The Calvin Cycle of C3 Photosynthesis

The Calvin Cycle proceeds in three stages:

1. Carboxylation - CO2 is covalently linked to a carbon skeleton (RuBP)
2. Reduction - carbohydrate is formed at the expense of ATP and NADPH
3. Regeneration - the CO2 acceptor RuBP reforms at the expense of ATP

Energy is required for reduction and regeneration:

Reduction requires 2 ATP + 2 NADPH for each CO2 incorporated
Regeneration requires 1 ATP for each CO2 incorporated

Regulation of the Calvin Cycle

The Calvin cycle is regulated by light-dependent activation of at least 5 enzymes:

NADP:glyceraldehyde-3-phosphate dehydrogenase
fructose-1,6-bisphosphate phosphatase
sedheptulose-1,7-bisphosphate phosphatase
ribulose-5-phosphate kinase

Of these 5 enzymes, all except rubisco are regulated by the ferrodoxin-thioredoxin system shown below:

Other processes that regulate Calvin cycle enzymes include:

Light-dependent ion movements:
Rubisco and other enzymes are activated by light-induced increase in stromal pH. All enzymes have an optimal pH range in which they function most effectively. Rubisco works best at a pH close to 8 but is relatively inactive at pH 7. As light-driven electron transport leads to the generation of a pH gradient across the thylakoid membrane, the stromal pH increases from around pH 7 to pH 8. As a result of the pH change, other ions move across the thylakoid membrane to compensate for charge differences. In particular, Mg++ moves out as H+ moves in. Rubisco requires Mg++ to function and the combination of increase in pH and Mg++ leads to a significant increase in rubisco activity.

Rubisco activase:
Rubisco is uniquely regulated by a separate protein called rubisco activase. Rubisco activase binds to inactive rubisco and upon ATP hydrolysis changes the conformation of rubisco into a highly active form. Rubisco activase works along with pH and Mg++ to tune rubisco function to demand.

Phosphate availability:
When triosphosphates move from the chloroplast to the cytoplasm for sucrose production (for storage & transport) or for respiration, it is necessary that the phosphate be recycled back to the chloroplast to allow for continued production of ATP. This is accomplished by a phosphate translocator enzyme that is localized in the inner envelope of the chloroplast.

When the rate of photosynthesis is very high and triose phosphates are being transported at a rate that is too high, it is possible for the chloroplast to become depleted of phosphate. Under such conditions, the triose phosphates are used to make starch in the chloroplast and the phosphates remain available in the stroma. Large starch grains can be seen to build up in chloroplasts on bright sunny days.