Plant lecture 5 - Regulation of Calvin cycle

Question 1

Reasons for regulation of the Calvin Cycle

Answer 1

• Adjustment to changes in light intensity/CO2 availability
• Switch off in dark to avoid depleting NADH/ATP
• Intermediates are shared w/ pathway of carbohydrate oxidation

Question 2

SBPase, FBPase, PRK

Answer 2

• Alkaline pH optimum + magnesium dependent (↑ [MG2+] ↑ activity] + redox state
• So activity goes from ↑ in light to ↓ in dark by 95%
• Thioredoxin reduce disulphide bridges in target e.g. PRK, 2 lys = oxidised w/ BS for ATP at AS. Reduced so open bs so ATP can bind. Changes conformation of the E
• Plants w/ 1/2 SBPase WT activity = significant but non-proportional inhibition (Cj = 0.1-0.2) due to ↓ regenerative capacity of TCA
• Transformants w/ 15% PRK activity had Cj = 0.06, even smaller than FBPase
• FBPase x effect photosynthesis until 60% removed
• Enzymes that thought to have ↑ control x but are ↑ regulatable

Question 3

Explanation of SBPase,FBPase,PRK activity

Answer 3

• PRK as an example
• PRK activity depends on relative conc. of reactants (+ve), products (-ve) + modulators (e.g. 3PGA -ve)
• If remove PRK, R5P→R1,5BP slows down
• But, R5P made at previous rate + R1,5BP removed at previous rate. R1,5BP levels ↓ + R5P levels ↑
• ↑ in R5P (product) activate E + ↓ in product (R1,5BP) + 3PGA also help activate

Question 4

Explanation of aldolase activity

/ Reversible E

Answer 4

• WATCH AGAIN PLEASE!!!!
• Previously thought inappropriate site that flux controlled through
• Reduction of aldolase below 30% of WT → significant ↓ in rate
• ↓ expression of aldolase inhibits photosynthesis in low light due to RuBP efficiently catalysing regenerative part of cycle → inhibition
• ↓ in high light by restricting regeneration of ATP. In high light Cj = 0.5 (more than 1/2 control in aldolase)