A Trichomonas vaginalis 120 kDa protein with identity to hydrogenosome pyruvate:ferredoxin oxidoreductase is a surface adhesin induced by iron

Trichomonas vaginalis, a human sexually transmitted protozoan, relies on adherence to the vaginal epithelium for colonization and maintenance of infection in the host. Thus, adherence molecules play a fundamental role in the trichomonal infection. Here, we show the identification and characterization of a 120 kDa surface glycoprotein (AP120) induced by iron, which participates in cytoadherence. AP120 is synthesized by the parasite when grown in 250 microM iron medium. Antibodies to AP120 and the electro-eluted AP120 inhibited parasite adherence in a concentration-dependent manner, demonstrating its participation in cytoadherence. In addition, a protein of 130 kDa was detected on the surface of HeLa cells as the putative receptor for AP120. By peptide matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), the AP120 adhesin showed homology with a hydrogenosomal enzyme, the pyruvate:ferredoxin oxidoreductase (PFO) encoded by the pfoa gene. This homology was confirmed by immunoblot and indirect immunofluorescence assays with an antibody to the carboxy-terminus region of the Entamoeba histolytica PFO. Reverse transcription polymerase chain reaction (RT-PCR) assays showed that a pfoa-like gene was better transcribed in trichomonads grown in iron-rich medium. In conclusion, the homology of AP120 to PFO suggests that this novel adhesin induced by iron could be an example of moonlighting protein in T. vaginalis.

[Carbon metabolism of filamentous anoxygenic phototrophic bacteria of the family Oscillochloridaceae]

The carbon metabolism of representatives of the family Oscillochloridaceae (Oscillochloris trichoides DG6 and the recent isolates Oscillochloris sp. R, KR, and BM) has been studied. Based on data from an inhibitory analysis of autotrophic CO2 assimilation and measurements of the activities of the enzymes involved in this process, it is concluded that, in all Oscillochloris strains, CO2 fixation occurs via the operation of the Calvin cycle. Phosphoenolpyruvate (PEP), which is formed in this cycle, can be involved in the metabolism via the following reaction sequence: PEP (+ CO2) --> oxalacetate --> malate --> fumarate --> succinate --> succinyl-CoA (+ CO2) --> 2-oxoglutarate (+ CO2) --> isocitrate. Acetate, utilized as and additional carbon source, can be carboxylated to pyruvate by pyruvate synthase and further involved in the metabolism via the above reaction sequence. Propionyl-CoA synthase and malonyl-CoA reductase, the key enzymes of the 3-hydroxypropionate cycle, have not been detected in Oscillochloris representatives.

[The mechanism of acetate assimilation in purple nonsulfur bacteria lacking the glyoxylate pathway: acetate assimilation in Rhodobacter sphaeroides cells]

The mechanism of acetate assimilation in the purple nonsulfur bacterium Rhodobacter sphaeroides, which lacks the glyoxylate pathway, is studied. It is found that the growth of this bacterium in batch and continuous cultures and the assimilation of acetate in cell suspensions are not stimulated by bicarbonate. The consumption of acetate is accompanied by the excretion of glyoxylate and pyruvate into the medium, stimulated by glyoxylate and pyruvate, and inhibited by citramalate. The respiration of cells in the presence of acetate is stimulated by glyoxylate, pyruvate, citramalate, and mesaconate. These data suggest that the citramalate cycle may function in Rba. sphaeroides in the form of an anaplerotic pathway instead of the glyoxylate pathway. At the same time, the low ratio of fixation rates for bicarbonate and acetate exhibited by the Rba. sphaeroides cells (approximately 0.1), as well as the absence of the stimulatory effect of acetate on the fixation of bicarbonate in the presence of the Calvin cycle inhibitor iodoacetate, suggests that pyruvate synthase is not involved in acetate assimilation in the bacterium Rba. sphaeroides.

Multiple orientations in a physiological complex: the pyruvate-ferredoxin oxidoreductase-ferredoxin system

Ferredoxin I from Desulfovibrio africanus (Da FdI) is a small acidic [4Fe-4S] cluster protein that exchanges electrons with pyruvate-ferredoxin oxidoreductase (PFOR), a key enzyme in the energy metabolism of anaerobes. The thermodynamic properties and the electron transfer between PFOR and either native or mutated FdI have been investigated by microcalorimetry and steady-state kinetics, respectively. The association constant of the PFOR-FdI complex is 3.85 x 10(5) M(-1), and the binding affinity has been found to be highly sensitive to ionic strength, suggesting the involvement of electrostatic forces in formation of the complex. Surprisingly, the punctual or combined neutralizations of carboxylate residues surrounding the [4Fe-4S] cluster slightly affect the PFOR-FdI interaction. Furthermore, hydrophobic residues around the cluster do not seem to be crucial for the PFOR-FdI system activity; however, some of them play an important role in the stability of the FeS cluster. NMR restrained docking associated with site-directed mutagenesis studies suggested the presence of various interacting sites on Da FdI. The modification of additional acidic residues at the interacting interface, generating a FdI pentamutant, evidenced at least two distinct FdI binding sites facing the distal [4Fe-4S] cluster of the PFOR. We also used a set of various small acidic partners to investigate the specificity of PFOR toward redox partners. The remarkable flexibility of the PFOR-FdI system supports the idea that the specificity of the physiological complex has probably been "sacrificed" to improve the turnover rate and thus the efficiency of bacterial electron transfer.

[Resistance of Russian isolates of Neisseria gonorrhoeae to fluoroquinolones]

Fluoroquinolones still belong to the drugs of choice in the treatment of uncomplicated gonorrhea. At the same time, there have been more data on the spreading N. gonorrhoeae strains resistant to fluoroquinolones. A variety of mechanisms, like modification of the target of antibiotic's action (point mutations in genes gyrA and parC), a decreasing permeability of the bacterial cell membrane (amino-acid changes Por protein) and a growing efflux of antibiotic (mutations in the promoter or in the coding region of mtrR) mediate in the shaping resistance of the drugs. The MIC values for four fluoroquinolone-series antibiotics were determined and the gyrA, parC, por and mtrR genes were examined for resistance-responsible mutations in 32 studied clinical strains of N. gonorrhoeae. Strains with high resistance to fluoroquinolones were detected; 3 of them had no common changes in GyrA or ParC, however, amino acid changes and mutations were detected in Por protein and promoter or gene mtrR encoding region, respectively. The paper contains priority data on the detection (in Russia) of N. gonorrhoeae strains with high resistance to fluoroquinolones. Involvement of different mechanisms in the process of resistance shaping is discussed. The results are of practical importance for planning the antibacterial therapy of gonorrhoeae; they point out the need in regional testing of resistance in the N. gonorrhoeae population encountered in Russia.

Non-mitochondrial complex I proteins in a hydrogenosomal oxidoreductase complex

Trichomonas vaginalis is a unicellular microaerophilic eukaryote that lacks mitochondria yet contains an alternative organelle, the hydrogenosome, involved in pyruvate metabolism. Pathways between the two organelles differ substantially: in hydrogenosomes, pyruvate oxidation is catalysed by pyruvate:ferredoxin oxidoreductase (PFOR), with electrons donated to an [Fe]-hydrogenase which produces hydrogen. ATP is generated exclusively by substrate-level phosphorylation in hydrogenosomes, as opposed to oxidative phosphorylation in mitochondria. PFOR and hydrogenase are found in eubacteria and amitochondriate eukaryotes, but not in typical mitochondria. Analyses of mitochondrial genomes indicate that mitochondria have a single endosymbiotic origin from an alpha-proteobacterial-type progenitor. The absence of a genome in trichomonad hydrogenosomes precludes such comparisons, leaving the endosymbiotic history of this organelle unclear. Although phylogenetic reconstructions of a few proteins indicate that trichomonad hydrogenosomes share a common origin with mitochondria, others do not. Here we describe a novel NADH dehydrogenase module of respiratory complex I that is coupled to the central hydrogenosomal fermentative pathway to form a hydrogenosomal oxidoreductase complex that seems to function independently of quinones. Phylogenetic analyses of hydrogenosomal complex I-like proteins Ndh51 and Ndh24 reveal that neither has a common origin with mitochondrial homologues. These studies argue against a vertical origin of trichomonad hydrogenosomes from the proto-mitochondrial endosymbiont.

Mutant P450 oxidoreductase causes disordered steroidogenesis with and without Antley-Bixler syndrome

Deficient activities of multiple steroidogenic enzymes have been reported without and with Antley-Bixler syndrome (ABS), but mutations of corresponding cytochrome P450 enzymes have not been found. We identified mutations in POR, encoding P450 oxidoreductase, the obligate electron donor for these enzymes, in a woman with amenorrhea and three children with ABS, even though knock-out of POR is embryonically lethal in mice. Mutations of POR also affect drug-metabolizing P450 enzymes, explaining the association of ABS with maternal fluconazole ingestion.

The importance of porE and porF in the anabolic pyruvate oxidoreductase of Methanococcus maripaludis

The operon of the anabolic pyruvate oxidoreductase (POR) of Methanococcus maripaludis encodes two genes ( porEF) whose functions are unknown. Because these genes possess sequence similarity to polyferredoxins, they may be electron carriers to the POR. To elucidate whether the methanococcal POR requires PorEF for activity, a deletion mutant, strain JJ150, lacking porEF was constructed. Compared to the wild-type strain JJ1, the mutant grew more slowly in minimal medium and minimal plus acetate medium, and pyruvate-dependent methanogenesis was inhibited. In contrast, the methyl-viologen-dependent pyruvate-oxidation activity of POR, carbon monoxide dehydrogenase, and hydrogenase activities of the mutant were similar to those of the wild-type. Upon genetic complementation of the mutant with porEF in the methanococcal shuttle vector pMEV2+ porEF, growth in minimal medium and pyruvate-dependent methanogenesis were restored to wild-type levels. Complementation with porE alone restored methanogenesis from pyruvate but not growth in minimal medium. Complementation with porF alone partially restored growth but not methanogenesis from pyruvate. Although the specific roles of porE and porF have not been determined, these results suggest that PorEF play important roles in the anabolic POR in vivo even though they are not required for the dye-dependent activity.

Attempts to inhibit ruminal methanogenesis by blocking pyruvate oxidative decarboxylation

The inhibition of pyruvate oxidative decarboxylation as a means of decreasing ruminal methanogenesis in vitro was studied. In the first experiment, the addition of adenosine and adenine (with and without ribose) to ruminal batch cultures did not decrease methanogenesis. In the second experiment, the addition of oxythiamin decreased methanogenesis by 23%. In the third experiment, three pyruvate derivatives did not inhibit methanogenesis, although hydroxypyruvate improved organic matter fermentation from 57.8% to 64.2%. The additives did not seem to inhibit pyruvate oxidative decarboxylation.Key words: methane, inhibition, rumen, pyruvate decarboxylation, thiamin.

The anabolic pyruvate oxidoreductase from Methanococcus maripaludis

In autotrophic methanogens, pyruvate oxidoreductase (POR) plays a key role in the assimilation of CO(2) and the biosynthesis of organic carbon. This enzyme has been purified to homogeneity, and the genes from Methanococcus maripaludis were sequenced. The purified POR contained five polypeptides with molecular masses of 47, 33, 25, 21.5 and 13 kDa. The N-terminal sequences of four of the polypeptides had high similarity to the subunits commonly associated with this enzyme from other archaea. However, the 21.5-kDa polypeptide had not been previously observed in PORs. Nucleotide sequencing of the gene cluster encoding the POR revealed six open reading frames ( porABCDEF). The genes porABCD corresponded to the subunits previously identified in PORs. On the basis of the N-terminal amino acid sequence, porE encoded the 21.5-kDa polypeptide and contained a high cysteinyl residue content and a motif indicative of a [Fe-S] cluster. porF also had a high sequence similarity to porE, a high cysteinyl residue content, and two [Fe-S] cluster motifs. Homologs to porE were also present in the genomic sequences of the autotrophic methanogens Methanocaldococcus jannaschii and Methanothermobacter thermautotrophicus. Based upon these results, it is proposed that PorE and PorF are components of a specialized system required to transfer low-potential electrons for pyruvate biosynthesis. Some biochemical properties of the purified methanococcal POR were also determined. This unstable enzyme was very sensitive to O(2 )and demonstrated high activity with pyruvate, oxaloacetate, and alpha-ketobutyrate. Methyl viologen, rubredoxin, FMN, and FAD were readily reduced. Activity was also observed with spinach and clostridial ferredoxins and cytochrome c. Coenzyme F(420) was not an electron acceptor for the purified enzyme.

Chemically gated electron transfer. A means of accelerating and regulating rates of biological electron transfer

Long-range protein electron transfer [ET] reactions may be relatively slow because of long ET distance and low driving force. It is possible to dramatically increase the rate of such nonadiabatic reactions by using an adiabatic chemical reaction to activate the system for rapid ET. Three such examples are discussed; nitrogenase, pyruvate:ferredoxin oxidoreductase, and the methylamine dehydrogenase-amicyanin complex. In each example, the faster activated ET reaction is gated (i.e., rate-limited) by the chemical reaction. However, the reaction rate is still orders of magnitude greater than that of the ungated true ET reaction in the absence of chemical activation. Models are presented to describe the mechanisms of activation in the context of ET theory, and the relevance of such chemically gated ET to the regulation of metabolism is discussed.

The roles of coenzyme A in the pyruvate:ferredoxin oxidoreductase reaction mechanism: rate enhancement of electron transfer from a radical intermediate to an iron-sulfur cluster

Pyruvate:ferredoxin oxidoreductase (PFOR) catalyzes the coenzyme A (CoA)-dependent oxidative decarboxylation of pyruvate. In many autotrophic anaerobes, PFOR links the Wood-Ljungdahl pathway to glycolysis and to cell carbon synthesis. Herein, we cloned and sequenced the M. thermoacetica PFOR, demonstrating strong structural homology with the structurally characterized D. africanus PFOR, including the presence of three [4Fe-4S] clusters per monomeric unit. The PFOR reaction includes a hydroxyethyl-thiamin pyrophosphate (HE-TPP) radical intermediate, which forms rapidly after PFOR reacts with pyruvate. This step precedes electron transfer from the HE-TPP radical intermediate to an intramolecular [4Fe-4S] cluster. We show that CoA increases the rate of this redox reaction by 10(5)-fold. Analysis by Marcus theory indicates that, in the absence of CoA, this is a true electron-transfer reaction; however, in its presence, electron transfer is gated by an adiabatic event. Analysis by the Eyring equation indicates that entropic effects dominate this rate enhancement. Our results indicate that the energy of binding CoA contributes minimally to the rate increase since the thiol group of CoA lends over 40 kJ/mol to the reaction, whereas components of CoA that afford most of the cofactor's binding energy contribute minimally. Major conformational changes also do not appear to explain the rate enhancement. We propose several ways that CoA can accomplish this rate increase, including formation of a highly reducing adduct with the HE-TPP radical to increase the driving force for electron transfer. We also consider the possibility that CoA itself forms part of the electron-transfer pathway.

Mechanisms of in vitro development of resistance to metronidazole in Trichomonas vaginalis

Development of resistance against metronidazole and mechanisms responsible for this process were studied in a sexually transmitted pathogen of humans, Trichomonas vaginalis. Monitoring of changes in metabolism and protein expression that accompanied increasing resistance of strains derived from a common drug-susceptible parent (TV 10-02) showed the multistep character of the process. The aerobic type of resistance known to occur in isolates from patients non-responsive to treatment appeared at the earliest stage, followed by development of the anaerobic type of resistance which was accompanied by gradual loss of hydrogenosomal proteins associated with drug-activating pathways [pyruvate:ferredoxin oxidoreductase (PFOR), hydrogenase, ferredoxin]. Unexpectedly, the loss of PFOR did not result in acquisition of full anaerobic resistance, thus indicating an alternative source of electrons required for the drug activation. These data suggest involvement of the oxidative decarboxylation of malate in hydrogenosomes, catalysed by NAD(+)-dependent malic enzyme and subsequent transfer of reduced equivalents to the drug via NADH:ferredoxin oxidoreductase and ferredoxin. Accordingly, all components of this pathway were eliminated before the resistance was fully developed. Resistant Trichomonas vaginalis compensated the impaired function of hydrogenosomes by enhanced conversion of pyruvate to lactate in the cytosol. Further analysis of the two key enzymes involved in metronidazole activation by Northern blotting and assay for nascent mRNA showed that the insufficient expression of the PFOR protein results from decreased gene transcription, while down-regulation of malic enzyme is controlled at the mRNA level.

Enzymes associated with reductive activation and action of nitazoxanide, nitrofurans, and metronidazole in Helicobacter pylori

Nitazoxanide (NTZ) is a redox-active nitrothiazolyl-salicylamide prodrug that kills Helicobacter pylori and also many anaerobic bacterial, protozoan, and helminthic species. Here we describe development and use of a spectrophotometric assay, based on nitroreduction of NTZ at 412 nm, to identify H. pylori enzymes responsible for its activation and mode of action. Three enzymes that reduce NTZ were identified: two related NADPH nitroreductases, which also mediate susceptibility to metronidazole (MTZ) (RdxA and FrxA), and pyruvate oxidoreductase (POR). Recombinant His-tagged RdxA, FrxA, and POR, overexpressed in nitroreductase-deficient Escherichia coli, each rapidly reduced NTZ, whereas only FrxA and to a lesser extent POR reduced nitrofuran substrates (furazolidone, nitrofurantoin, and nitrofurazone). POR exhibited no MTZ reductase activity either in extracts of H. pylori or following overexpression in E. coli; RdxA exhibited no nitrofuran reductase activity, and FrxA exhibited no MTZ reductase activity. Analysis of mutation to rifampin resistance (Rif(r)) indicated that NTZ was not mutagenic and that nitrofurans were only weakly mutagenic. Alkaline gel DNA electrophoresis indicated that none of these prodrugs caused DNA breakage. In contrast, MTZ caused DNA damage and was strongly mutagenic. We conclude that POR, an essential enzyme, is responsible for most or all of the bactericidal effects of NTZ against H. pylori. While loss-of-function mutations in rdxA and frxA produce a Mtz(r) phenotype, they do not contribute much to the innate susceptibility of H. pylori to NTZ or nitrofurans.

Crystal structure of the free radical intermediate of pyruvate:ferredoxin oxidoreductase

In anaerobic organisms, the decarboxylation of pyruvate, a crucial component of intermediary metabolism, is catalyzed by the metalloenzyme pyruvate: ferredoxin oxidoreductase (PFOR) resulting in the generation of low potential electrons and the subsequent acetylation of coenzyme A (CoA). PFOR is the only enzyme for which a stable acetyl thiamine diphosphate (ThDP)-based free radical reaction intermediate has been identified. The 1.87 A-resolution structure of the radical form of PFOR from Desulfovibrio africanus shows that, despite currently accepted ideas, the thiazole ring of the ThDP cofactor is markedly bent, indicating a drastic reduction of its aromaticity. In addition, the bond connecting the acetyl group to ThDP is unusually long, probably of the one-electron type already described for several cation radicals but not yet found in a biological system. Taken together, our data, along with evidence from the literature, suggest that acetyl-CoA synthesis by PFOR proceeds via a condensation mechanism involving acetyl (PFOR-based) and thiyl (CoA-based) radicals.

Characterization and cloning of an extremely thermostable, Pyrococcus furiosus-type 4Fe ferredoxin from Thermococcus profundus

An extremely thermostable [4Fe-4S] ferredoxin was isolated under anaerobic conditions from a hyperthermophilic archaeon Thermococcus profundus, and the ferredoxin gene was cloned and sequenced. The nucleotide sequence of the ferredoxin gene shows the ferredoxin to comprise 62 amino acid residues with a sequence similar to those of many bacterial and archaeal 4Fe (3Fe) ferredoxins. The unusual Fe-S cluster type, which was identified in the resonance Raman and EPR spectra, has three cysteines and one aspartate as the cluster ligands, as in the Pyrococcus furiosus 4Fe ferredoxin. Under aerobic conditions, a ferredoxin was purified that contains a [3Fe-4S] cluster as the major Fe-S cluster and a small amount of the [4Fe-4S] cluster. Its N-terminal amino acid sequence is the same as that of the anaerobically-purified ferredoxin up to the 26th residue. These results indicate that the 4Fe ferredoxin was degraded to 3Fe ferredoxin during aerobic purification. The aerobically-purified ferredoxin was reversibly converted back to the [4Fe-4S] ferredoxin by the addition of ferrous ions under reducing conditions. The anaerobically-purified [4Fe-4S] ferredoxin is quite stable; little degradtion was observed over 20 h at 100 degrees C, while the half-life of the aerobically-purified ferredoxin is 10 h at 100 degrees C. Both the anaerobically- and aerobically-purified ferredoxins were found to function as electron acceptors for the pyruvate-ferredoxin oxidoreductase purified from the same archaeon.

Spectroscopic analysis of desiccation-induced alterations of the chlorophyllide transformation pathway in etiolated barley leaves

Effects of water deficit on the chlorophyllide (Chlide) transformation pathway were studied in etiolated barley (Hordeum vulgare) leaves by analyzing absorption spectra and 77-K fluorescence spectra deconvoluted in components. Chlide transformations were examined in dehydrated leaves exposed to a 35-ms saturating flash triggering protochlorophyllide (Pchlide) and Chlide transformation processes. During the 90 min following the flash, we found that dehydration induced modifications of Chlide transformations, but no effect on Pchlide phototransformation into Chlide was observed. During this time, content of NADPH-Pchlide oxydoreductase in leaves did not change. Chlide transformation process in dehydrated leaves was characterized by the alteration of the Shibata shift process, by the appearance of a new Chlide species emitting at 692 nm, and by the favored formation of Chl(ide) A(668)F(676). The formation of Chl(ide) A(668)F(676), so-called "free Chlide," was probably induced by disaggregation of highly aggregated Chlide complexes. Here, we offer evidence for the alteration of photoactive Pchlide regeneration process, which may be caused by the desiccation-induced inhibition of Pchlide synthesis.

Pyruvate : NADP+ oxidoreductase from the mitochondrion of Euglena gracilis and from the apicomplexan Cryptosporidium parvum: a biochemical relic linking pyruvate metabolism in mitochondriate and amitochondriate protists

Most eukaryotes perform the oxidative decarboxylation of pyruvate in mitochondria using pyruvate dehydrogenase (PDH). Eukaryotes that lack mitochondria also lack PDH, using instead the O(2)-sensitive enzyme pyruvate : ferredoxin oxidoreductase (PFO), which is localized either in the cytosol or in hydrogenosomes. The facultatively anaerobic mitochondria of the photosynthetic protist Euglena gracilis constitute a hitherto unique exception in that these mitochondria oxidize pyruvate with the O(2)-sensitive enzyme pyruvate : NADP oxidoreductase (PNO). Cloning and analysis of Euglena PNO revealed that the cDNA encodes a mitochondrial transit peptide followed by an N-terminal PFO domain that is fused to a C-terminal NADPH-cytochrome P450 reductase (CPR) domain. Two independent 5.8-kb full-size cDNAs for Euglena mitochondrial PNO were isolated; the gene was expressed in cultures supplied with 2% CO(2) in air and with 2% CO(2) in N(2). The apicomplexan Cryptosporidium parvum was also shown to encode and express the same PFO-CPR fusion, except that, unlike E. gracilis, no mitochondrial transit peptide for C. parvum PNO was found. Recombination-derived remnants of PNO are conserved in the genomes of Saccharomyces cerevisiae and Schizosaccharomyces pombe as proteins involved in sulfite reduction. Notably, Trypanosoma brucei was found to encode homologs of both PFO and all four PDH subunits. Gene organization and phylogeny revealed that eukaryotic nuclear genes for mitochondrial, hydrogenosomal, and cytosolic PFO trace to a single eubacterial acquisition. These findings suggest a common ancestry of PFO in amitochondriate protists with Euglena mitochondrial PNO and Cryptosporidium PNO. They are also consistent with the view that eukaryotic PFO domains are biochemical relics inherited from a facultatively anaerobic, eubacterial ancestor of mitochondria and hydrogenosomes.

How does oxygen inhibit central metabolism in the obligate anaerobe Bacteroides thetaiotaomicron

The molecular basis of obligate anaerobiosis is not well established. Bacteroides thetaiotaomicron is an opportunistic pathogen that cannot grow in fully aerobic habitats. Because microbial niches reflect features of energy-producing strategies, we suspected that aeration would interfere with its central metabolism. In anaerobic medium, this bacterium fermented carbohydrates to a mixture of succinate, propionate and acetate. When cultures were exposed to air, the formation of succinate and propionate ceased abruptly. In vitro analysis demonstrated that the fumarase of the succinate-propionate pathway contains an iron-sulphur cluster that is sensitive to superoxide. In vivo, fumarase activity fell to < 5% when cells were aerated; virtually all activity was recovered after extracts were chemically treated to rebuild iron-sulphur clusters. Aeration minimally affected the remainder of this pathway. However, aeration reduced pyruvate:ferredoxin oxidoreductase (PFOR), the first enzyme in the acetate fermentation branch, to 3% of its anaerobic activity. This cluster-containing enzyme was damaged in vitro by molecular oxygen but not by superoxide. Thus, aerobic growth is precluded by the vulnerability of these iron-sulphur cluster enzymes to oxidation. Importantly, both enzymes were maintained in a stable, inactive form for long periods in aerobic cells; they were then rapidly repaired when the bacterium was returned to anaerobic medium. This result explains how this pathogen can easily recover from occasional exposure to oxygen.

Direct electrochemical characterization of hyperthermophilic Thermococcus celer metalloenzymes involved in hydrogen production from pyruvate

The reduction potentials of the metalloproteins pyruvate ferredoxin oxidoreductase (POR), ferredoxin, and hydrogenase isolated from hyperthermophilic Thermococcus celer (Topt = 88 degrees C) were determined as a function of temperature from 10 to 85 degrees C. Square-wave voltammetry experiments were carried out on 15 microL samples directly at an unmodified "edge-polished" pyrolytic graphite electrode using MgCl2 as an electrode promoter. POR exhibited two voltammetric waves with peaks at -280 and -403 mV at room temperature, indicating multiple redox centers, and a single wave at -420 mV at 85 degrees C. These waves displayed different temperature-dependent peak positions and peak heights, indicating that these redox centers have different thermodynamic and kinetic properties. Ferredoxin displayed a single linear temperature-dependent voltammetric wave at -280 mV at room temperature and -327 mV at 85 degrees C. Hydrogenase displayed a single biphasic temperature-dependent voltammetric wave at -197 mV at room temperature and -211 mV at 85 degrees C. Thermodynamic parameters associated with electron transfer, namely standard enthalpies and entropies for the redox centers in the various proteins, are reported.

Molecular evidence for the aerobic expression of nifJ, encoding pyruvate:ferredoxin oxidoreductase, in cyanobacteria

Pyruvate:ferredoxin (flavodoxin) oxidoreductase (PFO, EC 1.2.7.1) catalyses the oxidative cleavage of pyruvate and coenzyme A to acetylcoenzyme A and CO2 with the simultaneous reduction of ferredoxin or flavodoxin. PFO occurs in anaerobes and in some aerobic archaea and bacteria. For cyanobacteria, activity measurements indicated the occurrence of PFO in heterocystous forms. The completely sequenced genomes of the unicellular Synechocystis sp. PCC 6803 and the heterocystous Anabaena sp. PCC 7120 and Nostoc punctiforme revealed the existence of one PFO (encoded by nifJ) in Synechocystis 6803 and N. punctiforme but two different PFOs, encoded by nifJ1 and nifJ2, in Anabaena. Sequence comparison now indicates that all cyanobacterial PFOs are more closely related to those of anaerobes than to those of aerobes. Reverse transcription-polymerase chain reaction (RT-PCR) experiments show that nifJ is transcribed in the presence of saturating iron concentrations in aerobically grown cells of the unicellular Synechococcus sp. PCC 6301 and Synechocystis 6803. Both nifJ genes are transcribed in aerobically grown Anabaena 7120. These findings are corroborated by luciferase reporter gene analysis of nifJ in Synechococcus sp. PCC 7942. The occurrence of PFO in these cyanobacteria is enigmatic.

Loss of multiple hydrogenosomal proteins associated with organelle metabolism and high-level drug resistance in trichomonads

Land, K. M., Clemens, D. L., and Johnson, P. J. 2001. Loss of multiple hydrogenosomal proteins associated with organelle metabolism and high-level drug resistance in trichomonads. Experimental Parasitology 97, 102-110. In trichomonads, metronidazole is activated to its cytotoxic form in a specialized energy-producing organelle called the hydrogenosome. Electron transport components in the organelle, pyruvate:ferredoxin oxidoreductase and ferredoxin, donate a single electron to the drug, converting it to a cytotoxic free radical. Previous biochemical analyses of enzyme activities of highly resistant strains of both Trichomonas vaginalis and Tritrichomonas foetus reveal undetectable activity for pyruvate:ferredoxin oxidoreductase and another hydrogenosomal enzyme, hydrogenase. We have chosen to analyze a highly drug-resistant strain of T. foetus and its parental drug-sensitive strain from which it was derived to study the molecular basis for these enzyme defects. Quantitation of pyruvate:ferredoxin oxidoreductase and ferredoxin levels in sensitive and resistant cells shows a marked reduction of these proteins in the resistant strain. RNA analysis reveals an approximately 60% reduction in pyruvate:ferredoxin oxidoreductase mRNA and 90-98% reduction in mRNA levels encoding hydrogenosomal proteins hydrogenase, ferredoxin, and malic enzyme. We have measured the levels of transcription of these genes and observed 60% reduction of pyruvate:ferredoxin oxidoreductase gene transcription and 85% reduction in malic enzyme gene transcription in the resistant strain. The reduction or absence of these organellar proteins is likely to reduce or eliminate the ability of the cell to activate the drug, giving rise to the highly resistant phenotype. Ultrastructural analysis of thin sections revealed that resistant cells are 20% smaller in size and hydrogenosomes in resistant cells are approximately one-third the size of those in the drug-sensitive parental strain. These data suggest that altered gene expression of multiple hydrogenosomal proteins results in the modification of the organelle and leads to drug resistance.

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.