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

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 winepolyphosphates, e.g. phytic acid in plant seeds)metal ions, Zn(II), Ca(II) (competition at high concentrations)

Metabolism of iron

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)octahedronsstrong antiferromagnetic coupling between the iron(III) ionsphosphate: „cover layer" link between the iron core and the proteinuptake of iron: in the form of iron(II), then oxidation to iron(III)

Hemosiderin:it functions in case of iron overloadiron(III)-oxide-hidroxide-phosphate

- less ordered- higher iron (~ 40 %) and phosphate content

Storage of iron II.

Structure of ferritin from protein crystals Schematic structure of apoferritin

Channels for uptake/release of iron in ferritin

Storage of iron III.

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)

• Fe2+ + HCO3– + Tf = Fe3+-Tf-CO3

2– + e– + 3 H+

• The Fe3+ reaches the cell through the membrane by receptor mediated endocytosis:

cellmembranepH > 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 stabilitybut 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)

2. Storage of copperCopper 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. erythro-cuprein = CuZn-SOD.Today it is thought that metallothioneins are the primary copperstorage proteins.Thionein: low molecular mass Cys rich proteins (polypeptides)

extreme high soft metal ion affinity.

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.)

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 (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 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.

Metallothioneins

Extracellular proteins:

Osteocalcin plays a role in mineralisation of bones

Calcium binding proteins

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[PO4

3-]2 > L

(precipitation)

Role of collagen as matrix material

Mineralisation of bones

The processes of siliciphication

x H4SiO4 [SiO4]4-

Condensation may occur with alcoholic-OH groups too:

Si

OHHO

OHHO Si

OHHO

OO

Gly Ser Ser

+ 2 H2O

OHH2O

Gly Ser Ser

Formation of esters between silicic acid and serin

Diatoma

Processes of siliciphication

Processes of siliciphication

Ellenőrző kérdések

1. Jellemezze a vas anyagcseréjét! Milyen metallo-proteinek 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?