1

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

3

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

4

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

5

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-

6

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)

7

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+).

8

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

9

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)

10

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

?

11

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

12

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

13

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

14

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