This document summarizes iron metabolism and the key proteins involved. It discusses that iron is stored in the body bound to ferritin and hemosiderin, and is transported by transferrin. Ferritin stores iron within a hollow protein shell, while hemosiderin aggregates from degraded ferritin. Transferrin transports iron in the bloodstream. Hepcidin regulates iron levels by decreasing ferroportin, the iron exporter. The document outlines the roles and structures of these iron-transport and storage proteins in maintaining iron homeostasis.

1. IRON METABOLISM I
DR AKSHAYA TOMAR
DEPT OF IMMUNOHEMATOLOGY AND
BLOOD TRANSFUSION
AFMC,PUNE

2. INTRODUCTION
 Iron is one of the most abundant element on earth, yet
only trace amounts are present in living cells.
 The total body iron content
 Adult male - 50mg/kg
 Adult female - 40 mg/kg
 Functional iron -> About 80% - Hemoglobin,
myoglobin , iron containing enzymes.
 Storage pool -> About 15% to 20%. Ferritin and
Hemosiderin.

3. DISTRIBUTION OF BODY
IRON
AMOUNT OF IRON
IN AVERAGE
ADULT
MALE(g) FEMALE(g) PERCENTAGE OF
TOTAL
HAEMOGLOBIN 2.4 1.7 65
FERRITIN AND
HAEMOSIDERIN
1.0(0.3-1.5) 0.3(0- 1.0) 30
MYOGLOBIN 0.15 0.12 3.5
HAEM
ENZYMES(eg
cytochrome,
catalase,
peroxidases
0.02 0.015 0.5
TRANSFERRIN
BOUND IRON
0.004 0.003 0.1

4. PROTEINS IMPORTANT IN
IRON METABOLISM

5. FERRITIN
 Primary iron storage protein.
 Protein-iron complex.
 Consists of a spherical apoprotein shell
enclosing a core of ferric
hydroxyphosphate.
 Ferritin is found in all cells and in the
highest concentration in liver, spleen and
bone marrow.

6. FERRITIN
 In the liver, most ferritin is stored within the
parenchymal cells (source- plasma transferrin)
 In spleen and the bone marrow, mainly in
macrophages (source - breakdown of RBC)
 Intracellular ferritin is located in the cytosol and
in lysosomes.
 Partially degraded protein shells of ferritin
aggregate into hemosiderin granules .

7.  The outer polypeptide shell (apoferritin) composed of
24 symmetrically placed protein chains (subunits).
 There are 184 residues in each peptide subunit in
human ferritin.
 The sphere that is formed is approximately 80
Angstroms in diameter, and the walls are
approximately 10 Angstroms thick.
FERRITIN

8.  The inner core contains an electron-dense
and chemically inert inorganic ferric iron-
core in the form of ferric hydroxy
phosphate.
 The ferritins are extremely large proteins
(450kDa) which can store up to 4500 iron
atoms as hydrous ferric oxide.
FERRITIN

9. Structure of ferritin showing the
outer polypeptide shell with inner
iron-core

10. FERRITIN
 The internal cavity of the ferritin molecule
communicates with the exterior via 6
channels through which the ferrous ions
may enter or leave.
 These channels maybe be three fold
protein channels or four fold protein
channels.

12. UNIT CELL FOR
FERRIHYDRITE

15. Ferritin and Hemosiderin
 Partially degraded proteins of ferritin aggregate into
granules - hemosiderin granules.
 Water insoluble, crystaline, protein iron complex
visible by light microscopy when stained by Prussian
blue stain( Pearls’ reaction)
 With normal iron stores, only trace amounts of
hemosiderin are found in macrophages of bone
marrow, spleen, and liver.
 In iron-overloaded cells, the proportion present as
haemosiderin increases considerably.

16. FERRITIN
 Plasma ferritin is derived largely from the
storage pool of body iron, its levels correlate
well with body iron stores.
 Normal Ferritin levels – 15to 300 μg/L. higher in
men ( median about 90μg/L) than in
premenopausal women ( 30μg/L)
 Storage iron depletion – < 15μg/L
 >300μg/L may indicate iron overload

17. HAEMOGLOBIN
 About 65% to 70% total body iron is found in haem group of
haemoglobin.
 Contains four haem groups linked to four globin chains , and
can bind four molecules of oxygen.
 A haem group consists of iron (Fe2+) ion held in a heterocyclic
ring, known as a porphyrin.
 This porphyrin ring consists of four pyrrole molecules cyclically
linked together with the iron bound in the centre.
 The nitrogen atoms of the pyrrole molecules form covalent
bonds with four of the iron's six available positions which all lie
in one plane.

18. Structure of heme showing the four coordinate bonds
between ferrous ion and four nitrogen bases of the
porphyrin rings.

19. Porphyrin Haem

20. Hemoglobin
 The iron is bound covalently to the globular protein via
the imidazole ring of the the proximal histidine residue
of the porphyrin ring.
 A sixth position can reversibly bind oxygen by a
coordinate covalent bond.

21. Structure of heme showing the square planar tetrapyrrole along
with the proximal and the distal histidine.

23. MYOGLOBIN
 Myoglobin is an iron- and oxygen-binding protein
found in the muscle tissue.
 It is a single-chain globular protein of 153 or 154
amino acids.
 Containing a haem prosthetic group in the center
around which the remaining apoprotein folds.
 It has eight alpha helices and a hydrophobic core.
 It is the primary oxygen-carrying pigment of muscle
tissues

25. TRANSFERRIN
 Iron is transported in plasma by an iron binding
glycoprotein called transferrin, which is synthesized
in liver, synthesis being inversely related to iron stores.
 Plasma half life – 8-11 days.
 The serum transferrin is 2.0–3.0 g/l and 1 mg of
transferrin binds 1.4 μg of iron.
 Two atoms of ferric iron bind to each molecule.
 Major function of transferrin is to deliver iron to the
tissues.

26.  Single chain polypeptide with molecular weight of
approximately 80,000 Da.
 Consisting of a single polypeptide chain of 680 to 700
amino acids.
 It has a bilobar structure with two Iron binding sites
 Each lobe contains an Iron binding site buried below
the surface of the protein in a hydrophilic environment.
TRANSFERRIN

27. Bi lobar structure of Human
transferrin

28. TRANSFERRIN
 In the normal physiological state
 one-ninth - fully saturated with iron at both sides
 four-ninth - have iron at either site
 four-ninth - are free of iron.
 Transferrin delivers iron to cells by binding to specific
cell surface receptors - TfR
 The TfR is a transmembrane protein consisting of two
subunits of 90,000 Da each, joined by a disulfide
bond.
 Each molecule of two subunits binds one transferrin
molecule.

29. OTHER HAEM PROTEINS
 Certain enzymes also contain haem as part of their
prosthetic group.
 Catalase
 Peroxidases
 Tryptophan pyrrolase
 Nitric oxide synthase
 Microsomal and mitochondrial cytochromes.
 Cyclooxygenase

30. DIVALENT METAL
TRANSPORTER 1
 Electrogenic pump that requires proton
cotransport in order to transfer Fe2+ across cell
membranes.
 Present at the apical membrane and subapical
endosomes of the duodenal enterocytes which
have a low pH.
 Transports iron from the gut lumen into the
labile iron pool.

31. FERROPORTIN
 Transmembrane protein - basolateral
transporter of iron
 Essential for iron release from macrophages,
intestinal enterocytes , placental
syncytiotrophoblasts.

33. HEPCIDIN
 Hepcidin was originally identified as an antimicrobial peptide
isolated from human urine.
 The liver is the predominant source of hepcidin
 84-amino-acid prepropeptide is synthesized
 Cleaved to yield 20-and 25-amino-acid peptides
 Released into the circulation
 Bound in plasma to α2 macroglobulin
 Filtered by the kidney.
 Hepcidin acts as a systemic iron-regulatory hormone

34.  Preferentially accumulates in the proximal duodenum and
spleen
 High expression of FPN in these areas.
 Hepatocytes release or down regulate hepcidin according to the
iron status of the body.
 The hepcidin response is remarkably rapid.
 In humans, iron ingestion results in a sharp increase in urinary
hepcidin excretion within 12 – 24 hours of starting treatment.
HEPCIDIN

35. Schematic Diagram showing the regulation of circulating
iron levels by Hepcidin

37.  FPN (Ferroportin) is a major target of hepcidin’s
action.
 Hepcidin appears to regulate FPN expression by two
distinct mechanisms.
 The first is at the level of FPN transcripts, which are decreased
following stimulation of endogenous hepcidin production or
administration of recombinant hepcidin .
 The second involves binding of hepcidin to FPN at the cell
membrane, causing internalization and degradation of FPN, thus
diminishing iron transfer.
HEPCIDIN

39.  Hepcidin might also directly inhibit erythroid-progenitor
proliferation and survival.
 Hepcidin synthesis is increased in response to raised serum
iron, iron overload and inflammation.
 Anemia, hypoxia and increased erythropoeitic activity are
associated with a dramatic decrease in liver hepcidin gene
expression.
 This may account for the increase in iron release from
reticuloendothelial cells and increase in iron absorption
HEPCIDIN

41. SCHEME OF NEXT PRESENTATION
IRON TURNOVER IN THE BODY
INTRACELLULAR IRON HOMEOSTASIS
DIAGNOSTIC METHODS FOR INVESTIGATING IRON METABOLISM