Week 9 Johnson Lecture 6

Question 1

What is cytochrome b6f?

Answer 1

A complex which is a dimer and binds 2 x PQ, 1 x carotenoid, 1 x chlorophyll, 4 x haems and 1 x 2Fe2S cluster per monomer

Question 2

What are the cofactors used in cytochrome b6f complex?

Answer 2

Electron transfer complex contain redox active cofactors. Common components include haems and iron-sulphur clusters.

• Cytochrome b6f complex contains two c- type haems (covalently linked to protein) and 2 b-type haems (linked via coordination bonds to the central Fe ion).
• The iron -sulphur cluster is a 2Fe2S type and is coordinated by histidines and cysteines. Both co-factors are 1 electron carriers.

Question 3

How does bifurcated electron transfer occur?

Answer 3

In Step 1 one plastoquinone (PQH2) is oxidised, the two protons are deposited into the lumen and the two electrons follow different paths (bifurcate). One electron reduces Pc and one electron reduces a PQ molecule to PQ-. In Step 2 another PQH2 is oxidised, the two protons are again deposited into the lumen and the two electrons follow different paths (bifurcate). One electron reduces a second Pc and one electron reduces the PQ- molecule to PQ2-, which binds two protons from stroma to become PQH2

Question 4

Why is bifurcated electron transfer useful?

Answer 4

Recycling of 1 of the 2 electrons from each PQH2 doubles the number of protons transferred by the complex per PQH2 oxidised, boosting its contribution to the proton motive force (Δp).

Question 5

Where does electron transport occur in chloroplast?

Answer 5

Thylakoid membrane

Question 6

Where does CO2 fixation take place?

Answer 6

Stroma

Question 7

How many ATPs are formed per NADPH during the light reactions?

• How many ATPs per NADPH are needed for the Calvin cycle?
• How is this imbalance corrected?

Answer 7

6 H+/4.67 (i.e. H+/ATP ratio in chloroplast (14/3 = 4.67) = 1.28 ATPs formed per NADPH (remember NADPH is a 2 electron carrier)
Calvin Cycle requires 1.5 ATP per NADPH
An imbalance in the ATP/ NADPH ratio which must be corrected for the efficient functioning of photosynthesis. The ‘redox balance’ is attained by a second type of electron transport taking place in chloroplasts, cyclic electron transport, that produces only ATP.

Question 8

What is cyclic electron transport (CET)?

Answer 8

CET involves returning electrons from Ferredoxin to the PQ pool. Two pathways exist, one involves Photosynthetic complex I and the other involves the PGR5 protein. Both pathways contribute to pmf and so ATP formation without net NADPH synthesis. Maintaining redox balance.