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Electron transfer in biological systems

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Biological electron transfer

• http://highered.mcgraw-hill.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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• For G0 = - activation energy G# becomes = 0: “activationless” e-transfer

• Fast reactions if G0 and aresimilar to one another

there are “ideal” combinations of reaction enthalpy and reorganization energy

Often observed in biological systems: Small values for both

Marcus Theory: Key points

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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

Absorption spectra of oxidized (Fe(III)) and reduced (Fe(II)) horse cytochrome c.

• classified by 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

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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 with Cys)

• 1 Met and 1 His ligand (axial)

•Conserved from bacteria to Man

horse heart cytochrome cBushnell, G.W.,  Louie, G.V.,  Brayer, G.D. J.Mol.Biol. v214 pp.585-595 , 1990

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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 CN5, axial ligand is a CysCys

• 66thth site for site for substrate/oxygen substrate/oxygen bindingbinding

• Hydroxylates Hydroxylates camphorcamphor

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 stabilised than

Fe(III)

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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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Fe2S2(Cys-S)4

Fe2S2(Cys-S)2-(His-N)2: Rieske proteins

Fe4S4(Cys-S)4

Fe3S4(Cys-S)4

1 awd: CHLORELLA FUSCA

1fda: Azotobacter vinelandii

1rfs: Spinach

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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-only hydrogenase from Clostridium pasteurianum

• Activation of H2

• Active site (binuclear Fe cluster) on top

• The other five Fe-S clusters provide long-range electron transfer pathways

Pdb 1feh

Fe-S proteins often contain more than one cluster:

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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

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Redox potentials

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• 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

Tuning of redox potentials

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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

*) HiPIP: high potential iron-sulfur proteins

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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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2.9 Å

2.11 Å

Amicyanin (pdb 1aac) from Paracoccus denitrificans

Blue copper proteins: coordination geometry

Angles also deviate strongly from ideal tetrahedron(84-136°)

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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.