Transcript Absorption, transport, storage of metal ions. Biomineralisation A

Absorption,transport,storage
Biomineralisation
Absorption, transport, storage of metal ions.
Biomineralisation
A balanced distribution of the elements inside and outside the
cells requires:
(1) Mechanisms for selective capturing trace quantities of essential
mineral ions in the extracellular environment; e.g. solubilisation of
mineral precipitates.
(2) Carrying charged
ions accross
hydrophobic
membranes.
(3) Transport of ions
within the cell and
their storage for
later use.
Transport and storage of iron in human organism
Metabolism of iron
1. Absorption:
- Daily iron transport: 10 – 20 mg iron/adult human
- Daily absorbed amount:  1 mg Fe/adult human
(hemoglobin decomposition: a hem and a globin decomposed and
excreted,
while most part of the iron is stored in the storing proteins (t1/2  20-30 y)
- feedback mechanism:
the amount of the absorbed iron is determined by the
saturation level of the iron storing proteins.
- Site of absorption: duodenum, upper part of small intestine
- Affecting factors: pH, solubility, (Fe(OH)3 has very low solubility)
hem  Fe(II)  Fe(III)
promote: ascorbic acid, citric acid, amino acids (Cys)
hinder: stable complexants of iron(III)
(polyphenols, e.g. tannins, tee, red wine
polyphosphates, e.g. phytic acid in plant seeds)
metal ions, Zn(II), Ca(II) (competition at high concentrations)
Storage of iron I.
The transport/storage of iron in higher organisms are performed by
transferrin/ferritine, while in microorganisms by siderophores.
Apoferritin:
M ~ 450 000,
24 protein subunits (~ 175 amino acids/subunit)
diameter: 1300 nm, 2 channels for uptake and release of iron; formed
from the hydrophylic and hydrophobic side chains of the protein.
Binding of iron:
~ 4500 iron atoms/ ferritin  (~ 1 Fe/1 amino acid !, 25% iron content)
~ 700 nm d iron containing micelle
approx. composition: (FeOOH)8.FeO.H2PO4
oxo-, hidroxo-bridged iron(III)octahedrons
strong antiferromagnetic coupling between the iron(III) ions
phosphate: „cover layer"  link between the iron core and the protein
uptake of iron: in the form of iron(II), then oxidation to iron(III)
Hemosiderin:
it functions in case of iron overload
iron(III)-oxide-hidroxide-phosphate
- less ordered
- higher iron (~ 40 %) and phosphate content
Storage of iron II.
Schematic structure of apoferritin
Structure of ferritin from protein crystals
Storage of iron III.
Channels for
uptake/release of
iron in ferritin
Electronmicroscopic picture of ferritin
The more saturated the protein protein-hollow the more regular
octahedral structures are the iron(III)-oxide-phosphate clusters.
Transferrins I.
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Transferrins (ovotransferrin, lactoferrin and serum transferrin) are 
8 kDa molecule mass glycoproteins,
They consist of two subunits, 1-1 Fe binding sites (log K ~ 22)
Binding site: 2 Tyr-O-, 1 His-N, 1 Asp-COO-, 1 bidentate carbonate (in
H-bonding with Arg and Thr side chains and 2 peptide-NH groups)
Fe2+ + HCO3– + Tf = Fe3+-Tf-CO32– + e– + 3 H+
The Fe3+ reaches the cell through the membrane by receptor
mediated endocytosis:
cellmembrane
pH > 5.5
pH < 5.5
Transferrins II.
(structure of human lactoferrin)
It consists of two subunits, each of them containing 1 iron atom.
Transferrins III.
(iron binding site in human lactoferrin)
Absorption, transport and storage of copper
Metabilitic processes of copper is much less explored than that of
iron.
This might be explained by the less amount of the metal in the
organisms and its many different functions.
1. Absorption and transport of copper:
Absorption of copper occurs in the form of CuII in the GI tract in lmm
amino acid complexes and reaches the circulatory system bound to
human serum albumin. It is transported to the liver by albumin, where
ceruloplasmin is synthesised and bound to this protein copper partly
get back to the circulatory system.
stomach(CuII) → circulation [CuII(His)2 → CuII-albumin]
→ liver [CuII-ceruloplasmin → CuI-metallothionein] → circulation
[CuII-ceruloplasmin + CuII-albumin] → chaperonok →
cells [CuI/II-containing enzymes]
Distribution of copper in the circulation:
~ 0,1 % in Cu(His)2 complex
~ 5-10 % in Cu(II)-albumin complex
~ 90-95 % bound to ceruloplasmin.
Based on these data ceruloplasmin was considered earlier as the
copper transporter, but more recent data point to the role of
albumin.
Albumin binds copper unusually in an oligopeptide-like manner at
the N-terminus. This binding mode has high termodynamic stability
but kinetically labile, in contrast with the inert copper
ceruloplasmin bond.
Human albumin: AspAlaHis.........
(HIS at position 3 provides extreme stability.)
Dog albumin: GluAlaTyr.....
(The „copper tolerability” of dogs is significantly lower than
that of humen)
Albumin is the primary copper transporter for the cells. However,
other proteins may also play important roles in transferring copper
accross cell membranes and transporting copper in the cells. They
are called as „copper-chaperons”, which are specific and usually Cys
rich copper transporter proteins. (Similar roles are assumed in case
of the prion proteins.)
2. Storage of copper
Copper is stored mostly in the liver (spleen, bile).
The „cuprein” proteins had been considered earlier as copper stores,
but more recent results point to the role of certain enzymes, e.g. erythrocuprein = CuZn-SOD.
Today it is thought that metallothioneins are the primary copper
storage proteins.
Thionein: low molecular mass Cys rich proteins (polypeptides)
extreme high soft metal ion affinity.
Metallothioneins
A metallothioneins occur in humen, in
animals and plants (phytochelatins), they
are low molecular mass (6-7 kDa) proteins,
which bind soft metal ions (CuI, ZnII, CdII,
Hg2II, HgII, AgI és CoII) in cluster structure.
Their sulphur and metal contents are very
high, may reach 10%.
Generally they consisit of two clusters (3M3S és 4M-5S), in which the metal ions
coordinate through Cys-thiolates. The
polypeptid part features repeated Cys-X-Cys
sequents, in which X stands for a non-Cys
amino acid. In the middle of the Figure 12
terminal and 8 bridging CYS side chain bind
all together 7 Cd2+-ions, in a chair
conformation [3M-3S] cluster (Cd3S9) and an
adamantane conformation [4M-5S] cluster
(Cd4S11).
Metallothioneins
Their basic functions depend on the
organism and the peptide variants:
(1) As metal storing proteins they
participate in the homeostasis of metal
ions first of all that of copper and zinc.
(2) As detoxification molecules they are
active in the removal of detrimental soft
metal ions (such as CdII, HgII, AgI and
AuI).
(3) Their synthesis is induced by some
essential, Zn and Cu, but some toxic
metal ions, Cd, too.
(4) Inorganic-Hg does, but organic-Hg does
not induce formation of metallothioneins.
Calcium binding proteins
Extracellular proteins:
Osteocalcin
 plays a role in mineralisation of bones
Structure of bones: Ca2+, PO43-  the main inorganic
components of bones: Ca10(PO4)6(OH)2
Other constituents: Mg2+, Na+, CO32-, Cl-, F-, citrate, other
anions
Mineralisation of bones

Components of bones: Ca2+, PO43-  main
constituents of bones: Ca10(PO4)6(OH)2
other ions: Mg2+, Na+, CO32-, Cl-, F-, citrate, other anions
 Ca2+ accumulates in the calcification cells 
„vesicle"
 activation
of
ATPase,
pyrophosphatase

concentration of PO43- increases  [Ca2+]3[PO43-]2 > L
(precipitation)

Role of collagen as matrix material
Mineralisation of bones
The processes of siliciphication
[SiO4]4-
x H4SiO4
Condensation may occur with
alcoholic-OH groups too:
Diatoma
HO
OH
Si
HO
H2O
HO
OH
Ser
+ 2 H2O
Si
O
O
Ser
Ser
OH
Gly
Gly
OH
Ser
Formation of esters between silicic acid and serin
Processes of siliciphication
Processes of siliciphication
Ellenőrző kérdések
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Jellemezze a vas anyagcseréjét! Milyen metalloproteinek vesznek részt benne?
Hasonlítsa össze a transzferrin, a ferritin, a
chaperonok és a metallothioneinek fémion kötését
szerkezeti, termodinamikai és kinetikai szempontból!
Változott-e a létfontosságú elemek csoportja a kémiai
és biológiai evolúció során? Példákkal igazolja
állítását!
Milyen fontosabb biomineralizációs folyamatokat
ismer?
Jellemezze a csontképződés folyamatát!
Mi az a szilicifikációs folyamat és hol van jelentősége?