3) metals in biological systems i (1) - bcp.fu-berlin.de · - smaller ion radius of low-spin fe2+...
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3) Metals in biological systems I (1)
Sodium, potassium
- There are significant differences in the ion distribution between cytoplasma and
blood
- active and passive mechanisms are required to maintain the concentration
difference
Blood plasma Cytoplasma Stomach
- Two mechanisms for metal transport
passing membranes:
passive diffusion
active transport
- Problem: membrane passage
Membrane
3) Metals in biological systems I (2)
Sodium, potassium ion pump / ATPase
-Transport opposite to the concentration gradient
- limited speed (400 – 500 ions per second)
- much energy required (typical mammalian cell spend up to 30% of their energy on
this pump (ATPase)
- Other ion pumps exist for other ions
and molecules
-glucose, amino acids
- HCO3-/Cl- Antiport system as a part
of the CO2 transport out of the
erythrozytes
2
3) Metals in biological systems I (3)
Potassium channel
-Transport through channel is size controlled
- Hydratisation (size vs. energy) of the ions play an decisive role in ion separation
Ion Diameter (A) Hydratisation energy (kcal/mol)
Na+ 1.90 -105
Ca2+ 1.98 -397
K+ 2.66 -85
Cl- 3.62 -82
Inside
Outside
3) Metals in biological systems II, Iron treatment (1)
Iron mobilisation and uptake, Siderophores
- Iron resources in inorganic chemistry contain almost exclusively Fe(III) and/or are
almost insoluble
- For iron mobilisation, powerful ligand systems are required, the complex
constants of which are competitive with the solubility constants of the minerals
- Frequent structure motifs: catecholates, hydroxamates (also protein bonding
possible)
- formation in microorganisms (bacteria, funghi)
- Release after uptake by pH- and redox-controled mechanisms
COR
NR' O H
O H
O H
Hydroxam ate Catecholate
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3) Metals in biological systems II, Iron treatment (1)
Iron mobilisation and uptake, Siderophores
Typical siderophores
Siderophore ligand Log K1 (FeIII complex) E0 (mV), pH 7 Ligand type
Coprogen 30.2 -447 Hydroxamate
Desferrioxamine B 30.5 -468 Hydroxamate
Ferrichtom A 32.0 -448 Hydroxamate
Aerobactin 22.5 -336 Hydroxamate,
Carboxylate
Enterobactin ca. 52 -790 (pH 7.4) Catecholate
Mugeinic Acid
(phytosiderophore)
18.1 -102 Carboxylate,
Amino acids
3) Metals in biological systems II, Iron treatment (3)
Iron mobilisation and uptake, Siderophores
Typical siderophores
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3) Metals in biological systems II, Iron treatment (4)
Iron transport
General iron transport proteins: transferrin
Design:
- Proteins with two domains, each being
able to bind one Fe3+ ion together with
one HCO3- ion
Function:
- Transport of iron between place of
resorption and storage proteins of iron
using cells
-Transferrins are not ion-specific (bind
also other M3+ ions and even Fe2+)
- Coordination of the metal by:
- tyrosinate
- aspargate
- histidine
- HCO3- (protein-bonded via arginine)
Transferrin molecule with two Fe atoms
3) Metals in biological systems II, Iron treatment (5)
Iron transport
Transferrin: iron-binding site
Iron bonding:
- Protein-bonded metal ion
- Binding realized by side chains of
amino acids
- 2 x tyrosin (phenolic OH)
- aspargic acid (carboxylic)
- histidine (imidazol)
- HCO3- (hydrogen-bonded to the
protein via the acidic amino acid
arginine)
- Control of the HCO3- binding
controls iron uptake and
release
Arginine
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3) Metals in biological systems II, Iron treatment (6)
Iron transport
Transferrin: iron uptake and release
pH-dependent mechanism
- Protein-bonded metal ion
- relatively strong coordinative inter-
actions
- H-bonding of HCO3-, however,
is pH-dependent
- uptake and release of HCO3-
causes a change in the
conformation of the protein
Strict coordination chemical control
a biological function
Transport between Fe3+ transport protein transferrin and the Fe3+ storage protein ferritin is
pH and redox-controlled
- Fe2+ is good water-soluble, but toxic → strict control and masking is required
- Organic chemistry provides suitable reducing agents
- Coordination chemistry provides ‘designed’ ligands for this task
3) Metals in biological systems II, Iron treatment (7)
Iron management between transport and storage
Reduction of Fe3+ by dihydroxyfumaric acid and formation of [Fe(H2O)6]3+
OH
O
OH
OH
OH
O
Dihydroxyfum arsäureDihydroxyfumaric acid
- The small [Fe(H2O)6]2+ ions can
pass the hydrophilic (trigonal) pores
of the storage protein ferritin
- Fast complex formation with the
protecting ligand ferrozine
- Drastic decease of the concentration
of the free (toxic) Fe2+ ion
N N
N
N
H
H
HH
H H
HH
H
H
H
H
S
O
O
O
SO
O
O
Ferrozin 2- -L igand
Fe
NN
N
N
S
O
O
O
SO
O
O
N
N
N
N
S
OO
O
S
O
O
O
NN
NN
S
O
O
O
S
O
O
O
[Fe(Ferrozin)3]4-
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3) Metals in biological systems II, Iron treatment (8)
Iron storage with ferritin
Ferritin
- Large protein consisting of 24 subunits (Morganic ≈ 400,000)
- Stucture of an empty ball (outside diameter: 13 nm, inside diameter: 7.5 nm)
- can accommodate up to 4500 Fe atoms (mean value: 1200)
- Arrangement of the Fe atoms similar to the mineral ferrihydrite (5Fe2O3 x 9 H2O)
Ferritin subunit Arrangement of 24 sub units in ball like shape Part of ferrihydrite
Trigonal pores
Hydrophilic
Tetragonal pores
hydrophobic
Oxygen
iron
3) Metals in biological systems II, Iron functions (9)
Oxygen transport and storage – hemoglobin, myoglobin
Different oxygen species, wich may play a role
Bond
order
Bond
Length (pm)
Vibration
frequency (cm-1)
Orb
ital
en
erg
y
O2 has a triplett state
- Paramagnetic molecule
- relatively inert
- long reaction times with diamagnetic
substrates (spin-forbidden reactions)
However!
Spin-forbidden state does not apply for
paramagnetic species (radicals, transition
metal complexes with unpaired electrons,
excited triplett state (photochemistry)
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3) Metals in biological systems II, Iron functions (10)
Iron transport
Dioxygen as ligand
- Ligand with σ-donor and π-acceptor
properties
- Various bonding modes are possible
- species at various redox states may
play a role during the coordination
- Internal redox chemistry is possible by
distinct back donation mechanisms
Please remember !
- The ability to use the oxidizing species O2 for the production of energy is an evolutionary
advantage for organism.
- Oxidation of reduced carbon atoms (food) and production of H2O and CO2
- This, however, requires a suitable management of dioxygen and other oxygen species
Oxygen transport and storage – hemoglobin, myoglobin
3) Metals in biological systems II, Iron functions (11)
Myoglobin (monomeric) Hemoglobin (tetrameric)
Hemoglobin
- 4 haem units
- oxygen transport
Oxygen transport and storage – hemoglobin, myoglobin
Identical bonding centres (haem) !!!!
Myoglobin
- 1 haem unit
- oxygen storage
Low oxidation state of Fe2+ is maintained by tertiary structure of the protein. Free heam
would be oxidized immediately under formation of hematin (Fe3+).
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3) Metals in biological systems II, Iron functions (12)
Oxygen transport and storage – hemoglobin, myoglobin
The O2 affinity of hemoglobin
depends:
-The pH value
- The O2 load (1, 2 or 3)
This allows O2 uptake and release
by the same molecule
The compound for oxygen storage must have a higher affinity to iron than the compound for
iron transport
Partial pressure O2 (mm Hg)
Satu
rati
on
(p
er
cen
t)
Partial pressure
In muscle
Partial
pressure
In lung
3) Metals in biological systems II, Iron functions (13)
Oxygen transport and storage – hemoglobin, myoglobin
- Fe2+ is bonded to porphine ring by 4
Fe-N bonds (out of plane)
- One axial coordination position is
occupied by one histidine, which
connect the hem to the protein
- sixth coordination is empty and can
be occupied by O2
- Close to the sixth position an distinal
histidine residue is located, which
influences coordination mode of the
incomming ligand (bent coordination
preferred)
The chemistry of the O2-bonding
One of the four hem subunits
Histidin
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3) Metals in biological systems II, Iron functions (14)
Oxygen transport and storage – hemoglobin, myoglobin
- high-spin Fe2+ is bonded to porphine
ring by 4 Fe-N bonds (out of plane)
- Coordination of O2 results in change
of the spin-state (high-spin → low-spin)
- smaller ion radius of low-spin Fe2+
allows to fit the in-plane coordination
- Fe2+ ion moves into the ring
- This movement is transferred to the
protein by the axial histidine
-The protein changes its conformation
and brings the next hem ring into
position
Consequence: The more O2 is bonded,
the higher is the affinity and vice versa.
→ Cooperative Effect
The chemistry of the O2-bonding
3) Metals in biological systems II, Iron functions (15)
Oxygen transport and storage – hemoglobin, myoglobin
Possible oxidation and spin states in the system Hem/O2
Desoxy form
(paramagnetic) S = 2
Oxy form
(diamagnetic) S = 0
Fe2+ (d6 high-spin)
almost octahedral
symmetry
Fe3+ (d5 low-spin)
+ 2O2●- (bonded)
+ strong antiparallel
spin-spin coupling
Fe2+ (d6 low-spin)
+ 1O2 (bonded)
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3) Metals in biological systems II, Iron functions (16)
Possible oxidation and spin states in the system Hem/O2
Pauling Weiss
Oxygen transport and storage – hemoglobin, myoglobin
3) Metals in biological systems II, Iron functions (17)
Support of the bent coordination mode of O2 by an additional histidin residue
Oxygen transport and storage – hemoglobin, myoglobin
Proximal
histidine
Distinal
histidine
Hydrogen bond
?
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3) Metals in biological systems II, Iron proteins (18)
Iron proteins in the human body – function, size and structure
Protein M (kDa) Mass Fe
(g)
% of overall
Fe in
organism
Hem (h) or
not-Hem
(nh)
Number of
Fe atoms
per protein
Function
Hämoglobin 64.5 2.60 65 h 4 O2 transport in
blood
Myoglobin 17.8 0.13 6 h 1 O2-storage in
muscel
Transferrin 76 0.007 0.2 nh 2 Fe transport in
plasma
Ferritin 444 0.52 13 nh Up to 5000 Fe storage
Katalase 260 0.004 0.1 h 4 Metabolism of
H2O2
Peroxidases var. low low h Frequently 1 Metabolism of
H2O2
Cytochrom c 12.5 0.004 0.1 h 1 Elektron transfer
Cytochrom c-Oxidase >100 <0.02 <0.5 h 2 Terminal oxidation
(O2H2O)
Flavoprotein-Oxygenasen
(e. g. P-450-system)
ca. 50 low low h 1 O2 insertion
Iron/Sulphur Proteins var. ca 0.04 ca. 1 nh 2-8 Elektron transfer
Ribonukleotid-Reduktase 260
(E. coli)
low low nh 4 Conversion of
RNA into DNA
3) Metals in biological systems II, Iron proteins (19)
Biocatalysis with iron proteins
Typical Hem proteins:
- Katalase (H2O2 metabolism)
- Peroxidase (H2O2 metabolism)
- Cytochrome c (electron transfer)
- Cytochrom c oxidase (terminal O2 oxidation)
- P450 proteins (insertion of molecular oxygen)
General Functions:
- Electron transfer and electron accumulation
- Controlled conversion of oxygen-containing intermediated (O22-, NO2
-, SO32-)
- Stepwise, controlled metbolism of reduced carbon compounds under formation of
CO2 and H2O
- Frequently in cooperation with other metal centres
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3) Metals in biological systems II, Iron proteins (20)
Cytochroms
- Electron transfer proteins with hem centres contributing to dioxygen degradation
- relatively small proteins (12-13 kD)
- Coordination number 6 (4 x porphyrin, 1 x histidin, 1 x methionin)
- promotion of electron transfer by 3-dimensional orientation
3) Metals in biological systems II, Iron proteins (21)
Hem proteins in the degradation of partially reduced nitrogen or
sulphur compounds
- use of the non-occupied coordination position of the hem unit
e.g. nitrite reductase
NO2- + 6 e- + 8 H+ NH4
+ + 2 H2O
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3) Metals in biological systems II, Iron proteins (22)
Hem proteins in the degradation of partially reduced nitrogen or
sulphur compounds
-Hem centers often interplay with other metal centers (e.g. iron-sulphur clusters)
3) Metals in biological systems II, Iron proteins (23)
Non-hem iron proteins
- Frequent coordination sites: histidin, tyrosin, aspartat, gutamin
- Binuclear reaction centers frquently possess oxo bridges or are carboxylate bridges
Frequently catalysed reactions: Enzyme reaction
Hydrogenase 2 H+ + 2 e- ⇋ H2
Nitrogenase N2 + 10 H+ + 8 e- ⇋ 2 NH4+ + H2
Sulfit reductase SO32- + 7 H+ + 6 e- ⇋ HS- + 3 H2O
Aldehyde oxidase R-CHO + 2 OH- ⇋ R-COOH + H2O + 2 e-
NADP-Oxidoreductase NADP+ + H+ + 2 e- ⇋ NADPH
Ribonucleotid reductase
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3) Metals in biological systems II, Iron proteins (24)
Iron-sulphur proteins
- Essential components of electron transfer proteins with one or more Fe atoms
- coordinative bonds to proteins via cystein
- connection of Fe atoms via S2- ligands
- various structures are known
Ferredoxin:
[Fe4S4(Cyst)4]3- ⇋ [Fe4S4(Cyst)4]
2- ⇋ [Fe4S4(Cyst)4]-
3) Metals in biological systems II, Iron proteins (25)
Iron-sulphur proteins
Protein Cluster Centra Fe unit Oxidation state of Fe EPR (g value)
Rubredoxin 1Fe-0S ox. Fe3+
red. Fe2+
4,3; 9
keine
2Fe-Ferredoxin 2Fe-2S ox. 2Fe2+
red. 1Fe3+, 1Fe2+
keine
1,89; 1,95; 2,05
3Fe-Ferredoxin 3Fe-4S ox. 3Fe3+
red. 2Fe3+, 1Fe2+
1,97; 2,00; 2,02
keine
4Fe-Ferredoxine 4Fe-4S ox. 3Fe3+, 1Fe2+
int. 2Fe3+, 2Fe2+
red. 1Fe3+, 3Fe2+
2,04; 2,04; 2,12
keine
1,88; 1,92; 2,06