Iron-induced changes in pyruvate metabolism of Tritrichomonas foetus and involvement of iron in expression of hydrogenosomal proteins

The main function of the hydrogenosome, a typical organelle of trichomonads, is to convert malate or pyruvate to H(2), CO(2) and acetate by a pathway associated with ATP synthesis. This pathway relies on activity of iron-sulfur proteins such as pyruvate:ferredoxin oxidoreductase (PFOR), hydrogenase and ferredoxin. To examine the effect of iron availability on proper hydrogenosomal function, the metabolic activity of the hydrogenosome and expression of hydrogenosomal enzymes were compared in Tritrichomonas foetus maintained under iron-rich (150 microM iron nitrilotriacetate) or iron-restricted (180 microM 2,2-dipyridyl) conditions in vitro. The activities of PFOR and hydrogenase, and also production of acetate and H(2), were markedly decreased or absent in iron-restricted trichomonads. Moreover, a decrease in activity of the hydrogenosomal malic enzyme, which is a non-Fe-S protein, was also observed. Impaired function of hydrogenosomes under iron-restricted conditions was compensated for by activation of the cytosolic pathway, mediating conversion of pyruvate to ethanol via acetaldehyde. This metabolic switch was fully reversible. Production of hydrogen by iron-restricted trichomonads was restored to the level of organisms grown under iron-rich conditions within 3 h after addition of 150 microM iron nitrilotriacetate. Protein analysis of purified hydrogenosomes from iron-restricted cells showed decreased levels of proteins corresponding to PFOR, malic enzyme and ferredoxin. Accordingly, these cells displayed decreased steady-state level and synthesis of mRNAs encoding PFOR and hydrogenosomal malic enzyme. These data demonstrate that iron is essential for function of the hydrogenosome, show its involvement in the expression of hydrogenosomal proteins and indicate the presence of iron-dependent control of gene transcription in Tt. foetus.

Comparison of sequencing of the por gene and typing of the opa gene for discrimination of Neisseria gonorrhoeae strains from sexual contacts

Typing of gonococcal strains is a valuable tool for the biological confirmation of sexual contacts. We have developed a typing method based on DNA sequencing of two overlapping por gene fragments generated by a heminested PCR. We compared sequencing of the por gene (POR sequencing) and typing of the opa gene (OPA typing) for the characterization of strains from 17 sexual partnerships. Both methods were highly discriminatory. A different genotype was detected in 15 of the 17 epidemiologically unconnected couples by POR sequencing and in 16 of the 17 couples by OPA typing with restriction enzyme HpaII. Within partnerships, identical genotypes were obtained from 16 of the 17 known sex contacts by POR sequencing and from 15 of the 17 by OPA typing. Compared to OPA typing, which relies on interpretation of bands in a gel, DNA sequence data offer the advantage of being objective and portable. As costs for sequencing decline, the method should become affordable for most laboratory personnel who wish to type gonococcal strains.

The role of pyruvate ferredoxin oxidoreductase in pyruvate synthesis during autotrophic growth by the Wood-Ljungdahl pathway

Pyruvate:ferredoxin oxidoreductase (PFOR) catalyzes the oxidative decarboxylation of pyruvate to acetyl-CoA and CO(2). The catalytic proficiency of this enzyme for the reverse reaction, pyruvate synthase, is poorly understood. Conversion of acetyl-CoA to pyruvate links the Wood-Ljungdahl pathway of autotrophic CO(2) fixation to the reductive tricarboxylic acid cycle, which in these autotrophic anaerobes is the stage for biosynthesis of all cellular macromolecules. The results described here demonstrate that the Clostridium thermoaceticum PFOR is a highly efficient pyruvate synthase. The Michaelis-Menten parameters for pyruvate synthesis by PFOR are: V(max) = 1.6 unit/mg (k(cat) = 3.2 s(-1)), K(m)(Acetyl-CoA) = 9 micrometer, and K(m)(CO(2)) = 2 mm. The intracellular concentrations of acetyl-CoA, CoASH, and pyruvate have been measured. The predicted rate of pyruvate synthesis at physiological concentrations of substrates clearly is sufficient to support the role of PFOR as a pyruvate synthase in vivo. Measurements of its k(cat)/K(m) values demonstrate that ferredoxin is a highly efficient electron carrier in both the oxidative and reductive reactions. On the other hand, rubredoxin is a poor substitute in the oxidative direction and is inept in donating electrons for pyruvate synthesis.

Inhibition of pyruvate-ferredoxin oxidoreductase gene expression in Giardia lamblia by a virus-mediated hammerhead ribozyme

Giardia lamblia is a primitive eukaryotic microorganism that derives its metabolic energy primarily from anaerobic glycolysis. In trophozoites, pyruvate-ferredoxin oxidoreductase (PFOR) converts pyruvate to acetyl-CoA with the transfer of a pair of electrons to ferredoxin, which can then reduce metronidazole and activate it into a potent antigiardiasis agent. It is unclear, however, whether this anaerobic disposal of electrons is essential for the energy metabolism in Giardia. In the present study, cDNAs encoding hammerhead ribozyme flanked with various lengths of antisense PFOR RNA were cloned into a viral vector pC631pac derived from the genome of giardiavirus (GLV). RNA transcripts of the plasmids showed high cleavage activities on PFOR mRNA in vitro. They were introduced into GLV-infected G. lamblia trophozoites by electroporation and stablized in the transfected cells via serial passages under puromycin selection. PFOR mRNA and enzyme activity in the transfected cells were decreased by 46-60% with the ribozyme PRzS flanked with 20 nt PFOR antisense RNA on each arm and by 69-80% with the ribozyme PRzL flanked with 600 and 1500 nt PFOR antisense RNA. PRzS without the inserted ribozyme or ribozyme flanked with alcohol dehydrogenase E antisense RNA showed no effect on PFOR mRNA and activity. The ribozyme-transfected cells demonstrated significantly enhanced resistance to metronidazole and grew equally well under anaerobic and aerobic conditions. In contrast, the wild-type cells grew slightly better anaerobically than the transfectants but did not grow at all in aerobic conditions. Thus, the reduced PFOR expression enables Giardia to grow under molecular oxygen and the presence of PFOR enhances the anaerobic growth of Giardia with an increased susceptibility towards metronidazole. In addition, this study demonstrated for the first time the feasibility of using a viral RNA vector to express a ribozyme targeted at a specific mRNA in G. lamblia to reduce the expression of a specific gene.

Clostridial iron-sulphur proteins

Iron-sulfur proteins are ubiquitous catalysts of a wide range of biological reactions, and are particularly abundant in clostridia which lack the ability to synthesize hemes. The development of research on these metalloproteins has therefore been strongly associated with biochemical investigations of clostridial metabolism. Major breakthroughs in the field, from the first isolation of an iron-sulfur protein in 1962, to the recent determination of an Fe-hydrogenase structure, have been made with clostridia. These data, as well as others obtained through studies on clostridia, are transferable to many other bioenergetic machineries, due to the strong phylogenetic conservation of some important components. For instance, clear homologies exist between constituents of the anaerobic electron transfer chains in clostridia and aerobic respiratory chains. The contribution of iron-sulfur proteins to the biotechnological and medical significance of clostridia is also discussed. Structural and functional genomics are expected to bring forth a wealth of novel data on clostridia and iron-sulfur proteins.

Structure and electron transfer mechanism of pyruvate:ferredoxin oxidoreductase

The first crystal structure of pyruvate:ferredoxin oxidoreductase to be determined has provided significant new information on its structural organization and redox chemistry. Spectroscopic analyses of a radical reaction intermediate have shed more light on its thiamin-based mechanism of catalysis. Different approaches have been used to study the interaction between the enzyme and ferredoxin, its redox partner.

Rubredoxin from the green sulfur bacterium Chlorobium tepidum functions as an electron acceptor for pyruvate ferredoxin oxidoreductase

Rubredoxin (Rd) from the moderately thermophilic green sulfur bacterium Chlorobium tepidum was found to function as an electron acceptor for pyruvate ferredoxin oxidoreductase (PFOR). This enzyme, which catalyzes the conversion of pyruvate to acetyl-CoA and CO(2), exhibited an absolute dependence upon the presence of Rd. However, Rd was incapable of participating in the pyruvate synthase or CO(2) fixation reaction of C. tepidum PFOR, for which two different reduced ferredoxins are employed as electron donors. These results suggest a specific functional role for Rd in pyruvate oxidation and provide the initial indication that the two important physiological reactions catalyzed by PFOR/pyruvate synthase are dependent on different electron carriers in the cell. The UV-visible spectrum of oxidized Rd, with a monomer molecular weight of 6500, gave a molar absorption coefficient at 492 nm of 6.89 mM(-1) cm(-1) with an A(492)/A(280) ratio of 0.343 and contained one iron atom/molecule. Further spectroscopic studies indicated that the CD spectrum of oxidized C. tepidum Rd exhibited a unique absorption maximum at 385 nm and a shoulder at 420 nm. The EPR spectrum of oxidized Rd also exhibited unusual anisotropic resonances at g = 9.675 and g = 4.322, which is composed of a narrow central feature with broader shoulders to high and low field. The midpoint reduction potential of C. tepidum Rd was determined to be -87 mV, which is the most electronegative value reported for Rd from any source.

[Vitamin B1]

Vitamin B1 (thiamin), taken-up into cells, is converted to thiamin diphosphate (TDP), and TDP acts as a cofactor for several enzymes involving in carbohydrate metabolism. CoA-dependent oxidative decarboxylation of pyruvate is catalyzed by pyruvate dehydrogenase multienzyme complex (PDC) with NAD+ as an electron acceptor in most organisms involving mammals and higher plants. PDC consists of three component enzymes, one of which is pyruvate dehydrogenase (lipoamide) which contains TDP as a prosthetic group. Similar multienzyme complex for 2-oxoglutarate or branched chain 2-oxoacids is also found in mammals. In anaerobic bacteria, archaebacteria and anaerobic protozoa, pyruvate:ferredoxin oxidoreductase (PFOR) functions for the oxidative decarboxylation of pyruvate with ferredoxin in place of NAD+. PFOR contains TDP as a cofactor; however its structure is quite different from PDC and 1-3[4Fe-4S] clusters are involved as redox centers. Pyruvate:NADP+ oxidoreductase (PNOR), which catalyzes the oxidative decarboxylation of pyruvate with NADP+ as an electron acceptor, occurs in mitochondria of Euglena gracilis, a protist containing chloroplasts. PNOR consists of two functional domains, one of which contains TDP and 3[4Fe-4S] clusters and resembles PFOR. Another domain involves FMN and FAD as redox centers and its structure is similar to NADPH-cytochrom P450 reductase.

Structural and kinetic studies of the pyruvate-ferredoxin oxidoreductase/ferredoxin complex from Desulfovibrio africanus

The pyruvate-ferredoxin oxidoreductase (PFOR)/ferredoxin (Fd) system of Desulfovibrio africanus has been investigated with the aim of understanding more fully protein-protein interaction and the kinetic characteristics of electron transfer between the two redox partners. D. africanus contains three Fds (Fd I, Fd II and Fd III) able to function as electron acceptors for PFOR. The complete amino acid sequence of Fd II was determined by automatic Edman degradation. It revealed a striking similarity to that of Fd I. The protein consists of 64 residues and its amino acid sequence is in agreement with a molecular mass of 6822.5 Da as measured by electrospray MS. Fd II contains five cysteine residues of which the first four (Cys11, Cys14, Cys17 and Cys54) are likely ligands for the single [4Fe-4S] cluster. A covalently cross-linked complex between PFOR and Fd I or Fd II was obtained by using a water soluble carbodiimide. This complex exhibited a stoichiometry of one ferredoxin for one PFOR subunit and is dependent on the ionic strength. The second-order rate constants for electron transfer between PFOR and Fds determined electrochemically using cyclic voltammetry are 7 x 107 M-1.s-1 for Fd I and 2 x 107 M-1.s-1 for Fd II and Fd III. The Km values of PFOR for Fd I and Fd II measured both by the electrochemical and the spectrophotometric method have been found to be 3 microM and 5 microM, respectively. The three-dimensional modelling of Fd II and surface analysis of Fd I, Fd II and PFOR suggest that a protein-protein complex is likely to be formed between aspartic acid/glutamic acid invariant residues of Fds and lysine residues surrounding the distal [4Fe-4S] cluster of PFOR. All of these studies are indicative of the involvement of electrostatic interactions between the two redox partners.

A single eubacterial origin of eukaryotic pyruvate: ferredoxin oxidoreductase genes: implications for the evolution of anaerobic eukaryotes

The iron sulfur protein pyruvate: ferredoxin oxidoreductase (PFO) is central to energy metabolism in amitochondriate eukaryotes, including those with hydrogenosomes. Thus, revealing the evolutionary history of PFO is critical to understanding the origin(s) of eukaryote anaerobic energy metabolism. We determined a complete PFO sequence for Spironucleus barkhanus, a large fragment of a PFO sequence from Clostridium pasteurianum, and a fragment of a new PFO from Giardia lamblia. Phylogenetic analyses of eubacterial and eukaryotic PFO genes suggest a complex history for PFO, including possible gene duplications and horizontal transfers among eubacteria. Our analyses favor a common origin for eukaryotic cytosolic and hydrogenosomal PFOs from a single eubacterial source, rather than from separate horizontal transfers as previously suggested. However, with the present sampling of genes and species, we were unable to infer a specific eubacterial sister group for eukaryotic PFO. Thus, we find no direct support for the published hypothesis that the donor of eukaryote PFO was the common alpha-proteobacterial ancestor of mitochondria and hydrogenosomes. We also report that several fungi and protists encode proteins with PFO domains that are likely monophyletic with PFOs from anaerobic protists. In Saccharomyces cerevisiae, PFO domains combine with fragments of other redox proteins to form fusion proteins which participate in methionine biosynthesis. Our results are consistent with the view that PFO, an enzyme previously considered to be specific to energy metabolism in amitochondriate protists, was present in the common ancestor of contemporary eukaryotes and was retained, wholly or in part, during the evolution of oxygen-dependent and mitochondrion-bearing lineages.

Combination of methods used in the structure solution of pyruvate:ferredoxin oxidoreductase from two crystal forms

The structure of the homodimeric 267 kDa pyruvate:ferredoxin oxidoreductase (PFOR) of Desulfovibrio africanus was solved with data from two crystals forms, both containing two monomers per asymmetric unit. Phases were obtained from multiwavelength anomalous dispersion (MAD), solvent flattening (SF), molecular replacement (MR) using a 5 A resolution electron-density search model, multiple isomorphous replacement (MIR) and, finally, electron-density averaging (DA) procedures. It is shown how the combination of all these techniques was used to overcome problems arising from incompleteness of MAD data and weak phasing power of MIR data. A real-space refinement (RSR) procedure is described to improve MR solutions and obtain very accurate protein envelopes and non-crystallographic symmetry (NCS) transformations from 5 A resolution phase information. These were crucial for the phase extension to high resolution by DA methods.

Crystal structures of the key anaerobic enzyme pyruvate:ferredoxin oxidoreductase, free and in complex with pyruvate

Oxidative decarboxylation of pyruvate to form acetyl-coenzyme A, a crucial step in many metabolic pathways, is carried out in most aerobic organisms by the multienzyme complex pyruvate dehydrogenase. In most anaerobes, the same reaction is usually catalyzed by a single enzyme, pyruvate:ferredoxin oxidoreductase (PFOR). Thus, PFOR is a potential target for drug design against certain anaerobic pathogens. Here, we report the crystal structures of the homodimeric Desulfovibrio africanus PFOR (data to 2.3 A resolution), and of its complex with pyruvate (3.0 A resolution). The structures show that each subunit consists of seven domains, one of which affords protection against oxygen. The thiamin pyrophosphate (TPP) cofactor and the three [4Fe-4S] clusters are suitably arranged to provide a plausible electron transfer pathway. In addition, the PFOR-pyruvate complex structure shows the noncovalent fixation of the substrate before the catalytic reaction.

Alternative 2-keto acid oxidoreductase activities in Trichomonas vaginalis

We have induced high levels of resistance to metronidazole (1 mM or 170 microg ml(-1)) in two different strains of Trichomonas vaginalis (BRIS/92/STDL/F1623 and BRIS/92/STDL/B7708) and have used one strain to identify two alternative T. vaginalis 2-keto acid oxidoreductases (KOR) both of which are distinct from the already characterised pyruvate:ferredoxin oxidoreductase (PFOR). Unlike the characterised PFOR which is severely down-regulated in metronidazole-resistant parasites, both of the alternative KORs are fully active in metronidazole-resistant T. vaginalis. The first, KORI, localized in all membrane fractions but predominantly in the hydrogenosome fraction, is soluble in Triton X-100 and the second, KOR2, is extractable in 1 M acetate from membrane fractions of metronidazole-resistant parasites. PFOR and both KORI and KOR2 use a broad range of 2-keto acids as substrates (pyruvate, alpha-ketobutyrate, alpha-ketomalonate), including the deaminated forms of aromatic amino acids (indolepyruvate and phenylpyruvate). However, unlike PFOR neither KORI or KOR2 was able to use oz-ketoglutarate. Deaminated forms of branched chain amino acids (alpha-ketoisovalerate) were not substrates for T. vaginalis KORs. Since KOR I and KOR2 do not apparently donate electrons to ferredoxin, and are not down-regulated in metronidazole-resistant parasites, we propose that KORI and KOR2 provide metronidazole-resistant parasites with an alternative energy production pathway(s) which circumvents metronidazole activation.

Crystallization and preliminary crystallographic analysis of the pyruvate-ferredoxin oxidoreductase from Desulfovibrio africanus

For the first time, crystals of a pyruvate-ferredoxin oxidoreductase (PFOR) suitable for X-ray analysis have been obtained. This enzyme catalyzes, in anaerobic organisms, the crucial energy-yielding reaction of pyruvate decarboxylation to acetylCoA. Polyethylene glycol and divalent metal cations have been used to crystallize the PFOR from the sulfate-reducing bacterium Desulfovibrio africanus. Two different orthorhombic (P212121 ) crystal forms have been grown with unit-cell dimensions a = 86.1, b = 146.7, c = 212.5 A and a = 84.8, b = 144.9, c = 203.0 A. Both crystals diffract to 2.3 A resolution using synchrotron radiation.

Purification and characterization of pyruvate oxidoreductase from the photosynthetic bacterium Rhodobacter capsulatus

Pyruvate:ferredoxin (flavodoxin) oxidoreductase (POR) was purified 3050-fold to apparent homogeneity from the photosynthetic bacterium Rhodobacter capsulatus using ion-exchange, Reactive Red, and gel filtration chromatography. The isolated enzyme was sensitive to dilution and oxygen (especially when in dilute solution). The molecular mass of the native enzyme was determined by high performance liquid chromatography gel filtration to be 270+/-20 kDa. Since a subunit molecular mass of 130+/-5 kDa was found by denaturing gel electrophoresis, POR from R. capsulatus thus appears to be a homodimer. Electron paramagnetic resonance analysis showed that a free radical was formed upon the addition of pyruvate. This POR is shown to be an indiscriminate electron donor causing the full reduction of R. capsulatus flavodoxin (Fld), R. capsulatus ferredoxin I (FdI), R. capsulatus ferredoxin II (FdII), as well as the major plant-type ferredoxin (FdI) from Anabaena variabilis. The purified enzyme can couple the oxidation of pyruvate to the reduction of nitrogenase in a coupled system with either R. capsulatus ferredoxins or nif-specific flavodoxin, NifF; (Fld>FdI>FdII). Immunoblot analysis shows that R. capsulatus POR is constitutively synthesized, with synthesis augmented under nitrogen-fixing conditions (34+/-13%) and decreased in acetate and aerobically grown cells.

The delta-subunit of pyruvate ferredoxin oxidoreductase from Pyrococcus furiosus is a redox-active, iron-sulfur protein: evidence for an ancestral relationship with 8Fe-type ferredoxins

Pyruvate ferredoxin oxidoreductase (POR) from the hyperthermophilic archaeon Pyrococcus furiosus (Pf) catalyzes the final oxidative step in carbohydrate fermentation in which pyruvate is oxidized to acetyl-CoA and CO2, coupled to the reduction of ferredoxin (Fd). POR is composed of two 'catalytic units' of molecular mass approximately 120 kDa. Each unit consists of four subunits, alpha beta gamma delta, with masses of approximately 44, 36, 20, and 12 kDa, respectively, and contains at least two [4Fe-4S] clusters. The precise mechanism of catalysis and the role of the individual subunits are not known. The gene encoding the delta-subunit of Pf POR has been expressed in E. coli, and the protein was purified after reconstitution with iron and sulfide. The reconstituted delta-subunit (recPOR-delta) is monomeric with a mass of 11 879 +/- 1.2 Da as determined by mass spectrometry, in agreement with that predicted from the gene sequence. Purified recPOR-delta contains 8 Fe mol/mol and remained intact when incubated at 85 degreesC for 2 h, as judged by its visible absorption properties. The reduced form of the protein exhibited an EPR spectrum characteristic of two, spin-spin interacting [4Fe-4S]1+ clusters. When compared with the EPR properties of the reduced holoenzyme, the latter was shown to contain a third [4Fe-4S]1+ cluster in addition to the two within the delta-subunit. The reduction potential of the two 4Fe clusters in isolated recPOR-delta (-403 +/- 8 mV at pH 8.0 and 24 degreesC) decreased linearly with temperature (-1.55 mV/ degreesC) up to 82 degreesC. RecPOR-delta replaced Pf Fd as an in vitro electron carrier for two oxidoreductases from Pf, POR and Fd:NADP oxidoreductase, and the POR holoenzyme displayed a higher apparent affinity for its own subunit (apparent Km = 1.0 microM at 80 degreesC) than for Fd (apparent Km = 4.4 microM). The molecular and spectroscopic properties and amino acid sequence of the isolated delta-subunit suggest that it evolved from an 8Fe-type Fd by the addition of approximately 40 residues at the N-terminus, and that this extension enabled it to interact with additional subunits within POR.

Identification of a gene sequence encoding a putative pyruvate oxidoreductase in Serpulina pilosicoli

Serpulina pilosicoli is a recently described species of intestinal spirochaete which can be identified using a species-specific monoclonal antibody BJL/AC1 reactive with a 29-kDa protein located in the cell envelope. A genomic library of the type strain of S. pilosicoli P43/6/78T was created in lambda zap express and screened using BJL/AC1. Single positive clones were isolated and excised into the phagemid vector pBK-CMV. Phagemid DNA was purified and a single clone was selected for sequencing. The size of spirochaetal DNA insert was determined by digestion with restriction endonucleases EcoRI and PstI as being approximately 2.6 kb. The nucleotide sequence of the gene encoding the protein with which the antibody reacted was determined by cycle sequencing. The insert contained an open reading frame of 285 nucleotides. Translation of the nucleotide sequence into amino acid (aa) residues showed a sequence of 275 aa. Comparison of this sequence with databases revealed homology to pyruvate oxidoreductases from various organisms found in the gastroinestinal tract. These included the pyruvate ferredoxin oxidoreductase (POR) alpha submit of Helicobacter pylori (38.8% identity in 250 aa), pyruvate-flavodoxin oxidoreductase of Escherichia coli (28.7% identify in 258 aa) and Giardia intestinalis (25.1% identity in 251 aa). A significant level of homology was also observed with hyperthermophilic bacteria such as the POR of Thermatoga maritima (38.6% in 254 aa) and the 2-ketovalerate-ferredoxin oxidoreductase of Pyrococcus furiosus (34% in 262 aa).

The pyruvate:ferredoxin oxidoreductase enzyme is located in the plasma membrane and in a cytoplasmic structure in Entamoeba

This work investigated the cellular location of the pyruvate:ferredoxin oxidoreductase (PFO) enzyme in Entamoeba. A 1.9 kb fragment located at the 3' end of the Ehpfo gene was cloned in the pRSETB vector and expressed. The recombinant peptide was purified and inoculated in rabbits. By Western blot assays the antibodies detected a single 130 kDa band in all E. histolytica strains tested and in E. moshkovskii. By immunofluorescence, the antibodies showed the presence of PFO in the plasma membrane and in a cytoplasmic structure that appeared as a ring or as a compact small body in E. histolytica strains. In E. invadens and E. moshkovskii (strains FIC and Laredo) PFO was located in the plasma membrane showing different fluorescence patterns. Immunofluorescence on E. histolytica synchronized cultures showed that the cytoplasmic structure appeared in 85, 60, 20 and 10% of the trophozoites in mitosis, G1, S and G2 phases, respectively. By in situ hybridization the Ehpfo gene was found in the nuclei and the trophozoites of the clone A, strain HM1:IMSS, differed in the Ehpfo gene content.

The pyruvate dehydrogenase complex from thermophilic organisms: thermal stability and re-association from the enzyme components

Examples of pyruvate dehydrogenase complexes, and of its probable precursors, the pyruvate ferredoxin oxidoreductases, both isolated from thermophilic organisms, are described. The pyruvate ferredoxin oxidoreductases are mostly characterized from thermophilic archaea like Sulfolobus solfataricus and Pyrococcus furiosus. They retain their catalytic activity up to 60 and 90 degreesC, respectively. Characteristic for the thermophilic nature is a biphasic temperature behavior, reflecting a more stable low temperature and a metastable high temperature form. Another feature is the strong binding of the cofactor thiamin diphosphate. Detailed analysis of thermostable pyruvate dehydrogenase complexes so far only exist for the enzymes from Bacillus stearothermophilus and Thermus flavus. In most respects, especially in the structural features, the enzyme complex from B. stearothermophilus resembles its mesophilic counterparts and only an elevated temperature maximum for the catalytic activity reveals the thermophilic nature. In contrast to this, the more thermostable enzyme complex from T. flavus shows a quite distinct behavior. One single protein chain (Mr=100 kDa) instead of an alpha2beta2 aggregate was found for the pyruvate dehydrogenase (E1) subunits of this enzyme complex. Its catalytic activity is controlled by allosteric regulation, while the enzyme complex from B. stearothermophilus shows no such regulation. Reversible phosphorylation as a regulatory principle of pyruvate dehydrogenase complexes from higher organisms does not take place in the thermophilic enzyme complexes. The overall activity of the enzyme complex from B. stearothermophilus remains stable at 60 degreesC for 50 min while that from T. flavus is active up to 83 degreesC. Thermophilic pyruvate dehydrogenase complexes do not spontaneously renature from their separated enzyme components. However, chaperonins from Thermus thermophilus stimulate the reactivation of the enzyme complex from T. flavus.

Effect of iron-sulfur cluster environment in modulating the thermodynamic properties and biological function of ferredoxin from Pyrococcus furiosus

The ferredoxin (7.5 kDa) of the hyperthermophilic archaeon, Pyrococcus furiosus, contains a single [4Fe-4S]1+,2+ cluster that is coordinated by three Cys and one Asp residue rather than the expected four Cys. The role of this Asp residue was investigated using a series of mutants, D14X, where X = C, S, H, N, V, and Y, prepared by heterologous gene expression in Escherichia coli. While the recombinant form of the wild-type and the D14S and D14C mutants contained a [4Fe-4S]1+,2+ cluster, the D14V, D14H, D14Y, and D14N proteins contained a [3Fe-4S]0,+ center, as determined by visible spectroscopy and electrochemistry. The redox potentials (at pH 7.0, 23 degrees C) of the D14C and D14S mutants were decreased by 58 and 133 mV, respectively, compared to those of the wild-type 4Fe-ferredoxin (Em -368 mV), while those of the 3Fe-protein mutants (including the 3Fe-form of the D14S, generated by chemical oxidation) were between 15 and 118 mV more positive than that of wild-type 3Fe-form (obtained by chemical oxidation, Em -203 mV). The reduction potentials of all of the 3Fe-forms, except the D14S mutant, showed a pH response over the range 3.0-10.0 with a pK of 3.3-4.7, and this was assigned to cluster protonation. The D14H mutant and the wild-type 3Fe-proteins showed an additional pK (both at 5.9) assumed to arise from protonation of the amino acid side chain. With the 4Fe-proteins, there was no dramatic change in the potentials of the wild-type or D14C form, while the pH response of the D14S mutant (pK 4.75) was ascribed to protonation of the serinate. While the ferredoxin variants exhibited a range of thermal stabilities (measured at 80 degrees C, pH 2.5), none of them showed any temperature-dependent transitions (0-80 degrees C) in their reduction potentials, and there was no correlation between the calculated DeltaS degrees' values and the absorbance maximum, reduction potential, or hydrophobicity of residue 14. In contrast, there was a linear correlation between the DeltaH degrees' value and reduction potential. Kinetic analyses were carried out at 80 degrees C using the ferredoxin as either an electron acceptor to pyruvate oxidoreductase (POR) or as an electron donor to ferredoxin:NADP oxidoreductase (FNOR, both from P. furiosus). The data showed that the reduction potential of the ferredoxin, rather than cluster type or the nature of the residue at position 14, appears to be the predominant factor in determining efficiency of electron transfer in both systems. However, compared to all the variants, the reduction potential of WT Fd makes it the most appropriate protein to both accept electrons from POR and donate them to FNOR.

Involvement of superoxide dismutase and pyruvate:ferredoxin oxidoreductase in mechanisms of metronidazole resistance in Entamoeba histolytica

Metronidazole resistance has been induced in an axenic strain of Entamoeba histolytica (HTH-56:MUTM) following continuous exposure to steadily increasing drug concentrations. The drug-resistant line is routinely maintained in normally lethal levels of metronidazole (10 microM). Resistance to this concentration of drug was developed over 177 days. Decreased pyruvate:ferredoxin oxidoreductase (PFOR) activity in anaerobic organisms is one mechanism of metronidazole resistance but in entamoeba, PFOR activity was not decreased in metronidazole-resistant parasites as determined by immunofluorescent assays and immunoblotting studies. 2-Oxoacid oxidoreductase activity, which appeared to be due to a single enzyme, PFOR, was evident with pyruvate as well as the alternative substrates, alpha-ketobutyrate, alpha-ketoglutarate and oxaloacetate. A marked increase in superoxide dismutase (SOD) activity was detected in metronidazole-resistant E. histolytica. Increased SOD activity has not previously been documented as a mechanism of drug resistance although SOD has been associated with a range of stress situations in other organisms.

In vitro Tn7 mutagenesis of Haemophilus influenzae Rd and characterization of the role of atpA in transformation

Haemophilus influenzae Rd is a gram-negative bacterium capable of natural DNA transformation. The competent state occurs naturally in late exponential growth or can be induced by a nutritional downshift or by transient anaerobiosis. The genes cya, crp, topA, and sxy (tfoX) are known to function in the regulation of competence development. The phosphoenolpyruvate:carbohydrate phosphotransferase system functions to maintain levels of cyclic AMP necessary for competence development but is not directly involved in regulation. The exact signal(s) for competence and the genes that mediate the signal(s) are still unknown. In an effort to find additional regulatory genes, H. influenzae Rd was mutated by using an in vitro Tn7 system and screened for mutants with a reduced ability to induce the competence-regulatory gene, comA. Insertions in atpA, a gene coding for the alpha subunit of the F1 cytoplasmic domain of the ATP synthase, reduce transformation frequencies about 20-fold and cause a significant reduction in expression of competence-regulatory genes, while the expression of constitutive competence genes is only minimally affected. In addition, we found that an insertion in atpB, which encodes the a subunit of the F0 membrane-spanning domain, has a similar effect on transformation frequencies.

The iron-sulfur centers of the pyruvate:ferredoxin oxidoreductase from Methanosarcina barkeri (Fusaro)

The iron-sulfur clusters of a pyruvate:ferredoxin oxidoreductase isolated from a methanogenic archaeon, Methanosarcina barkeri (Fusaro), have been unambiguously identified for the first time. In agreement with the estimated iron and sulfur contents (Bock and Schonheit, Eur. J. Biochem., 237 (1996) 35-44), the enzyme is shown to contain three [4Fe-4S](2+/1+) clusters, which in the reduced state give a complex EPR spectrum resulting from three distinct centres, magnetically interacting. The catalytic cycle of the enzyme was studied by visible and EPR spectroscopies. A thiamine diphosphate based radical is also an intermediate in the M. barkeri enzyme catalytic cycle. However, under anaerobic conditions, the enzyme or Clostridium pasteurianum ferredoxin iron-sulfur clusters are reduced only in the presence of both substrates, pyruvate and coenzyme A.