Regulation of photosynthesis

Question 1

What are most Calvin cycle enzymes activated by?

Answer 1

The reducing environment generated as a result of the light reactions.

Question 2

How is Rubisco sensitive to alkaline pH?

Answer 2

In alkaline pH, the inorganic form of carbon dioxide is hydrogencarbonate - therefore alkaline pH encourages formation of lysine carbamate at lysine 201.

Question 3

How is Rubisco activity favoured by high Mg2+ concentrations?

Answer 3

Lysine carbamate coordinates the magnesium ion which coordinates the substrate.

Question 4

What is Rubisco activase?

Answer 4

A 43kda chaperone protein that activates Rubisco, using its ATPase activity.

Question 5

Describe the inactive state of Rubisco.

Answer 5

Active site loop 6 blocks the active site.

Question 6

What is catalysed by Rubisco activase?

Answer 6

The opening of active site loop 6 to allow RuBP binding to Rubisco.

Question 7

How does Rubisco activase activate Rubisco?

Answer 7

Uses ATP hydrolysis to open active site loop 6 of Rubisco. Rubisco activase-ADP dissociates from Rubisco, allowing RuBP to bind the active site.

Question 8

How are misfire products produced by Rubisco?

Answer 8

Rubisco performs an isomerisation reaction instead of carboxylation or oxygenation. Isomerisation of RuBP to XuBP or KABP.

Question 9

How does production of misfire products regulate Rubisco activity?

Answer 9

Misfire products act as competitive inhibitors of Rubisco

Question 10

What is fallover?

Answer 10

When Rubisco activity plateaus as a result of all the active sites becoming filled with misfire products.

Question 11

How is Rubisco activity recovered following inhibition by misfire products?

Answer 11

Rubisco activase catalyses the opening of the Rubisco active site to release the misfire products.

Question 12

How is Rubisco activase activated?

Answer 12

By redox activation, involving a 46kda isoform subunit.

Question 13

How does the 46kDa isoform subunit regulate Rubisco activase activity?

Answer 13

Binds the 43kDa monomer. When there is disfulfide bond formation in the C-terminal region of the 46kDa subunit, Rubisco activase is inactive. When there are free SH groups, Rubisco activase is activated.

Question 14

When do some plants make their own rubisco inhibitors?

Answer 14

When there is too much light and sufficient glucose has been produced.

Question 15

What inhibitor is produced by some plants?

Answer 15

CA1P

Question 16

How is CA1P inhibition reversible?

Answer 16

Rubisco activase can open the active site of Rubisco to release the inhibitor. CA1P can then be degraded by CA1P phosphatase.

Question 17

What are the two proteins that mediate redox regulation of rubisco activase?

Answer 17

Thioredoxin and CP12

Question 18

What other small protein is thioredoxin coupled to?

Answer 18

Ferredoxin

Question 19

What type of FeS cluster is there in ferredoxin/thioredoxin reductase?

Answer 19

4Fe-4S

Question 20

How are FBPase and SBPase regulated?

Answer 20

By S-S bond formation - they are redox regulated

Question 21

Describe CP12.

Answer 21

12 kDa protein

Has an intrinsically disordered region of around 70 regions, and this region contains conserved cysteines.

Question 22

Where is CP12 found?

Answer 22

It is conserved across plants and Cyanobacteria.

Question 23

Describe CP12 in the dark.

Answer 23

S-S bonds become oxidised via slow reaction with oxygen. This gives order to the disordered region of the protein. This enables CP12 to bind GAPDH and PRK, forming an inactive complex and inactivating the Calvin cycle.

Question 24

Describe CP12 in light.

Answer 24

CP12 is reduced in light and the inactive complex falls apart as the disordered region becomes disordered again. This releases GAPDH and PRK.