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Pilani Campus
CHEM F343: Inorganic Chemistry III
Lecture-6
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Transport of metal ions in vivo
• At physiological O2 concentration, Fe2+ is readily oxidized to Fe3+,
which is highly insoluble in aqueous solution at neutral pH
Fe3+ (aq) + 3OH- (aq)
Fe(OH)3 (s) Ksp ~ 10-39 M4
• To overcome such serious solubility problem, trace transition metals
require special carrier proteins to bind and transport the ions in vivo
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Iron transport: Transferrin
• Serum transferrin,
melanotransferrin
ovotransferrin,
lactoferrin
and
• Lactoferrin is found in milk, while
• Ovotransferrin in egg white
• Tf isolated from blood plasma is occasionally referred as
serotransferrin while Ovotransferrin as Conalbumin
• Tf are glycoproteins, that bind two Fe(III) ions in similar
but structurally distinct sites termed as N-terminal and Cterminal domains
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Transferrin: a protein
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Features of Tf during iron binding
• The iron binding site is rich in hard ligands, suitable for
Fe(III) binding
• When the iron atom enters the active site, it is
coordinated by one η1-aspartyl, one histidyl and two
tyrosinate side-chains; a non-protein ligand carbonate
(CO32−)
• Carbonate group is held in place within the protein via
hydrogen bonding interactions
• The carbonate coordinates to the iron in an η2 fashion; in
other words, it is a chelating ligand.
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Overall structure of Tf
• Both iron atoms gets deeply buried within the protein structure,
which might be protecting the iron atom from microbial
siderophores
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Iron transport
• Carbonate ion is required for strong
Fe3+ coordination
• Neither Fe3+ nor CO32- binds Tf strongly
on its own, but together binding favours
• Such cooperative behavior is termed as
synergism
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UV-Visible studies of Fe-Tf
(a) Tf at pH 6 (A)
(b) Fe-Tf at pH 6 (B)
(c) Tf at pH 11 (C)
(d) Fe-Tf at pH 11 (D)
245nm
Fe3+ + Tf + CO32-
(Fe3+)Tf(CO32-) + 3H+
~103 s-1
3H+ are from Tyr and Asp.
Step wise:
Fe2+ + Tf + CO32(Fe2+)Tf(CO32-) + xH+
~107 s-1
(Fe2+)Tf(CO32-) +O2
(Fe3+)Tf(CO32-) + O2-
•Tf exhibits two absorption bands
•Fe binding enhances the absorption
Absorption spectrum of native transferrin
showing the tyrosinate-Fe3+ charge
transfer band that develops at 245nm
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UV-Visible studies of Fe-Tf
UV-VIS spectroscopy can be convenient
method
The red color of the Fe3+ protein: due to
a tyrosinate → Fe3+ LMCT
There is also an increased absorbance in the
UV region ca 245 nm and 295nm from the
deprontonated tyrosine
• Change
in
absorbance
monitored at 245 nm
Titration of Conalbumin (Tf)
with Fe(III)
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• Reveals the binding of two
iron/Tf
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Iron transport
• Since all the coordinating atoms in transferrin is hard in
nature, Tf forms stable complex with iron(III).
• The stability constant of the Fe(III)–transferrin complex
is in the order of 1020 mol−1
• Strong stability constant protects the iron(III) against
the low concentration of any siderophores present
• Transferrins show mild antibacterial properties to prevent
extensive iron chelation by siderophores
• Transferrin also forms relatively stable complexes with
other hard metals
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Stability constants of M-Tf complexes
Metal
Log10(stability constant)
Cadmium(II)
5.95
Zinc(II)
7.80
Aluminium(III)
13.50
Iron(III)
22.80
• These data suggests that other metals can also be
transported by transferrin into cells, where they are
potentially harmful.
• Aluminium, is of major concern because it is used widely
in cooking utensils
• Accumulation of Aluminium leads to Alzheimer's disease
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Iron translocation by Tf
• Endocytosis:
Process by
which cell breaks off to form
an internal vesicle
• Membrane bound H+-ATPase
pumps the protons into the
vesicle to drop the pH by 5.5
to release iron
•The free iron then being
transported to the cytosol
aqu. Component of cytoplasm
through the channels
•Fe can then be used for heme
formation, stored in ferritin
or bound by other enzymes
•Fusion with cell membrane and
release of apo-Tf
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Metal Storage: Ferritins
Class of storage protein
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Metal Storage: Ferritins
• In mammals, iron is stored in liver, spleen and bone morrow
• An average person contains 3-4 gm of iron, 35 mg is used
biological process like oxygen transport, redox and enzymatic
reactions
• Major part of Fe is stored in liver, spleen and bone marrow for
mammals.
• Ferritins are a class of iron storage proteins found among the
animal, plant and microbial kingdoms
• Ferritin is the major repository for iron storage in eukaryotes
• This avoids the buildup of large quantities of free iron with
accompanying solubility problems and provides a reserve of iron
in a stable form that is readily accessible
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Structure of Ferritin
• Horse spleen ferritin is composed of 24 subunits assembled into
hollow spherical structure within which iron is stored as a
hydrous ferric oxide mineral core, can hold upto 4500 Fe3+
centers.
•
The core diameter lies between 60-80 Å
• The structure of horse spleen ferritin and apoprotein have been
determined by X-ray crystallography
• The figure shows a schematic drawing of the ferritin molecule
and a diagram of an individual subunit showing the bundle of
helices from which it derives its tertiary structure
• These subunits form a shell around the hollow core that
accommodates the iron ions
Ferritin
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Structure of Ferritin (contd)
• Access to this core is by way of a set of 8 funnel-shaped
channels that possess a 3-fold axis of symmetry and are
lined with hydrophilic residues (Asp, Glu, Ser)
• Another series of channels possesses a fourfold axis of
symmetry and are lined with 12 hydrophobic leucine
residues which provide access for organic reductants
(NADH- Nicotinamide adenine dinucleotide or FADH:
coenzyme) or chelating agents that are required for
release of iron
• In common with other iron-binding ligands, ferritin takes
up iron as M2+ because
1. First, Fe3+ is very insoluble at intracellular pHs
2. Second, Fe2+ is kinetically labile relative to Fe3+ [k (Fe2+) ~107s-1;
k (Fe3+) ~103s-1] which is essential for rapid complexation.
Structure of Ferritin (contd)
•
•
•
•
•
•
•
Iron enters the ferritin core as Fe2+, is subsequently oxidized to
Fe3+ and taken up by the growing crystalline lattice within the
protein core
Oxidation to inert Fe3+also stabilizes iron for long–term storage or
transport
The ferritin core is composed of a microcrystalline array of
ferrihydride phosphate [Fe(O)OH)8(FeOPO3H2)∙xH2PO4] formed
by oxidation of Fe2+ and
hydrolysis of Fe2+ and
uptake of orthophosphate
Experiments with 18O2 and H218O have shown that the oxygen atoms
derive from water, and so oxidation and hydrolysis are discrete
reactions
4Fe2+ + O2 + 4H+ = 4Fe3+ + 2H2O
4Fe3+ + 8 H2O = 4Fe(O)OH + 12H+
Ferritin
• Ferritin possesses eight funnel-shaped hydrophilic channels
• Channels are lined with hydrophilic residues like Asp, Glu and Ser
• Ferritin
core
is
composed
[Fe(O)(OH)8(FeOPO3H2).xH2PO4]
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of
ferrihydrite
phosphate
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Next class
✓ Oxygen carrier and transport
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