How Calvin Cycle is regulated in light ?

The Calvin cycle is only active in the light, being regulated by thioredoxin and by stromal pH and Mg2+ ion conc. Regulatory mechanisms must be in place to prevent futile cycle from happening.

(i) An important regulator of chloroplast enzymes is thioredoxin. Thioredoxins are small proteins, found in all organisms and occur in most organelles. Four different forms (f,m,x &y) of thioredoxins occur in chloroplast. Among which, f and m are involved in regulating carbon assimilation enzymes. All thioredoxins contain the same sequence of four amino acids- cys-gly-pro-cys, within the active site. In the oxidised form, the two cysteines are linked by a disulfide bridge is replaced by two sulfhydryl groups (SH) in the cys residues. Reduced thioredoxins can reduce the disulfide bonds of a large number of target proteins.

In the light, e are passed from water via PSI and PSII to ferredoxin. Reduced ferredoxin passes e to the enzyme, ferredoxin-thioredoxin reductase, which reduces thioredoxin. Reduced thioredoxin then can directly reduce disulfide groups on target enzymes, causing activation of some enzymes and inactivation of others. Certain enzymes are activated by reduction in light and inactivated by oxiadtion in the dark. Some OPPP enzymes are inactivated by reduction in light and activated by oxidation in dark (e.g., Glucose-6-phosphate-dehydrogenase, transaldolase etc.) In this way, futile cycling is prevented and Calvin cycle is made to function only in light.

(ii) Thioredoxins also play a part in controlling some forms of the activase enzyme that activates Rubisco. CP12, a protein complex of Glyceraldehyde-3-phosphate-dehydrogenase and phosphoribulokinase. Both of these enzymes are inactive when embedded in the complex. In the light, reducing conditions within the chloroplast stroma (e.g., NADPH and reduced thioredoxin) cause the CP12 complex to dissociate and the enzymes become active. This mechanism is important in Cyanobacteria and green algae.

(iii) Another means of increasing the activity of Calvin cycle enzymes in the light is provided by light-dependent changes in stromal pH and Mg2+ conc. The stromal pH increases from 7.2 to 8 when transferred from dark to light. Increases in stromal Mg2+ conc. also take place in light. These ion movements occur as a consequence of light-driven transfer of H+ from stroma to the thylakoid lumen during photosynthetic e transport. This results in an increase in conc. from 1-3 mM in the dark to 3-6 mM in light. Several Calvin cycle enzymes become activated under these conditions. Fructose-1.6 bisphosphatase, sedoheptulose 1,7-bisphosphhatase and phosphoribulokinase are all active at alkaline pH and are stimulated by increases in Mg2+ conc.