The role of ferric enterochelin esterase in enterochelin-mediated iron transport and ferrochelatase activity in Escherichia coli

A metaproteomics approach to elucidate host and pathogen protein expression during catheter-associated urinary tract infections (CAUTIs)

Long-term catheterization inevitably leads to a catheter-associated bacteriuria caused by multispecies bacterial biofilms growing on and in the catheters. The overall goal of the presented study was (1) to unravel bacterial community structure and function of such a uropathogenic biofilm and (2) to elucidate the interplay between bacterial virulence and the human immune system within the urine. To this end, a metaproteomics approach combined with in vitro proteomics analyses was employed to investigate both, the pro- and eukaryotic protein inventory. Our proteome analyses demonstrated that the biofilm of the investigated catheter is dominated by three bacterial species, that is, Pseudomonas aeruginosa, Morganella morganii, and Bacteroides sp., and identified iron limitation as one of the major challenges in the bladder environment. In vitro proteome analysis of P. aeruginosa and M. morganii isolated from the biofilm revealed that these opportunistic pathogens are able to overcome iron restriction via the production of siderophores and high expression of corresponding receptors. Notably, a comparison of in vivo and in vitro protein profiles of P. aeruginosa and M. morganii also indicated that the bacteria employ different strategies to adapt to the urinary tract. Although P. aeruginosa seems to express secreted and surface-exposed proteases to escape the human innate immune system and metabolizes amino acids, M. morganii is able to take up sugars and to degrade urea. Most interestingly, a comparison of urine protein profiles of three long-term catheterized patients and three healthy control persons demonstrated the elevated level of proteins associated with neutrophils, macrophages, and the complement system in the patient's urine, which might point to a specific activation of the innate immune system in response to biofilm-associated urinary tract infections. We thus hypothesize that the often asymptomatic nature of catheter-associated urinary tract infections might be based on a fine-tuned balance between the expression of bacterial virulence factors and the human immune system.

The Bordetella bfe system: growth and transcriptional response to siderophores, catechols, and neuroendocrine catecholamines

Ferric enterobactin utilization by Bordetella bronchiseptica and Bordetella pertussis requires the BfeA outer membrane receptor. Under iron-depleted growth conditions, transcription of bfeA is activated by the BfeR regulator by a mechanism requiring the siderophore enterobactin. In this study, enterobactin-inducible bfeA transcription was shown to be TonB independent. To determine whether other siderophores or nonsiderophore catechols could be utilized by the Bfe system, various compounds were tested for the abilities to promote the growth of iron-starved B. bronchiseptica and induce bfeA transcription. The BfeA receptor transported ferric salmochelin, corynebactin, and the synthetic siderophores TRENCAM and MECAM. Salmochelin and MECAM induced bfeA transcription in iron-starved Bordetella cells, but induction by corynebactin and TRENCAM was minimal. The neuroendocrine catecholamines epinephrine, norepinephrine, and dopamine exhibited a remarkable capacity to induce transcription of bfeA. Norepinephrine treatment of B. bronchiseptica resulted in BfeR-dependent bfeA transcription, elevated BfeA receptor production, and growth stimulation. Pyrocatechol, carbidopa, and isoproterenol were similarly strong inducers of bfeA transcription, whereas tyramine and 3,4-dihydroxymandelic acid demonstrated low inducing activity. The results indicate that the inducer structure requires a catechol group for function and that the ability to induce bfeA transcription does not necessarily correlate with the ability to stimulate bacterial growth. The expanded range of catechol siderophores transported by the BfeA receptor demonstrates the potential versatility of the Bordetella Bfe iron retrieval system. The finding that catecholamine neurotransmitters activate bfeA transcription and promote growth suggests that Bordetella cells can perceive and may benefit from neuroendocrine catecholamines on the respiratory epithelium.

Functions of the siderophore esterases IroD and IroE in iron-salmochelin utilization

The siderophore salmochelin is produced under iron-poor conditions bySalmonellaand many uropathogenicEscherichia colistrains. The production of salmochelin, a C-glucosylated enterobactin, is dependent on the synthesis of enterobactin and theiroBCDENgene cluster. AnE. coliIroD protein with an N-terminal His-tag cleaved cyclic salmochelin S4 to the linear trimer salmochelin S2, the dimer salmochelin S1, and the monomers dihydroxybenzoylserine and C-glucosylated dihydroxybenzoylserine (salmochelin SX, pacifarinic acid). The periplasmic IroE protein was purified as a MalE–IroE fusion protein. This enzyme degraded salmochelin S4 and ferric-salmochelin S4 to salmochelin S2 and ferric-salmochelin S2, respectively. InE. coli, uptake of ferric-salmochelin S4 was dependent on the cleavage by IroE, and independent of the FepBDGC ABC transporter in the cytoplasmic membrane. IroC, which has similarities to ABC-multidrug-resistance proteins, was necessary for the uptake of salmochelin S2 from the periplasm into the cytoplasm. IroE did not function as a classical binding protein since salmochelin S2 was taken up in the absence of a functional IroE protein. IroC mediated the uptake of iron via enterobactin in afepBmutant. IroE was also necessary in this case for the uptake of ferric-enterobactin, which indicated that only the linear degradation products of enterobactin were taken up via IroC. PfeE, thePseudomonas aeruginosaIroE homologue, was cloned, and its enzymic activity was shown to be very similar to that of IroE. It is suggested that homologues in other bacteria are also periplasmic IroE-type esterases of siderophores.

Chrysobactin-dependent iron acquisition in Erwinia chrysanthemi. Functional study of a homolog of the Escherichia coli ferric enterobactin esterase

Under iron limitation, the plant pathogen Erwinia chrysanthemi produces the catechol-type siderophore chrysobactin, which acts as a virulence factor. It can also use enterobactin as a xenosiderophore. We began this work by sequencing the 5'-upstream region of the fct-cbsCEBA operon, which encodes the ferric chrysobactin receptor and proteins involved in synthesis of the catechol moiety. We identified a new iron-regulated gene (cbsH) transcribed divergently relative to the fct gene, the translated sequence of which is 45.6% identical to that of Escherichia coli ferric enterobactin esterase. Insertions within this gene interrupt the chrysobactin biosynthetic pathway by exerting a polar effect on a downstream gene with some sequence identity to the E. coli enterobactin synthase gene. These mutations had no effect on the ability of the bacterium to obtain iron from enterobactin, showing that a functional cbsH gene is not required for iron removal from ferric enterobactin in E. chrysanthemi. The cbsH-negative mutants were less able to utilize ferric chrysobactin, and this effect was not caused by a defect in transport per se. In a nonpolar cbsH-negative mutant, chrysobactin accumulated intracellularly. These defects were rescued by the cbsH gene supplied on a plasmid. The amino acid sequence of the CbsH protein revealed characteristics of the S9 prolyl oligopeptidase family. Ferric chrysobactin hydrolysis was detected in cell extracts from a cbsH-positive strain that was inhibited by diisopropyl fluorophosphate. These data are consistent with the fact that chrysobactin is a d-lysyl-l-serine derivative. Mössbauer spectroscopy of whole cells at various states of (57)Fe-labeled chrysobactin uptake showed that this enzyme is not required for iron removal from chrysobactin in vivo. The CbsH protein may therefore be regarded as a peptidase that prevents the bacterial cells from being intracellularly iron-depleted by chrysobactin.

Disruption of tonB in Bordetella bronchiseptica and Bordetella pertussis prevents utilization of ferric siderophores, haemin and haemoglobin as iron sources

The Bordetella bronchiseptica tonB gene was cloned by detection of a chromosomal restriction fragment hybridizing with each of two degenerate oligonucleotides that corresponded to Pro-Glu and Pro-Lys repeats characteristic of known TonB proteins. The tonB(Bb) gene was situated upstream of exbB and exbD homologues and downstream of a putative Fur-regulated promoter. Hybridization results indicated that the tonB operon and flanking regions were highly conserved between B. bronchiseptica, Bordetella pertussis and Bordetella parapertussis. Disruption of tonB in B. bronchiseptica resulted in inability to grow in iron-limiting media, and inability to utilize alcaligin, enterobactin, ferrichrome, desferroxamine B, haemin and haemoglobin. Although it was not possible to inactivate tonB in a clinical B. pertussis isolate, tonB was disrupted in a laboratory B. pertussis strain previously selected for the ability to grow on Luria-Bertani medium. This B. pertussis tonB mutant shared a similar iron complex utilization deficient phenotype with the B. bronchiseptica tonB mutant. The B. bronchiseptica tonB operon present on a plasmid did not complement an Escherichia coli tonB mutant, but inefficient reconstitution of enterobactin utilization was observed in one fepA mutant harbouring plasmid copies of the B. pertussis fepA homologue and tonB(Bb) operon.

An iron-regulated outer-membrane protein specific to Bordetella bronchiseptica and homologous to ferric siderophore receptors

The bfrA (Bordetella bronchiseptica ferric iron repressed outer-membrane protein) gene was cloned from Bordetella bronchiseptica by screening a library of TnphoA insertion mutants for iron-repressed fusions to phoA. The bfrA gene encoded an 80 kDa outer-membrane protein with a high level of amino acid sequence identity to several bacterial proteins belonging to the family of Ton B-dependent outer-membrane receptors. BfrA was especially homologous to Cir of Escherichia coli, IrgA of Vibrio cholerae and to three previously characterized ferric enterobactin receptors. DNA hybridization results indicated that bfrA was not present in other Bordetella species. Expression of the bfrA gene was induced by low iron availability from a promoter overlapped by a sequence resembling a consensus Fur-binding sequence, and bfrA expression was derepressed in a B. bronchiseptica fur mutant. Utilization of the Bordetella siderophore alcaligin and the exogenous siderophore enterobactin was unaffected in bfrA mutants. Upon attempting to find the specificity of BfrA, 2,3-dihydroxybenzoylserine (DHBS) was shown to be utilized in a bfeA (Bordetella ferric enterobactin receptor gene)-dependent manner by B. bronchiseptica and B. pertussis. In addition, the hydroxamate siderophores ferrichrome and desferrioxamine B, and the iron source haemin were shown to be utilized independently of bfeA and bfrA in B. bronchiseptica and B. pertussis.

Identification of enterobactin and linear dihydroxybenzoylserine compounds by HPLC and ion spray mass spectrometry (LC/MS and MS/MS)

The production of catecholate siderophores was studied in some selected species of Enterobacter (Enterobacteriaceae). The extracted catecholates were separated as iron-free compounds by HPLC on a C18 reversed-phase column using methanol/0.1% phosphoric acid or methanol/0.1% formic acid as a solvent system and identified by ion spray mass spectrometry (LC/MS, MS/MS). Five catecholate compounds were identified which include 2,3-dihydroxybenzoylserine, its linear dimer and trimer, the cyclic enterobactin and an unidentified isomer of enterobactin. In addition, a new large-scale method for the isolation of catecholate siderphores is described which is based on adsorption on XAD-2 and subsequent purification on Sephadex LH20.

Antibacterial effect of the scandium complex of enterochelin. Studies of the mechanism of action

There is good evidence to show that ferric enterochelin is an essential growth factor for a number of Gram-negative pathogenic bacteria exposed to the host iron binding proteins, transferrin and lactoferrin. Tests of nineteen complexes of enterochelin as potential antibacterial agents showed that only those containing either indium (In3+) or scandium (Sc3+) inhibited bacterial growth. In this study, further evidence is presented which demonstrates a competition between the Sc3+ and Fe3+ complexes. The uptake of both complexes is energy dependent and is also repressed in iron-replete cells. The Sc3+ complex accumulates within the cells at 20% of the rate of the Fe3+ complex. The main components of the ferric enterochelin transport system are required for the transport of the Sc3+ complex although some Sc3+ appears to enter the cell by another route. The accumulation, within the cell, of 14C-labelled enterochelin complexes depends on the growth medium. The relationship of the size of the metal ion to the biological activity of the complex is discussed and possible mechanisms of action of the Sc3+ complex are considered.

Critical examination for the presence of a low molecular weight fraction in serum iron

Native human pool serum and individual sera were ultrafiltered by Pellicon ultrafilters (Millipore) and the ultrafiltrates were extracted by an ammonium pyrrolidinedithiocarbamate/methylisobutylketone system after addition of different internal iron standards to three of four identical ultrafiltrates. The extracts were examined for iron content by atomic absorption spectrometry. During ultrafiltration pH 7.4 was maintained by a constant atmosphere of a CO2/air mixture. Low molecular weight iron in native human sera from normal, normal orally iron substituted and siderotic individuals was found to be less than 0.05 micrograms/100 ml. Elevating serum citrate to 3-fold normal had no effect on this result. More iron became ultrafiltrable if the serum pH were lowered by citric acid as compared with hydrochloric acid.

The role of ferrichrome reductase in iron metabolism of Ustilago sphaerogena

Ferrichrome, the ferric ionophore for Ustilago sphaerogena, can serve as a source of iron for the enzyme ferrochelatase (protoheme ferrolyase, EC 4.99.1.1) in this organism, but only after enzymatic removal of the iron from its carrier. U. sphaerogena contains a specific ferrichrome reductase (NADH:ferrichrome oxidoreductase) which catalyzes cellular dissociation of the complex by reduction of the metal to the ferrous state. A spectrophotometric assay was developed based on trapping of the ferrous ion produced by ferrozine. There is an apparent inhibition by oxygen which is thought to be due to re-oxidation of the metal under the assay conditions. The close structural analogue, ferrichrome A, is not a substrate, nor is the ester type siderochrome ferric hexahydro-N,N',N"-triacetylfusarinine C. Aluminum desferriferrichrome is inhibitory. The importance of this enzyme for the metabolism of iron in this organism is discussed.

Enzymatic hydrolysis of enterochelin and its iron complex in Escherichia Coli K-12. Properties of enterochelin esterase

Properties of the enzyme which hydrolyses enterochelin (a cyclic trimer of 2,3-dihydroxy-N-benzoyl-L-serine) to 2,3-dihydroxybenzoylserine have been investigated with a view to resolving discrepancies between earlier reports. Enterochelin esterase, previously reported to consists of two components (O'Brien, I.G., Cox, G.B. and Gibson, F. (1971) Biochim. Biophys. Acta 237, 537-549), has been shown to be fully active in the absence of the so-called A component. The hydrolase described previously (Bryce, G.F. and Brot, N. (1972) Biochemistry 11, 1708-1715) as being able to break down enterochelin but not its iron complex, ferric-enterochelin, appears to be identical with the B component of enterochelin esterase. The single component enterochelin esterase corresponding to what was previously described as component B, hydrolyses both enterochelin and ferric-enterochelin. Under the assay conditions used, enterochelin is hydrolysed 2.5 times faster than the complex. Enzymatic activity is inhibited by N-ethylmaleimide and is lost rapidly at 37 degrees C. Activity is stabilized in the presence of ferric-enterochelin, enterochelin, dithiothreitol or certain protein fractions.

Purification and characterization of the membrane-bound ferrochelatase from Spirillum itersonii

The membrane-bound enzyme ferrochelatase (protoheme ferro-lyase, EC 4.99.1.1) was purified from isolated membrane fragments of Spirillum itersonii approximately 490-fold. Purification was achieved by solubilization with chaotropic salts followed by ammonium sulfate fractionation, diethylaminoethyl-cellulose chromatography, and gel filtration on Sephadex G-200. The purified enzyme has an apparent minimum molecular weight of approximately 50,000, as determined by gel filtration in the presence of 0.1% Brij 35 and 1 mM dithiothreitol but forms high-molecular-weight aggregates in the absence of detergent. Purified ferrochelatase is strongly stimulated in the presence of copper. The apparent Km for Fe2+ is 20 micrometer in the absence of copper and 9.5 micrometer in the presence of 20 micrometer CuCl2. The apparent Km for protoporphyrin is 50 micrometer, and it is unaltered by copper. Ferrochelatase has a single pH optimum of 7.50, and it is inhibited 50% by 20 micrometer heme. Certain divalent cations and sulfhydryl reagents also inhibit the enzyme.

Reduction of iron and synthesis of protoheme by Spirillum itersonii and other organisms

Membranes from Spirillum itersonii reduce ferric iron to ferrous iron with reduced nicotinamide adenine dinucleotide or succinate as a source of reductant. Iron reduction was measured spectrophotometrically at 562 nm using ferrozine, which chelates ferrous iron specifically. Reduced nicotinamide adenine dinucleotide or succinate was also effective as a source of iron. The effects of respiratory inhibitors suggested that reduction of iron occurs at one or more sites on the respiratory chain before cytochrome c. Reduction of iron and synthesis of protoheme with the physiological reductants were also observed with crude extracts of other bacteria, including Rhodopseudomonas spheroides, Rhodopseudomonas capsulata, Paracoccus denitrificans, and Escherichia coli. The effect of oxygen upon reduction of iron and formation of protoheme was examined with membranes from S. itersonii, using succinate as a source of reductant. Both systems were inhibited by oxygen, but this effect was completely reversed by addition of antimycin A. We conclude that reduced components of the respiratory chain serve as reductants for ferric iron, but with oxygen present they are oxidized preferentially by the successive members of the chain. This could be a mechanism for regulating synthesis of heme and cytochrome by oxygen.

Fungal ornithine esterases: relationship to iron transport

Extracts of Fusarium roseum (ATCC 12822) contain an enzyme which hydrolyzes the ornithine ester bonds of fusarinine C, a cyclic trihydroxamic acid produced by this organism. The methyl ester of Ndelta-dinitrophenyl-L-ornithine is also a substrate for the enzyme, and an assay was devised using this substrate. The enzyme exhibits a sharp maximum of activity at pH 7.5 and is extremely temperature sensitive. It is strongly inhibited by HgCl2 and p-chloromercuribenzoate, and it is competitively inhibited by Ndelta-dinitrophenyl-D-ornithine methyl ester (Ki = 0.3mM). Methyl esters of glycine, L-alanine, dinitrophenyl-L-alanine, dinitrophenyl-beta-alanine, and Ndelta-dinitrophenyl-Nalpha-acetyl-L-ornithine are not substrates, although Nepsilon-dinitrophenyl-L-lysine methyl ester is as effective as the ornithine derivative. Nonspecific lipases do not hydrolyze ornithine esters, nor does trypsin. The three ester bonds of fusarinine C are progressively hydrolyzed by the enzyme to eventually yield the monomer, fusarinine. The ferric chelate of fusarinine C is not hydrolyzed. An enzyme from Penicillium sp. was isolated with identical properties toward Nbeta-dinitro-phenyl-L-ornithine methyl ester as substrate. It also hydrolyzes N,N',N"-triacetylfusarinine C, a cyclic trihydroxamate containing Nalpha-acetylornithine ester bonds, which is produced by this organism. This substrate is hydrolyzed to Nalpha-acetylfusarine. In contrast to the Fusarium enzyme, this enzyme is fully active toward the ferric trihydroxamate chelate. However, replacement of iron by aluminum leads to a completely inactive substrate. Production of the enzyme is severely suppressed by iron in the growth medium. It is proposed that these specific ornithylesterases provide a mechanism of cellular iron release by hydrolysis of the ferric ionophores, and that an iron-exchange step occurs prior to, and is a prerequisite for, hydrolysis of the ester bonds.

Derepression of certain aromatic amino acid biosynthetic enzymes of Escherichia coli K-12 by growth in Fe3+-deficient medium

3-Deoxy-arabino-heptulosonic acid 7-phosphate synthase, prephenate dehydratase, tryptophan synthase, and 2,3-dihydroxybenzoylserine synthase enzyme activities are derepressed in wild-type Escherichia coli K-12 cells grown on Fe3+-deficient medium. This derepression is reversed when FeSO4 is added to the growth medium. Addition of shikimic acid to the Fe3+-deficient growth medium caused repression of the first three enzyme activities but not of 2,3-dihydroxybenzoylserine synthase activity. Addition of 2,3-dihydroxybenzoic acid to the Fe3+-deficient growth medium has no effect on any of the above-mentioned enzyme activities. The Fe3+ deficiency-mediated derepression of 3-deoxyarabino-heptulosonic acid 7-phosphate synthase activity is due to an elevation of the tyrosine-sensitive isoenzyme; the phenylalanine-sensitive isoenzyme is not derepressed under these conditions.

An other look at iron: role in host pathogen interaction

Iron, as participant of many biological processes is a prerequisite for life. Uptake, internal transport and storage by organisms is handled by highly specialized chemical systems endowed with strong metal binding affinities. Apart from the homeostatic function of iron-binding compounds they appear of significance for inter-species interactions. Thus, by tight binding transferrin withholds the iron from invading microorganisms required for their optimal growth. This bacteriostatic property of the iron transport protein is however partially overcome by small molecular substances synthesized by bacteria and successfully competing for the metal. The balance of such interaction is a complex one. Yet, strong evidence points to the crucial importance of the amount of iron offered by a host to infecting agents for determining the fate of bacterial disease.

The reconstitution of oxidase activity in membranes derived from a 5-aminolaevulinic acid-requiring mutant of Escherichia coli

1. The reconstitution of oxidase activity in cell-free extracts of a mutant of Escherichia coli K12Ymel, that require 5-aminolaevulinic acid for growth on non-fermentable carbon sources, is described. 2. The reconstitution is dependent on haematin or a haem extract from a prototrophic strain of E. coli, and the product of the reaction has been identified as NADH-reducible cytochrome b. 3. The requirement for haematin cannot be replaced by four other porphyrins. Coproporphyrin III does not inhibit the haematin-dependent reconstitution, mesoporphyrin IX and protoporphyrin IX apparently compete with haematin for a binding site on the cytochrome apoprotein(s) and deuteroporphyrin IX binds to cytochrome apoprotein(s) and cannot be subsequently replaced by haematin. 4. The properties of electron-transport particles from cell-free extracts of the mutant strain, grown aerobically in the presence or absence of 5-aminolaevulinic acid, are described. In the absence of 5-aminolaevulinic acid no detectable cytochromes are produced, and oxidase activities are lowered but there is no apparent effect on the activities of the NADH dehydrogenase and d-lactate dehydrogenase. 5. The reconstitution of oxidase activity by electron-transport particles from cells grown in the absence of 5-aminolaevulinic acid requires ATP and haematin, and the product of the reaction was identified as NADH-reducible cytochrome b. 6. It is concluded that the cytochrome apoproteins are synthesized and incorporated into the cytoplasmic membrane of E. coli in the absence of haem synthesis. The subsequent reconstitution of functional cytochrome(s) requires protohaem, but the nature of the side chain on the 2 and 4 positions of the porphyrin appears to be important.