subclusters.The first subcluster contains the nematode, arthropod, and protozoan AKs.P. westermani PK falls in the second subcluster together with S. mansoni PK, molluscan AKs and sipunculid HTK.

Expression of recombinant P. westermani PK and enzyme kinetics
The full-length and truncated domains of P. westermani PK were expressed in E. coli as MBP fusion proteins.Specific primers with EcoRI and PstI restriction sites were designed to amplify the open reading frames (ORFs) of P. westermani PK.The forward primer PwKorPK5D1EcoRI (5'TTGAATTCGCCGATGTCATGCCTGTTGAG-3') and reverse primer PwKorPK3D1PstI (5'-TTCTGCAGTTACGGAGCATCCTTGTTGTAA-3') were used to amplify D1 while the primers PwKorPK5D2EcoRI (5'-TTGAATTCG CCGATG TCATGCCTGTTGAG-3') and PwKorPK3D2PstI (5'-TTCTGCAGTCAAA GTGACTGTTCGATAGC-3') were used for D2.PwKorPK5D1EcoRI and PwKorPK3D2PstI were used to amplify the ORF of the full-length construct.PCR was done in a total volume of 50 µL using KOD DNA polymerase for high fidelity amplification.The thermal cycling condition used was similar as described above.The components of the PCR reaction mixture are shown on Table 4. Since KOD DNA polymerase produces blunt-ended DNA products, an A-tail was added to the 3' end of the PCR product.Amplified products were first purified using the QIA quick PCR purification columns (QIAGEN, GmbH, Hilden, Germany).The 30 µL A-tailing reaction mixture contained purified KOD PCR product, 15 U of Gene Taq DNA Polymerase (Wako Nippon Gene, Japan), 3 µL of 10X Gene Taq Buffer, and 1.2 µL of 5 mM dATP.The mixture was incubated at 70°C for 30 minutes.A-tailing product was purified, subcloned, and sequenced as described above.

Components
To isolate the ORFs of P. westermani PK D1, D2, and D1D2, plasmids with verified sequence were restriction digested and cloned into the EcoRI/PstI site of pMAL-c2 (New England Biolabs, MA, USA).Plasmids from selected clones were isolated and sequenced as above for final verification of orientation and sequence of the inserts.The maltose binding protein (MBP)-phosphagen kinase fusion protein was expressed in E. coli TB1 cells by induction with 1 mM IPTG at 25°C for 24 h.The cells were resuspended in 5X TE Buffer, sonicated, and the soluble protein was extracted.The recombinant enzymes were obtained as soluble fractions, and successfully purified by affinity chromatography using amylose resin (New England Biolabs, MA, USA).SDS-PAGE was used to determine the purity of the expressed protein.A single 120 kDa band (PwTK D1D2+MBP) was obtained from SDS-PAGE of the full-length recombinant protein and 80 kDa band (truncated domain+MBP) for each of the truncated domain.
The purified enzymes were placed on ice until enzyme activity assay within 12 h.The enzyme activity of the recombinant proteins was measured with an NADH-linked assay spectrophotometric assay at 25°C (Morrison & James, 1965) and determined for the forward reaction or phosphagen synthesis (Fujimoto et al., 2005).The following available substrates were used to determine specificity: L-arginine, D-arginine, creatine, glycocyamine, and taurocyamine.The MBP tag was not removed from the recombinant enzymes due to possible enzyme inactivation if the tag was to be digested.Tada et al. (2008), based on results of previous studies, concluded that the presence of MBP tag had no significant effect on the substrate binding properties of AK or CK activity.The full-length and truncated domains 1 and 2 showed significant activity for the substrate taurocyamine (0.715 -32.857 umol/min*mg protein) (Table 1).Therefore, it was concluded that the PK of P. westermani is a taurocyamine kinase (Jarilla et al., 2009).(Jarilla et al., 2009).

Substrate
Taurocyamine kinase was previously suggested to be exclusively found in marine annelids (Uda et al., 2005).This enzyme was first purified from the lugworm Arenicola marina by sequential ammonium sulfate precipitation and gel-sieve chromatography (Thoai et al., 1963 as cited in Surholt, 1979) and inferred to be localized in the cytosol and mitochondria (Surholt, 1979).Indeed, cytoplasmic and mitochondrial isoforms of TK were found in A. brasiliensis (Uda et al., 2005) and the tubeworm Riftia pachyptila (Uda et al., 2005).It has been observed that cytoplasmic isoforms are more specific to taurocyamine compared to the mitochondrial isoforms (Uda et al., 2005).On the basis that P. westermani TK showed activity to taurocyamine only, it can be surmised that this TK is cytoplasmic.But this cannot be confirmed at this point, since the 5' UTR of the cDNA was not succesfully amplified; thus, the absence or presence of the mitochondrial targeting sequence cannot be determined.
The kinetic parameters (Km, K d , and k cat ) and V max of the MBP-tagged P. westermani TK were also obtained for the forward reaction with various concentrations of the substrate taurocyamine and ATP.Shown in Table 6 are the components of the reaction mixture (total 1.0 ml).The reaction was started by adding 0.05 ml of an appropriate concentration of guanidine substrate made up in 100 mM Tris-HCl (pH 8).The initial velocity values were obtained by varying the concentration of guanidine substrate (taurocyamine) versus fixed concentrations of the ATP.The K m Tc value was determined from the enzyme reaction using nine different substrate concentrations of taurocyamine around the rough K m Tc value.To determine the K d value, the above reactions were done at four different concentrations of ATP (10 mM, 7 mM, 5 mM, and 3 mM).For the estimation of kinetic constants (K m and K cat ), a Lineweaver-Burk plot was made and fitted by the least-square method in Microsoft Excel.
The kinetics of phosphagen kinase can be explained as a random-order, rapid-equilibrium kinetic mechanism (Morrison and James, 1965), and the K d , the dissociation constant, was obtained graphically as described by Suzuki et al. (1997) or by fitting data directly according to the method of Cleland (Cleland, 1979), using the software written by Dr. R. Viola (Enzyme Kinetics Programs, ver.2.0).Protein concentration was estimated from the absorbance at 280 nm (0.77 AU at 280 nm in a 1 cm cuvette corresponds to 1 mg protein/ml).X-ray crystal structures of substrate-free as well as transition state forms of both AKs and CKs showed that these enzymes can be divided into two structural domains, a smaller amino-terminal (N-terminal) domain and a carboxyl-terminal (C-terminal) domain (Zhou et al., 1998, Lahiri et al., 2002, Gattis et al., 2004).During substrate binding, a flexible loop from each domain folds over the substrate at the active site resulting to large conformational changes (Zhou et al., 1998) which appear to be necessary in aligning the two substrates for catalysis, configuring the active site only when productive phosphoryl transfer is possible, and excluding water from the active site to avoid wasteful ATP hydrolysis (Zhou et al., 2000).These conformational changes are elicited by the combination of Mg 2+ + ADP or ATP which are substrates common to all PKs (Forstner et al., 1998).

Components
Recent studies by Yousef et al. (2003) and Fernandez et al. (2007) on crystal structures of AKs from Limulus polyphemus and Trypanosoma cruzi, respectively, suggested that instead of the movement of two domains, the differences in substrate binding can be attributed to the motion of three domains relative to a fixed one.Dynamic domain 1 comprises the aminoterminal globular domain, as well as other elements of the active site that are critical to substrate specificity and catalysis.It also contains the active-site cysteine conserved in phosphagen kinases that is proposed to mediate the synergism in substrate binding (Yousef et al., 2003) that appears to be a common feature in PKs (Wu et al., 2008).The substrate synergism may be associated with substrate-induced conformational changes within the tertiary complex (Maggio et al., 1977;Zhou et al., 1998).Gattis et al. (2004) suggested that the active-site cysteine is relevant to catalysis and that one of its roles is enhancing the catalytic rate through electrostatic stabilization of the transition state.Also included in the dynamic domain 1 is the highly conserved segment "NEEDH" regarding which interactions link conformational changes to phosphagen binding.The other two dynamic domains, together with the dynamic domain 1, close the active site with separate hinge rotations relative to the fixed domain (Yousef et al. 2003).
The complex conformational changes during substrate binding may be affected by the presence of two or more catalytic domains on a single polypeptide chain (Compaan & Ellington, 2003).Contiguous dimeric AKs from Ensis and C. japonica have high sequence conservation in both domains of the protein but only their second domain showed activity (Compaan & Ellington, 2003;Suzuki al., 2003).However, this was not the case for P. westernai TK since both truncated domains exhibited activity for taurocyamine (Jarilla et al, 2009).
Table 7 shows the kinetic parameters of P. westermani TK and annelid TKs.The second domain and the contiguous domain of P. westermani TK have stronger affinity for taurocyamine than D1 as indicated by their lower K m Tc values.However, D1 has stronger affinity for ATP.P. westermani T K a l s o h a s l o w e r K m Tc compared to the Arenicola cytoplasmic and mitochondrial TKs and Riftia mitochondrial TK.All P. westermani TK constructs exhibit synergism during substrate binding since the K d Tc /K m Tc and K d ATP /K m ATP obtained for the three recombinant enzymes were greater than one.It appears that the full-length P. westermani TK is catalytically more efficient than the truncated domains since the k cat value (a measure of the number of substrate molecules converted to product per enzyme molecule per unit time) for D1D2 accounts for the k cat values of the truncated domains.This is further corroborated by the values obtained for the V max and k cat /K m Tc .

Role of the amino acids on the guanidino specificity (GS) region
The guanidine specificity (GS) region has been proposed by Suzuki et al. (1997) as a potential candidate for the guanidine substrate recognition site based on amino acid sequence analysis results.This is the substrate specifity loop (residues 61-68), included in the part of the N-terminal domain, which moves substantially closer to the phosphagen substrate-binding site (Yousef et al., 2003).It has been suggested that there is a proportional relationship between the size of the deletion in the GS region and the mass of the guanidine substrate used.For instance, CK and GK, which use the smallest substrate, have no deletion while LK, AK, and TK, which recognize relatively large guanidine substrates, have a fiveamino acid deletion (Suzuki et al., 1997;Uda et al., 2005).Previous studies on AKs of Nautilus and Stichopus, CK of Danio and LK of Eisenia showed that introduction of mutations on the GS region significantly reduced the activity of the said enzymes (Suzuki et al., 2000;Suzuki et al., 2000;Uda & Suzuki, 2004;Suzuki & Yamamoto, 2000;Tanaka and Suzuki, 2004).Unlike cytoplasmic TKs from A. brasiliensis and R. Pachyptila which have five amino acid deletions on the GS region (Uda et al., 2005;Uda et al., 2005), the two domains of P. westermani TK have six amino acid deletions in the GS region (Fig. 4) (Jarilla et al., 2009).Elucidation of the amino acids involved in substrate binding and maintenance of substratebound structure in P. westermani TK are important for the exploration of inhibitors against this enzyme.Currently, the functional properties and substrate binding mechanisms in TK are not well known.To characterize the substrate recognition system in P. westermani TK we introduced mutations (D1: Gly 58 Arg, Ala 59 Gly, Ile 60 Val, Tyr 84 His, and Tyr 84 Ile; D2: Arg 61 Leu, Ala 62 Gly, Ile 63 Val, Tyr 87 His, and Tyr 87 Ile) on the amino acids on and near the GS region of pMAL/P.westermani TK template by PCR-based mutagenesis.The PCR products were digested with DpnI and the target DNA was purified using the QIA quick purification column (Qiagen, USA).The mutated cDNA was self-ligated after blunting and phosphorylation and sequenced as described above to check for mutations.After the mutation has been confirmed, enzyme expression and determination of kinetic properties were done as above and the obtained values are shown on Table 8.
The conformational change or the change from open to close structure during substrate binding in PKs is reflected by the kinetic parameters K m P (the value comparable to the dissociation constant of the phosphagen substrate in the absence of ATP) and K d P (the dissociation constant of the phosphagen substrate in the absence of ATP) (Suzuki et al., 2003).For Domain 1, mutations on the GS region caused a decrease in the affinity for taurocyamine as seen in the increase in K m Tc values.The most significant decrease was observed for the Ala 59 to Gly mutant which had a K m Tc value 4 times greater than the wild type.The K d Tc /K m Tc of this mutant was almost equal to 1 indicating the absence of synergism upon ATP substrate binding.Moreover, the V max value for the Ala 59 to Gly mutant was reduced to only 50% of the wild type.Mutation of the equivalent position in Domain 2 (Ala 62 Gly mutant) resulted in the loss of detectable enzyme activity.These suggest that the replacement of Ala on the GS region may have affected the stabilization of the closed structure implying that this amino acid may play an important role in taurocyamine binding.In Danio rerio CK, Arg-96 which is located close to the substrate-binding site has been proposed to have a key role in substrate recognition and in organizing a hydrogen-bond network for active center configuration (Uda et al., 2009).For annelid TKs, the equivalent amino acid is His (Uda et al., 2005;Uda et al., 2005); however, it has been replaced with tyrosine in P. westermani TK similar to that in AKs (Jarilla et al., 2009).Replacement of this amino acid with isoleucine (Tyr 84 Ile and Tyr 87 Ile mutants) did not change the substrate specificity from taurocyamine to glycocyamine but caused a decrease in affinity and V max in domain 1 and loss of activity in domain 2 (Table 8).Similar results were observed when Tyr was replaced with His.These imply that, in the case of P. westermani TK, Tyr may play a role in substrate binding but it may not be the key residue for substrate recognition.

Gene arrangement of P. westemani TK
We have determined the exon/intron organization of P. westermani TK (Fig. 5) to further elucidate the phylogentic relationship with other PKs.Genomic DNA was extracted from an adult worm of P. westermani using Easy-DNA TM Kit (Invitrogen, USA).PCR was performed with Ex Taq TM (Takara, Tokyo, Japan) and primers designed from the cDNA.PCR conditions were as follows: initial denaturation at 94 o C for 2min, followed by 35 cycles of 94 o C for 30 sec, annealing at 50 o C for 30 sec and extension at 72 o C for 3 min and a final extension at 72 o C for 4min.The PCR products were purified, cloned in T-vector, and sequenced as described above.
Thus far, for Domain 1, we have identified four exons and three introns with intron sizes ranging from 88 bp to 3,627 bp.Domain 2 has five exons and four introns with sizes ranging from 99 bt to 3,145 bp.We have also identified a bridge intron (2,806 bp) between the two domains.All of these introns typically began with gt and ended with ag.Interestingly, no intron positions are conserved between P. westermani TK and TK from the annelid A. brasiliensis.Instead, P. westermani TK share more intron positions with the molluscan AKs (D1: 300.0,D2: 97.1, 300.0, 366.1) suggesting that P. westermani TK evolved from an AK gene, consistent with the phylogenetic analysis of amino acid sequence.On the contrary, several intron positions are conserved among annelid TKs, AKs, GKs and vertebrate CKs.Annelid TKs and other annelid-specific PKs are hypothesized to have evolved from a MiCK-like ancestor early in the divergence of the protostome metazoans (Tanaka et al., 2007) with cytoplasmic TKs diverging earliest together with cytoplasmic AKs and LKs (Suzuki et al., 2009).Three intron positions were also shared with the sipunculid HTK which was suggested to have evolved from an AK gene (Uda et al., 2005).It should also be noted that less intron positions are conserved in the first domain which could imply that it has recently diverged.However, the 5' half of P. westermani TK gene still needs to be further amplified to confirm for the presence of intron.

Conclusion
The phosphagen kinase system found in the lung fluke P. westermani, a taurocyamine kinase, appears to be different from those identified in annelids.This TK, which probably evolved from an AK gene, consists of two enzymatically active domains that may have a unique substrate recognition and binding mechanisms.Since TK is not present in mammalian hosts, P. westermani TK could be a potential novel chemotherapeutic target for the effective control and eradication of paragonimiasis.This enzyme could also be utilized in the development of diagnostic tools specific for pulmonary paragonimiasis to avoid misdiagnoses especially in regions where tuberculosis is also enedemic.However, for the validation of this enzyme as a therapeutic target, there is still a need for further studies to determine the specific role of TK in the metabolic routes of the lung fluke.Further elucidation of the structure and substrate binding mechanisms are also necessary for the subsequent search of specific TK inhibitors.

P
. westermani PK cDNA comprises 2, 305 bp with 163 bp of 3' UTR; the 5'UTR was not successfully amplified.The ORF consisting of 2, 142 bp codes for 713 amino acid residues and the translated protein has a calculated mass of 80, 216 Da and an estimated pI of 7.86.Alignment of P. westermani PK amino acid sequence with other PKs indicated that this enzyme has a contiguous two-domain structure potentially encoding for two distinct PK enzymes.Domain 1 (D1) (Fig.1) consists of 360 amino acids with a calculated mass of 40, 422 Da and an estimated pI of 8.47.Domain 2 (D2) consists of 353 amino acids with a calculated mass of 39, 583 Da and an estimated pI of 7.63.

Fig. 4 .
Fig. 4. Alignment of the guanidino specificity region of P. westermani PK with other selected PK.

Fig. 5 .
Fig. 5. Comparison of the gene strucrures (exon/inton organization) of P. westermani TK with PKs from other sources.The intron postions are based on aligned amino acid sequences.Intron phases are indicated by ".0", ".1", or ".2" followed by the amino acid sequence position.The conserved introns are shown by vertical lines

Table 1
. Components of the PCR reaction mixture for the amplification of P. westermani PK cDNA.

Table 2 .
Thermal cycling conditions for the amplification of P. westermani PK cDNA.

Table 4 .
Components of the PCR reaction mixture for amplification of P. westermani PK ORF.

Table 5 .
Enzyme activity of P. westermani phosphagen kinase for various guanidine compounds

Table 6 .
Components of the reaction mixture used for enzyme kinetics assays.

Table 8
. Kinetic parameter of wild-type and mutant P. westermani TK.