Siderophores
The siderophores ( large iron carriers ) are a group of around 200 iron-binding low molecular weight compounds and oligopeptides that are formed by aerobic bacteria , fungi and plant roots and excreted in the surrounding medium. After the iron ions have been complexed , the loaded siderophores are taken up again by the producer's cells via specific transport systems. A special feature of the siderophores is their low molar mass of 300 to 2,000 Daltons . Some plants also form iron-complexing substances. These are called phytosiderophores .
Occurrence in nature
Well-known siderophores are the enterobactins , which are produced by enterobacteria , the ferrioxamines of actinobacteria , the pyoverdines in Pseudomonas , the mycobactins and exochelins of mycobacteria , the ferrichromes of fungi and the citrate-containing siderophores in fungi and bacteria. Siderophores are only formed by aerobic bacteria , fungi and plant roots and could be detected in partly high concentrations in the soil and seawater .
Properties and biological importance
Siderophores are small molecules that belong to the catecholates , hydroxamates or α-hydroxy / α- ketocarboxylic acids . There are also mixed-functional siderophores that also have nitrogen ligands, as well as short peptides with unusual modifications (such as the pyoverdine ). What they all have in common is that they form a multidentate ligand that binds a metal ion with six, rarely four centers. The small size of the molecules allows them to cross the cell wall of bacteria, fungi or plants twice through relatively narrow channels. The iron transport proteins of higher living beings (e.g. transferrin ) have a molar mass of about 80,000 Dalton (siderophores 300 to 2,000).
Iron transport and storage
Siderophores bind iron (III) ions very selectively, which can then be transported into the cells. Despite the large amount of naturally occurring iron, this has a very low bioavailability, since it is usually present as an insoluble hydroxo complex. The high affinity of the siderophores for Fe (III) ions allows them to compensate for the low bioavailability of this ion. Since the complex binding constant of the siderophores for Fe (II) ions is much lower than for trivalent iron, the metal ion is released from the complex in the cell after reduction to iron (II). These molecules are also important as a storage system for iron; d. H. the iron-loaded siderophores can be stored inside the cells.
pathology
| complex | Stability constant at pH 7 |
|---|---|
| Fe (III) enterobactin | 10 56 |
| Fe (III) ferrioxamine E. | 10 32 |
| Fe (III) -Ferrichrome A | 10 29 |
| Fe (III) transferrin A | 10 24 |
| Fe (III) - EDTA | 10 25 |
Enterobactin and other bacterial siderophores form much more stable complexes with iron (III) than do human siderophores. They are therefore able to “steal” iron from hemoglobin (after hemolysis ) and transferrin in the human body .
biosynthesis
The production of the siderophores is linked to the iron concentration within the cell via a special control circuit. The protein FUR ( ferric uptake regulator ) binds zinc (II) and iron (II) ions. If there is no longer enough divalent iron in the cell, the siderophore production is stimulated and the cell transport channels for iron are opened.
For the synthesis as such, amino acids are mostly used as the basis, which are assembled like a conveyor belt by large modular multi-enzyme complexes of the non-ribosomal peptide synthetases (NRPS) to form the non-ribosomal peptide ; the process is similar to the synthesis of fatty acids by carrier enzymes.
Use as chelators
It is known that pyoverdines, which can have a very variable structure, can also act as chelators for heavy metals such as uranium and thorium .
Examples
literature
- Miethke M, Marahiel MA: Siderophore-based iron acquisition and pathogen control . In: Microbiol. Mol. Biol. Rev. . 71, No. 3, September 2007, pp. 413-51. doi : 10.1128 / MMBR.00012-07 . PMID 17804665 . PMC 2168645 (free full text).
- Crosa JH, Walsh CT: Genetics and assembly line enzymology of siderophore biosynthesis in bacteria . In: Microbiol. Mol. Biol. Rev. . 66, No. 2, June 2002, pp. 223-49. PMID 12040125 . PMC 120789 (free full text).
- Gobin J, Horwitz MA: Exochelins of Mycobacterium tuberculosis remove iron from human iron-binding proteins and donate iron to mycobactins in the M. tuberculosis cell wall . In: J. Med.. . 183, No. 4, April 1996, pp. 1527-32. PMID 8666910 . PMC 2192514 (free full text).
- Gobin J, Moore CH, Reeve JR, Wong DK, Gibson BW, Horwitz MA: Iron acquisition by Mycobacterium tuberculosis: isolation and characterization of a family of iron-binding exochelins . In: Proc. Natl. Acad. Sci. USA . 92, No. 11, May 1995, pp. 5189-93. PMID 7761471 . PMC 41874 (free full text).
- Snow GA: Mycobactins: iron-chelating growth factors from mycobacteria . In: Bacteriol Rev . 34, No. 2, June 1970, pp. 99-125. PMID 4918634 . PMC 408312 (free full text).
- N. Noinaj, NC Easley et al. a .: Structural basis for iron piracy by pathogenic Neisseria. In: Nature. Volume 483, number 7387, March 2012, pp. 53-58, doi: 10.1038 / nature10823 . PMID 22327295 . PMC 3292680 (free full text).
Individual evidence
- ↑ SM Kraemer, DE Crowley, R. Kretzschmar: Geochemical Aspects of Phytosiderophore-Promoted Iron Acquisition by Plants , 2006, Advances in Agronomy, doi: 10.1016 / S0065-2113 (06) 91001-3 .
- ^ Joseph W. Lengeler, G. Drews, Hans Günter Schlegel: Biology of the prokaryotes . Thieme, Stuttgart 1999, ISBN 3-13-108411-1 , p. 183 ff .
- ↑ Brainard JR, Strietelmeier BA, Smith PH, and Langston-Unkefer PJ: actinides binding and solubilization by microbial siderophores . In: Radiochim. Acta . 58-59, 1992, pp. 357-363. doi : 10.1524 / ract.1992.5859.2.357 .
Web links
- Siderophores at the University of Tübingen