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 (t
1/2
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 hem
Fe(II)
Fe(III) has very low solubility) 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 .
H
2
PO
4 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.
•
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)
• Fe 2+ + HCO
3
–
+ Tf = Fe 3+ -Tf-CO
3
2 –
+ e
–
+ 3 H +
• The Fe 3+ 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 Cu II 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(Cu II ) → circulation [Cu II (His)
2
→ Cu II -albumin]
→ liver [Cu II -ceruloplasmin → Cu I -metallothionein] → circulation
[Cu II -ceruloplasmin + Cu II -albumin ]
→ chaperonok → cells [Cu I/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 (Cu I , Zn II , Cd II ,
Hg
2
II , Hg II , Ag I és Co II ) in cluster structure.
Their sulphur and metal contents are very high, may reach 10%.
Generally they consisit of two clusters (3M-
3S
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 Cd 2+ -ions, in a chair conformation [3M-3S] cluster (Cd
3
S
9
) and an adamantane conformation [4M-5S] cluster
(Cd
4
S
11
).
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 Cd II , Hg II , Ag I and
Au I ).
(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
Osteocalcin
plays a role in mineralisation of bones
Structure of bones: Ca 2+ , PO
4
3-
the main inorganic components of bones: Ca
10
(PO
4
)
6
(OH)
2
Other constituents: Mg 2+ , Na + , CO
3
2, Cl , F , citrate, other anions
Mineralisation of bones
Components of bones: Ca 2+ , PO
4
3-
main constituents of bones: Ca
10
(PO
4
)
6
(OH)
2 other ions: Mg 2+ , Na + , CO
3
2, Cl , F , citrate, other anions
Ca 2+ accumulates in the calcification cells
„vesicle"
activation of ATPase, pyrophosphatase
concentration of PO
4
3increases
[Ca 2+ ] 3 [PO
4
3] 2 > L
(precipitation)
Role of collagen as matrix material
Mineralisation of bones
x H
4
SiO
4
The processes of siliciphication
[SiO
4
] 4-
Condensation may occur with alcoholic-OH groups too:
HO OH
Si
HO OH
OH
Gly
H
2
O
Ser Ser
Gly
HO
Si
O
OH
O
Ser
Diatoma
Ser
+ 2 H
2
O
Formation of esters between silicic acid and serin
Processes of siliciphication
Processes of siliciphication
1.
Jellemezze a vas anyagcseréjét! Milyen metalloproteinek vesznek részt benne?
2.
Hasonlítsa össze a transzferrin, a ferritin, a chaperonok és a metallothioneinek fémion kötését szerkezeti, termodinamikai és kinetikai szempontból!
3.
Változott-e a létfontosságú elemek csoportja a kémiai
és biológiai evolúció során? Példákkal igazolja
állítását!
4.
Milyen fontosabb biomineralizációs folyamatokat ismer?
5.
Jellemezze a csontképződés folyamatát!
6.
Mi az a szilicifikációs folyamat és hol van jelentősége?