Electrontransfer proteins. Electrontransfer proteins. In biological systems the elecetrontransfer proteins make possible to carry out oxidation and reduction separated from each other in space. Complex I. Complex III. Complex IV.
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In biological systems the elecetrontransfer proteins make possible to carry out oxidation and reduction separated from each other in space.
A schematic drawing of the mitochondrium respiratory chain (FMN: flavin mononucleotide, FeS: iron-sulphur protein, Q: ubiquinone, Cyt: cytochrome, CuA: type 1 blue copper protein).
Function: Transport electrons to oxidise the substrate and
reduce molecular oxygen. →
„One-electron” change of a metal ion with saturated
coordination sphere in the interactions with electron-
donor or acceptor molecules.
Condition: Molecules with easy oxidability or reducibility.
The electrontransfer molecules should cover wide redox
small organic molecules:
e.g. NAD/NADH, FAD/FADH2,....
A schematic representation of the biological fuel cell
The gross process:
2H2 + O2 → 2H2O
Oxidation of H2 inside
the membrane, reduct-
ion of the O2 outside
the membrane →
electrons are trans-
ferred by cytochromes
while the energy is
stored in the form
of proton/ATP gradient.
Fe-S → cytochrome b → cytochrome c → ciyochrome a → blue copper
Electrontransfer proteins containing hem prosthetic groups,
i.e. they are in relation with hemo- and myoglobins.
Nomenclature: colouring material of the cells,
characteristic absorption band in the range 400-450 (600) nm
Types: based on the hem group:
Main types of iron-sulphur proteins:
[FeIII2(S2−)2]2+ RS−)4 : plant ferredoxin
[FeII2FeIII2(S2−)4]2+(RS−)4 : bacterial ferredoxin and HIPIP
[FeIIFeIII2(S2−)4](RS−)3: „irregular” clusters
(One edge of the cube is empty, which can be occupied by other metal ions, e.g. Ni, V, Mo,...
Further clusters are also possible, e.g. Fe7S8, etc.)
Structures of the iron-sulphur proteins:
2. Plant-type ferredoxin: [FeIII2(S2−)2]2+(RS−)4
In resting state 2 FeIII, but only one of
them is reduced to FeII
FeII−FeIII (1 unpaired electron,
significant Fe-Fe interaction, but not
complete electron delocalisation
3. Fe4-S4 clusters:
[FeIII3FeIIS4]3+ [FeIII2FeII2S4]2+ [FeIIIFeII3S4]+
HIPIPresting state bacterial ferredoxin
+ 350 mV −400 mV
In resting state both Fe4-S4 clusters are in FeIII2FeII2 state and can take up or release only a single electron.
The large difference in the redox potential is explained by the differences in the amino acid sequence of the two proteins.
Occur mostly in plants (preparation from algae)
Participate primarily in photosynthesis, as electrontransfer proteins
(plastocyanin, azurin, cytochrome c oxidase CuA centre).
- low molecular mass (M ~ 10 000 ~ 100 amino acid + 1 db CuII)
- Intense blue colour λ ~ 600 nm, ε ~ 3000 - 5000
- EPR active, low coupling constant (A||)
- high redox potential (ε ~ + 0.3-0.7 V)
(easy to reduce)
Cu(II) - SR Cu(I) +.SR
Cu(II) in unusual chemical environment distorted tetrahedron
(usually: 2 His + 1 Cys + 1Met)
Relatively low molecular mass. M ~ 10.500 (99 amino acids)
Globular protein, cylindrical shape.
420 x 320 x 280 nm → CuII ion in 60 nm depth.
Coordination geometry of CuII-ion: distorted tetrahedron
Bond lengths (nm)
Bonds CuII CuI CuI
Cu-S(Cys) 2.13 2.17 2.13
Cu-S(Met) 2.90 2.87 2.51
Cu-N(His37) 2.04 2.13 2.12
Cu-N(His87) 2.10 2.39 < 4