Section 2: Transport and Storage of Iron Flashcards
Iron: ion status
-Essential trace element
(most important trace metal in humans)
-Deficiency-> anemia
-Use O2 transport and storage, e- transfer
-in oxidation, hydroxylation and h-transfer enzymes
Amounts of Iron in the human body
- around 4.2 grams in human body
- daily intake 5-40 mg
- only metal not lost during excretion
- can only be lost by bleeding
- only a small percentage actually in use at a time
2 main difficulties with iron in organisms
- low bioavailability
- toxicity when unbound
Difficulty: Bioavailability
- Most abundant TM in earths crust
- Bioavailabillity is poor due to fe(III) compounds being insoluable at neutral pH
- most minerals contain fe(iii) and in air Fe forms polymeric oxide bridged Fe(III) (rust)
Difficulty: Unbound toxicity
- In compoudns Fe(II) or Fe(III)
- excess of ‘free’/unbound iron is dangerous to organisms since it can react readily to form free radicals
high spin Fe(II) +O2 –> Fe(III) + O2(-)
Fe(II) +H2O2 –> Fe(III) +OH- +OH
What is the iron transport system of micro-organisms?
eg. yeast, fungi and bacteria
siderophores
How do siderophores work? (general)
absorbs Fe from the aqueous environment in the microorganisms (Fe3+ precipitates as Fe(OH)3 in aq media)(minuscule amounts of Fe actually dissolved dissolved)
Siderophores secreted from cells into external aqeous medium where they bind to Fe(3+) to give a soluble complex which can re-enter the organism at a specific ATP-driven receptor site
Siderophore structure
there are over 200 different siderophores
the majority can be divided into two groups
hydroxamates and catecholates
PICTURES
Chemistry of the siderophore
-All Fe(III)-siderophore complexes have contain the Fe centre in a
high spin state with octahedral coordination.
-they also have a strong chelate effect that results in high association constants (R+L rev RL)
-The ligands have either hard O or N atoms and are negetively charged and therefore have a high affinity for Fe(III)
studying the structure of siderophore complexes
high spin Fe(III) complexes are kinetically labile. Therefore in order to study the structure of the complexes. Fe3+ ion can be exchanged with Cr3+ ion to give kinetically inert complexes that can be studied
WHY WHY WHY WHY
Siderophorin complexes: what happens after?
Once inside the cell, the iron is released from the complex. The mechanism of this release is still not known
3 hypothesis:
1. Fe(III) converted to Fe(II)- reducing stability contant by several magnitudes
2. Ligand hydrolysis occurs in cyctoplasm, resulting in release if Fe
3. An intracellular iron binding ligand strips the metal from the siderophore (unknown mechanism possibly redox)
What is the iron transport mechanism for higher organisms?
more complex,
involves transferrins
initial stage involves passage of Fe(II) through the stomach lining and uptake in the blood as a Fe(III) transferrin complex - Fe-Tf
What is a transferrin?
- its a glycoprotein (consists of protein and carbohydrates
- has a very high binding constant for Fe3+
- found in blood plasma, milk and egg whites
- molecular mass of 80kDa
the binding sites of transferrin description
-2 similar but seperate binding sites that are both around 10A below the protein surface
- each active site coordinates to the Fe3+
via 1 x O- carboxylate (Asp-)
2x O- phenolate(Tyr-)
1x N imdazole (His)
2x O from bound carbonate
to produce a distorted octrahedral environment
binding sites of transferring binding mechanism
-complexation of Fe3+ at each site involves simulataneous binding of HCO3- or CO32- and a release of H+
Apo-TF +FE(III) +HCO3- -> Fe-TF + H+
How does transferrin structure change w complexation
- contain considerable proportion of alpha helix which allow for flexibility.
- On complexation of Fe(III), a conformation change based around a hinge occurs.
- transferrin receptor proteins can ‘differentiate’ between Fe-Tf and apo-Tf due to this
Transferrin and other ions
Tf binds to Fe(III) much more strongly than Fe(II) due to its hard ionic ligands.
Al(III) can also bind strongly to Tf and is not readily released- the main reason that makes Al toxic
What is the transferrin receptor?
A receptor that is situated in the plasma membrane
it can selectively bind Fe-Tf
Release of iron into the cell mechanism for higher organisms
- Fe-Tf binds to TfR selectively
- The bound TfR and a section of the cell membrane “pinches off” to form a vesicle
- The pH of the vesicle is lowered by a membrane bound protein pump
- Fe(III) occurs due to protonation of carbonate and Tyr at low pH)
- Vesicle “merges” with cell membrane
process takes approximately 15 minutes
What happens once the iron is inside the cell?
The iron is quickly utilized for enzyme and haem manufacture or for storage by ferritin
What is the function of ferritin?
Principle store of iron in animals and many plants.
In humans, ferritin is found in the liver, bone marrow and spleen
-can contain up to 20% Fe by mass and able to store iron in non toxic and water soluble form.
What is the structure of ferritin?
Ferritin consists of two parts:
- a Fe oxide ‘mineral core’
- a protein shell 13 nm in diameter with a central cavity of 7.5 nm
Component 1 of ferritin (inner)
- can contain up to 4500 high spin Fe3+ centres
- core composition [FeO(OH)8][FeO(PO4H2)]
- Structure of which is similar to the mineral ferrihydrite (Fe2O3.nH2O)and rust.
- These iron oxide particles are coated with phosphate groups- hypothesized to achor the iron oxide to the protein shell
Component 2 of ferritin (outer)
-Consists of 24 subunits that link to form a hollow sphere.
-The 3 or 4 subunits can come together at a joint to produce 3 fold axis channels/pores or 4 fold acis channels or pores.
3 fold hydrophilic
4 fold hydrophobic
3 fold pores lined w/polar AA’s Asp and Glu
4 fold pores lined w/ AA Leu
What is the composition of Rust?
Double layers of closely packed O(2-) and OH- ions WITH Fe3+ occupying the interstitial sites between layers
PICTURE
How does Fe3+ get in an out of the core?
Fe(III) oxides and hydroxides are insoluble
to get in and out of the core
- the most likely mechanism is transported in and out of the core as Fe(II) along hydrophilic pores as aqueous ion
When inside the ion it can be oxidised
Fe(II) + O2 +H+ –> Fe(III)O(OH)
The (III) ions can then be reduced to (II) when iron is required to travel out of protein.
EVIDENCE
experiment: ferritin +reducing agent (sodium thionite) and a Fe(II) chelating agent (2,2 bipy)
gives apo-ferritin
What does apo mean>?
apo means the biological species without its (mainly metal) ligand or prothetic group
Siderophore summary
to do
Transferrin summary
to do
Ferritin summary
to do
