Cytochrome bd confers nitric oxide resistance to Escherichia coli

The aerobic respiratory chain of Escherichia coli has two terminal quinol oxidases: cytochrome bo and cytochrome bd. Cytochrome bd was thought to function solely to facilitate micro-aerobic respiration. However, it has recently been shown to be overexpressed under conditions of nitric oxide (NO) stress; we show here that cytochrome bd is crucial for protecting E. coli cells from NO-induced growth inhibition by virtue of its fast NO dissociation rate.

Characterization of the Shewanella oneidensis MR-1 decaheme cytochrome MtrA: expression in Escherichia coli confers the ability to reduce soluble Fe(III) chelates

Shewanella oneidensis MR-1 has the metabolic capacity to grow anaerobically using Fe(III) as a terminal electron acceptor. Growth under these conditions results in the de novo synthesis of a number of periplasmic c-type cytochromes, many of which are multiheme in nature and are thought to be involved in the Fe(III) respiratory process. To begin a biochemical study of these complex cytochromes, the mtrA gene that encodes an approximate 32-kDa periplasmic decaheme cytochrome has been heterologously expressed in Escherichia coli. Co-expression of mtrA with a plasmid that contains cytochrome c maturation genes leads to the assembly of a correctly targeted holoprotein, which covalently binds ten hemes. The recombinant MtrA protein has been characterized by magnetic circular dichroism, which shows that all ten hemes have bis-histidine axial ligation. EPR spectroscopy detected only eight of these hemes, all of which are low spin and provides evidence for a spin-coupled pair of hemes in the oxidized state. Redox titrations of MtrA have been carried out with optical- and EPR-monitored methods, and the hemes are shown to reduce over the potential range -100 to -400 mV. In intact cells of E. coli, MtrA is shown to obtain electrons from the host electron transport chain and pass these onto host oxidoreductases or a range of soluble Fe(III) species. This demonstrates the promiscuous nature of this decaheme cytochrome and its potential to serve as a soluble Fe(III) reductase in intact cells.

Purification and magneto-optical spectroscopic characterization of cytoplasmic membrane and outer membrane multiheme c-type cytochromes from Shewanella frigidimarina NCIMB400

Two membranous c-type cytochromes from the Fe(III)-respiring bacterium Shewanella frigidimarina NCIMB400, CymA and OmcA, have been purified and characterized by UV-visible, magnetic circular dichroism, and electron paramagnetic resonance spectroscopies. The 20-kDa CymA is a member of the NapC/NirT family of multiheme cytochromes, which are invariably anchored to the cytoplasmic membrane of Gram-negative bacteria, and are postulated to mediate electron flow between quinols and periplasmic redox proteins. CymA was found to contain four low-spin c-hemes, each with bis-His axial ligation, and midpoint reduction potentials of +10, -108, -136, and -229 mV. The 85-kDa OmcA is located at the outer membrane of S. frigidimarina NCIMB400, and as such might function as a terminal reductase via interaction with insoluble Fe(III) substrates. This putative role is supported by the finding that the protein was released into solution upon incubation of harvested intact cells at 25 degrees C, suggesting an attachment to the exterior face of the outer membrane. OmcA was revealed by magneto-optical spectrocopies to contain 10 low-spin bis-His ligated c-hemes, with the redox titer indicating two sets of near iso-potential components centered at -243 and -324 mV.

Voltammetry of a flavocytochrome c(3): the lowest potential heme modulates fumarate reduction rates

Iron-induced flavocytochrome c(3), Ifc(3), from Shewanella frigidimarina NCIMB400, derivatized with a 2-pyridyl disulfide label, self-assembles on gold electrodes as a functional array whose fumarate reductase activity as viewed by direct electrochemistry is indistinguishable from that of Ifc(3) adsorbed on gold or graphite electrodes. The enhanced stability of the labeled protein's array permits analysis at a rotating electrode and limiting catalytic currents fit well to a Michaelis-Menten description of enzyme kinetics with K(M) = 56 +/- 20 microM, pH 7.5, comparable to that obtained in solution assays. At fumarate concentrations above 145 microM cyclic voltammetry shows the catalytic response to contain two features. The position and width of the lower potential component centered on -290 mV and corresponding to a one-electron wave implicates the oxidation state of the lowest potential heme of Ifc(3) as a defining feature in the mechanism of fumarate reduction at high turnover rates. We propose the operation of dual pathways for electron transfer to the active site of Ifc(3) with the lowest potential heme acting as an electron relay on one of these pathways.

Open conformation of a flavocytochrome c3 fumarate reductase

Fumarate reductases and succinate dehydrogenases play central roles in the metabolism of eukaryotic and prokaryotic cells. A recent medium resolution structure of the Escherichia coli fumarate reductase (Frd) has revealed the overall organization of the membrane-bound complex. Here we present the first high resolution X-ray crystal structure of a water-soluble bacterial fumarate reductase in an open conformation. This structure reveals a mobile domain that modulates substrate access to the active site and provides new insights into the mechanism of this widespread and important family of FAD-containing respiratory proteins.

Characterization of a flavocytochrome that is induced during the anaerobic respiration of Fe3+ by Shewanella frigidimarina NCIMB400

A 63.9 kDa periplasmic tetrahaem flavocytochrome c(3), designated Ifc(3), was found to be expressed in Shewanella frigidimarina NCIMB400 grown anaerobically with ferric citrate or ferric pyrophosphate as the sole terminal electron acceptor, but not in anaerobic cultures of the bacterium with other respiratory substrates. Ifc(3) was purified to homogeneity and revealed by biochemical, spectroscopic and primary structure analyses to contain four low-spin bis-His-ligated c(3)-haems, with midpoint reduction potentials of -73, -141, -174 and -259 mV. A low-potential flavin was present in the form of non-covalently bound FAD; the protein possessed a unidirectional fumarate reductase activity. Disruption of the chromosomal gene encoding Ifc(3), ifcA, did not lead to a significant change in the rate of Fe(3+) reduction in batch culture. However, during such growth the Ifc(3)-deficient mutant produced both a 35 kDa periplasmic c-type cytochrome and a 45 kDa membrane-associated c-type cytochrome at markedly higher levels than did the parent strain. Nucleotide sequencing data from directly upstream of ifcA indicated the presence of an open reading frame encoding a putative outer-membrane beta-barrel protein of 324 amino acid residues.

Dissimilatory Fe(III) reduction by Clostridium beijerinckii isolated from freshwater sediment using Fe(III) maltol enrichment

A microorganism which reduces Fe(III) during the fermentation of glucose was isolated from freshwater sediment. The Fe(III) was supplied to enrichment cultures as a soluble complex with the bidentate ligand maltol (3-hydroxy-2-methyl-4-pyrone). Advantages that were afforded by the use of Fe(III)(maltol)3 over previously published methods included negation of the requirement for assays of Fe(II) formation. Because Fe(III)(maltol)3 has a characteristic deep red colour, Fe(III) reduction could be quantified spectrophotometrically by monitoring the disappearance of the complex in liquid cultures. Furthermore, Fe(III) reduction on agar plates containing the complex was apparent by zones of decolourisation around the bacterial colonies. 16S rRNA gene sequencing indicated the isolate to be a strain of Clostridium beijerinckii. Growth experiments were performed on the isolate in batch cultures with varying concentrations of Fe(III) citrate and 50 mM glucose. Increasing the level of Fe(III) citrate present was found to alter the fermentation balance, with less acidic products being formed. The presence of Fe(III) led to increases in the growth rate and growth yield, which were both approximately doubled when the supply of the cation reached 25 mM. A NAD(P)H-dependent Fe(III) reductase activity was localised to the bacterial membrane and found not to be sensitive to respiratory inhibitors. Taken together, these data suggest that dissimilatory Fe(III) reduction by the isolate provides a means of utilising the cation as an electron sink, thus facilitating pyridine nucleotide to be recycled during fermentative metabolism.

Crystallization and preliminary X-ray crystallographic analysis of a periplasmic tetrahaem flavocytochrome c3 from Shewanella frigidimarina NCIMB400 which has fumarate reductase activity

The fumarate reductase of Escherichia coli and other bacteria is a membrane-bound enzyme consisting of four subunits. A soluble periplasmic 64 kDa tetrahaem flavocytochrome c3 from Shewanella frigidimarina NCIMB400 which possesses a catalytic fumarate reductase activity has been crystallized. The crystals belong to space group P212121 with unit-cell parameters a = 72.4, b = 110.1, c = 230.2 A. Assuming a molecular dimer in the asymmetric unit, the crystals contain 65% solvent and, when cryocooled to 100 K, the crystals diffract to at least 3.0 A resolution. The crystals, however, display an inherent lack of isomorphism and the plausibility of a MAD phasing experiment has therefore been investigated by measuring the iron K absorption edge from a single crystal.

Dissimilatory iron(III) reduction by Rhodobacter capsulatus

The photosynthetic proteobacterium Rhodobacter capsulatus was shown to be capable of dissimilatory Fe(III) reduction. Activity was expressed during anaerobic phototrophic and microaerobic growth with malate as the carbon source, but not during equivalent aerobic growth. A variety of Fe(III) complexes were demonstrated to act as substrates for intact cells and membrane fractions of strain N22DNAR+ using a ferrozine assay for Fe(II) formation. Rates of reduction appeared to be influenced by the reduction potentials of the Fe(III) complexes. However, Fe(III) complexed by citrate, which is readily reduced by Shewanella putrefaciens, was a poor substrate for dissimilation by R. capsulatus. The Fe(III)-reducing activity of R. capsulatus was located solely in the membrane fraction. The reduction of Fe(III) complexes by intact cells was inhibited by 2-heptyl-4-hydroxyquinoline-N-oxide (HQNO), suggesting the involvement of ubiquinol: cytochrome c oxidoreductases in the electron transport chain. Lack of sensitivity to myxothiazol plus data from mutant strains implies that the cytochrome bc 1 complex and cytochrome c 2 are not obligatory for dissimilation of Fe(III)(maltol)3. Alternative pathways of electron transfer to Fe(III) must hence operate in R. capsulatus. Using strain N22DNAR+, the reduction rate of Fe(III) complexed by nitrilotriacetic acid (NTA) was elevated compared to that of Fe(III)(maltol)3, and moreover was sensitive to myxothiazol. However, these differences were not observed in the absence of the electron donor malate. The governing factor for the reduction rate of Fe(III)(maltol)3 thus appears to be the limited Fe(III)-reducing activity, whilst the reduction rate of Fe(III) complexed by NTA is controlled by the flux of electrons through the respiratory chain. The use of mutant strains confirmed that the role of the cytochrome bc 1 complex in Fe(III) reduction becomes apparent only with the superior substrate. The energy-conserving nature of Fe(III) reduction by R. capsulatus was demonstrated by electrochromic measurements, with the endogenous carotenoid pigments being employed as indicators of membrane potential generation in intact cells. Using Fe(III)EDTA as electron acceptor, periods of membrane potential generation were directly proportional to the quantity of complex added, and were extended in the presence of HQNO. Fe(III)-dependent carotenoid bandshifts were abolished by addition of the protonophoric uncoupler carbonyl cyanide p-trifluoromethoxy-phenylhydrazone.

Structures of 3-hydroxy-1-(2-methoxyethyl)-2-methyl-4-pyridinone, its hydrochloride and 1-ethyl-3-hydroxy-2-methyl-4-pyridinone hydrochloride hydrate

3-Hydroxy-1-(2-methoxyethyl)-2-methyl-4(1H)-pyridinone (1) exists mainly in the quinoid form with a small contribution from the aromatic zwitterion form, while 3-hydroxy-1-(2-methoxyethyl)-2-methyl-4(1H)-pyridinone hydrochloride (2) and 1-ethyl-3-hydroxy-2-methyl-4(1H)-pyridinone hydrochloride hydrate (3) are observed to occur in the aromatic form with a minor contribution from the quinoid resonance form. Different substituents at the ring N position do not have significant geometric effects in either the neutral molecules, as a group, or in the hydrochloride salts.

The effect of a synthetic hexadentate iron chelator (CP130) and desferrioxamine on rabbit kidneys exposed to cold and warm ischaemia

The effect of CP130 (a synthetic hexadentate pyridinone iron chelator) on the formation of two markers of lipid peroxidation (TBA-reactive material and Schiff's bases) in rabbit kidneys following a 72 h period of cold (0-4 degrees C) ischaemia was investigated by either adding CP130 to the flush/storage solution (hypertonic citrate solution) or by administering the agent intravenously 15 min before removal of the organs. In both cases, CP130 blocked the adverse rises in lipid peroxidation caused by ischaemia and subsequent reoxygenation of the homogenates in vitro. Both CP130 and desferrioxamine (DFX) (administered intravenously 15 min before ischaemia and 5 min before reperfusion) were also found to significantly reduce post-ischaemic rates of in vitro lipid peroxidation in kidneys rendered warm ischaemic for 90 min followed by reperfusion for 5 or 60 min in situ. Kidneys exposed to warm ischaemia and reperfusion developed interstitial and intracellular oedema, congestion and haemorrhage. DFX administration had little effect on the histological outcome, whereas CP130 significantly reduced interstitial oedema (at 5 min reperfusion compared to the DFX-treated group), intracellular oedema (at 60 min reperfusion compared to the DFX-treated group) and congestion (at 5 min reperfusion compared with a control group not given any agent). It is concluded that while CP130 and DFX exhibited similar antioxidant properties, CP130 provided better protection from ischaemia/reperfusion injury at the histological level. Synthetic iron chelators may therefore be of benefit in clinical organ transplantation by protecting against tissue damage caused by prolonged ischaemia.

Synthesis, physicochemical properties, and biological evaluation of N-substituted 2-alkyl-3-hydroxy-4(1H)-pyridinones: orally active iron chelators with clinical potential

The synthesis of a range of novel bidentate ligands containing the chelating moiety 3-hydroxy-4(1H)-pyridinone is described. The pKa values of the ligands and the stability constants of their iron(III) complexes have been determined. The crystal structures of one of the ligands and one of the iron(III) complexes are presented. The distribution coefficients of the ligands are reported and are related to the ability of the ligands to remove iron from hepatocytes. The influence of 3-hydroxy-4(1H)-pyridinones on oxidative damage to cells is described. In contrast to the iron chelator in current therapeutic use, desferrioxamine-B, many of the bidentate ligands described in this study are orally active in iron-overloaded mice.

3-[1-(Ethylamino)ethylidene]-6-methyl-3H-pyran-2,4-dione

The structural results clearly indicate that 3-[1-(ethylamino)ethylidene]-6-methyl-3H-pyran-2,4-dione exists as a keto-enamine tautomer in the solid state. The H atom bonded to N(9) refines with a normal temperature factor and the bond distance of the keto group [C(4)--O(13)] of 1.263 (1) A is elongated due to resonance. This resonance is also indicated by an averaging of the single bonds [C(2)--C(3), C(3)--C(4), C(4)--C(5) and C(8)--N(9)] and double bonds [C(3)--C(8) and C(4)--O(13)] in the crystal structure and by the fact that not only is the pyran ring planar [r.m.s. deviation 0.0212 (9) A] but also there is planarity for the whole molecule [r.m.s. deviation 0.074 (1) A]. A strong intramolecular hydrogen bond is found between the 4-oxo group and the amine H atom.

Urinary metabolic profiles in human and rat of 1,2-dimethyl- and 1,2-diethyl-substituted 3-hydroxypyridin-4-ones

The urinary metabolic profiles of two novel orally active iron chelators, 1,2-dimethyl-3-hydroxypyridin-4-one (CP20 or L1) and 1,2-diethyl-3-hydroxypyridin-4-one (CP94), have been studied in rats. The metabolism of CP20 was also studied in humans. Four novel metabolites of CP20, and a further three metabolites of CP94 were characterized. CP20 was found to undergo extensive phase II metabolism at the 3-hydroxy position, forming predominantly the O-glucuronide, which accounted for 44% of the dose administered in rat and greater than 85% of the dose administered in man. The 3-O-methylated CP20 metabolite (metabolite I) accounted for 1% of the administered dose in both species, whereas the unmetabolized CP20 amounted to 10.5% and 4% of the dose administered in the rats and man, respectively. In contrast, CP94 was extensively hydroxylated at the 2-ethyl position to give its 2-(1-hydroxyethyl) metabolite in the rat, which accounted for 40% of the administered dose. The O-glucuronide metabolite of CP94 accounted for 13.8% of the administered dose, whereas the unmetabolized CP94 amounted to 6.9% of the administered dose. At 72 hr, urinary levels of CP20 and CP94 and their metabolites in the rat accounted for about 55-60% of the administered dose. A large portion of the dose is therefore probably eliminated via the bile. The identity of the above metabolites was established using a combination of two or more of the following techniques: fast atom bombardment-mass spectroscopy, LC-MS, UV-VIS spectroscopy, NMR spectroscopy, specific enzyme hydrolysis assays, and chemical synthesis of compounds.