Uptake and storage of iron

Question 1

Why does iron have a low bioavailability, despite its high abundance in the Earth’s crust?

Answer 1

The predominant state of iron in aqueous, non-acidic, oxygenated environments is Fe3+
An oxidising atmosphere produces Fe(OH)3 which is insoluble

Question 2

What is the growth-limiting factor in the oceans?

Answer 2

The availability of iron

Question 3

What are the 3 ways metal (complexes) can pass through cell membranes?

Answer 3

1. Passive diffusion of neutral complexes e.g. cis-platin
2. Migration through ion channels (cationic/anionic)
3. Specific receptor proteins complex to a specific ion and pass the metal into the cell –> this is used for Fe uptake

Question 4

What properties must a receptor protein that passes iron into cells have?

Answer 4

Form water-soluble Fe(III) complexes
High complexation constant
Easily produced for release into the environment

Question 5

What do microorganisms use for iron uptake?

Answer 5

Siderophores
Fe/siderophore complexes are recognised by receptors on the microorganism surface and reabsorbed
(Fe(III) then reduced to Fe(II) inside the microorganism, which is less tightly held and becomes available for use)

Question 6

Siderophores

Answer 6

Amongst the strongest Fe3+ binders know
Solubilise Fe(III)

Question 7

Enterobactin

Answer 7

High affinity siderophore (K = 10^49 M^-1)
Highly selective for iron over other metals
Primarily found in Gram-negative bacteria e.g. E. coli

Question 8

Enterobactin amide groups

Answer 8

Responsible for recognition by the receptor

Question 9

Enterobactin catechol groups

Answer 9

Required for Fe chelation, not membrane recognition

Question 10

How is the biosynthesis of enterobactin controlled?

Answer 10

By Ferric Uptake Regulator (FUR) protein at the transcriptional level

Question 11

How is the biosynthesis of enterobactin/siderophores controlled?

Answer 11

By Ferric Uptake Regulator (FUR) protein at the transcriptional level
Excess Fe(II) binds to FUR protein
The resulting complex binds to DNA, blocking transcription of the siderophore gene

Question 12

Hemosiderin

Answer 12

Even larger iron storage complex for Fe overload
Complex of ferritin, denatured ferritin and other material
Can store >20000 Fe atoms (35 % wt. Fe)
Insoluble - only intracellular, doesn’t circulate
Iron within hemosiderin is poorly bioavailable

Question 13

Structure of ferritin

Answer 13

Protein shell - 10 Å thick, 100 Å diameter
Fe bound by Asp, Glu and Tyr residues
Most of the Fe in an Fe2O3 core that may contain various anions e.g. hydroxide, phosphate

Question 14

Apoferritin

Answer 14

Iron-free ferritin
Filled with water molecules
Hydrophilic and hydrophobic channels

Question 15

Uptake of Fe by ferritin

Answer 15

Fe is introduced as Fe(II) then oxidised by O2 inside ferritin
12Fe2+ + 3O2 + 12H2O 6Fe2O3 + 24H+

> 10000 H+ released during filling of one ferritin protein
H+ must diffuse slowly in and out of ferritin (subtle pH control)
Therefore oxidation of Fe(II) is slow

Question 16

Iron transport proteins

Answer 16

Transferrin
Lactoferrin
= iron-binding blood plasma glycoproteins that control the level of free iron in biological fluids
Keep iron soluble, allow it to be transferred across cell membranes

Question 17

Transferrin

Answer 17

80 kDa, 6 % sugar
Two high affinity Fe(III)-binding sites
Binds iron tightly but reversibly
Bound by Asp, Tyr and His residues as well as a carbonate anion (anion required for Fe to bind)

Question 18

Mechanism of release of Fe from transferrin

Answer 18

When a transferrin protein loaded with iron encounters a transferrin receptor on a cell surface, it is transported into the cell by receptor-mediated endocytosis
The vesicle interior is acidified to pH 5.5, leading to displacement of Fe from the carbonate by H+ (H+ is a better Lewis acid)

Question 19

Regulation of ferritin and transferrin biosynthesis

Answer 19

Controlled at the translation stage by [Fe]

Low [Fe] = slow translation of ferritin, fast translation of transferrin
High [Fe] = fast translation of ferritin, slow translation of transferrin

Question 20

IRE

Answer 20

Iron Response Element
Bound by Iron Response Proteins (IRPs)
Located in the UTRs of the mRNA of proteins involved in iron metabolism

Question 21

IREs in regulation of ferritin and transferrin biosynthesis

Answer 21

IRE at 5’ UTR of ferritin mRNA
At low [Fe], IRPs bind to the IRE leading to a reduced rate of translation

IRE at 3’ UTR of transferrin mRNA
At low [Fe], IRP binding to IREs increases the stability of the mRNA, so translation continues

Question 22

How is the affinity of IRPs for IREs regulated?

Answer 22

By [Fe]
At low [Fe], K = 10^11 M^-1 (i.e. high affinity)
So less ferritin, more transferrin
At high [Fe], K = 5 x 10^9 M^-1 (i.e. lower affinitY)
So more ferritin, less transferrin

Question 23

What are the 5 processes involved in iron homeostasis?

Answer 23

1. Uptake - active or passive during food ingestion
2. Transport - selective movement of Fe ions through membranes into particular cells
3. Utilisation - in cellular processes e.g. incorporation into a protein
4. Storage - of excess iron in a non-toxic, easily accessible form
5. Elimination - controlled removal of iron from the system by excretion

Question 24

Iron deficiency

Answer 24

Can lead to anaemia, which can cause acute health problems

Question 25

Iron excess

Answer 25

= haemochromatosis
Excess iron can deposit in tissues and cause radical damage
Implicated in coronary disease

Question 26

Treatment for iron poisoning

Answer 26

Chelation therapy to remove excess iron via the kidneys
Desferrioxamine = ligand of choice
(enterobactin too easily hydrolysed)

Question 27

How do pathogenic microorganisms scavenge a supply of iron from the host?

Answer 27

Absord iron directly from the blood plasma, leading to a decrease in blood iron levels
Treated with iron-scavenging ligands e.g. desferrioxamine (so no free iron available to pathogen)

Question 28

Where do animals absorb iron from?

Answer 28

The gut