Ferric reductases in Escherichia coli: the contribution of the haemoglobin-like protein

Free flavins accelerate release of ferrous iron from iron storage proteins by both free flavin-dependent and -independent ferric reductases in Escherichia coli

Ferredoxin NADP⁺ oxidoreductase (Fpr) and oxygen-insensitive NAD(P)H nitroreductase (NfnB) are purified from Escherichia coli JM109 (E. coli JM109) as a predominant free flavin-independent ferric reductase. In the present study, we prepared natural iron storage proteins, E. coli ferritin A (FtnA) and bacterioferritin (Bfr), to show the effective ferrous iron release from these proteins by Fpr and NfnB in the presence of free flavins. Fpr and NfnB showed flavin reductase activity for flavin adenine dinucleotide (FAD), flavin mononucleotide (FMN) and riboflavin, and their ferrous iron release activities were positively associated with the catalytic efficiencies (k_cat/K_m) for individual flavins. The ferrous iron release activity of E. coli cell-free extracts was affected by flavin reductase activity of the extracts. The Butyl TOYOPEARL column chromatography of the extracts, on the basis of NAD(P)H-dependent flavin reductase activity, resulted in the separation of six active fractions containing Fpr, NfnB, NAD(P)H-quinone oxidoreductase (QOR), flavin reductase (Fre) or alkyl hydroperoxide reductase subunit F (AhpF) as major components. Like Fpr and NfnB, recombinant QOR, Fre, and AhpF showed flavin reductase activity and ferrous iron release activity in the presence of free flavins, indicating an association of flavin reductase activity with ferrous iron releasing activity. Taken together, both free flavin-dependent and free flavin-independent ferric reductases in E. coli require free flavins to mediate an electron transfer from NAD(P)H to ferric iron in the iron storage proteins for the effective ferrous iron release.

Maintenance of Nitric Oxide and Redox Homeostasis by the Salmonella Flavohemoglobin Hmp

Intracellular pathogens must resist the antimicrobial actions of nitric oxide (NO.) produced by host cells. To this end pathogens possess several NO.-metabolizing enzymes. Here we show that the flavohemoglobin Hmp is the principal enzyme responsible for aerobic NO. metabolism by Salmonella enterica serovar typhimurium. We further show that Hmp is required for Salmonella virulence in mice, in contrast to S-nitrosoglutathione reductase, flavorubredoxin, or cytochrome c nitrite reductase. Abrogation of murine-inducible NO. synthase restores virulence to hmp mutant bacteria. In the presence of nitrosative stress, Hmp-deficient Salmonella exhibits reduced NO. consumption, impaired growth, increased protein S-nitrosylation, and filamentous morphology. However, under aerobic conditions in the absence of nitrosative stress, elevated hmp expression increases S. typhimurium susceptibility to hydrogen peroxide. Both the heme binding and flavoreductase domains are required for resistance to NO., whereas the flavoreductase domain is responsible for iron-dependent susceptibility to oxidative stress. This provides a rationale for the regulation of hmp expression by the transcriptional repressor NsrR in response to both nitrosative stress and intracellular free iron concentration. The Hmp flavohemoglobin plays a central role in the response of Salmonella to nitrosative stress but requires precise regulation to avoid the exacerbation of oxidative stress that can result if electrons are shuttled to extraneous iron.

Bacterial hemoglobins and flavohemoglobins: versatile proteins and their impact on microbiology and biotechnology

In response to oxygen limitation or oxidative and nitrosative stress, bacteria express three kinds of hemoglobin proteins: truncated hemoglobins (tr Hbs), hemoglobins (Hbs) and flavohemoglobins (flavo Hbs). The two latter groups share a high sequence homology and structural similarity in their globin domain. Flavohemoglobin proteins contain an additional reductase domain at their C-terminus and their expression is induced in the presence of reactive nitrogen and oxygen species. Flavohemoglobins detoxify NO in an aerobic process, termed nitric oxide dioxygenase reaction, which protects the host from various noxious nitrogen compounds. Only a small number of bacteria express hemoglobin proteins and the best studied of these is from Vitreoscilla sp. Vitreoscilla hemoglobin (VHb) has been expressed in various heterologous hosts under oxygen-limited conditions and has been shown to improve growth and productivity, rendering the protein interesting for biotechnology industry. The close interaction of VHb with the terminal oxidases has been shown and this interplay has been proposed to enhance respiratory activity and energy production by delivering oxygen, the ultimate result being an improvement in growth properties.

Microbial ferric iron reductases

Almost all organisms require iron for enzymes involved in essential cellular reactions. Aerobic microbes living at neutral or alkaline pH encounter poor iron availability due to the insolubility of ferric iron. Assimilatory ferric reductases are essential components of the iron assimilatory pathway that generate the more soluble ferrous iron, which is then incorporated into cellular proteins. Dissimilatory ferric reductases are essential terminal reductases of the iron respiratory pathway in iron-reducing bacteria. While our understanding of dissimilatory ferric reductases is still limited, it is clear that these enzymes are distinct from the assimilatory-type ferric reductases. Research over the last 10 years has revealed that most bacterial assimilatory ferric reductases are flavin reductases, which can serve several physiological roles. This article reviews the physiological function and structure of assimilatory and dissimilatory ferric reductases present in the Bacteria, Archaea and Yeast. Ferric reductases do not form a single family, but appear to be distinct enzymes suggesting that several independent strategies for iron reduction may have evolved.

Microbial globins

Globins are an ancient and diverse superfamily of proteins. The globins of microorganisms were relatively ignored for many decades after their discovery by Warburg in the 1930s and rediscovery by Keilin in the 1950s. The relatively recent focus on them has been fuelled by recognition of their structural diversity and fine-tuning to fulfill (probably) discrete functions but particularly by the finding that a major role of certain globins is in protection from the stresses caused by exposure to nitric oxide (NO)--itself a molecule that has attracted intense curiosity recently. At least three classes of microbial globin are recognised, all having features of the classical globin protein fold. The first class is typified by the myoglobin-like haemprotein Vgb from the bacterium Vitreoscilla, which has attracted considerable attention because of its ability to improve growth and metabolism for biotechnological gain in a variety of host cells, even though its physiological function is not fully understood. The truncated globins are widely distributed in bacteria, microbial eukaryotes as well as plants and are characterised by being 20-40 residues shorter than Vgb. The polypeptide is folded into a two-over-two helical structure while retaining the essential features of the globin superfamily. Roles in oxygen and NO metabolism have been proposed. The third and best understood class comprises the flavohaemoglobins, which were first discovered and partly characterised in yeast. These are distinguished by the presence of an additional domain with binding sites for FAD and NAD(P)H. Widely distributed in bacteria, these proteins undoubtedly confer protection from NO and nitrosative stresses, probably by direct consumption of NO. However, a bewildering array of enzymatic capabilities and the presence of an active site in the haem pocket reminiscent of peroxidases hint at other functions. A full understanding of microbial globins promises advances in controlling the interactions of pathogenic bacteria with their animal and plant hosts, and manipulations of microbial oxygen transfer with biotechnological applications.

Flavohemoglobin detoxifies nitric oxide in aerobic, but not anaerobic, Escherichia coli. Evidence for a novel inducible anaerobic nitric oxide-scavenging activity

Nitric-oxide dioxygenase (NOD) and reductase (NOR) activities of flavohemoglobin (flavoHb) have been suggested as mechanisms for NO metabolism and detoxification in a variety of microbes. Mechanisms of NO detoxification were tested in Escherichia coli using flavoHb-deficient mutants and overexpressors. flavoHb showed negligible anaerobic NOR activity and afforded no protection to the NO-sensitive aconitase or the growth of anoxic E. coli, whereas the NOD activity and the protection afforded with O(2) were substantial. A NO-inducible, O(2)-sensitive, and cyanide-resistant NOR activity efficiently metabolized NO and protected anaerobic cells from NO toxicity independent of the NOR activity of flavoHb. flavoHb possesses nitrosoglutathione and nitrite reductase activities that may account for the protection it affords against these agents. NO detoxification by flavoHb occurs most effectively via O(2)-dependent NO dioxygenation.

Nitric-oxide dioxygenase activity and function of flavohemoglobins. sensitivity to nitric oxide and carbon monoxide inhibition

Widely distributed flavohemoglobins (flavoHbs) function as NO dioxygenases and confer upon cells a resistance to NO toxicity. FlavoHbs from Saccharomyces cerevisiae, Alcaligenes eutrophus, and Escherichia coli share similar spectra, O(2), NO, and CO binding kinetics, and steady-state NO dioxygenation kinetics. Turnover numbers (V(max)) for S. cerevisiae, A. eutrophus, and E. coli flavoHbs are 112, 290, and 365 NO heme(-1) s(-1), respectively, at 37 degrees C with 200 microm O(2). The K(M) values for NO are low and range from 0.1 to 0.25 microm. V(max)/K(M)(NO) ratios of 900-2900 microm(-1) s(-1) indicate an extremely efficient dioxygenation mechanism. Approximate K(M) values for O(2) range from 60 to 90 microm. NO inhibits the dioxygenases at NO:O(2) ratios of > or =1:100 and makes true K(M)(O(2)) values difficult to determine. High and roughly equal second order rate constants for O(2) and NO association with the reduced flavoHbs (17-50 microm(-1) s(-1)) and small NO dissociation rate constants suggest that NO inhibits the dioxygenase reaction by forming inactive flavoHbNO complexes. Carbon monoxide also binds reduced flavoHbs with high affinity and competitively inhibits NO dioxygenases with respect to O(2) (K(I)(CO) = approximately 1 microm). These results suggest that flavoHbs and related hemoglobins evolved as NO detoxifying components of nitrogen metabolism capable of discriminating O(2) from inhibitory NO and CO.

Steady-state and transient kinetics of Escherichia coli nitric-oxide dioxygenase (flavohemoglobin). The B10 tyrosine hydroxyl is essential for dioxygen binding and catalysis

Escherichia coli expresses an inducible flavohemoglobin possessing robust NO dioxygenase activity. At 37 degrees C, the enzyme shows a maximal turnover number (V(max)) of 670 s(-1) and K(m) values for NADH, NO, and O(2) equal to 4.8, 0.28, and approximately 100 microM, respectively. Individual reduction, ligand binding, and NO dioxygenation reactions were examined at 20 degrees C, where V(max) is approximately 94 s(-1). Reduction by NADH occurs in two steps. NADH reduces bound FAD with a rate constant of approximately 15 microM(-1) s(-1), and heme iron is reduced by FADH(2) with a rate constant of 150 s(-1). Dioxygen binds tightly to reduced flavohemoglobin, with association and dissociation rate constants equal to 38 microM(-1) s(-1) and 0.44 s(-1), respectively, and the oxygenated flavohemoglobin dioxygenates NO to form nitrate. NO also binds reversibly to reduced flavohemoglobin in competition with O(2), dissociates slowly, and inhibits NO dioxygenase activity at [NO]/[O(2)] ratios of 1:100. Replacement of the heme pocket B10 tyrosine with phenylalanine increases the O(2) dissociation rate constant approximately 80-fold and reduces NO dioxygenase activity approximately 30-fold, demonstrating the importance of the tyrosine hydroxyl for O(2) affinity and NO scavenging activity. At 37 degrees C, V(max)/K(m)(NO) is 2,400 microM(-1) s(-1), demonstrating that the enzyme is extremely efficient at converting toxic NO into nitrate under physiological conditions.

Expression of Alcaligenes eutrophus flavohemoprotein and engineered Vitreoscilla hemoglobin-reductase fusion protein for improved hypoxic growth of Escherichia coli

Expression of the vhb gene encoding hemoglobin from Vitreoscilla sp. (VHb) in several organisms has been shown to improve microaerobic cell growth and enhance oxygen-dependent product formation. The amino-terminal hemoglobin domain of the flavohemoprotein (FHP) of the gram-negative hydrogen-oxidizing bacterium Alcaligenes eutrophus has 51% sequence homology with VHb. However, like other flavohemoglobins and unlike VHb, FHP possesses a second (carboxy-terminal) domain with NAD(P)H and flavin adenine dinucleotide (FAD) reductase activities. To examine whether the carboxy-terminal redox-active site of flavohemoproteins can be used to improve the positive effects of VHb in microaerobic Escherichia coli cells, we fused sequences encoding NAD(P)H, FAD, or NAD(P)H-FAD reductase activities of A. eutrophus in frame after the vhb gene. Similarly, the gene for FHP was modified, and expression cassettes encoding amino-terminal hemoglobin (FHPg), FHPg-FAD, FHPg-NAD, or FHP activities were constructed. Biochemically active heme proteins were produced from all of these constructions in Escherichia coli, as indicated by their ability to scavenge carbon monoxide. The presence of FHP or of VHb-FAD-NAD reductase increased the final cell density of transformed wild-type E. coli cells approximately 50 and 75%, respectively, for hypoxic fed-batch culture relative to the control synthesizing VHb. Approximately the same final optical densities were achieved with the E. coli strains expressing FHPg and VHb. The presence of VHb-FAD or FHPg-FAD increased the final cell density slightly relative to the VHb-expressing control under the same cultivation conditions. The expression of VHb-NAD or FHPg-NAD fusion proteins reduced the final cell densities approximately 20% relative to the VHb-expressing control. The VHb-FAD-NAD reductase-expressing strain was also able to synthesize 2.3-fold more recombinant beta-lactamase relative to the VHb-expressing control.

Identification and characterization of a novel ferric reductase from the hyperthermophilic Archaeon Archaeoglobus fulgidus

Archaeoglobus fulgidus, a hyperthermophilic sulfate-reducing Archaeon, contains high Fe(3+)-EDTA reductase activity in its soluble protein fraction. The corresponding enzyme, which constitutes about 0.75% of the soluble protein, was purified 175-fold to homogeneity. Based on SDS-polyacrylamide gel electrophoresis, the ferric reductase consists of a single subunit with a M(r) of 18,000. The M(r) of the native enzyme was determined by size exclusion chromatography to be 40,000 suggesting that the native ferric reductase is a homodimer. The enzyme uses both NADH and NADPH as electron donors to reduce Fe(3+)-EDTA. Other Fe(3+) complexes and dichlorophenolindophenol serve as alternative electron acceptors, but uncomplexed Fe(3+) is not utilized. The purified enzyme strictly requires FMN or FAD as a catalytic intermediate for Fe(3+) reduction. Ferric reductase also reduces FMN and FAD, but not riboflavin, with NAD(P)H which classifies the enzyme as a NAD(P)H:flavin oxidoreductase. The enzyme exhibits a temperature optimum of 88 degrees C. When incubated at 85 degrees C, the enzyme activity half-life was 2 h. N-terminal sequence analysis of the purified ferric reductase resulted in the identification of the hypothetical gene, AF0830, of the A. fulgidus genomic sequence. The A. fulgidus ferric reductase shares amino acid sequence similarity with a family of NAD(P)H:FMN oxidoreductases but not with any ferric reductases suggesting that the A. fulgidus ferric reductase is a novel enzyme.

The flavohemoglobin of Escherichia coli confers resistance to a nitrosating agent, a "Nitric oxide Releaser," and paraquat and is essential for transcriptional responses to oxidative stress

Escherichia coli possesses a flavohemoglobin (Hmp), product of hmp, the first microbial globin gene to be sequenced and characterized at the molecular level. Although related proteins occur in numerous prokaryotes and eukaryotic microorganisms, the function(s) of these proteins have been elusive. Here we report construction of a defined hmp mutation and its use to probe Hmp function. As anticipated from up-regulation of hmp expression by nitric oxide (NO), S-nitrosoglutathione (GSNO) or sodium nitroprusside (SNP), the hmp mutant is hypersensitive to these agents. The hmp promoter is more sensitive to SNP and S-nitroso-N-penicillamine (SNAP) than is the soxS promoter, consistent with the role of Hmp in protection from reactive nitrogen species. Additional functions for Hmp are indicated by (a) parallel sensitivity of the hmp mutant to the redox-cycling agent, paraquat, (b) inability of the mutant to up-regulate fully the soxS and sodA promoters in response to oxidative stress caused by paraquat, GSNO and SNP, and (c) failure of the mutant to accumulate reduced paraquat radical after anoxic growth. We conclude that Hmp plays a role in protection from nitrosating agents and NO-related species and oxidative stress. This protective role probably involves direct detoxification of those species and sensing of NO-related and oxidative stress.

Nitrosative stress: metabolic pathway involving the flavohemoglobin

Nitric oxide (NO) biology has focused on the tightly regulated enzymatic mechanism that transforms L-arginine into a family of molecules, which serve both signaling and defense functions. However, very little is known of the pathways that metabolize these molecules or turn off the signals. The paradigm is well exemplified in bacteria where S-nitrosothiols (SNO)-compounds identified with antimicrobial activities of NO synthase-elicit responses that mediate bacterial resistance by unknown mechanisms. Here we show that Escherichia coli possess both constitutive and inducible elements for SNO metabolism. Constitutive enzyme(s) cleave SNO to NO whereas bacterial hemoglobin, a widely distributed flavohemoglobin of poorly understood function, is central to the inducible response. Remarkably, the protein has evolved a novel heme-detoxification mechanism for NO. Specifically, the heme serves a dioxygenase function that produces mainly nitrate. These studies thus provide new insights into SNO and NO metabolism and identify enzymes with reactions that were thought to occur only by chemical means. Our results also emphasize that the reactions of SNO and NO with hemoglobins are evolutionary conserved, but have been adapted for cell-specific function.

Expression and characterization of a heme oxygenase (Hmu O) from Corynebacterium diphtheriae. Iron acquisition requires oxidative cleavage of the heme macrocycle

A full-length heme oxygenase gene from the pathogenic bacterium Corynebacterium diphtheriae has been subcloned and expressed in Escherichia coli. The enzyme is expressed at high levels as a soluble catalytically active protein that results in the accumulation of biliverdin within the E. coli cells. The purified heme oxygenase forms a 1:1 complex with heme (Kd = 2.5 +/- 1 microM) and has hemeprotein spectra similar to those previously reported for the purified eukaryotic heme oxygenases. In the presence of an E. coli NADPH-dependent reductase isolated during the purification of Hmu O, the heme-Hmu O complex is catalytically turned over to yield biliverdin IXalpha and carbon monoxide. A number of redox partners were investigated for their ability to reconstitute Hmu O activity in vitro. Of these the most efficient appeared to be the recombinant NADH-dependent putidaredoxin/putidaredoxin reductase from Pseudomonas putida. As with the E. coli NADPH-dependent reductase the final products of the reaction were biliverdin IXalpha and carbon monoxide. This is the first bacterial heme oxygenase to be described to date. The close relationship between iron acquisition and pathogenesis suggests that the release of iron from heme by heme oxygenase may play a crucial role in the pathogenicity of C. diphtheriae.

Escherichia coli flavohaemoglobin (Hmp) reduces cytochrome c and Fe(III)-hydroxamate K by electron transfer from NADH via FAD: sensitivity of oxidoreductase activity to haem-bound dioxygen

Escherichia coli flavohaemoglobin (Hmp) reduced purified mitochondrial cytochrome c aerobically in a reaction that was not substantially inhibited by superoxide dismutase, demonstrating that superoxide anion, the product of O2 reduction by Hmp, did not contribute markedly to cytochrome c reduction. Cytochrome c was reduced by Hmp even in the presence of 0.5 mM CO, when the haem B was locked in the ferrous, low-spin state, demonstrating that electron transfer to cytochrome c from NADH was via FAD, not haem. Hmp also reduced the ferrisiderophore complex Fe(III)-hydroxamate K from Rhizobium leguminosarum bv. viciae anaerobically in a CO-insensitive manner, but at low rates and with low affinity for this substrate. The NADH-cytochrome c oxidoreductase activity of Hmp was slightly sensitive to the binding and reduction of O2 at the haem. The Vmax of cytochrome c reduction fell from 7.1 s-1 in the presence of 0.5 mM CO to 5.0 s-1 in the presence of 100 microM O2, with no significant change in K(m) for cytochrome c (6.8 to 7.3 microM, respectively). O2 at near-micromolar concentrations diminished cytochrome c reduction to a similar extent as did 100 microM O2. Thus, Hmp acts as a reductase of broad specificity, apparently without involvement of electron transfer via the globin-like haem. These data are consistent with the hypothesis that Hmp could act as an intracellular sensor of O2 since, in the absence of O2, electron flux from FAD to other electron acceptors increases. However, the nature of such acceptors in vivo is not known and alternative models for O2 sensing are also considered.

Nitric oxide, nitrite, and Fnr regulation of hmp (flavohemoglobin) gene expression in Escherichia coli K-12

Escherichia coli possesses a soluble flavohemoglobin, with an unknown function, encoded by the hmp gene. A monolysogen containing an hmp-lacZ operon fusion was constructed to determine how the hmp promoter is regulated in response to heme ligands (O2, NO) or the presence of anaerobically utilized electron acceptors (nitrate, nitrite). Expression of the phi (hmp-lacZ)1 fusion was similar during aerobic growth in minimal medium containing glucose, glycerol, maltose, or sorbitol as a carbon source. Mutations in cya (encoding adenylate cyclase) or changes in medium pH between 5 and 9 were without effect on aerobic expression. Levels of aerobic and anaerobic expression in glucose-containing minimal media were similar; both were unaffected by an arcA mutation. Anaerobic, but not aerobic, expression of phi (hmp-lacZ)1 was stimulated three- to four-fold by an fnr mutation; an apparent Fnr-binding site is present in the hmp promoter. Iron depletion of rich broth medium by the chelator 2'2'-dipyridyl (0.1 mM) enhanced hmp expression 40-fold under anaerobic conditions, tentatively attributed to effects on Fnr. At a higher chelator concentration (0.4 mM), hmp expression was also stimulated aerobically. Anaerobic expression was stimulated 6-fold by the presence of nitrate and 25-fold by the presence of nitrite. Induction by nitrate or nitrite was unaffected by narL and/or narP mutations, demonstrating regulation of hmp by these ions via mechanisms alternative to those implicated in the regulation of other respiratory genes. Nitric oxide (10 to 20 microM) stimulated aerobic phi (hmp-lacZ)1 activity by up to 19-fold; soxS and soxR mutations only slightly reduced the NO effect. We conclude that hmp expression is negatively regulated by Fnr under anaerobic conditions and that additional regulatory mechanisms are involved in the responses to oxygen, nitrogen compounds, and iron availability. Hmp is implicated in reactions with small nitrogen compounds.

Reactions of the Escherichia coli flavohaemoglobin (Hmp) with NADH and near-micromolar oxygen: oxygen affinity of NADH oxidase activity

The soluble flavohaemoglobin (Hmp) of Escherichia coli, product of the hmp gene, contains haem B and FAD in a single polypeptide of molecular mass 44 kDa. The function of this protein (and of the similar proteins identified in several bacteria and yeast) is unknown, but the observation that the binding of oxygen to haem modulates the reduction level of FAD has suggested that Hmp could act as an oxygen sensor. Here, stopped-flow, rapid-scan spectroscopy has shown that the oxidized protein reacts rapidly with NADH to form an oxygenated species, even when efforts are made to reduce oxygen concentrations to sub-micromolar levels, suggesting a high affinity for this ligand. As is the case at high oxygen concentrations (130 microM), oxygenated species formation was kinetically and spectrally heterogeneous. Between 12 ms and 1 s after mixing, following transient formation of the deoxy form and its reaction with dioxygen, a steady-state level of the oxygenated species was attained. During the oxygenated steady state, the flavin remained largely oxidized, as observed previously at 130 microM oxygen. Hmp is an NADH oxidase; on exhaustion of oxygen by reduction (in < 10 s under these conditions), the oxygenated species disappeared to generate the deoxy Fe(II) haem, whereupon the flavin was reduced. The affinity for oxygen during NADH oxidation was measured by continuous dual-wavelength monitoring of the deoxygenation of oxymyoglobin. The Km for oxygen was 2.6 microM, much higher than the Km values determined, using the same method, for the membrane-bound terminal oxidases cytochromes bo' and bd. These results show that the oxidase activity of Hmp, but not necessarily oxygen binding, would be minimal at oxygen concentrations that limit terminal oxidase function.

The flavohaemoglobin (HMP) of Escherichia coli generates superoxide in vitro and causes oxidative stress in vivo

Purified flavohaemoglobin (HMP) of Escherichia coli reduces Fe(III) in a superoxide dismutase (SOD)-sensitive reaction, demonstrating superoxide anion generation during aerobic NADH oxidation. In vivo, sodA-lacZ fusion activity was increased 3-fold by introducing plasmid pPL341, containing the hmp gene, or by growth with paraquat. The effects were additive and SOXS-dependent. Thus HMP activity causes oxidative stress in vivo. Activities of sodA-lacZ and hmp-lacZ fusions were stimulated in a himA mutant, demonstrating repression of both promoters by integration host factor (IHF), but the effects of pPL341 on sodA-lacZ activity were not due to titration of IHF by the hmp promoter.

Oxygen reactions with bacterial oxidases and globins: binding, reduction and regulation

Oxygen is favoured as terminal electron acceptor in aerobic and facultative microorganisms because of its appropriate physical state, satisfactory solubility and its desirable combinations of kinetic and thermodynamic properties. Oxygen is generally reduced by four electrons to yield oxygen, but there are important biological consequences of, and roles for, the partial reduction to superoxide and peroxide. Complex and multiple regulatory networks ensure (i) the utilization of oxygen in preference to other oxidants, (ii) the synthesis of oxygen-consuming enzymes with appropriate properties (particularly affinity for the ligand), and (iii) appropriate cellular protection in the event of oxidative stress. This contribution reviews the terminal respiratory oxidases of selected Gram-negative bacteria and microbial haemoglobin-like proteins. Recent studies of the cytochrome bd-type oxidases of Escherichia coli and Azotobacter vinelandii suggest that, despite probable similarity at the amino acid level, the reactivities of these oxidases with oxygen are strikingly different. The respiratory protection afforded to nitrogenase in the obligately aerobic diazotroph A. vinelandii by the cytochrome bd complex appears to be accompanied by, and may be the result of, a low affinity for oxygen and a high Vmax. The poorly characterized cytochrome o-containing oxidase in this bacterium is not required for respiratory protection. In E. coli, the cytochrome bd-type oxidase has a remarkably high affinity for oxygen, consistent with the view that this is an oxygen-scavenging oxidase utilized under microaerobic conditions. The demonstration of substrate (i.e. oxygen) inhibition in this complex suggests a mechanism whereby wasteful electron flux through a non-proton-pumping oxidase is avoided at higher dissolved oxygen tensions. The demonstration of two ligand-binding sites (haems d and b595) in oxidases of this type suggests plausible mechanisms for this phenomenon. In E. coli, assembly of the cytochrome bd-type oxidase (and of periplasmic cytochromes b and c) requires the presence of an ABC transporter, which may serve to export haem or some "assembly factor" to the periplasm. There is at least one additional oxygen-consuming protein in E. coli-the flavohaemoglobin encoded by the hmp gene. Globin-like proteins are also widely distributed in other bacteria, fungi and protozoa, but most have unknown functions. The function of HMP and the related chimaeric flavohaemoglobins in other bacteria and yeast is unknown; one of several possibilities for HMP is that its relatively low affinity for oxygen during turnover with NADH as substrate could enable it to function as a sensor of failing (or rising) cytoplasmic oxygen concentrations.