![]() ![]() ![]() Typically, the water is replaced completely by siderophore in an entropically-favorable way. Siderophores surround this complex, usually in a hexadentate manner that results in octahedral geometry. 9įe(III) is typically found in an environment with an aqueous solution, meaning the naturally-occurring complex is the octahedral Fe(H 2O) 6 3+. 8 (C) A staphyloferrin ligand, a type of carboxylate from a Staphylococci. 7 (B) An enterobactin ligand, a type of catecholate from a Bacillus subtilis cell. (A) A ferrichrome ligand, a type of hydroxamate from an Esherichia coli cell. The light gray represents the variable groups. The colored portions highlight the iron and siderophore functional groups. 3 Examples of hexadentate siderophores are in Figure 2.įigure 2: Examples of the three types of siderophores in a hexadentate formation. The resulting hexadentate complexes are also more stable and less labile, which makes sense, since one iron-bound ligand–instead of two or three–increases the entropy. Hexadentate siderophores tend to bind to Fe(III) more so than tetradentate or bidentate siderophores, due to the chelate effect. ![]() Hexadentate and tetradentate structures are the most common. 6 The functional groups act as bidentate ligands, but naturally occurring siderophores can arrange multiples of these groups through the variable R-groups into a higher chelating structure. Some distortions can occur and other atoms like nitrogen or sulfur can be substituted as one of the interacting atoms (although this has a tendency to reduce Fe(III) affinity). These bidentate functional groups interact with iron through two negatively-charged oxygen atoms, but this is not a steadfast rule. 2 The functional groups of these structures that are the main component in iron chelation are shown below, in Figure 1.įigure 1 2: Functional groups of siderophores and the complexes that form upon binding to Fe(III). The most common siderophores fall into three main categories: hydroxamates, catecholates, and carboxylates. Siderophores are an incredibly diverse group of biocompounds, with hundreds of different types categorized. 5 Siderophores have the highest specificity and affinity for Fe(III), but are capable of forming complexes with other metallic ions, like Fe 2+, Zn 2+, Ga 3+, and Cr 3+. Siderophores are low molecular weight compounds that chelate iron and transport it into cells under low iron environments. 3, 4 To circumvent the lacking bioavailability of iron, these organisms synthesize and secrete siderophores. The insolubility means there is a low Fe(III) concentration (10 -18 M) of available iron too low to provide sustenance to microorganisms, which require a minimum concentration of 10 -8 M to live and grow. This is due to the aerobic nature of the environment, which causes iron to be present in the highly insoluble form, Fe(OH) 3. 2 This mechanism is more difficult, despite the prominence of iron in the atmosphere. The simpler of the two, is iron diffusion across cellular membranes. 1 There are two main mechanisms by which cells acquire iron. Animals receive this nutrient from diet, but plants and single-celled organisms must utilize different strategies. Most all living organisms require iron for crucial processes that allow for the life and growth of cells.
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