A novel scenario for the evolution of haem-copper oxygen reductases

The increasing sequence information on oxygen reductases of the haem-copper superfamily, together with the available three-dimensional structures, allows a clear identification of their common, functionally important features. Taking into consideration both the overall amino acid sequences of the core subunits and key residues involved in proton transfer, a novel hypothesis for the molecular evolution of these enzymes is proposed. Three main families of oxygen reductases are identified on the basis of common features of the core subunits, constituting three lines of evolution: (i) type A (mitochondrial-like oxidases), (ii) type B (ba3-like oxidases) and (iii) type C (cbb3-type oxidases). The first group can be further divided into two subfamilies, according to the helix VI residues at the hydrophobic end of one of the proton pathways (the so-called D-channel): (i) type A1, comprising the enzymes with a glutamate residue in the motif -XGHPEV-, and (ii) type A2, enzymes having instead a tyrosine and a serine in the alternative motif -YSHPXV-. This second subfamily of oxidases is shown to be ancestor to the one containing the glutamate residue, which in the Bacteria domain is only present in oxidases from Gram-positive or purple bacteria. It is further proposed that the Archaea domain acquired terminal oxidases by gene transfer from the Gram-positive bacteria, implying that these enzymes were not present in the last common ancestor before the divergence between Archaea and Bacteria. In fact, most oxidases from archaea have a higher amino acid sequence identity and similarity with those from bacteria, mainly from the Gram-positive group, than with oxidases from other archaea. Finally, a possible relation between the dihaemic subunit (FixP) of the cbb3 oxidases and subunit II of caa3 oxidases is discussed. As the families of haem-copper oxidases can also be identified by their subunit II, a parallel evolution of subunits I and II is suggested.

Uric acid in chronic heart failure: a marker of chronic inflammation

BACKGROUND

Chronic heart failure is associated with hyperuricaemia and elevations in circulating markers of inflammation. Activation of xanthine oxidase, through free radical release, causes leukocyte and endothelial cell activation. Associations could therefore be expected between serum uric acid level, as a marker of increased xanthine oxidase activity, and markers of inflammation. We have explored these associations in patients with chronic heart failure, taking into account the hyperuricaemic effects of diuretic therapy and insulin resistance.

METHODS AND RESULTS

Circulating uric acid and markers of inflammation were measured in 39 male patients with chronic heart failure and 16 healthy controls. All patients underwent a metabolic assessment, which provided a measure of insulin sensitivity (intravenous glucose tolerance tests and minimal modelling analysis). Compared to controls, patients with chronic heart failure had significantly higher levels of circulating uric acid, interleukin-6, soluble tumour necrosis factor receptor (sTNFR)-1, soluble intercellular adhesion molecule-1 (ICAM-1, all P<0.001), E-selectin and sTNFR2 (both P<0.05). In patients with chronic heart failure, serum uric acid concentrations correlated with circulating levels of sTNFR1 (r=0.74), interleukin-6 (r=0.66), sTNFR2 (r=0.63), TNFa (r=0.60) (all P<0.001), and ICAM-1 (r=0.41, P<0.01). In stepwise regression analyses, serum uric acid emerged as the strongest predictor of ICAM-1, interleukin-6, TNF, sTNFR1 and sTNFR2, independent of diuretic dose, age, body mass index, alcohol intake, serum creatinine, plasma insulin and glucose, and insulin sensitivity.

CONCLUSIONS

Serum uric acid is strongly related to circulating markers of inflammation in patients with chronic heart failure. This is consistent with a role for increased xanthine oxidase activity in the inflammatory response in patients with chronic heart failure.

Structure of a dioxygen reduction enzyme from Desulfovibrio gigas

Desulfovibrio gigas is a strict anaerobe that contains a well-characterized metabolic pathway that enables it to survive transient contacts with oxygen. The terminal enzyme in this pathway, rubredoxin:oxygen oxidoreductase (ROO) reduces oxygen to water in a direct and safe way. The 2.5 A resolution crystal structure of ROO shows that each monomer of this homodimeric enzyme consists of a novel combination of two domains, a flavodoxin-like domain and a Zn-beta-lactamase-like domain that contains a di-iron center for dioxygen reduction. This is the first structure of a member of a superfamily of enzymes widespread in strict and facultative anaerobes, indicating its broad physiological significance.

The three classes of hydrogenases from sulfate-reducing bacteria of the genus Desulfovibrio

Three types of hydrogenases have been isolated from the sulfate-reducing bacteria of the genus Desulfovibrio. They differ in their subunit and metal compositions, physico-chemical characteristics, amino acid sequences, immunological reactivities, gene structures and their catalytic properties. Broadly, the hydrogenases can be considered as 'iron only' hydrogenases and nickel-containing hydrogenases. The iron-sulfur-containing hydrogenase ([Fe] hydrogenase) contains two ferredoxin-type (4Fe-4S) clusters and an atypical iron-sulfur center believed to be involved in the activation of H2. The [Fe] hydrogenase has the highest specific activity in the evolution and consumption of hydrogen and in the proton-deuterium exchange reaction and this enzyme is the most sensitive to CO and NO2-. It is not present in all species of Desulfovibrio. The nickel-(iron-sulfur)-containing hydrogenases [( NiFe] hydrogenases) possess two (4Fe-4S) centers and one (3Fe-xS) cluster in addition to nickel and have been found in all species of Desulfovibrio so far investigated. The redox active nickel is ligated by at least two cysteinyl thiolate residues and the [NiFe] hydrogenases are particularly resistant to inhibitors such as CO and NO2-. The genes encoding the large and small subunits of a periplasmic and a membrane-bound species of the [NiFe] hydrogenase have been cloned in Escherichia (E.) coli and sequenced. Their derived amino acid sequences exhibit a high degree of homology (70%); however, they show no obvious metal-binding sites or homology with the derived amino acid sequence of the [Fe] hydrogenase. The third class is represented by the nickel-(iron-sulfur)-selenium-containing hydrogenases [( NiFe-Se] hydrogenases) which contain nickel and selenium in equimolecular amounts plus (4Fe-4S) centers and are only found in some species of Desulfovibrio. The genes encoding the large and small subunits of the periplasmic hydrogenase from Desulfovibrio (D.) baculatus (DSM 1743) have been cloned in E. coli and sequenced. The derived amino acid sequence exhibits homology (40%) with the sequence of the [NiFe] hydrogenase and the carboxy-terminus of the gene for the large subunit contains a codon (TGA) for selenocysteine in a position homologous to a codon (TGC) for cysteine in the large subunit of the [NiFe] hydrogenase. EXAFS and EPR studies with the 77Se-enriched D. baculatus hydrogenase indicate that selenium is a ligand to nickel and suggest that the redox active nickel is ligated by at least two cysteinyl thiolate and one selenocysteine selenolate residues.(ABSTRACT TRUNCATED AT 400 WORDS)

Epidemiology of visceral leishmaniasis in northeast Brazil

Epidemiologic aspects of the relationship between infection with Leishmania chagasi and development of clinical visceral leishmaniasis (VL) were studied in all children < 11 years old in a defined, endemic, rural area of the state of Ceará in northeast Brazil. Antileishmanial antibodies were measured in the same subjects by ELISA on six occasions between May 1987 and August 1989. Seroconversion was documented during this period in 108 children, with a cumulative annual incidence of 4.6%. Twelve (11.1%) of these children developed VL. Age < 4 years, hematocrit < 33%, and living in the mountains predicted the development of clinically apparent VL after seroconversion. Despite a high percentage of dogs serologically positive in the region (38%), there was no increased risk of infection for children living in the same household with dogs. Since children in households with a prior case of VL had a threefold increased risk of infection, human-sandfly-human transmission might have been important.

A nested PCR for the ssrRNA gene detects Trypanosoma binneyi in the platypus and Trypanosoma sp. in wombats and kangaroos in Australia

Trypanosome infections in their natural hosts are frequently difficult to detect by microscopy, and culture methods are unreliable and not suitable for all species of Trypanosoma. A nested PCR strategy for detecting and identifying Trypanosoma species, suitable for detecting both known and unknown trypanosomes, is presented. Thirty-two blood samples from 23 species of Australian birds and mammals were screened by a nested PCR for the presence of Trypanosoma sp. ssrRNA. Three infections were detected, one in an eastern grey kangaroo (Macropus giganteus), one in a common wombat (Vombatus ursinus) and one in a platypus (Ornithorhynchus anatinus). The kangaroo and wombat are new host records for Trypanosoma sp.; the platypus parasite was Trypanosoma hinneyi. The three parasites could be distinguished by restriction fragment length polymorphisms of the amplified fragment of the ssrRNA gene. The kangaroo and wombat parasites were also isolated in a semi-solid blood agar medium. The culture forms of the kangaroo trypanosome had an expanded flagellar sheath in which structures similar to hemidesmosomes were detected by EM. The nested PCR was at least as sensitive as culture, and analysis of the PCR products gave parasite-specific fingerprints. Therefore this method could be suitable for rapidly screening host animals for the presence of trypanosomes and identifying the infecting strain.

Studies on the redox centers of the terminal oxidase from Desulfovibrio gigas and evidence for its interaction with rubredoxin

Rubredoxin-oxygen oxidoreductase (ROO) is the final component of a soluble electron transfer chain that couples NADH oxidation to oxygen consumption in the anaerobic sulfate reducer Desulfovibrio gigas. It is an 86-kDa homodimeric flavohemeprotein containing two FAD molecules, one mesoheme IX, and one Fe-uroporphyrin I per monomer, capable of fully reducing oxygen to water. EPR studies on the native enzyme reveal two components with g values at approximately 2.46, 2.29, and 1.89, which are assigned to low spin hemes and are similar to the EPR features of P-450 hemes, suggesting that ROO hemes have a cysteinyl axial ligation. At pH 7.6, the flavin redox transitions occur at 0 +/- 15 mV for the quinone/semiquinone couple and at -130 +/- 15 mV for the semiquinone/hydroquinone couple; the hemes reduction potential is -350 +/- 15 mV. Spectroscopic studies provided unequivocal evidence that the flavins are the electron acceptor centers from rubredoxin, and that their reduction proceed through an anionic semiquinone radical. The reaction with oxygen occurs in the flavin moiety. These data are strongly corroborated by the finding that rubredoxin and ROO are located in the same polycistronic unit of D. gigas genome. For the first time, a clear role for a rubredoxin in a sulfate-reducing bacterium is presented.

Nickel-[iron-sulfur]-selenium-containing hydrogenases from Desulfovibrio baculatus (DSM 1743). Redox centers and catalytic properties

The hydrogenase from Desulfovibrio baculatus (DSM 1743) was purified from each of three different fractions: soluble periplasmic (wash), soluble cytoplasmic (cell disruption) and membrane-bound (detergent solubilization). Plasma-emission metal analysis detected in all three fractions the presence of iron plus nickel and selenium in equimolecular amounts. These hydrogenases were shown to be composed of two non-identical subunits and were distinct with respect to their spectroscopic properties. The EPR spectra of the native (as isolated) enzymes showed very weak isotropic signals centered around g approximately 2.0 when observed at low temperature (below 20 K). The periplasmic and membrane-bound enzymes also presented additional EPR signals, observable up to 77 K, with g greater than 2.0 and assigned to nickel(III). The periplasmic hydrogenase exhibited EPR features at 2.20, 2.06 and 2.0. The signals observed in the membrane-bound preparations could be decomposed into two sets with g at 2.34, 2.16 and approximately 2.0 (component I) and at 2.33, 2.24, and approximately 2.0 (component II). In the reduced state, after exposure to an H2 atmosphere, all the hydrogenase fractions gave identical EPR spectra. EPR studies, performed at different temperatures and microwave powers, and in samples partially and fully reduced (under hydrogen or dithionite), allowed the identification of two different iron-sulfur centers: center I (2.03, 1.89 and 1.86) detectable below 10 K, and center II (2.06, 1.95 and 1.88) which was easily saturated at low temperatures. Additional EPR signals due to transient nickel species were detected with g greater than 2.0, and a rhombic EPR signal at 77 K developed at g 2.20, 2.16 and 2.0. This EPR signal is reminiscent of the Ni-signal C (g at 2.19, 2.14 and 2.02) observed in intermediate redox states of the well characterized Desulfovibrio gigas hydrogenase (Teixeira et al. (1985) J. Biol. Chem. 260, 8942]. During the course of a redox titration at pH 7.6 using H2 gas as reductant, this signal attained a maximal intensity around -320 mV. Low-temperature studies of samples at redox states where this rhombic signal develops (10 K or lower) revealed the presence of a fast-relaxing complex EPR signal with g at 2.25, 2.22, 2.15, 2.12, 2.10 and broad components at higher field. The soluble hydrogenase fractions did not show a time-dependent activation but the membrane-bound form required such a step in order to express full activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Purification and characterization of an iron superoxide dismutase and a catalase from the sulfate-reducing bacterium Desulfovibrio gigas

The iron-containing superoxide dismutase (FeSOD; EC 1.15.1.1) and catalase (EC 1.11.1.6) enzymes constitutively expressed by the strictly anaerobic bacterium Desulfovibrio gigas were purified and characterized. The FeSOD, isolated as a homodimer of 22-kDa subunits, has a specific activity of 1,900 U/mg and exhibits an electron paramagnetic resonance (EPR) spectrum characteristic of high-spin ferric iron in a rhombically distorted ligand field. Like other FeSODs from different organisms, D. gigas FeSOD is sensitive to H(2)O(2) and azide but not to cyanide. The N-terminal amino acid sequence shows a high degree of homology with other SODs from different sources. On the other hand, D. gigas catalase has an estimated molecular mass of 186 +/- 8 kDa, consisting of three subunits of 61 kDa, and shows no peroxidase activity. This enzyme is very sensitive to H(2)O(2) and cyanide and only slightly sensitive to sulfide. The native enzyme contains one heme per molecule and exhibits a characteristic high-spin ferric-heme EPR spectrum (g(y,x) = 6.4, 5.4); it has a specific activity of 4,200 U/mg, which is unusually low for this class of enzyme. The importance of these two enzymes in the context of oxygen utilization by this anaerobic organism is discussed.

Desulfovibrio Gigas hydrogenase: redox properties of the nickel and iron-sulfur centers

Below 30 K, oxidized Desulfovibrio gigas hydrogenase presents an intense electron paramagnetic resonance (EPR) signal centered at g = 2.02, typical of an iron-sulfur center. In addition a rhombic EPR signal, attributed to Ni(III) species, is also observed [LeGall, J., Ljungdahl, P., Moura, I., Peck, H.D., Jr, Xavier, A.V., Moura, J.J.G., Teixeira, M., Huynh, B.H., and DerVartanian, D.V. (1982) Biochem. Biophys. Res. Commun. 106, 610-616; and Cammack, R., Patil, D., Aguirre, R., and Hatchikian, E.C., (1982) FEBS Lett. 142, 289-292]. At higher temperatures (77 K) the iron-sulfur EPR signal is broader and all the EPR features of the rhombic nickel signal can easily be observed. We have now obtained additional information concerning the redox properties of these EPR active centers, using an EPR redox titration method in the presence of dye mediators at pH = 8.5. The mid-point potential was determined to be -70 mV for the Fe,S cluster and -220 mV for the Ni center. Intermediate oxidation states were obtained upon partial reduction with either dithionite or hydrogen. Although upon dithionite reduction the centers are reduced in the order of decreasing mid-point reduction potentials, under a hydrogen atmosphere the nickel center reduces preferentially. This suggests a catalytic involvement of the nickel redox center in the binding of hydrogen. Preliminary Mössbauer studies on Desulfovibrio gigas hydrogenase reveal the presence of a paramagnetic 3 Fe center and two 4 Fe centers. The 3 Fe center is responsible for the g = 2.02 EPR signal but the two 4 Fe centers have been so far undetectable by EPR.

Characterization of a heme c nitrite reductase from a non-ammonifying microorganism, Desulfovibrio vulgaris Hildenborough

A cytochrome c nitrite reductase (NiR) was purified for the first time from a microorganism not capable of growing on nitrate, the sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough. It was isolated from the membranes as a large heterooligomeric complex of 760 kDa, containing two cytochrome c subunits of 56 and 18 kDa. This complex has nitrite and sulfite reductase activities of 685 micromol NH(4)(+)/min/mg and 1.0 micromol H(2)/min/mg. The enzyme was studied by UV-visible and electron paramagnetic resonance (EPR) spectroscopies. The overall redox behavior was determined through a visible redox titration. The data were analyzed with a set of four redox transitions, with an E(0)' of +160 mV (12% of total absorption), -5 mV (38% of total absorption), -110 mV (38% of total absorption) and -210 mV (12% of total absorption) at pH 7.6. The EPR spectra of oxidized and partially reduced NiR show a complex pattern, indicative of multiple heme-heme magnetic interactions. It was found that D. vulgaris Hildenborough is not capable of using nitrite as a terminal electron acceptor. These results indicate that in this organism the NiR is not involved in the dissimilative reduction of nitrite, as is the case with the other similar enzymes isolated so far. The possible role of this enzyme in the detoxification of nitrite and/or in the reduction of sulfite is discussed.

The 'strict' anaerobe Desulfovibrio gigas contains a membrane-bound oxygen-reducing respiratory chain

Sulfate-reducing bacteria are considered as strict anaerobic microorganisms, in spite of the fact that some strains have been shown to tolerate the transient presence of dioxygen. This report shows that membranes from Desulfovibrio gigas grown in fumarate/sulfate contain a respiratory chain fully competent to reduce dioxygen to water. In particular, a membrane-bound terminal oxygen reductase, of the cytochrome bd family, was isolated, characterized, and shown to completely reduce oxygen to water. This oxidase has two subunits with apparent molecular masses of 40 and 29 kDa. Using NADH or succinate as electron donors, the oxygen respiratory rates of D. gigas membranes are comparable to those of aerobic organisms (3.2 and 29 nmol O(2) min(-1) mg protein(-1), respectively). This 'strict anaerobic' bacterium contains all the necessary enzymatic complexes to live aerobically, showing that the relationships between oxygen and anaerobes are much more complex than originally thought.

In vitro and in vivo Leishmanicidal activity of 2-hydroxy-3-(3-methyl-2-butenyl)-1,4-naphthoquinone (lapachol)

This study aims to evaluate the in vitro and in vivo leishmanicidal activity of lapachol, a naphthoquinone found in the seeds and heartwood of certain tropical plants, and to compare its efficacy with a reference drug, sodium stibogluconate (Pentostam(R)). These compounds (0.0125-4.0 mg/mL) were evaluated in vitro against intracellular amastigotes of Leishmania (Viannia) braziliensis (LVb), then tested in an animal model (hamster) to try to reproduce the leishmanicidal activity. In vitro, lapachol exhibited an anti-amastigote effect, whereas in vivo it did not prevent the development of LVb-induced lesions at an oral dose of 300 mg/kg/day for 42 days. Pentostam(R) demonstrated a significant anti-amastigote effect in vitro for LVb and apparent clinical cure in vivo (60 mg/kg/day). However, it could not completely eradicate parasites from the tissues of infected animals. The observation that lapachol exerts leishmanicidal activity in vitro without offering significant protection against LVb-infected lesions in hamsters suggests that lapachol in vivo might possibly inhibit the microbicidal functioning of macrophages. Alternatively, it might be transformed into an inactive metabolite(s) or neutralized, losing its leishmanicidal activity. It is also possible that an optimal and sustained plasma level of the drug could not be achieved at the dose used in this study.

A blue non-heme iron protein from Desulfovibrio gigas

A novel iron-containing blue protein, named neelaredoxin, was isolated from the sulfate-reducing bacterium Desulfovibrio gigas. It is a monomeric protein with a molecular mass of 15 kDa containing two iron atoms/molecule. The N-terminal sequence of neelaredoxin has similarity to the second domain of desulfoferrodoxin, a protein purified from Desulfovibrio vulgaris Hildenborough. This finding supports the hypothesis that the gene coding for desulfoferrodoxin (rbo) might have arisen from a gene fusion [Brumlik, M. J., Leroy, G., Bruschi, M. & Voordouw, G. (1990) J. Bacteriol. 172, 7289-7292]. The visible spectrum exhibits a single band at 666 nm, responsible for the blue color of the protein, which is completely bleached upon reduction with sodium ascorbate. In the oxidized state the EPR spectrum is complex, exhibiting well-resolved features at g = 7.6, 7.0, 5.9, and 5.8 which are assigned to two high-spin (S = 5/2) mononuclear-iron (III) centers with different rhombic distortions (E/D approximately 0.05 and approximately 0.08). The two iron atoms contribute identically to the visible spectrum as judged from visible redox titrations, from which a reduction potential of +190 mV was determined for both iron sites at pH 7.5. At high pH the visible and the EPR spectra become pH-dependent with a pKa above 9: the 666-nm band shifts to 590 nm and the EPR signals are converted into a signal with gmax approximately 4.7. Neelaredoxin is readily reduced both by H2/hydrogenase/cytochrome c3 and by NADH/NADH-rubredoxin oxidoreductase.

Spectroscopic studies of cobalt and nickel substituted rubredoxin and desulforedoxin

The single iron site of rubredoxin was replaced by nickel and cobalt. The near-infrared/visible/UV spectra of these metal derivatives show ligand-field transitions and charge-transfer bands which closely resemble those of simple tetrathiolate complexes, indicating a tetrahedral arrangement of the sulfur cysteinyl ligands around the metal core. The 1H NMR spectra of the nickel and cobalt derivatives reveal extremely low-field contact shifted resonances of one proton intensity assigned to beta-CH2 and alpha-CH cysteinyl protons. Other well resolved resonances shifted out of the main protein spectral envelope are also observed and probably arise from contact plus pseudocontact shift mechanisms. Rubredoxins from different sulfate reducers were metal substituted and assignments of aliphatic protons are tentatively proposed, taking advantage of the amino acid sequence homologies. The present data is promising in terms of structural analysis of the coordination sphere of the metal core. It was also shown that replacement of the iron atom of desulforedoxin, a close analogue of rubredoxin, by cobalt and nickel was possible.

A seven-iron ferredoxin from the thermoacidophilic archaeon Desulfurolobus ambivalens

A seven-iron ferredoxin was isolated from aerobically grown cells of the hyperthermoacidophilic archaeon Desulfurolobus ambivalens (DSM 3772). The protein is monomeric, with an apparent molecular mass of 15 kDa and contains 7 iron atoms/molecule. The N-terminal sequence shows a large similarity (70% identity) with that of the ferredoxin isolated from the archaeon Sulfolobus acidocaldarius. The EPR characteristics in both the native (oxidized) and dithionite-reduced states of this protein allowed an unequivocal identification of a [3Fe-4S]1+/0 center, with a reduction potential of -270 +/- 20 mV, at pH 7.5. The protein also contains a [4Fe-4S]2+/1+ center with a very low reduction potential (Eo = -540 mV, pH 7.0), which yields a rhombic EPR spectrum upon reduction with sodium dithionite at high pH. The reduction potentials of both centers are slightly pH dependent between pH 6 and 9. The [3Fe-4S] ferredoxin center is able to accept electrons from pyruvate oxidase and NADH oxidase isolated from D. ambivalens. This ferredoxin is present in large amounts (at least 130 mg/kg wet cells), which allowed the unequivocal observation of oxidized [3Fe-4S] clusters in intact D. ambivalens cells.

Nitrite reductase from Desulfovibrio desulfuricans (ATCC 27774)--a heterooligomer heme protein with sulfite reductase activity

The membrane bound cytochrome c nitrite reductase from the sulfate reducer Desulfovibrio desulfuricans (ATCC 27774) was found to have a high specific activity in the reduction of sulfite, producing stoichiometric amounts of sulfide. The K(m) for sulfite in the MV+.:sulfite oxidoreductase assay is 0.75 mM, and the specific activity 2.06 mumolH2/min/mg. Visible and EPR spectroscopies studies indicate that the enzyme high-spin heme reacts with sulfite in the oxidised state, and that sulfide partially reduces the enzyme. The redoxcycled enzyme, using H2/Hydrogenase/MV+. as a reductant, is identical to the resting enzyme. This is the first time that a c-type nitrite reductase has been shown to reduce sulfite. These findings, besides revealing a new function for the nitrite reductase, raise a major question regarding the sulfur metabolism in the sulfate reducing bacteria as well as the cellular localization of the enzymatic activities involved in the dissimilatory reduction of sulfate. The purified nitrite reductase is a heterooligomer, containing two types of subunits of 62 kDa (+/- 5 kDa) and 18.8 kDa (+/- 1 kDa), and forms a complex or aggregate with a molecular mass of approximately 750 kDa.

Membrane-bound electron transfer chain of the thermohalophilic bacterium Rhodothermus marinus: a novel multihemic cytochrome bc, a new complex III

A novel multihemic cytochrome bc complex was isolated from the membranes of Rhodothermus marinus. It is a complex with a minimum of three subunits (43, 27, and 18 kDa), containing five low-spin heme centers of the B and C types, in a 1:4 ratio. All the C-type hemes are in the same subunit (27 kDa). Three distinct redox transitions, at 235, 80, and -45 mV, were observed by visible redox titrations. The first involves one B- and one C-type hemes, and in the other two transitions one and two C-type hemes are involved, respectively. Spectroscopic data strongly suggest that the two hemes intervening in the last transition are in van der Waals contact, yielding a split Soret band. Electron paramagnetic resonance spectra of the oxidized complex show resonances of five low-spin ferric heme centers. Upon reduction with ascorbate, all these resonances vanish and a new one attributed to the last pair of hemes appears. A [3Fe-4S]1+/0 center copurifies with this complex, having a high reduction potential of +140 mV. No Rieske-type centers are detected in R. marinus and no effect is observed in the respiratory rates when the typical bc1 complex inhibitors are present, suggesting that such a complex is absent in R. marinus [Pereira et al. (1994) FEBS Lett. 352, 327-330]. The newly isolated cytochrome bc complex has quinol:cytochrome c or high-potential iron-sulfur protein (HiPIP) oxidoreductase activity, being a functional analogue of the canonical bc1 complexes; i.e., it is the complex III in R. marinus. This complex plays a central role in this bacterium's electron-transfer chain, coupling the electron transfer between the quinols reduced by the dehydrogenases and the HiPIP, the final electron donor to the terminal oxidases [Pereira, M. M., Carita, J. N., and Teixeira, M. (1999) Biochemistry 38, 1276-1283].

Oxygen detoxification in the strict anaerobic archaeon Archaeoglobus fulgidus: superoxide scavenging by neelaredoxin

Archaeoglobus fulgidus is a hyperthermophilic sulphate-reducing archaeon. It has an optimum growth temperature of 83 degrees C and is described as a strict anaerobe. Its genome lacks any homologue of canonical superoxide (O2.-) dismutases. In this work, we show that neelaredoxin (Nlr) is the main O2.- scavenger in A. fulgidus, by studying both the wild-type and recombinant proteins. Nlr is a 125-amino-acid blue-coloured protein containing a single iron atom/molecule, which in the oxidized state is high spin ferric. This iron centre has a reduction potential of +230 mV at pH 7.0. Nitroblue tetrazolium-stained gel assays of cell-soluble extracts show that Nlr is the main protein from A. fulgidus which is reactive towards O2.-. Furthermore, it is shown that Nlr is able to both reduce and dismutate O2.-, thus having a bifunctional reactivity towards O2.-. Kinetic and spectroscopic studies indicate that Nlr's superoxide reductase activity may allow the cell to eliminate O2.- quickly in a NAD(P)H-dependent pathway. On the other hand, Nlr's superoxide dismutation activity will allow the cell to detoxify O2.- independently of the cell redox status. Its superoxide dismutase activity was estimated to be 59 U mg-1 by the xanthine/xanthine oxidase assay at 25 degrees C. Pulse radiolysis studies with the isolated and reduced Nlr proved unambiguously that it has superoxide dismutase activity; at pH 7.1 and 83 degrees C, the rate constant is 5 x 106 M-1 s-1. Besides the superoxide dismutase activity, soluble cell extracts of A. fulgidus also exhibit catalase and NAD(P)H/oxygen oxidoreductase activities. By putting these findings together with the entire genomic data available, a possible oxygen detoxification mechanism in A. fulgidus is discussed.