Cytochrome B6f

Question 1

What actually is CytB6f?

Answer 1

It is a Plastoquinol-Plastocyanin Oxidoreductase. It also brings a H+ to the lumen from the stroma.

Question 2

What is the structure of CytB6f and what does it bind?

Answer 2

CytB6f is a functional dimer, which has bound:
4 Haem types (2x C-type or coordination-bond, and 2x B-type or covalent)
2x PQs
1x 2Fe-2S Cluster (Coordinated by 3x Cys, 1x His residues)
1x Chlorophyll
1x Carotenoid

Question 3

Describe the Proton-Motive ‘Q-Cycle’ catalysed by CytB6f

Answer 3

Bifurcated electron transfer (2e- are removed from PQH2) along two ‘chains’ occurs:

1) The ‘high potential’ chain, often labelled P, which leads to Fd reduction and 2H+ movement from PQH2 to the lumen
2) The ‘low potential’ chain, often labelled N, which leads to PQH2 regeneration

This ultimately leads to double the H+ pumping by recycling a PQH2, as H+ is taken from the stroma to form it (so ultimately, 4H+ are pumped by CytBf6f for every 2 electrons).

Question 4

What is the overall NET reaction catalysed by CytB6f?

Answer 4

PQH2 + 2Pc (oxidised) + 2H+ (stroma) –> PQ + 2Pc (reduced) + 4H+ (lumen)

Question 5

Describe the two methods of Cyclic Electron Transfer (CET)

Answer 5

CET can be conducted by the PGR5 protein route or the Photosynthetic Complex I route:

1) PGR5 route uses Ferredoxin-NADPH-oxidoreductase/Ferredoxin complex to recycle the e-, so it goes back to the PQ pool (PQ –> PQH2) instead of to NADP+

2) 2x Ferredoxins donates electrons to the PQ to regenerate PQH2 of the PQ pool

This ultimately leads to more proton pumping by CytB6f, into the lumen

Question 6

Why is Cyclic Electron Transfer (CET) necessary?

Answer 6

CET is necessary to increase the ATP:NADPH ratio to 1.5, as Linear Electron Transport (LET) only creates a ratio of 1.28 which isn’t sufficient for the Calvin cycle (and thus photosynthesis) to function.