BioMetals (v.22, #4)

Siderocalins: siderophore-binding proteins of the innate immune system by Matthew C. Clifton; Colin Corrent; Roland K. Strong (557-564).
Recent studies have revealed that the mammalian immune system directly interferes with siderophore-mediated iron acquisition through siderophore-binding proteins and that the association of certain siderophores, or siderophore modifications, with virulence is a direct response of pathogens to evade these defenses.
Keywords: Siderophores; Siderophore-binding proteins; Bacterial virulence; Bacterial iron acquisition; Innate immunity

Siderophores of Marinobacter aquaeolei: petrobactin and its sulfonated derivatives by Vanessa V. Homann; Katrina J. Edwards; Eric A. Webb; Alison Butler (565-571).
Siderophores are low molecular weight, high-affinity iron(III) ligands, produced by bacteria to solubilize and promote iron uptake under low iron conditions. Two prominent structural features characterize the majority of the marine siderophores discovered so far: (1) a predominance of suites of amphiphilic siderophores composed of an iron(III)-binding headgroup that is appended by one or two of a series of fatty acids and (2) a prevalence of siderophores that contain α-hydroxycarboxylic acid moieties (e.g., β-hydroxyaspartic acid or citric acid) which are photoreactive when coordinated to Fe(III). Variation of the fatty acid chain length affects the relative amphiphilicity within a suite of siderophores. Catecholate sulfonation is another structural variation that would affect the hydrophilicity of a siderophore. In addition to a review of the marine amphiphilic siderophores, we report the production of petrobactin disulfonate by Marinobacter aquaeolei VT8.
Keywords: Marine siderophores; Petrobactins; Bacillibactins; Sulfonated siderophores

Multiple roles of siderophores in free-living nitrogen-fixing bacteria by A. M. L. Kraepiel; J. P. Bellenger; T. Wichard; F. M. M. Morel (573-581).
Free-living nitrogen-fixing bacteria in soils need to tightly regulate their uptake of metals in order to acquire essential metals (such as the nitrogenase metal cofactors Fe, Mo and V) while excluding toxic ones (such as W). They need to do this in a soil environment where metal speciation, and thus metal bioavailability, is dependent on a variety of factors such as organic matter content, mineralogical composition, and pH. Azotobacter vinelandii, a ubiquitous gram-negative soil diazotroph, excretes in its external medium catechol compounds, previously identified as siderophores, that bind a variety of metals in addition to iron. At low concentrations, complexes of essential metals (Fe, Mo, V) with siderophores are taken up by the bacteria through specialized transport systems. The specificity and regulation of these transport systems are such that siderophore binding of excess Mo, V or W effectively detoxifies these metals at high concentrations. In the topsoil (leaf litter layer), where metals are primarily bound to plant-derived organic matter, siderophores extract essential metals from natural ligands and deliver them to the bacteria. This process appears to be a key component of a mutualistic relationship between trees and soil diazotrophs, where tree-produced leaf litter provides a living environment rich in organic matter and micronutrients for nitrogen-fixing bacteria, which in turn supply new nitrogen to the ecosystem.
Keywords: Azotobacter vinelandii ; Metalophore; Metal uptake; Molybdenum; Vanadium; Tungsten

Iron trafficking as an antimicrobial target by Rosanne E. Frederick; Jeffery A. Mayfield; Jennifer L. DuBois (583-593).
Iron is essential for the survival of most organisms. Microbial iron acquisition depends on multiple, sometimes complex steps, many of which are not shared by higher eukaryotes. Depriving pathogenic microbes of iron is therefore a potential antimicrobial strategy. The following minireview briefly describes general elements in microbial iron uptake pathways and summarizes some of the current work aiming at their medicinal inhibition.
Keywords: Iron; Antibiotic; Siderophore

This review describes the design, synthesis and evaluation of novel catechol based anchors for surface modification. The anachelin chromophore, the catecholate fragment of the siderophore anachelin from the cyanobacterium Anabaena cylindrica, allows for the immobilization of polyethylene glycol (PEG) on titania and glass surfaces thus rendering them protein resistant and antifouling. It is proposed that catecholate siderophores constitute a class of natural products useful for surface modification similar to dihydroxyphenylalanine and dopamine derived compounds found in mussel adhesive proteins. Second-generation dopamine derivatives featuring a quaternary ammonium group were found to be equally efficient in generating antifouling surfaces. The anachelin chromophore, merged via a PEG linker to the glycopeptide antibiotic vancomycin, allowed for the generation of antimicrobial surfaces through an operationally simple dip-and-rinse procedure. This approach offers an option for the prevention of nosocomial infections through antimicrobial implants, catheters and stents. Consequences for the mild generation of functional biomaterials are discussed and novel strategies for the immobilization of complex natural products, proteins and DNA on surfaces are presented.
Keywords: Natural products; Siderophores; Surface chemistry; Biomaterials; Organic synthesis

Coupled biogeochemical cycling of iron and manganese as mediated by microbial siderophores by Owen W. Duckworth; John R. Bargar; Garrison Sposito (605-613).
Siderophores, biogenic chelating agents that facilitate Fe(III) uptake through the formation of strong complexes, also form strong complexes with Mn(III) and exhibit high reactivity with Mn (hydr)oxides, suggesting a pathway by which Mn may disrupt Fe uptake. In this review, we evaluate the major biogeochemical mechanisms by which Fe and Mn may interact through reactions with microbial siderophores: competition for a limited pool of siderophores, sorption of siderophores and metal–siderophore complexes to mineral surfaces, and competitive metal-siderophore complex formation through parallel mineral dissolution pathways. This rich interweaving of chemical processes gives rise to an intricate tapestry of interactions, particularly in respect to the biogeochemical cycling of Fe and Mn in marine ecosystems.
Keywords: Dissolution; Ligand exchange; Sorption; Siderophore; Iron; Manganese

Siderophores as drug delivery agents: application of the “Trojan Horse” strategy by Ute Möllmann; Lothar Heinisch; Adolf Bauernfeind; Thilo Köhler; Dorothe Ankel-Fuchs (615-624).
The outer membrane permeability barrier is an important resistance factor of bacterial pathogens. In combination with drug inactivating enzymes, target alteration and efflux, it can increase resistance dramatically. A strategy to overcome this membrane-mediated resistance is the misuse of bacterial transport systems. Most promising are those for iron transport. They are vital for virulence and survival of bacteria in the infected host, where iron depletion is a defense mechanism against invading pathogens. We synthesized biomimetic siderophores as shuttle vectors for active transport of antibiotics through the bacterial membrane. Structure activity relationship studies resulted in siderophore aminopenicillin conjugates that were highly active against Gram-negative pathogens which play a crucial role in destructive lung infections in cystic fibrosis patients and in severe nosocomial infections. The mechanism of action and the uptake of the compounds via specific iron siderophore transport routes were demonstrated. The novel conjugates were active against systemic Pseudomonas aeruginosa infections in mice with ED50 values comparable to the quinolone ofloxacin and show low toxicity.
Keywords: Siderophores; Aminopenicillin conjugates; Permeability barrier; Efflux; Pseudomonas aeruginosa

Siderophore production by marine-derived fungi by Brian Holinsworth; Jessica D. Martin (625-632).
Siderophore production by marine-derived fungi has not been extensively explored. Three studies have investigated the ability of marine-derived fungi to produce siderophores in response to iron limitation [(Vala et al. in Indian J Mar Sci 29:339–340, 2000; Can J Microbiol 52:603–607, 2006); Baakza et al. in J Exp Mar Biol Ecol 311:1–9, 2004]. In all, 24 of 28 marine fungal strains were found to secrete hydroxamate or carboxylate siderophores; no evidence was found for production of catecholate siderophores. These studies did not determine the structures of the iron-binding compounds. More recently, a study of the natural products secreted by a marine Penicillium bilaii revealed that this strain produced the rare catecholate siderophore pistillarin when grown under relatively high iron concentrations (Capon et al. J Nat Prod 70:1746–1752, 2007). Additionally, the production of rhizoferrin by a marine isolate of Cunninghamella elegans (ATCC36112) is reported in this manuscript. The current state of knowledge about marine fungal siderophores is reviewed in light of these promising results.
Keywords: Siderophores; Fungi; Marine

The recent rise in drug resistance found amongst community acquired infections has sparked renewed interest in developing antimicrobial agents that target resistant organisms and limit the natural selection of immune variants. Recent discoveries have shown that iron uptake systems in bacteria and fungi are suitable targets for developing such therapeutic agents. The use of siderophore-drug conjugates as “Trojan Horse” drug delivery agents has attracted particular interest in this area. This review will discuss efforts in our research group to study the salmycin class of “Trojan Horse” antibiotics. Inspired by the natural design of the salmycins, a series of desferridanoxamine-antibiotic conjugates were synthesized and tested in microbial growth inhibition assays. The results of these studies will be related to understanding the role of drug release in siderophore-mediated drug delivery with implications for future siderophore-drug conjugate design.
Keywords: Siderophores; Salmycins; Danoxamine; Antibiotics; Resistance; “Trojan Horse”; Iron transport; Drug delivery

Siderophore sorption to clays by Patricia A. Maurice; Elizabeth A. Haack; Bhoopesh Mishra (649-658).
Siderophores are low molecular weight organic ligands exuded by some aerobic organisms and plants to acquire Fe under Fe-limited conditions. The hydroxamate siderophores may sorb to aluminosilicate clays through a variety of mechanisms depending upon the nature of the clay and of the siderophore along with solution conditions such as pH, ionic strength, and presence of metal cations. They may also affect metal binding to clays. Here, we review previous studies of siderophore sorption to aluminosilicate clays; briefly discuss how the techniques of X-ray diffractometry, Fourier-transform infrared spectroscopy, and X-ray absorption spectroscopy may be applied to such studies; review effects of siderophores on metal sorption to clays; and highlight some areas for future research.
Keywords: Siderophore; Clay; Sorption; Metals; XRD; FTIR; EXAFS

The role of siderophores in iron acquisition by photosynthetic marine microorganisms by Brian M. Hopkinson; François M. M. Morel (659-669).
The photosynthetic picocyanobacteria and eukaryotic microorganisms that inhabit the open ocean must be able to supply iron for their photosynthetic and respiratory needs from the subnanomolar concentrations available in seawater. Neither group appears to produce siderophores, although some coastal cyanobacteria do. This is interpreted as an adaptation to the dilute oceanic environment rather than a phylogenetic constraint, since there are cases in which related taxa from different environments have the capacity to produce siderophores. Most photosynthetic marine microorganisms are presumably, however, capable of accessing iron from strong chelates since the majority of dissolved iron in seawater is complexed by organic ligands, including siderophores. Rather than direct internalization of siderophores and other iron chelates, marine organisms primarily appear to use uptake pathways that involve a reduction step to free bound iron, closely coupled with transport into the cell.
Keywords: Siderophores; Phytoplankton; Ocean; Iron uptake; Cyanobacteria

FpvA is the primary outer membrane transporter required for iron acquisition via the siderophore pyoverdine (Pvd) in Pseudomonas aeruginosa. FpvA, like other ferrisiderophore transporters, consists of a membrane-spanning β-barrel occluded by a plug domain. The β-strands of the barrel are connected by large extracellular loops and periplasmic turns. Like some other TonB-dependent transporters, FpvA has a periplasmic domain involved in a signalling cascade that regulates expression of genes required for ferrisiderophore transport. Here, the structures of FpvA in different loading states are analysed in light of mutagenesis data. This analysis highlights the roles of different protein domains in Pvd-Fe uptake and the signalling cascade and reveals a strong correlation between Pvd-Fe transport and activation of the signalling cascade. It is likely that conclusions drawn for FpvA will be relevant to other TonB-dependent ferrisiderophore transport and signalling proteins.
Keywords: Siderophore; Iron uptake; Outer membrane transporter; Pyoverdine; FpvA; Structure

The redox hypothesis in siderophore-mediated iron uptake by James M. Harrington; Alvin L. Crumbliss (679-689).
The viability of iron(III/II) reduction as the initial step in the in vivo release of iron from its thermodynamically stable siderophore complex is explored.
Keywords: Iron bioavailability; Iron transport; Redox potentials; Redox kinetics

Salmochelin, the long-overlooked catecholate siderophore of Salmonella by Silke I. Müller; Marianne Valdebenito; Klaus Hantke (691-695).
Salmochelin is a C-glucosylated enterobactin produced by Salmonella species, uropathogenic and avian pathogenic Escherichia coli strains, and certain Klebsiella strains. It was the first glucosylated siderophore described. The glucosylation has been interpreted as a bacterial evasion mechanism against the mammalian catecholate siderophore-binding protein siderocalin (NGAL-lipocalin). The synthesis, excretion, and uptake of salmochelin requires five genes, iroBCDEN, and also the enterobactin biosynthesis and utilization system. Some salmochelin-producing strains also secrete microcins, which possess a C-terminal, linear glucosyl-enterobactin moiety. These microcins recognize the catecholate siderophore receptors IroN, Cir, Fiu, and FepA, and may inhibit the growth of competitors for catecholate siderophores.
Keywords: Salmochelin; Iron transport; Siderocalin; Salmonella ; C-glucosylation; Microcin E492