Transcript Power Point Presentation

More Siderophore Stuff
Steven “Babyface” Backues
Donnie “Big D” Berkholz
Brooks “Mad Dog” Maki
Overview of Iron Uptake
Two basic strategies:
- Reduction before uptake
- Reduction after uptake
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)
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
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 DG°’ but not any more
positive than in many other NADH or NADPH
driven reactions
- Below pH of 7.9, decreasing pH favors reduction
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.
Use and Storage of Iron
• After reduction, Fe(II) is always bound to
carrier proteins until used
• Iron is always stored as Fe(III)
Ferritin
Ferritin is found in most
animals, plants, and some
bacteria.
It can store up to 5,000
atoms of Fe(III) as
[FeO(OH)]8[FeO(H2PO4)].
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.
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
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
Photoreactivity
• Light mediated decarboxylation of an alphahydroxy 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.
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.
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)
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
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.
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
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.
“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.”
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
More references
• Photochemical cycling of iron in the surface ocean mediated by
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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