Section 4: Biological electron transfer Flashcards
Where does the energy for life come from?
Mainly from the sun
Directly from photosynthesis
indirectly using photosynthesising organisms as fuel.
How can energy be viewed from an electrochemical perspective?
Considered as a flow of electrons
Fuels: fats, sugars, H2
Oxidants: O2, nitrate, H+
How does nature achieve this electron flow?
proteins- 3 main protein types
1) Blue Copper proteins
2) Iron-sulphur proteins
3) Cytochromes
The number and type of ligands varies which electrochemical property? Why?
Reduction potential of a redox couple eg Fe w diff ligands varies the E0=+n V value.
Strong donors stabilise high oxidation states and lower the reduction potential.
Weak donors, pi-acceptors and protons all stabilise low oxidation states and raise the reduction potential.
Reduction potential can also be affected by relative permittivity, neighbouring charges and H bonding.
How can the rate of e- transfer be explained?
using Marcus Theory
-takes into account the reorganisation energy
-the lower this is, the faster the reaction
(the energy change that occurs as the charge is distributed throughout the donor-acceptor)
What is a Blue Copper Protein
Small proteins with bind a single Cu atom
-Are called blue due to their intense blue colour in the oxidised state
Example of Blue copper protein
-Chloroplastic plastocyanins, in plants
used in the photosynthetic pathway to transport electrons between Photostem I and II,
- Azurin, in bacteria,
- involved in the e- transport for the conversion of [NO3-] to N2
Structure of plastocyanins
-Three closely bound donors (one Cys and two His residues) and one weaker bound Met donor
Cu centre is shielded from oxygen
Structure of Azurin
-Three closely bound donors (one Cys and two His residues) and one weaker bound Met donor
-additional weak coordination from Gly O atom
Cu centre is shielded from oxygen
What are the distinctive characteristics of blue copper proteins?
1) an intense blue colour in the Cu(II) state at 600nm
due to S(cys) to Cu(II) LMCT (high intensity-not d-d)
2) A high but variable reduction potential Eo~350 mV compare to under 100 mV for typical Cu complexes
3) An EPR spectrum with a small hyperfine coupling to the CU nuceleus in the Cu(II) state
How does the protein structure help e transfer?
The protein has a beta-barrel structure
-holds Cu in very rigid geometry.
-coordination sphere applicable to both Cu(I) and Cu(II)
this facilitates rapid electron transfer
-Bond lengths increase by only 5-10 pm from II to I
What is an Iron Sulphur protein?
-Many different types know
-Nearly all contrain high spin FE(III) or FE(II)
-tetrahedrally coordinated by sulphur ligands [S(2-) or S-(Cys)]
-They are classified to the number of iron and sulphide (S(2-)) atoms they contain
Rubredoxins one iron centre
Ferrodoxins contain di-, tri- or tetra-
Functions of Iron Sulphur proteins
they are essential componants in
1) e transfer proccesses including photosynthesis and cell respiration
2) nitrogen fixation
3) catalytic sites in hydrogenases
What are the 5 types of Fe-S protein
- [1Fe-0S] Rubredoxins
- [2Fe-2S] Ferrodoxins
- [2Fe-2S] Rieske protein
- [4Fe-4S] Ferrodoxins
- [3Fe-4S] Ferrodoxins
[1Fe-0S] Rubredoxins
Picture Fe-(S(Cys))4 -found in some bacteria -contain high spin Fe -coordinated to four Cys residues in a distorted tetrahedral fashion undergo a single electron redox: Fe(III) +e <=> Fe(II)
E0 around 0 can be postive or negative 0.05
sensitive to the conformation of the protein chain as this can change geometry and bond distances (pm) which can change e transfer
-[2Fe-2S] Ferrodoxins
(S(Cys))2-Fe-S2-Fe-(S(Cys))2 Picture -found in mammals, plants and bacteria -tetrahedral Fe bridged by two S(2-) Undergo a single electron redox: Fe(III).Fe(III) + e <=> Fe(III).Fe(II) negative E potential
More than one Fe centre allows a greater range of reduction potentials
Negative reduction potentials mean that in their reduced form they are good reduction agents- are likely to give off an electron to something else
Characterised magnetically
Oxidised form- 2 x high spin d5 Fe - coule anti ferromagnetically to give a diamagnetic complex S=0
Reduced form high spin d6-d5 complex paramagnetic complex S=1/2
-[2Fe-2S] Rieske protein
Important class of ferrodoxin
Carries out electron transfer in the photosynthetic pathway.
(S(Cys))2-Fe-S2-Fe-(His)2
This minor structural change stabilises the iron as Fe(II) and raises the reduction potential to +290 mV
-[4Fe-4S] Ferrodoxins
Picture
no known biological occuring 4FE structures
Cubic with Fe and S(2- in alternate corners
Fe further coordinated by S(Cys) or N(His)
single e transfer
2Fe(III).2Fe(II) +e <=> Fe(III).3Fe(II) E0= negative
The e’s are delocalised over the 4 Fe centres. resulting in minimal bond length changes, decreasing the organisation energy leading to faster electron transfer
What is a HiPIP?
HIgh potential protein
-used in anaerobic e transport which occurs in photosynthetic bacteria.
4Fe-4S structure BUT
3Fe(III).Fe(II) + e <=> 2Fe(III).2Fe(II) E0=postive reduction potential
no single ferrodoxin can acess all three states
-[3Fe-4S] Ferrodoxins
structure bacteria cubic with one fe missing Single electron transfer 3Fe(III) + e <=> 2.Fe(III).Fe(II) E0=pos or neg S=0.5 to 2
How does the mechanism work on a protein scale?
an enzyme (eg. hydrogenase) with an active site. an electron transfer protein will bind to the protein via diff interactions The ET protein contains a series of Fe-S clusters that provide a long range e transfer pathway from the outside of the protein to the active site in the hydrogenase the speed therefore not only depends on lack of a change of geometry but also distance between the clusters
Explain the whole geometry thing
The rate of e transfer reactions can be explained using marcus theory.
Includes rearrangement energy, so if electron transfer causes a change in configuration, the larger the reorganisation energy
the larger the reorganisation energy, the slower the reaction
How have these Fe S structures been determined
study of a whole metallocene v difficult
model studies using Fe-S compounds have been extensively used to gain an undertanding of the properties of these protons
The synthesis of these models are challenging due to the tendancy of iron thiolate complexes to undergo redox or polymerization reactions.
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What is a cyctochrome
-Haem proteins which are able to act as 1 e transfer centres.
e transfer is typically between Fe(II) and Fe(III) forms with a redox potential in range -0.3 to +0.4V
Fe is usually six coordinate w two stable ligands and 4 pyrrole from ring
Very little change in conformation for cyctochromes during redox resulting in extremely fast e transfer process
How does the bonding in cyctochromes facilitate e transfer.
the t2g non bonding orbitals of Fe overlap w/ the pi* antibondin orbitals of the ring system, effectively extending the d orbitals out to the edge of the ring, enhancing e transfer
this also reduces the distance over which an e must transfer between redox centres
