Section 4: Biological electron transfer

Question 1

Where does the energy for life come from?

Answer 1

Mainly from the sun
Directly from photosynthesis
indirectly using photosynthesising organisms as fuel.

Question 2

How can energy be viewed from an electrochemical perspective?

Answer 2

Considered as a flow of electrons
Fuels: fats, sugars, H2
Oxidants: O2, nitrate, H+

Question 3

How does nature achieve this electron flow?

Answer 3

proteins- 3 main protein types

1) Blue Copper proteins
2) Iron-sulphur proteins
3) Cytochromes

Question 4

The number and type of ligands varies which electrochemical property? Why?

Answer 4

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.

Question 5

How can the rate of e- transfer be explained?

Answer 5

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)

Question 6

What is a Blue Copper Protein

Answer 6

Small proteins with bind a single Cu atom

-Are called blue due to their intense blue colour in the oxidised state

Question 7

Example of Blue copper protein

Answer 7

-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

Question 8

Structure of plastocyanins

Answer 8

-Three closely bound donors (one Cys and two His residues) and one weaker bound Met donor
Cu centre is shielded from oxygen

Question 9

Structure of Azurin

Answer 9

-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

Question 10

What are the distinctive characteristics of blue copper proteins?

Answer 10

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

Question 11

How does the protein structure help e transfer?

Answer 11

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

Question 12

What is an Iron Sulphur protein?

Answer 12

-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-

Question 13

Functions of Iron Sulphur proteins

Answer 13

they are essential componants in

1) e transfer proccesses including photosynthesis and cell respiration
2) nitrogen fixation
3) catalytic sites in hydrogenases

Question 14

What are the 5 types of Fe-S protein

Answer 14

• [1Fe-0S] Rubredoxins
• [2Fe-2S] Ferrodoxins
• [2Fe-2S] Rieske protein
• [4Fe-4S] Ferrodoxins
• [3Fe-4S] Ferrodoxins

Question 15

[1Fe-0S] Rubredoxins

Answer 15

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

Question 16

-[2Fe-2S] Ferrodoxins

Answer 16

(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

Question 17

-[2Fe-2S] Rieske protein

Answer 17

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

Question 18

-[4Fe-4S] Ferrodoxins

Answer 18

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

Question 19

What is a HiPIP?

Answer 19

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

Question 20

-[3Fe-4S] Ferrodoxins

Answer 20

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

Question 21

How does the mechanism work on a protein scale?

Answer 21

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

Question 22

Explain the whole geometry thing

Answer 22

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

Question 23

How have these Fe S structures been determined

Answer 23

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.

Question 24

zn

Answer 24

nz

Question 25

What is a cyctochrome

Answer 25

-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

Question 26

How does the bonding in cyctochromes facilitate e transfer.

Answer 26

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