Electrontransfer proteins

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

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

Electrontransfer proteins I.

Function: Transport electrons to oxidise the substrate andreduce 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 redoxpotential 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 insidethe membrane, reduct-ion of the O2 outside the membrane →electrons are trans-ferred by cytochromeswhile the energy isstored 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 changesin 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 changein the structure of the metal complex, e.g. it should be a low energyprocess.

Electrontransfer proteins IV.

Types and structures of electrontransfer proteins:

1. 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).

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 saturatedaxial 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 hemand 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

[FeII2FeIII

2(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 asignificant 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 notcomplete electron delocalisation

Iron-sulphur proteins IV.

3. Fe4-S4 clusters:

[FeIII3FeIIS4]3+ [FeIII

2FeII2S4]2+ [FeIIIFeII

3S4]+

HIPIP resting state bacterial ferredoxin+ 350 mV −400 mV

In resting state both Fe4-S4 clusters are in FeIII2FeII

2 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)

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) + .SRfast process

Structure:Cu(II) in unusual chemical environment distorted tetrahedron(usually: 2 His + 1 Cys + 1 Met)

Blue copper proteins

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 2.13Cu-S(Met) 2.90 2.87 2.51Cu-N(His37) 2.04 2.13 2.12Cu-N(His87) 2.10 2.39 < 4

Ellenőrző kérdések

1. Mi az elektronszállító fehérjék funkciója a biológiai rendszerekben? Milyen elektronszállító fehérjéket ismer?

2. Jellemezze a citokromokat szerkezeti szempontból!3. Hasonlítsa össze a vas-kén proteineket és a

citokrómokat szerkezeti sajátságaikat tekintve!4. A vas-kén proteinek milyen kéntartalommal

rendelkeznek?5. Mi a jellemzője a réztartalmú elektrontranszfer

fehérjéknek? 6. A kémiai evolúcióban való keletkezésük szempontjából

elemezze a vas-kén fehérjék, a citokrómok és a kékréz fehérjék családját!