Section 2: Transport and Storage of Iron

Question 1

Iron: ion status

Answer 1

-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

Question 2

Amounts of Iron in the human body

Answer 2

• 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

Question 3

2 main difficulties with iron in organisms

Answer 3

• low bioavailability

- toxicity when unbound

Question 4

Difficulty: Bioavailability

Answer 4

• 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)

Question 5

Difficulty: Unbound toxicity

Answer 5

• 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

Question 6

What is the iron transport system of micro-organisms?

Answer 6

eg. yeast, fungi and bacteria

siderophores

Question 7

How do siderophores work? (general)

Answer 7

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

Question 8

Siderophore structure

Answer 8

there are over 200 different siderophores
the majority can be divided into two groups
hydroxamates and catecholates
PICTURES

Question 9

Chemistry of the siderophore

Answer 9

-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)

Question 10

studying the structure of siderophore complexes

Answer 10

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

Question 11

Siderophorin complexes: what happens after?

Answer 11

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)

Question 12

What is the iron transport mechanism for higher organisms?

Answer 12

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

Question 13

What is a transferrin?

Answer 13

• 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

Question 14

the binding sites of transferrin description

Answer 14

-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

Question 15

binding sites of transferring binding mechanism

Answer 15

-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+

Question 16

How does transferrin structure change w complexation

Answer 16

• 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

Question 17

Transferrin and other ions

Answer 17

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

Question 18

What is the transferrin receptor?

Answer 18

A receptor that is situated in the plasma membrane

it can selectively bind Fe-Tf

Question 19

Release of iron into the cell mechanism for higher organisms

Answer 19

1. Fe-Tf binds to TfR selectively
2. The bound TfR and a section of the cell membrane “pinches off” to form a vesicle
3. The pH of the vesicle is lowered by a membrane bound protein pump
4. Fe(III) occurs due to protonation of carbonate and Tyr at low pH)
5. Vesicle “merges” with cell membrane

process takes approximately 15 minutes

Question 20

What happens once the iron is inside the cell?

Answer 20

The iron is quickly utilized for enzyme and haem manufacture or for storage by ferritin

Question 21

What is the function of ferritin?

Answer 21

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.

Question 22

What is the structure of ferritin?

Answer 22

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

Question 23

Component 1 of ferritin (inner)

Answer 23

• 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

Question 24

Component 2 of ferritin (outer)

Answer 24

-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

Question 25

What is the composition of Rust?

Answer 25

Double layers of closely packed O(2-) and OH- ions WITH Fe3+ occupying the interstitial sites between layers
PICTURE

Question 26

How does Fe3+ get in an out of the core?

Answer 26

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

Question 27

What does apo mean>?

Answer 27

apo means the biological species without its (mainly metal) ligand or prothetic group

Question 28

Siderophore summary

Answer 28

to do

Question 29

Transferrin summary

Answer 29

to do

Question 30

Ferritin summary

Answer 30

to do