Week 9 Johnson Lecture 5

Question 1

What is the function of PSI?

Answer 1

PSI functions as a light-dependent plastocyanin (E ’ =+370 mV)-ferredoxin (E ’ =-420 mV) oxidoreductase

• Catalyses the overall reaction: with 1 photon moved across the reaction
• Pcred + Fdox——>Pcox + Fdred

Question 2

What is the redox scheme for PSI?

-What is the function of ferredoxin do?

Answer 2

The redox potential of the P700+/P700 redox couple is +480 mV, which is insufficient to oxidise water. However, the P700*/P700+ redox potential is -1320 mV which is sufficient to reduce the 2Fe-2S cluster of ferredoxin (-420 mV). Ferredoxin is a powerful reductant that can reduce NADP+ to NADPH and NO3- to NH4+

Question 3

What are the energetics of the PSI reaction?

Answer 3

Overall reaction:
Pcred + Fdox —–> Pcox + Fdred
PSI oxidises plastocyanin (E ’ = +370 mV) and reduces ferredoxin (E ’=-420 mV), a 1 electron transfer reaction.
ΔE ’ = -420 - (+370)
ΔE ’ = -790 mV
ΔG = -nFΔE ’(n = moles of electrons transferred, F= Faraday constant)
ΔG = -1 × 96.5 × 10-3 × -790
ΔG = +76 kJ mol-1
This free energy input is provided by a 700 nm photon 171 kJ mol-1 each
75/171 = 44% efficiency

Question 4

What is plastocyanin?

Answer 4

Plastocyanin (Pc) is a small (~10 kDa) soluble electron transfer protein. A copper ion bound at its active site (coordinated by 2 His, a Cys and a Met ) which acts as the electron carrier. The copper ion is oxidised from Cu+ to Cu2+ by PSI, and reduced back by cytochrome b6f.

Question 5

What is ferredoxin?

Answer 5

Ferredoxin is also a small (~10kDa) soluble electron transfer protein that binds a 2Fe-2S cluster at its active site bound by several cysteine residues that acts as the electron carrier.

Question 6

What are the co-factors which take part in electron transfer reactions from plastocyanin to ferredoxin?

Answer 6

Include
4Fe4S (FB) 
4Fe4S (FA)
4Fe4S (Fx) 
Phylloquinone (QKA)
Phylloquinone (QKB)
Chlorophyll aB2 (ChlB2)
Chlorophyll aB1 (ChlB1)
Chlorophyll aA2 (ChlA2)
Chlorophyll aA1 (ChlA1)
Chlorophyll a P700A / P700B

Question 7

How does the photochemical reaction happen in PSI reaction centre?

Answer 7

Excitation and therefore charges separate. The electron goes through any branch, A or B, until it reaches ferredoxin. P700 is re-reduced from plastocyanin

Question 8

Why does photosynthesis happen?

Answer 8

in both PSII and PSI the most energetically favourable reaction would appear to be direct recombination (and loss of the energy as heat) (direct recombination is when the two charges come back together) of the primary and secondary radical pairs e.g. P680+ChlA- / P700+Chl1-. This is because electron transfer rates have an optimum energy to have a high yield. This is as when an electron is transferred, the reactions environment that is around the electron need to be moved/ changed to accommodate the change in charge (amino acids, water, etc.) = reorganisation energy. In Marcus’s theory, the electron transfer rate is optimum when the driving force matches the reorganisation energy.