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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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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

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).


Electrontransfer proteins I.

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

potential range.


small organic molecules:

e.g. NAD/NADH, FAD/FADH2,....



Electrontransfer proteins II.

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.


Electrontransferproteins III.

  • Conditions of electron transfer:
  • The various proteins should cover a wide redox potential range
  • (e.g. Fe-S → blue copper proteins ~ − 0.4 - + 0.7 V)
  • The coordination sphere of the metal ion should be saturated and
      • should not change practically during electron transfer.
  • Change in the oxidation state should not be accompanied by changes
      • in the coordination geometry, bond length/bond angles. →
    • the specific coordination geometry should be suitable for both
    • oxidation states of the metal ion.
    • Electron uptake and removal should not result in significant change
    • in the structure of the metal complex, e.g. it should be a low energy
    • process.

Electrontransferproteins IV.

  • Types and structures of electrontransfer proteins:
  • Cytochromes: hem proteinsmay be: cytochrome a, b, c, (f): they differ in the type of the hem, the form of the hem-protein binding and the value of the redox potential (−200 - + 500 mV).
  • Iron-sulphur proteins:[FeII/IIIa(S2−)b(RS−)c]redox potential: (−700) − 400 - 0.0 (+400) mV
  • Blue copper proteins:distorted terahedral CuII complexes [CuIIN(His),N(His),S(Cys),S(Met)]redox potential: + 300 - +700 mV

Oxidation chain:

Fe-S → cytochrome b → cytochrome c → ciyochrome a → blue copper


Cytochromes I.

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:


Cytochromes II.

  • Structural features of cytochromes:
  • The coordination sphere of FeIII/II ions should be saturated axial amino acids: His, Met, (Lys, Cys, Tyr)
  • In the case of cytochrome a and b the hem is bound strongly but not
      • covalently to the protein. In the case of cytochrome c the hem
      • and the protein bind covalently.
      • Most of the cytochromes are 1:1 units (1 hem + 1 protein), but there are cytochromes with multiple hem units.
  • The cytochromes always participate in one electron processes: FeII → FeIII reversible electron transfers.
  • Redox potential values: (fairly wide range) characteristic values: cytochrome a: ~ + 400 mV cytochrome b: ~ + 0.0 mV cytochrome c: ~ + 260 mV

Iron-sulphur proteins I.

  • They can be characterised by the following general composition:
  • [FeII/IIIaS2−b](RS−)c
  • where: S2−: sulphide ion (inorganic sulphur)
  • RS−: protein Cys side chain (organic sulphur)
  • General feature:
  • widely spread in mammals and in plants
  • participate in one electron steps
  • the reduction potential is usually negative (0 - − 400 with the exception of HIPIP: High Potential Iron Protein = +350 mV)
  • - usually electrontransfer coenzymes of enzyme systems, but e.g. the aconitase itself is an enzyme.

Iron-sulphurproteins II.

Main types of iron-sulphur proteins:

[FeIII(RS−)4]: rubredoxin

[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.)


Iron-sulphurproteins III.

Structures of the iron-sulphur proteins:

  • rubredoxin, [FeIII(RS−)4]:
  • Iron(III) ion is in a little distorted tetrahedral
  • environment. Reduction is not accompanied by a
  • significant change in geometry.
  • Electrontransfer component of the sulphur-bacteria.

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


Iron-sulphurproteins IV.

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.


Blue copper 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

fast process


Cu(II) in unusual chemical environment distorted tetrahedron

(usually: 2 His + 1 Cys + 1Met)


Plastocyanin I.

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.


Plastocyanin II.

Coordination geometry of CuII-ion: distorted tetrahedron

Bond lengths (nm)

Bonds CuII CuI CuI

pH=7.0 pH=3.8

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

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