Iron metabolism Flashcards
Name many functions of iron in the body
o O2 transport (Hb)
o Electron transfer (Cytochromes) (Fe2+ –> Fe3+ helps for catalysis of biochemical reactions)
o DNA synthesis (Ribonucleotide reductase)
o Neurotransmitter production (tyrosine hydroxylase)
Is iron soluble?
- Insoluble (10-17M) Under physiological conditions
What are the 2 types of iron within proteins?
- Heme centers (hemoproteins) – Most abundant iron-containing proteins
o Hemoglobin
o myoglobin - Non-heme, mostly Fe-S clusters
o Mitochondrial complexes I-III (electron transport chain)
o Ferrochelatase
o Xanthine oxidase (enzyme related to generation of uric acid)
Why is iron scarcity so common on the planet?
o Most occurring iron in the environment is in the ferric form (Fe3+) which is insoluble
- Low solubility of naturally occurring oxidized forms of iron;
- For humans, iron is inefficiently absorbable from plant-based foods;
What quantity of iron is there in the human body and in which compartments?
The average adult human contains ~3-4 g iron, most of which in erythrocyte hemoglobin (~2-3g iron)
- Muscles contain iron predominantly in myoglobin;
- Iron is distributed to the tissues through blood plasma which contains only 2- 4 mg iron (tranferrin);
- Plasma iron turns over every few hours as ~20-25mg iron a day move through this compartment (senescent red blood cells).
How much iron do we absorb from food on average per day?
1-2mg… same for losses (most of which come from epithelial desquamation, nail loss etc)
Which part of the intestine absorbs iron?
duodenum
How is iron recycled?
Macrophages from liver and spleen recycle iron from senescent RBCs
When iron is absorbed from ferritin and heme , which form is it in?
ferric
Iron in ferric form crosses apical membrane of erythrocytes via divalent metal transporter (DMT1)
However evidence strongly suggests that independent of the uptake pathway, heme and ferritin derived iron exit the enterocyte in the ferric iron form to the plasma.
What happens once Fe3+ enters an enterocyte?
it can be stored in ferritin or out of basolateral membrane through ferroportin –> blood plasma by transferrin
What is DMT1
- DMT1 imports ferrous iron (Fe2+) as well as several other divalent metals (e.g. Zn) but not trivalent (ferric) iron;
- DMT1 is an integral membrane protein predicted to have 12 transmembrane domains;
Where is DMT1 expressed?
- DMT1 is expressed on the brush-border membrane of duodenal enterocytes and also abundant in erythrocyte precursors;
- DMT1 and the natural resistance associated macrophage protein-1 (NRAMP- 1), are also involved in iron transport in macrophages;
How is DMT1 transcription regulated?
• DMT1 contain 3’ iron-responsive element (3’IRE) – post-transcription regulation of DMT1
How can DMT1 absorb iron despite most iron available being ferric?
it needs ferric reductase enzyme to add an electron. In acid environments, brushborder region will more easily add an electron to Fe3+ so it can be absorbed
How is heme iron absorbed?
• Heme transporters are separate: Heme Carrier Protein 1 (HCP1) and Heme responsive gene-1
What is ferritin?
- Ferritin is a spherical heteropolymeric protein composed of 24 subunits of heavy (H) and light (L) type;
- Ferritin storage large amounts of iron in its interior;
Explain the H subunits of ferritin
The H-ferritin subunits function as ferroxidases to facilitate the conversion of cytoplasmatic Fe+2 to an oxidized mineral form for storage (i.e. iron is stored as Fe3+)
How is ferrous iron delivered to ferritin in the cells?
Ferrous iron is delivered to ferritin by cytoplasmatic chaperones (PCBP-1);
Where is ferritin located?
It is found in most tissues as a cytosolic protein - A soluble relatively iron poor form of ferritin in found in blood plasma;
What affects ferritin’s synthesis?
Ferritin synthesis is stimulated by increased Fe
Where is iron recycled?
As getting close to end of life, specific markers are recognized by the macrophages of the liver and spleen which are gonna bind RBCs and phagocytose them
Explain the iron recycling process and the enzymes involved
- As getting close to end of life, specific markers are recognized by the macrophages of the liver and spleen which are gonna bind RBCs and phagocytose them
- Inside, RBCs will breakdown and heme will be released.
- Free heme will be broken down by heme oxygenases releasing biliverdin, carbon monoxide and iron
- Iron (ferrous) can either be stored in ferritin inside macrophage or transported out of macrophage through ferroportin to the plasma
What is HO and what are its isoforms?
Catalyses the only physiological mechanism for heme degradation; Two well characterized isoforms (HO1 and HO2); o HO1 (inducible) protective; o HO2 (constitutive) O2, NO and CO sensor; HO uses O2 and NADPH to recycle heme to iron, biliverdin (--> bilirubin after) and CO
What happens to HO1 KO mice?
Hmox1-/- are anemic
low serum iron
liver and kidney iron accumulation
HO1 crucial for hemoglobin iron recycling
At what level is HO1 regulated?
Regulation mostly at the transcriptional level
How is OH1 expression regulated?
- Regulation mostly at the transcriptional level
- Promoter of HO-1 bound to Bach1 (which suppresses its expression)
- Increase in IC heme levels (e.g. recycling of RBC) –> heme tells Nrf2 to unbind Keap1 and bind to Bach1 on binding site and decreases its affinity to the HO-1 promoter. Bach 1 is released and degraded in the cytoplasm
What is transferrin? Its function?
- Transports iron between sites of absorption, storage and utilization;
- Glycoprotein; m.w. ~80,000; 2 specific high affinity Fe(III) binding sites;
Where is transferrin produced?
Produced largely by the hepatocytes of the liver;
What regulates Fe affinity to Tf?
pH
1022 M-1 at pH 7.4, affinity ↓ with ↓ pH
How is Tf’s saturation, normally?
- Under normal conditions, Tf is 30% saturated.
o Plasma has a “buffering capacity” to deal with an increase in plasma iron
Where are transferrin receptors expressed?
Transferrin receptors (TfRs) are expressed in all cells except from mature erythrocytes TfRs are highly expressed in hemoglobin synthetizing cells, placenta and rapidly dividing cells
What is the structure of TfR
TfR consists of disulfide-linked transmembrane glycoprotein homodimer, and each subunit (90KDa) binds one molecule of transferrin
In what aspects are erythroid cells different in terms of Tf-R?
- Erythroid cells take up iron ONLY via Tf- TfR dependent pathway;
- In non-erythroid cells, TfR synthesis is stimulated by Fe
When there is an increase in intracellular iron, what happens to the expression of ferritin? of TfR?
increase of the intracellular iron pool leads to stimulation of ferritin synthesis and also decrease in the expression of TfRs –> done by a single genetic regulation system
How are ferritin and TfR regulated (iron-dependant mechanism)?
Iron-dependent regulation of both ferritin and TfRs occurs post- transcriptionally and is mediated by virtually identical iron-responsive elements (IREs) (non-erythroid cells);
- IREs are cis-acting nucleotide sequences, forming a stem-loop structure
- IREs are recognized by trans-acting cytosolic RNA-binding proteins known as iron-regulatory proteins (IRPs).
Explain IRE’s structure
Lower stem of variable length
Bulge that is always a cytosine residue
Upper stem = 5 base pairs
Loop: CAGUG + pyrimidine
Differentiate IRP1 from IRP 2
IRP1
- Iron-sulfur cluster enzyme
- Aconitase activity (Conversion of citrate to isocitrate in kreb’s cycle)
o Low iron = binds IRE in low affinity
o High iron = binds IRE with high affinity
IRP2
- 61% of homology with IRP1
- Functions solely as an RNA-binding protein
- Regulation by iron is mediated by specific proteolysis
How does IRP1 regulate ferritin translation?
IRP-1 is bound to IRE in 5’ region low iron environments –> no ferritin is translated. When iron is present, IRP-1 unbinds which allows ferritin to be translated.
How does IRP1 regulate ferritin translation?
IRP-1 is bound to 3x IREs in 3’ region. Low iron –> IRP protects against degradation by denucleases = increased stability of mRNA, more translation occurs
Name 3 proteins which have an IRE on 5’UTR for which high iron decreases synthesis
o Ferritin o Ferroportin (FPN1A) o Erythroid 5-aminolevunic acid synthase (ALA-S2) --> important in heme synthesis
Name 2 proteins which have an IRE on 3’UTR for which low iron prevents mRNA degradation
o Transferrin receptor 1
o DMT1
What is hepcidin and where is it produced? where is it encoded on human genes?
- Hepcidin is 25 AA protein produced in the liver;
- Human hepcidin is encoded by a single 3 exon gene on chromosome 19;
- Hepcidin acts by posttranscriptionally controlling the membrane concentration of Ferroportin.
What is ferroportin? Where is it expressed and what is its function?
Ferroportin (FPN) is expressed at all sites involved in iron transfer to plasma;
Macrophages of spleen and liver also express ferroportin
FPN is thought to be a 12 transmembrane domain protein with both termini in the cytoplasm;
Ferroportin exports Fe+2 and Zn+2;
What are the 2 transcripts that encode ferroportin?
- FPN1A (IRE)
- FPN1B (no IRE; dueodenum and erythroid cells) – not modulated by iron
Describe the interraction between hepcidin and ferroportin
Hepcidin internalizes ferroportin as needed to decrease iron leaving cell.
Which signals modulate hepcidin expression (3)? Name examples for each.
- Inflammatory cytokines like IL-6 and Activin B will act on hepatocytes and induce production of hepcidin
o Same for plasma transferrin
o Same for iron stores in hepatocytes
–> done to control multiplication of pathogens! - Erythroid regulators (e.g. in anemia) GDF15 and erythroferrone (hormone produced by erythroid precursors; is a signal from erythroid precursor to liver) to increase RBC synthesis
- Plasma Fe-Tf
Where is hepcidin regulated?
Transcriptionally (with TFs) - not known if also regulated after.
Mainly regulated by IL6R-JAK-STAT3 and BMPR-SMAD
Where is hepcidin regulated?
Transcriptionally (with TFs) - not known if also regulated after.
Mainly regulated by IL6R-JAK-STAT3 and BMPR-SMAD
Most signals come from erythroid compartments and will interact with specific TFs in hepatocytes and induce expression of hepcidin
