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ABSTRACT: Glutamate dehydrogenases (GDHs) play key roles in cellular redox, amino acid, and energy metabolism, thus representing potential targets for pharmacological interventions. Here we studied the functional network provided by the three known glutamate dehydrogenases of the malaria parasite Plasmodium falciparum. The recombinant production of the previously described PfGDH1 as hexahistidyl-tagged proteins was optimized. Additionally, PfGDH2 was cloned, recombinantly produced, and characterized. Like PfGDH1, PfGDH2 is an NADP(H)-dependent enzyme with a specific activity comparable to PfGDH1 but with slightly higher K(m) values for its substrates. The three-dimensional structure of hexameric PfGDH2 was solved to 3.1 Å resolution. The overall structure shows high similarity with PfGDH1 but with significant differences occurring at the subunit interface. As in mammalian GDH1, in PfGDH2 the subunit-subunit interactions are mainly assisted by hydrogen bonds and hydrophobic interactions, whereas in PfGDH1 these contacts are mediated by networks of salt bridges and hydrogen bonds. In accordance with this, the known bovine GDH inhibitors hexachlorophene, GW5074, and bithionol were more effective on PfGDH2 than on PfGDH1. Subcellular localization was determined for all three plasmodial GDHs by fusion with the green fluorescent protein. Based on our data, PfGDH1 and PfGDH3 are cytosolic proteins whereas PfGDH2 clearly localizes to the apicoplast, a plastid-like organelle specific for apicomplexan parasites. This study provides new insights into the structure and function of GDH isoenzymes of P. falciparum, which represent potential targets for the development of novel antimalarial drugs.
Molecular and Biochemical Parasitology 02/2012; 183(1):52-62. · 2.55 Impact Factor
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ABSTRACT: There are five species of the apicomplexan genus Plasmodium which cause human malaria. The diseases are distinct entities, and may require different drugs for their treatment. For the different intervention goals, specific parasitized cells and tissues must be considered as targets of the antimalarial agents. As suggested by large-scale experimenta naturae, such as the effects of pro-oxidant glucose-6-phosphate dehydrogenase (G6PD) alleles and pro-oxidant foodstuffs, the compromised antioxidant capacity of erythrocytes protects against severe malaria. One mechanism underlying this protection is the IgG-mediated, early phagocytosis of parasitized erythrocytes. As illustrated here for glutathione reductase (GR) deficiency, this mechanism cannot be observed in cultures of parasitized erythrocytes. Pro-oxidant agents such as the GR inhibitor and subversive substrate methylene blue (MB), dapsone, or primaquine most likely result in pharmacological phenocopies of natural protection mechanisms.
02/2011: pages 115 - 136; , ISBN: 9783527633883
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Dirk Sibbing,
Arne Pfeufer,
Tamara Perisic,
Alexander M Mannes, Karin Fritz-Wolf,
Sarah Unwin,
Moritz F Sinner,
Christian Gieger,
Christian Johannes Gloeckner,
Heinz-Erich Wichmann, [......],
Axel Walch,
Martin Hinterseer,
Michael Näbauer,
Stefan Kääb,
Adnan Kastrati,
Albert Schömig,
Thomas Meitinger,
Georg W Bornkamm,
Marcus Conrad,
Nicolas von Beckerath
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ABSTRACT: Cardiac energy requirement is met to a large extent by oxidative phosphorylation in mitochondria that are highly abundant in cardiac myocytes. Human mitochondrial thioredoxin reductase (TXNRD2) is a selenocysteine-containing enzyme essential for mitochondrial oxygen radical scavenging. Cardiac-specific deletion of Txnrd2 in mice results in dilated cardiomyopathy (DCM). The aim of this study was to investigate whether TXNRD2 mutations explain a fraction of monogenic DCM cases.
Sequencing and subsequent genotyping of TXNRD2 in patients diagnosed with DCM (n = 227) and in DCM-free (n = 683) individuals from the general population sample KORA S4 was performed. The functional impact of observed mutations on Txnrd2 function was tested in mouse fibroblasts. We identified two novel amino acid residue-altering TXNRD2 mutations [175G > A (Ala59Thr) and 1124G > A (Gly375Arg)] in three heterozygous carriers among 227 patients that were not observed in the 683 DCM-free individuals. Both DCM-associated mutations result in amino acid substitutions of highly conserved residues in helices contributing to the flavin-adenine dinucleotide (FAD)-binding domain of TXNRD2. Functional analysis of both mutations in Txnrd2(-/-) mouse fibroblasts revealed that contrasting to wild-type (wt) Txnrd2, neither mutant did restore Txnrd2 function. Mutants even impaired the survival of Txnrd2 wt cells under oxidative stress by a dominant-negative mechanism.
For the first time, we describe mutations in DCM patients in a gene involved in the regulation of cellular redox state. TXNRD2 mutations may explain a fraction of human DCM disease burden.
European Heart Journal 01/2011; 32(9):1121-33. · 10.48 Impact Factor
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ABSTRACT: Thioredoxin reductase 1 (TrxR1) is a homodimeric flavoprotein crucially involved in the regulation of cellular redox homeostasis, growth, and differentiation. Its importance in various diseases makes TrxR1 a highly interesting drug target. Here we present the first crystal structures of human TrxR1 in complex with its substrate thioredoxin (Trx). The carboxy-terminal redox centre is found about 20 Å apart from the amino-terminal redox centre, with no major conformational changes in TrxR or Trx. Thus, our structure confirms that the enzyme uses a flexible C-terminal arm for electron transport to its substrates, which is stabilized by a guiding bar for controlled transfer. This notion is supported by mutational analyses. Furthermore, essential residues of the interface region were characterized both structurally and functionally. The structure provides templates for future drug design, and contributes to our understanding of redox regulatory processes in mammals.
Nature Communications 01/2011; 2:383. · 7.40 Impact Factor
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ABSTRACT: Ornithine δ-aminotransferase (OAT) of the malaria parasite Plasmodium falciparum catalyzes the reversible conversion of ornithine into glutamate-5-semialdehyde and glutamate and is-in contrast to its human counterpart-activated by thioredoxin (Trx) by a factor of 10. Trx, glutaredoxin, and plasmoredoxin are redox-active proteins that play a crucial role in the maintenance and control of redox reactions, and were shown to interact with P. falciparum OAT. OAT, which is involved in ornithine homeostasis and proline biosynthesis, is essential for mitotic cell division in rapidly growing cells, thus representing a potential target for chemotherapeutic intervention. Here we report the three-dimensional crystal structure of P. falciparum OAT at 2.3 Å resolution. The overall structure is very similar to that of the human OAT. However, in plasmodial OAT, the loop involved in substrate binding contains two cysteine residues, which are lacking in human OAT. Site-directed mutagenesis of these cysteines and functional analysis demonstrated that Cys154 and Cys163 mediate the interaction with Trx. Interestingly, the Cys154→Ser mutant has a strongly reduced specific activity, most likely due to impaired binding of ornithine. Cys154 and Cys163 are highly conserved in Plasmodium but do not exist in other organisms, suggesting that redox regulation of OAT by Trx is specific for malaria parasites. Plasmodium might require a tight Trx-mediated control of OAT activity for coordinating ornithine homeostasis, polyamine synthesis, proline synthesis, and mitotic cell division.
Journal of Molecular Biology 09/2010; 402(2):445-59. · 4.00 Impact Factor
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ABSTRACT: The cancer chemopreventive agent ellagic acid (EA) is a known inhibitor of glutathione S-transferases (GSTs) and possesses antiplasmodial activities in the upper-nanomolar range. In the recent drug development approach, the properties of the active site of Plasmodium falciparum GST were exploited for inhibitor design by introducing one or two additional hydroxyl groups into EA, yielding flavellagic acid (FEA) and coruleoellagic acid (CEA), respectively. Indeed, the inhibition of P. falciparum GST was improved with the increasing hydrophilicity of the planar polyaromatic ring system. Studying the effects of the two compounds on the central redox enzymes of Plasmodium revealed that glutathione reductase and thioredoxin reductase also are inhibited in the lower-micromolar range. Both compounds had strong antiplasmodial activity in the lower-nanomolar range and were particularly effective against chloroquine (CQ)-resistant P. falciparum strains. Neither FEA nor CEA showed cytotoxic effects on human cells. This was supported by negligible changes in transcript levels and enzyme activities of redox enzymes in human A549 cells upon treatment with the compounds. In Plasmodium, however, CEA treatment resulted in a marked downregulation of most antioxidant genes studied and impaired mainly the trophozoite stage of the parasites. In addition, EA, CEA, and FEA were found to strongly inhibit in vitro heme aggregation. In vitro and preliminary in vivo studies indicated that, compared to CQ, CEA is a slowly acting compound and is able to significantly improve the survival of Plasmodium berghei-infected mice. We conclude that FEA and CEA are promising antimalarial compounds that deserve to be studied further.
Antimicrobial Agents and Chemotherapy 12/2008; 53(2):622-30. · 4.84 Impact Factor
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ABSTRACT: Human thioredoxin reductase (hTrxR) is a homodimeric flavoprotein crucially involved in the regulation of cellular redox reactions, growth and differentiation. The enzyme contains a selenocysteine residue at its C-terminal active site that is essential for catalysis. This redox center is located on a flexible arm, solvent-exposed and reactive towards electrophilic inhibitors, thus representing a target for antitumor drug development. During catalysis reducing equivalents are transferred from the cofactor NADPH to FAD, then to the N-terminal active site cysteine residues and from there to the flexible C-terminal part of the other subunit to be finally delivered to a variety of second substrates at the molecule's surface. Here we report the first crystal structure of hTrxR1 (Sec-->Cys) in complex with FAD and NADP(+) at a resolution of 2.8 A. From the crystals three different conformations of the carboxy-terminal arm could be deduced. The predicted movement of the arm is facilitated by the concerted action of the three side-chain residues of N418, N419 and W407, which act as a guiding bar for the C-terminal sliding process. As supported by previous kinetic data, the three visualized conformations might reflect different stages in enzymatic catalysis. Comparison with other disulfide reductases including human glutathione reductase revealed specific inhibitor binding sites in the intersubunit cavity of hTrxR that can be exploited for structure-based inhibitor development.
Journal of Molecular Biology 06/2007; 370(1):116-27. · 4.00 Impact Factor
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ABSTRACT: Glutathione reductase is an important housekeeping enzyme for redox homeostasis both in human cells and in the causative agent of tropical malaria, Plasmodium falciparum. Glutathione reductase inhibitors were shown to have anticancer and antimalarial activity per se and to contribute to the reversal of drug resistance. The development of menadione chemistry has led to the selection of 6-[2'-(3'-methyl)-1',4'-naphthoquinolyl]hexanoic acid, called M(5), as a potent reversible and uncompetitive inhibitor of both human and P. falciparum glutathione reductases. Here we describe the synthesis and kinetic characterization of a fluoromethyl-M(5) analogue that acts as a mechanism-based inhibitor of both enzymes. In the course of enzymatic catalysis, the suicide substrate is activated by one- or two-electron reduction, and then a highly reactive quinone methide is generated upon elimination of the fluorine. Accordingly the human enzyme was found to be irreversibly inactivated with a k(inact) value of 0.4 +/- 0.2 min(-1). The crystal structure of the alkylated enzyme was solved at 1.7 A resolution. It showed the inhibitor to bind covalently to the active site Cys58 and to interact noncovalently with His467', Arg347, Arg37, and Tyr114. On the basis of the crystal structure of the inactivated human enzyme and stopped-flow kinetic studies with two- and four-electron-reduced forms of the unreacted P. falciparum enzyme, a mechanism is proposed which explains naphthoquinone reduction at the flavin of glutathione reductase.
Journal of the American Chemical Society 09/2006; 128(33):10784-94. · 9.91 Impact Factor
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ABSTRACT: The substrate spectrum of human thioredoxin reductase (hTrxR) is attributed to its C-terminal extension of 16 amino acids carrying a selenocysteine residue. The concept of an evolutionary link between thioredoxin reductase and glutathione reductase (GR) is presently discussed and supported by the fact that almost all residues at catalytic and substrate recognition sites are identical. Here, we addressed the question if a deletion of the C-terminal part of TrxR leads to recognition of glutathione disulfide (GSSG), the substrate of GR. We introduced mutations at the putative substrate binding site to enhance GSSG binding and turnover. However, none of these enzyme species accepted GSSG as substrate better than the full length cysteine mutant of TrxR, excluding a role of the C-terminal extension in preventing GSSG binding. Furthermore, we show that GSSG binding at the N-terminal active site of TrxR is electrostatically disfavoured.
FEBS Letters 07/2006; 580(15):3595-600. · 3.54 Impact Factor
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Angewandte Chemie International Edition 04/2006; 45(12):1881-6. · 13.45 Impact Factor
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ABSTRACT: Glutathione S-transferase of the malarial parasite Plasmodium falciparum (PfGST) represents a novel class of GST isoenzymes. Since the architecture of the PfGST substrate binding site differs significantly from its human counterparts and there is only this one isoenzyme present in the parasite, PfGST is considered a highly attractive target for antimalarial drug development. Here we report the mechanistic, kinetic, and structural characterization of PfGST as well as its interaction with different ligands. Our data indicate that in solution PfGST is present as a tetramer that dissociates into dimers in the presence of glutathione (GSH). Fluorescence spectroscopy shows that in the presence of GSH GST serves as ligandin for parasitotoxic ferriprotoporphyrin IX with a high- and a low-affinity binding site. This is supported by a clear uncompetitive inhibition type. Site-directed mutagenesis studies demonstrate that neither Cys 86 nor Cys 101 contribute to the peroxidase activity of the enzyme, which is thus performed GSH-dependently at the active site. Tyr 9 is responsible for the deprotonation of GSH and Lys 15, but also Gln 71 are involved in GSH binding. We furthermore report the 2.4 A resolution X-ray structure of PfGST cocrystallized with the inhibitor S-hexylglutathione. In comparison with a previously reported structure obtained by crystal soaking, differences occur at the C-terminal end of helix alpha4 and at the S-hexylmoiety of the inhibitor. We furthermore show that, in contrast to previous reports, the antimalarial drug artemisinin is not metabolized by PfGST.
Protein Science 03/2006; 15(2):281-9. · 2.80 Impact Factor
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Marcel Deponte,
Sabine Urig,
L David Arscott, Karin Fritz-Wolf,
Régis Réau,
Christel Herold-Mende,
Sasa Koncarevic,
Markus Meyer,
Elisabeth Davioud-Charvet,
David P Ballou,
Charles H Williams,
Katja Becker
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ABSTRACT: The homodimeric flavoprotein glutathione reductase (GR) is a central player of cellular redox metabolism, connecting NADPH to the large pool of redox-active thiols. In this work, the inhibition of human GR by a novel gold-phosphole inhibitor (GoPI) has been studied in vitro. Two modes of inhibition are observed, reversible inhibition that is competitive with GSSG followed by irreversible inhibition. When approximately 1 nm GoPI is incubated with NADPH-reduced GR (1.4 nm) the enzyme becomes 50% inhibited. This appears to be the most potent stable inhibitor of human GR to date. Analyzing the monophasic oxidative half-reaction of reduced GR with GSSG at pH 6.9 revealed a K(d)((app)) for GSSG of 63 microm, and a k((obs)max) of 106 s(-1) at 4 degrees C. The reversible inhibition by the gold-phosphole complex [{1-phenyl-2,5-di(2-pyridyl)phosphole}AuCl] involves formation of a complex at the GSSG-binding site of GR (K(d) = 0.46 microm) followed by nucleophilic attack of an active site cysteine residue that leads to covalent modification and complete inactivation of the enzyme. Data from titration spectra, molecular modeling, stopped-flow, and steady-state kinetics support this theory. In addition, covalent binding of the inhibitor to human GR was demonstrated by mass spectrometry. The extraordinary properties of the compound and its derivatives might be exploited for cell biological studies or medical applications, e.g. as an anti-tumor or antiparasitic drug. Preliminary experiments with glioblastoma cells cultured in vitro indicate an anti-proliferative effect of the inhibitor in the lower micromolar range.
Journal of Biological Chemistry 06/2005; 280(21):20628-37. · 4.77 Impact Factor
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ABSTRACT: GSTs catalyze the conjugation of glutathione with a wide variety of hydrophobic compounds, generally resulting in nontoxic products that can be readily eliminated. In contrast to many other organisms, the malarial parasite Plasmodium falciparum possesses only one GST isoenzyme (PfGST). This GST is highly abundant in the parasite, its activity was found to be increased in chloroquine-resistant cells, and it has been shown to act as a ligandin for parasitotoxic hemin. Thus, the enzyme represents a promising target for antimalarial drug development. We now have solved the crystal structure of PfGST at a resolution of 1.9 A. The homodimeric protein of 26 kDa per subunit represents a GST form that cannot be assigned to any of the known GST classes. In comparison to other GSTs, and, in particular, to the human isoforms, PfGST possesses a shorter C-terminal section resulting in a more solvent-accessible binding site for the hydrophobic and amphiphilic substrates. The structure furthermore reveals features in this region that could be exploited for the design of specific PfGST inhibitors.
Proceedings of the National Academy of Sciences 12/2003; 100(24):13821-6. · 9.68 Impact Factor
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ABSTRACT: Glutarylamidase is an important enzyme employed in the commercial production of 7-aminocephalosporanic acid, a starting compound in the synthesis of cephalosporin antibiotics. 7-aminocephalosporanic acid is obtained from cephalosporin C, a natural antibiotic, either chemically or by a two-step enzymatic process utilizing the enzymes D-amino acid oxidase and glutarylamidase. We have investigated possibilities for redesigning glutarylamidase for the production of 7-aminocephalosporanic acid from cephalosporin C in a single enzymatic step. These studies are based on the structures of glutarylamidase, which we have solved with bound phosphate and ethylene glycol to 2.5 A resolution and with bound glycerol to 2.4 A. The phosphate binds near the catalytic serine in a way that mimics the hemiacetal that develops during catalysis, while the glycerol occupies the side-chain binding pocket. Our structures show that the enzyme is not only structurally similar to penicillin G acylase but also employs essentially the same mechanism in which the alpha-amino group of the catalytic serine acts as a base. A subtle difference is the presence of two catalytic dyads, His B23/Glu B455 and His B23/Ser B1, that are not seen in penicillin G acylase. In contrast to classical serine proteases, the central histidine of these dyads interacts indirectly with the O(gamma) through a hydrogen bond relay network involving the alpha-amino group of the serine and a bound water molecule. A plausible model of the enzyme-substrate complex is proposed that leads to the prediction of mutants of glutarylamidase that should enable the enzyme to deacylate cephalosporin C into 7-aminocephalosporanic acid.
Protein Science 02/2002; 11(1):92-103. · 2.80 Impact Factor
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Flavins and Flavoproteins (1991).
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Nature, v.381, 341-345 (1996).
