Download Fe 4 S 4 Cys 4

Electron transfer in biological
systems
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Biological electron transfer
• http://highered.mcgrawhill.com/sites/0072437316/student_view0/
chapter9/animations.html#
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Kinetics of electron transfer reactions
• Electron transfer between 2 metal centers in
metalloproteins is always via outer-sphere
mechanism (no bridging ligand, coordination
spheres remain essentially the same for both
metal ions)
• Reasonably fast (> 10 s-1) over large distances
(up to 30 Å)
• Can be rationalised by Marcus Theory
(see Shriver/Atkins, 4th edition p. 516ff)
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Marcus Theory: Key points
• For DG0 = - l , activation energy DG#
becomes = 0: “activationless” e-transfer
• Fast reactions if DG0 and l are similar to
one another
 there are “ideal” combinations of
reaction enthalpy and reorganization
energy
Often observed in biological systems:
Small values for both
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e- transfer proteins
Cytochromes
Fe-S proteins
Blue copper proteins
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Examples for efficient electron
transfer units (1): Cytochromes
• Name comes from the fact that they are coloured
• Differ by axial ligands and whether covalently
bound
• Involved in electron transfer (a,b,c) or oxygen
activation (P450)
• Essential for many redox reactions
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UV-Vis Spectra of cytochromes
• classified by a bands:
• a: 580-590 nm
• b: 550-560 nm
• c: 548-552 nm
• (there’s also d and f)
• all involved in electron
transfer, all CN6
• P450: 450 nm:
• Oxygen activation; CN5
Absorption spectra of oxidized (Fe(III)) and reduced (Fe(II)) horse cytochrome c. 8
Cytochrome c
• Small soluble proteins
(ca. 12 kDa)
• Near inner membrane of
mitochondria
• Transfers electrons
between 2 membrane
proteins ( for respiration)
• Heme is covalently linked
to protein via vinyl
groups (thioether bonds
horse heart cytochrome c
Bushnell, G.W., Louie, G.V., Brayer,
with Cys)
G.D. J.Mol.Biol. v214 pp.585-595 ,
1990
• 1 Met and 1 His ligand
•Conserved from bacteria to (axial)
Man
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Cytochromes b
• Heme has no covalent
link to protein
• Two axial His ligands
•
Shown is only soluble domain;
the intact protein is bound to
membrane
F Arnesano, L Banci, I Bertini, IC Felli:
The solution structure of oxidized
rat microsomal
cytochrome b5. Biochemistry (1998) 37, 173-84.
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Why e- transfer in cytochromes is
efficient
• Porphyrin ring provides rigid scaffold:
No significant changes in structure
between Fe(II) and Fe(III) forms:
relatively small reorganisation energy
• Electron is delocalised over porphyrin
ring: can be transferred efficiently over
edge of ring
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Not for electron transfer:
the cytochromes P450 are oxygenases
• CN5, axial ligand is
a Cys
• 6th site for
substrate/oxygen
binding
• Hydroxylates
camphor
P450Cam
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Tuning of heme function
• In (deoxy)hemoglobin, Fe(II) is 5-coordinate
• Must avoid oxidation to Fe(III) (Met-hemoglobin)
• Neutral His ligand: His-Fe(II)-porphyrin is
uncharged: Favourable
• P450: Catalyses hydroxylation of hydrophobic
substrates. Also 5-coordinate
• 1 axial Cys thiolate ligand (negatively charged):
Resting state is Fe(III), also uncharged
• In cytochromes, CN=6: No binding of additional
ligand, but very effective 1 e- transfer
• Neutral ligands (Met or His): Fe(II) more
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stabilised than Fe(III)
Examples for efficient electron
transfer units (2): Fe-S proteins
• Probably amongst the first enzymes
• Generally, Fe (II) and (III), Cys thiolate and
sulfide
• Main function: fast e- transfer
• At least 13 Fe-S clusters in mitochondrial
respiration chain
• Rubredoxins: mononuclear FeCys4 site
• Ferredoxins: 2,3 or 4 irons
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Rubredoxins: FeCys4
X-ray Structure of
RUBREDOXIN from
Desulfovibrio gigas at 1.4 A
resolution.
FREY, M., SIEKER, L.C.,
PAYAN, F.
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1rfs: Spinach
Fe2S2(Cys-S)4
1 awd: CHLORELLA FUSCA
Fe2S2(Cys-S)2-(His-N)2: Rieske
proteins
Fe3S4(Cys-S)4
Fe4S4(Cys-S)4
1fda: Azotobacter vinelandii
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Fe-S clusters can be easily synthesised from
Fe(III), sulfide and organic thiols, but are prone
to rapid oxidation in air
Richard Holm
Self-assembly of Fe-S clusters
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Delocalisation of electrons: Mixed
valence
localized Fe3+ (red) and
localized Fe2+ (blue)
sites, and
delocalized Fe2.5+Fe2.5+
pairs (green)
Why e- transfer is fast:
• Clusters can delocalize
the “added” electron
• minimizes bond length
changes
• decreases
reorganization energy
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Fe-S proteins often contain more than one cluster:
Fe-only hydrogenase
from Clostridium
pasteurianum
• Activation of H2
• Active site (binuclear
Fe cluster) on top
• The other five Fe-S
clusters provide longrange electron transfer
pathways
Pdb 1feh
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Nitrogenase (Klebsiella pneumoniae)
• Catalyses
nitrogen fixation
• P cluster
• FeMoCo cofactor
cluster
N2 + 8H+ + 8e- + 16 ATP → 2NH3 + H2 + 16ADP + 16 Pi 20
Redox potentials
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Tuning of redox potentials
• For both heme proteins and Fe-S clusters,
ligands coarsely tune redox potential
• In [4Fe-4S] clusters, proteins can stabilise a
particular redox couple through:
(a) solvent exposure of the cluster
(b) specific hydrogen bonding networks
especially NH-S bonds
(c) the proximity and orientation of protein
backbone and side chain dipoles
(d) the proximity of charged residues to the
cluster
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Tuning of redox potentials:
Stabilisation of different redox states
via weak interactions
• Bacterial ferredoxins and HiPIPs: Both have
Fe4S4Cys4 clusters
• -400 mV vs. +350 mV
• Ferredoxins: [Fe4S4Cys4]3- → [Fe4S4Cys4]2• HiPIPs: [Fe4S4Cys4]2- → [Fe4S4Cys4]1• HiPIPs are more hydrophobic: Favours -1
• NH...S bonds: 8-9 in Fd, only 5 in HiPIPs
• Compensate charge on cluster; -3 favoured
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*) HiPIP: high potential iron-sulfur proteins
Examples for efficient e- -transfer (3):
Blue copper proteins
• Azurin, stellacyanin, plastocyanin
• Unusual coordination geometry: Another
example for how proteins tune metal
properties
• Consequences:
– Curious absorption and EPR spectra
– High redox potential (Cu(I) favoured)
– No geometric rearrangement for redox reaction:
Very fast
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Blue copper proteins: coordination geometry
2.11 Å
2.9 Å
Angles also deviate strongly from ideal
tetrahedron
(84-136°)
Amicyanin (pdb 1aac) from Paracoccus denitrificans
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Key points
• Properties such as redox potentials are
tuned by proteins
• Coarse tuning by metal ligands
• Charge imposed by ligand can favour
particular oxidation state
• Geometry can be imposed by protein: Can
favour particular oxidation state, and also
increase reaction rate
• Fine tuning by “second shell”:
hydrophobicity, hydrogen bonds, charges
and dipoles in vicinity etc.
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