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
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 electrondonor or acceptor molecules.
Condition: Molecules with easy oxidability or reducibility.
The electrontransfer molecules should cover wide redox
potential range.
Structure:
small organic molecules:
e.g. NAD/NADH, FAD/FADH2,....
metalloproteins
Electrontransfer proteins II.
A schematic representation of the biological fuel cell
The gross process:
2H2 + O2 → 2H2O
Oxidation of H2 inside
the membrane, reduction of the O2 outside
the membrane →
electrons are transferred by cytochromes
while the energy is
stored in the form
of proton/ATP gradient.
Electrontransfer proteins 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.
Electrontransfer proteins IV.
Types and structures of electrontransfer proteins:
1. Cytochromes: hem proteins
may be: cytochrome a, b, c, (f):
they differ in the type of the hem, the form of the hemprotein binding and the value of the redox potential (−200 + 500 mV).
2. Iron-sulphur proteins: [FeII/IIIa(S2−)b(RS−)c]
redox potential: (−700) − 400 - 0.0 (+400) mV
3. 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-sulphur proteins 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-sulphur proteins III.
Structures of the iron-sulphur proteins:
1. 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-sulphur proteins IV.
3. Fe4-S4 clusters:
[FeIII3FeIIS4]3+
HIPIP
[FeIII2FeII2S4]2+
resting state
+ 350 mV
[FeIIIFeII3S4]+
bacterial ferredoxin
−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.
Fe4-S4 Iron-sulphur protein (HIPIP)
Blue copper proteins
Occur mostly in plants (preparation from algae)
Participate primarily in photosynthesis, as electrontransfer proteins
(plastocyanin, azurin, cytochrome c oxidase CuA centre).
Characteristics:
- 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)
Action:
Cu(II) - SR
 Cu(I) + .SR
fast process
Structure:
Cu(II) in unusual chemical environment  distorted tetrahedron
(usually:
2 His + 1 Cys + 1 Met)
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
Cu-S(Cys)
Cu-S(Met)
Cu-N(His37)
Cu-N(His87)
2.13
2.90
2.04
2.10
CuI
CuI
pH=7.0 pH=3.8
2.17
2.87
2.13
2.39
2.13
2.51
2.12
<4
Ellenőrző kérdések
1.
2.
3.
4.
5.
6.
Mi az elektronszállító fehérjék funkciója a biológiai
rendszerekben? Milyen elektronszállító fehérjéket
ismer?
Jellemezze a citokromokat szerkezeti szempontból!
Hasonlítsa össze a vas-kén proteineket és a
citokrómokat szerkezeti sajátságaikat tekintve!
A vas-kén proteinek milyen kéntartalommal
rendelkeznek?
Mi a jellemzője a réztartalmú elektrontranszfer
fehérjéknek?
A kémiai evolúcióban való keletkezésük szempontjából
elemezze a vas-kén fehérjék, a citokrómok és a kékréz
fehérjék családját!