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The liver fluke zoonoses, Fasciola spp. are parasitic helminths infecting humans and animals globally. Recent sequencing of the genome of Fasciola gigantica has provided a basis to understand the biochemistry of this parasite. Here, we identified the cytosolic malate dehydrogenase in F. gigantica (FgMDH) and characterized the enzyme biochemically and structurally. F. gigantica encodes a single cytosolic MDH, a key enzyme of the citric acid cycle. It catalyzes the reversible oxidation of malate to oxaloacetate using NAD + . The Fgmdh gene was amplified and cloned for expression of the recombinant protein. The purified protein showed a molecular weight of ~ 36 kDa that existed in a dimeric form in solution. The recombinant enzyme was catalytically active as it catalyzed both forward and reverse reactions efficiently. The kinetic parameters were determined for both directions. The structure of FgMDH and human MDH were modeled and validated. The superimposition of both the model structures showed overall structural similarity in the active site loop region, however, the conformation of the residues was different. Molecular docking elucidated the binding sites and affinities of the substrates and cofactors to the enzyme. Simulation of molecular dynamics and principal component analysis indicated the stability of the systems and collective motions, respectively. Understanding the structural and functional properties of MDH is important to better understand the roles of this enzyme in the biochemistry of the parasite.

Glutathione S-transferases (GSTs) are enzymes that are involved in the detoxification of harmful electrophilic endogenous and exogenous compounds by conjugating with glutathione (GSH). The liver fluke GSTs have multifunctional roles in the host-parasite interaction, such as general detoxification and bile acid sequestration to synthase activity. The GSTs have been highlighted as vaccine candidates towards parasitic flukes. In this study, we have thoroughly examined the urea-induced unfolding of a mu-class Fasciola gigantica GST1 (FgGST1) using spectroscopic techniques and molecular dynamic simulations. FgGST1 is a highly cooperative molecule, because during urea-induced equilibrium unfolding, a concurrent unfolding of the protein without stabilization of any folded intermediate was observed. The protein was stabilized with conformational free energy of about ~12.36 kcal/mol. The protein loses its activity with increasing urea concentration, as the GSH molecule is not able to bind to the protein. We also studied the fluorescence quenching of Trp residues and the obtained K SV data that provided additional information on the unfolding of FgGST1. Molecular dynamic trajectories simulated in different urea concentrations and temperatures indicated that urea destabilizes FgGST1 structure by weakening hydrophobic interactions and the hydrogen bond network. We observed a precise correlation between the in vitro and in silico studies., (© 2018 Wiley Periodicals, Inc.)

Fasciola gigantica is an important food-borne trematode responsible for the hepatobiliary disease, commonly known as fascioliasis. In F. gigantica, the glyceraldehyde 3-phosphate dehydrogenase (FgGAPDH) is a key enzyme of the glycolytic pathway and catalyzes the reversible oxidative phosphorylation of D-glyceraldehyde-3-phosphate (G-3-P) to 1,3-bisphosphoglycerate (1,3-BPG), with the simultaneous reduction of NAD + to NADH. In the present study, we analyzed the sequence of FgGAPDH and investigated its structural, binding, and catalytic properties. Sequence alignment of FgGAPDH showed 100% identity with the sister fluke Fasciola hepatica GAPDH. The gapdh gene was cloned and expressed in Escherichia coli, and the recombinant protein was purified. The purified FgGAPDH exists as a homo-tetramer, composed of a ~ 37-kDa subunit under non-dissociating conditions at 300 mM salt concentration indicating that higher salt stabilizes the tetrameric state. The binding of the cofactor NAD + caused a conformational rearrangement in the enzyme structure, leading to the stabilization of the enzyme. A homology model of FgGAPDH was constructed, the cofactor (NAD + ) and substrate (G-3-P) were docked, and the binding sites were identified in a single chain. The inter-subunit cleft of GAPDH that has been exploited for structure-based drug design in certain protozoan parasites is closed in the case of FgGAPDH, similar to the human GAPDH. Thus, the conformation of FgGAPDH in this region is similar to the human enzyme. Therefore, GAPDH may not be a suitable target for drug discovery against fascioliasis. Still, the analysis of the structural and functional attributes of GAPDH will be significant in understanding the various roles of this enzyme in the parasite as well as provide new insights into the biochemistry of flukes.

The asparaginyl-tRNA synthetase (NRS) catalyzes the attachment of asparagine to its cognate tRNA during translation. NRS first catalyzes the binding of Asn and ATP to form the NRS-asparaginyl adenylate complex, followed by the esterification of Asn to its tRNA. We investigated the role of constituent domains in regulating the structure and activity of Fasciola gigantica NRS (FgNRS). We cloned the full-length FgNRS, along with its various truncated forms, expressed, and purified the corresponding proteins. Size exclusion chromatography indicated a role of the anticodon-binding domain (ABD) of FgNRS in protein dimerization. The N-terminal domain (NTD) was not essential for cognate tRNA binding, and the hinge region between the ABD and the C-terminal domain (CTD) was crucial for regulating the enzymatic activity. Molecular docking and fluorescence quenching experiments elucidated the binding affinities of the substrates to various domains. The molecular dynamics simulation of the modeled protein showed the presence of an unstructured region between the NTD and ABD that exhibited a large number of conformations over time, and further analysis indicated this region to be intrinsically disordered. The present study provides information on the structural and functional regulation, protein-substrate(s) interactions and dynamics, and the role of non-catalytic domains in regulating the activity of FgNRS., (© 2018 The Author(s). Published by Portland Press Limited on behalf of the Biochemical Society.)

The thiol-disulfide redox metabolism in platyhelminth parasites depends entirely on a single selenocysteine (Sec) containing flavoenzyme, thioredoxin glutathione reductase (TGR) that links the classical thioredoxin (Trx) and glutathione (GSH) systems. In the present study, we investigated the catalytic and structural properties of different variants of Fasciola gigantica TGR to understand the role of Sec. The recombinant full-length Sec containing TGR (FgTGRsec), TGR without Sec (FgTGR) and TGRsec without the N-terminal glutaredoxin (Grx) domain (∆NTD-FgTGRsec) were purified to homogeneity. Biochemical studies revealed that Sec597 is responsible for higher thioredoxin reductase (TrxR) and glutathione reductase (GR) activity of FgTGRsec. The N-terminal Grx domain was found to positively regulate the DTNB-based TrxR activity of FgTGRsec. The FgTGRsec was highly sensitive to inhibition by auranofin (AF). The structure of FgTGR was modeled, and the inhibitor AF was docked, and binding sites were identified. Unfolding studies suggest that all three proteins are highly cooperative molecules since during GdnHCl-induced denaturation, a monophasic unfolding of the proteins without stabilization of any intermediate is observed. The Cm for GdnHCl induced unfolding of FgTGR was higher than FgTGRsec and ∆NTD-FgTGRsec suggesting that FgTGR without Sec was more stable in solution than the other protein variants. The free energy of stabilization for the proteins was also determined. To our knowledge, this is also the first report on unfolding and stability analysis of any TGR., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Fascioliasis is caused by the helminth parasites of genus Fasciola. Thioredoxin glutathione reductase (TGR) is an important enzyme in parasitic helminths and plays an indispensable role in its redox biology. In the present study, we conducted a structure-based virtual screening of natural compounds against the Fasciola gigantica TGR (FgTGR). The compounds were docked against FgTGR in four sequential docking modes. The screened ligands were further assessed for Lipinski and ADMET prediction so as to evaluate drug proficiency and likeness property. After refinement, three potential inhibitors were identified that were subjected to 50 ns molecular dynamics simulation and free energy binding analyses to evaluate the dynamics of protein-ligand interaction and the stability of the complexes. Key residues involved in the interaction of the selected ligands were also determined. The results suggested that three top hits had a negative binding energy greater than GSSG (-91.479 KJ · mol -1 ), having -152.657, -141.219, and -92.931 kJ · mol -1 for compounds with IDs ZINC85878789, ZINC85879991, and ZINC36369921, respectively. Further analysis showed that the compound ZINC85878789 and ZINC85879991 displayed substantial pharmacological and structural properties to be a drug candidate. Thus, the present study might prove useful for the future design of new derivatives with higher potency and specificity., (© 2017 Wiley Periodicals, Inc.)

Glutathione S‒transferases (GSTs) play an important role in the detoxification of xenobiotics. They catalyze the nucleophilic addition of glutathione (GSH) to nonpolar compounds, rendering the products water-soluble. In the present study, we investigated the catalytic and structural properties of a mu-class GST from Fasciola gigantica (FgGST1). The purified recombinant FgGST1 formed a homodimer composed of 25 kDa subunit. Kinetic analysis revealed that FgGST1 displays broad substrate specificity and shows high GSH conjugation activity toward 1-chloro-2,4-dinitrobenzene, 4-nitroquinoline-1-oxide, and trans-4-phenyl-3-butene-2-one and peroxidase activity towards trans-2-nonenal and hexa-2,4-dienal. The FgGST1 was highly sensitive to inhibition by cibacron blue. The cofactor (GSH) and inhibitor (cibacron blue) were docked, and binding sites were identified. The molecular dynamics studies and principal component analysis indicated the stability of the systems and the collective motions, respectively. Unfolding studies suggest that FgGST1 is a highly cooperative molecule because, during GdnHCl-induced denaturation, a simultaneous unfolding of the protein without stabilization of any partially folded intermediate is observed. The protein is stabilized with a conformational free energy of about 10 ± 0.3 kcal mol -1 . Additionally, the presence of conserved Pro-53 and structural motifs such as N-capping box and hydrophobic staple, further aided in the stability and proper folding of FgGST1.

• Urea-induced conformational changes in F. gigantica MDH were studied using spectroscopic methods and MD simulations. • Non-superimposable Trp fluorescence and far-UV CD curves indicate a partially folded intermediate state. • Unfolding of tertiary structure preceded the secondary structure, linked to loss of enzyme activity. • Intermediate state destabilized due to disrupted hydrogen-bond networks and hydrophobic interactions. • Strong correlation was observed between the data obtained from in-vitro and in-silico approaches. Malate dehydrogenase enzyme plays an essential role in energy metabolism in all eukaryotes. It catalyzes the interconversion of malate to oxaloacetate, producing NAD+/NADH. In the current study, we monitored the urea-induced conformational changes in Fasciola gigantica malate dehydrogenase (FgMDH) structure using multiple spectroscopic techniques combined with all-atom molecular dynamics (MD) simulations. FgMDH showed a two-state cooperative unfolding revealed by intrinsic tryptophan (Trp) fluorescence and CD spectroscopy; however, their non-superimposable curves indicate a partially folded intermediate state formation. The study showed that the unfolding of tertiary structures preceded the secondary structure unfolding. The binding of the fluorescence probe 1-anilino-8-naphthalene sulfonate (ANS) at 1.5 M urea confirmed stabilization of the intermediate state. The free energy of stabilization determined from Trp fluorescence and CD spectroscopy were 11.4 and 13.2 kcal.mol−1, respectively. The loss of enzymatic activity was observed before any significant loss in structural components. This suggested that low urea concentrations induce minor structural changes in the substrate or cofactor binding site leading to activity loss. Quenching of fluorescence intensity also provides information on unfolding and activity loss. MD simulations confirmed that FgMDH formed an intermediate state at 1.5 M urea at 300 K, which destabilized at higher urea concentration and temperature due to disrupted hydrogen-bond networks and hydrophobic interactions. We conclude that a partially folded metastable state is stabilized during the unfolding of cytosolic FgMDH. A strong correlation was observed between the data obtained from in-vitro and in-silico approaches. [ABSTRACT FROM AUTHOR]