text: SBPase is involved in the regeneration of 5-carbon sugars during the Calvin cycle. Although SBPase has not been emphasized as an important control point in the Calvin cycle historically, it plays a large part in controlling the flux of carbon through the Calvin cycle.  Additionally, SBPase activity has been found to have a strong correlation with the amount of photosynthetic carbon fixation.  Like many Calvin cycle enzymes, SBPase is activated in the presence of light through a ferredoxin/thioredoxin system.  In the light reactions of photosynthesis, light energy powers the transport of electrons to eventually reduce ferredoxin. The enzyme ferredoxin-thioredoxin reductase uses reduced ferredoxin to reduce thioredoxin from the disulfide form to the dithiol. Finally, the reduced thioredoxin is used to reduced a cysteine-cysteine disulfide bond in SBPase to a dithiol, which converts the SBPase into its active form.  
  SBPase has additional levels of regulation beyond the ferredoxin/thioredoxin system. Mg2+ concentration has a significant impact on the activity of SBPase and the rate of the reactions it catalyzes.  SBPase is inhibited by acidic conditions (low pH). This is a large contributor to the overall inhibition of carbon fixation when the pH is low inside the stroma of the chloroplast.  Finally, SBPase is subject to negative feedback regulation by sedoheptulose-7-phosphate and inorganic phosphate, the products of the reaction it catalyzes.