Siderophores (Greek: "iron carrier") are small, high-affinity iron-chelating compounds that are secreted by microorganisms such as bacteria and fungi. They help the organism accumulate iron. Although a widening range of siderophore functions is being appreciated, siderophores are among the strongest known (highest affinity) Fe3+ binding agents. Phytosiderophores are siderophores produced by plants.
Scarcity of soluble iron Despite being one of the most abundant elements in the Earth's crust, iron is not readily bioavailable. In most aerobic environments, such as the soil or sea, iron exists in the ferric (Fe3+) state, which tends to form insoluble rust-like solids. To be effective, nutrients must not only be available, they must be soluble. Microbes release siderophores to scavenge iron from these mineral phases by formation of soluble Fe3+ complexes that can be taken up by active transport mechanisms. Many siderophores are nonribosomal peptides, although several are biosynthesised independently.
Siderophores are also important for some pathogenic bacteria for their acquisition of iron. In mammalian hosts, iron is tightly bound to proteins such as hemoglobin, transferrin, lactoferrin and ferritin. The strict homeostasis of iron leads to a free concentration of about 10−24 mol L−1, hence there are great evolutionary pressures put on pathogenic bacteria to obtain this metal. For example, the anthrax pathogen Bacillus anthracis releases two siderophores, bacillibactin and petrobactin, to scavenge ferric ion from iron containing proteins. While bacillibactin has been shown to bind to the immune system protein siderocalin, petrobactin is assumed to evade the immune system and has been shown to be important for virulence in mice.
Siderophores are amongst the strongest binders to Fe3+ known, with enterobactin being one of the strongest of these. Because of this property, they have attracted interest from medical science in metal chelation therapy, with the siderophore desferrioxamine B gaining widespread use in treatments for iron poisoning and thalassemia.
Besides siderophores, some pathogenic bacteria produce hemophores (heme binding scavenging proteins) or have receptors that bind directly to iron/heme proteins. In eukaryotes, other strategies to enhance iron solubility and uptake are the acidification of the surroundings (e.g. used by plant roots) or the extracellular reduction of Fe3+ into the more soluble Fe2+ ions.
Structure Siderophores usually form a stable, hexadentate, octahedral complex preferentially with Fe3+ compared to other naturally occurring abundant metal ions, although if there are fewer than six donor atoms water can also coordinate. The most effective siderophores are those that have three bidentate ligands per molecule, forming a hexadentate complex and causing a smaller entropic change than that caused by chelating a single ferric ion with separate ligands. Fe3+ is a strong Lewis acid, preferring strong Lewis bases such as anionic or neutral oxygen atoms to coordinate with. Microbes usually release the iron from the siderophore by reduction to Fe2+ which has little affinity to these ligands.
Siderophores are usually classified by the ligands used to chelate the ferric iron. The major groups of siderophores include the catecholates (phenolates), hydroxamates and carboxylates (e.g. derivatives of citric acid). Citric acid can also act as a siderophore. The wide variety of siderophores may be due to evolutionary pressures placed on microbes to produce structurally different siderophores which cannot be transported by other microbes' specific active transport systems, or in the case of pathogens deactivated by the host organism.
Diversity Examples of siderophores produced by various bacteria and fungi:
Hydroxamate siderophores Siderophore Organism ferrichrome Ustilago sphaerogena desferrioxamine B (deferoxamine) Streptomyces pilosus Streptomyces coelicolor desferrioxamine E Streptomyces coelicolor fusarinine C Fusarium roseum ornibactin Burkholderia cepacia rhodotorulic acid Rhodotorula pilimanae coprogen fungi aerobactin Escherichia coli
Catecholate siderophores Siderophore Organism enterobactin Escherichia coli enteric bacteria bacillibactin Bacillus subtilis Bacillus anthracis vibriobactin Vibrio cholerae mycobactin Mycobacterium tuberculosis salmochelin Salmonella sp.
Mixed ligands Siderophore Organism azotobactin Azotobacter vinelandii pyoverdine Pseudomonas aeruginosa yersiniabactin Yersinia pestis
Amino carboxylate ligands Siderophore Organism Mugineic acid Hordeum vulgare barley Nicotianamine rice
A comprehensive list of siderophore structures (over 250) is presented in Appendix 1 in reference.
Biological function
Bacteria and fungi In response to iron limitation in their environment, genes involved in microbe siderophore production and uptake are derepressed, leading to manufacture of siderophores and the appropriate uptake proteins. In bacteria, Fe2+-dependent repressors bind to DNA upstream to genes involved in siderophore production at high intracellular iron concentrations. At low concentrations, Fe2+ dissociates from the repressor, which in turn dissociates from the DNA, leading to transcription of the genes. In gram-negative and AT-rich gram-positive bacteria, this is usually regulated by the Fur (ferric uptake regulator) repressor, whilst in GC-rich gram-positive bacteria (e.g. Actinomycetota) it is DtxR (diphtheria toxin repressor), so-called as the production of the dangerous diphtheria toxin by Corynebacterium diphtheriae is also regulated by this system.
This is followed by excretion of the siderophore into the extracellular environment, where the siderophore acts to sequester and solubilize the iron. Siderophores are then recognized by cell specific receptors on the outer membrane of the cell. In fungi and other eukaryotes, the Fe-siderophore complex may be extracellularly reduced to Fe2+, while in many cases the whole Fe-siderophore complex is actively transported across the cell membrane. In gram-negative bacteria, these are transported into the periplasm via TonB-dependent receptors, and are transferred into the cytoplasm by ABC transporters.
Once in the cytoplasm of the cell, the Fe3+-siderophore complex is usually reduced to Fe2+ to release the iron, especially in the case of "weaker" siderophore ligands such as hydroxamates and carboxylates. Siderophore decomposition or other biological mechanisms can also release iron, especially in the case of catecholates such as ferric-enterobactin, whose reduction potential is too low for reducing agents such as flavin adenine dinucleotide, hence enzymatic degradation is needed to release the iron.
Plants
Although there is sufficient iron in most soils for plant growth, plant iron deficiency is a problem in calcareous soil, due to the low solubility of iron(III) hydroxide. Calcareous soil accounts for 30% of the world's farmland. Under such conditions graminaceous plants (grasses, cereals and rice) secrete phytosiderophores into the soil, a typical example being deoxymugineic acid. Phytosiderophores have a different structure to those of fungal and bacterial siderophores having two α-aminocarboxylate binding centres, together with a single α-hydroxycarboxylate unit. This latter bidentate function provides phytosiderophores with a high selectivity for iron(III). When grown in an iron -deficient soil, roots of graminaceous plants secrete siderophores into the rhizosphere. On scavenging iron(III) the iron–phytosiderophore complex is transported across the cytoplasmic membrane using a proton symport mechanism. The iron(III) complex is then reduced to iron(II) and the iron is transferred to nicotianamine, which although very similar to the phytosiderophores is selective for iron(II) and is not secreted by the roots. Nicotianamine translocates iron in phloem to all plant parts.
Chelating in Pseudomonas aeruginosa Iron is an important nutrient for the bacterium Pseudomonas aeruginosa, however, iron is not easily accessible in the environment. To overcome this problem, P. aeruginosa produces siderophores to bind and transport iron. But the bacterium that produced the siderophores does not necessarily receive the direct benefit of iron intake. Rather all members of the cellular population are equally likely to access the iron-siderophore complexes. The production of siderophores also requires the bacterium to expend energy. Thus, siderophore production can be looked at as an altruistic trait because it is beneficial for the local group but costly for the individual. This altruistic dynamic requires every member of the cellular population to equally contribute to siderophore production. But at times mutations can occur that result in some bacteria producing lower amounts of siderophore. These mutations give an evolutionary advantage because the bacterium can benefit from siderophore production without suffering the energy cost. Thus, more energy can be allocated to growth. Members of the cellular population that can efficiently produce these siderophores are commonly referred to as cooperators; members that produce little to no siderophores are often referred to as cheaters. Research has shown when cooperators and cheaters are grown together, cooperators have a decrease in fitness while cheaters have an increase in fitness. It is observed that the magnitude of change in fitness increases with increasing iron-limitation. With an increase in fitness, the cheaters can outcompete the cooperators; this leads to an overall decrease in fitness of the group, due to lack of sufficient siderophore production.
Pyoverdine and siderophore production in Pseudomonas aeruginosa In a recent study, the production of pyoverdine (PVD), a type of siderophore, in the bacterium Pseudomonas aeruginosa has been explored. This study focused on the construction, modeling, and dynamic simulation of PVD biosynthesis, a virulence factor, through a systemic approach. This approach considers that the metabolic pathway of PVD synthesis is regulated by the phenomenon of quorum-sensing (QS), a cellular communication system that allows bacteria to coordinate their behavior based on their population density.
The study showed that as bacterial growth increases, so does the extracellular concentration of QS signaling molecules, thus emulating the natural behavior of P. aeruginosa PAO1. To carry out this study, a metabolic network model of P. aeruginosa was built based on the iMO1056 model, the genomic annotation of the P. aeruginosa PAO1 strain, and the metabolic pathway of PVD synthesis. This model included the synthesis of PVD, transport reactions, exchange, and QS signaling molecules.
The resulting model, called CCBM1146, showed that the QS phenomenon directly influences the metabolism of P. aeruginosa towards the biosynthesis of PVD as a function of the change in QS signal intensity. This work is the first in silico report of an integrative model that comprises the QS gene regulatory network and the metabolic network of P. aeruginosa, providing a detailed view of how the production of pyoverdine and siderophores in Pseudomonas aeruginosa are influenced by the quorum-sensing phenomenon
Furthermore, intratumor P. aeruginosa may scavenge iron by producing pyoverdine, which indirectly protect tumor cells from ferroptosis ('iron death'), emphasizing the need for ferroptosis inducers (thiostrepton) for cancer treatment.
Ecology Siderophores become important in the ecological niche defined by low iron availability, iron being one of the critical growth limiting factors for virtually all aerobic microorganisms. There are four major ecological habitats: soil and surface water, marine water, plant tissue (pathogens) and animal tissue (pathogens).
Soil and surface water