Regulation of Calvin cycle by light

Regulation of Calvin cycle by light

1. Photosynthesis induces higher pH and higher Mg⁺⁺ in stroma. Therefore, stromal enzymes adapt to higher pH and higher Mg⁺⁺. Since Calvin cycle takes place in stroma, enzymes involved in Calvin cycle are also regulated in this manner.

2. Enzymes : (1) Ru-5-P kinase, (2) F-1,6- BPase, (3) SH-1,7-BPase and (4) G-3-P dehydrogenase are activated by light driven reduction of disulphide bonds of the enzymes; enzymes containing free –SH groups are active, whereas enzymes containing –S-S- are inactive.
   
   Electrons from PSI reduce Fd , reduced Fd_red reduces Thioredoxin which reduces the disulphide linkage of enzymes to active forms. Therefore, in the day time enzymes of Calvin cycle are activated resulting in its full activity.
   
   Starch is only synthesized during light. In night , -SH hroups are reoxidized to disulphide bonds. As a result above 4 enzymes are inactivated ; ATP is not expended, but starch synthesized and stored in day time is degraded to fuel glycolysis.
   
   On the other hand, Glu-6-P dehydrogenase is oppositely regulated by light. In the day time when plenty of NADPH is available this enzyme is inactivated, but in the night time this enzymes becomes active and produce much needed NADPH.