more siderophore stuff steven “babyface” backues donnie “big d” berkholz brooks “mad...
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More Siderophore Stuff
Steven “Babyface” Backues
Donnie “Big D” Berkholz
Brooks “Mad Dog” Maki
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Overview of Iron Uptake
Two basic strategies:
- Reduction before uptake
- Reduction after uptake
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Reduction Before Uptake
• Release of “reductants” into environment, or reducing enzyme bound to cell surface
• Advantages:• No need for permeases, which can be used by
pathogens such as phages
• Disadvantages• Less specific, and can lead to toxicity from
other metals (Cu(II), Cd(II), Co(II), Ni(II)
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Reduction After Uptake
• Uses siderophores to bind Fe(III), which is released inside the cell, usually via reduction of iron from Fe(III) to Fe(II)
• Highly specific, but requires more energy to form the siderophores and uptake system
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Is Reduction Difficult?
For various fungal siderophores, reduction potential of Fe(III) is around –400mV
The reduction potential of NAD+/NADH or NADP+/NADPH is around –320mV
- So, there is a positive G°’ but not any more positive than in many other NADH or NADPH driven reactions
- Below pH of 7.9, decreasing pH favors reduction
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Other Release Mechanisms?
• Degradation of the siderophore?
• Release without reduction?• For Fe(III) only partially coordinated by a
siderophore, Cl- ions can increase dissociation rates 100-1000 fold.
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Use and Storage of Iron
• After reduction, Fe(II) is always bound to carrier proteins until used
• Iron is always stored as Fe(III)
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Ferritin
Ferritin is found in mostanimals, plants, and some bacteria.
It can store up to 5,000 atoms of Fe(III) as [FeO(OH)]8[FeO(H2PO4)].
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Siderophores as Iron Storage
• Mössbauer spectroscopy shows that reduction is not rate-limiting for siderophore uptake.
• Experiments with 55Fe and a fluorescent ferrichrome analogue showed that while loaded siderophores were taken up within minutes, the iron was not fully released for up to 16 hours after uptake.
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In some fungi, one type of siderophore is used for uptake and another for storage
- in N. crassa, coprogen shuttles, while ferricrocin stores- in R. minuta, Rhodotorulic acid used only for storage, not for uptake
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Amphiphilic Siderophores
• Prior to binding, these siderophores are micelles with hydrophobic centers
• With the addition of Fe(III) they form vesicles.
• Vesicles are approx. 100 nm across with hydrophobic ring lined with hydrophilic heads
• This structure is important in photoreactivity
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Photoreactivity
• Light mediated decarboxylation of an alpha-hydroxy acid complexed to a transition metal ion is well known.
• It has been found that this reaction also occurs in Fe-siderophore complexes.
• Fe(III) petrobactin was readily photolyzed in this way under ocean surface conditions.
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Photoreactivity, the sequel
• Photolysis is mediated by light in the ultraviolet spectrum
• Therefore these reactions occur deep into the euphotic zone (80 m)
• Fe-siderophore complexes are structurally stable in sterile sea water.
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Photoreactivity, the final chapter
Two main products of photochemical reaction:
hydrophobic
(fatty acid tail)
hydrophilic
(head group - peptide)
Fe (III) is reduced to Fe(II)
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Fe Cycling
• What happens to Fe(II)?• Direct biological uptake• Oxidation back to Fe (III) (possibly complexed by
another siderophore)• Possible chelation by organic ligands?
• The photo-oxidized ligand continues to bind Fe(III)
• Iron bound by these ligands may be more available for uptake, as stability is reduced from original siderophoreReferences
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Iron Scavenging by Pathogens
• Within animals, all of the iron is generally complexed and being used, so bacteria must steal it, often by use of siderophores.
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Exochelins
• Exochelins are released by M. tuberculosis.
• They scavenge metal primarily from transferrin and lactoferrin, human iron binding proteins; less effectively from ferritin
• They transfer their iron to mycobactins in the M. tuberculosis cell wall
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Heme Acquisition System A
• This is a protein, not a siderophore
• It or similar proteins are produced from many gram negative bacteria
• It binds an entire heme molecule, extracting it from hemoglobin, then releasing it to the bacterial membrane receptor HasR.
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“The heme binding site is made up of some hydrophobic residues and is held by the two ligands: residue His32 lies on one side while Tyr75 completes the coordination of the heme iron.”
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References
Ardon, Orly et. al. Microbiology, 1997, 143 3625-3631 Boukhalfa, Hakim; Crumbliss, Alvin, Inorganic Chemistry 2001, 40 4183-4190 Czjzek, Mirjam et. al. AFMB Activity Report 1996-1999 (http://afmb.cnrs-mrs.fr/subjects/pdf/21.pdf) De Luca, Nicoala; Wood, Paul Advances in Microbial Physiology, 2000, 43 39-74 Gobin, Jovana; Horwitz, Marcus Journal of Experimental Medicine, 1996, 183 1527-1532 Matzanke, Berthold; Winkelmann, Günther FEBS Letters, 1981, 130 50-53
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More references
• Photochemical cycling of iron in the surface ocean mediated by microbial iron(iii) binding ligands. K. Barbeau, E.L. Rue,
K.W. Bruland, A. Butler. Letters to Nature 27 Sep. 2001
• Scientists Chart Iron Cycle in Ocean. National Science Foundation 27 Sep. 2001
• Sunlight Affects Iron Cycles. Pamela Zurer Biogeochemisty 1 Oct. 2001
• Marine Bacteria Foster Iron Cycling. Jacquelyn Savani University of California, Santa Barbara
• Petrobactin, a Photoreactive Siderophore K. Barbeau, G. Zhang, D. Live, A. Butler American Chemical Society 7 Aug. 2001