The illumination of the chloroplast leads to light driven pumping of protons from the stroma into the
thylakoid lumen. The stromal pH rises, the lumen pH decreases. The stromal pH rises to around 8 pH
units. At this pH, Lys201 reacts with CO2 to form the carbamylated lysine that coordinates to the
magnesium ion. In addition as the pH rises in the stroma , the activities of Fructose 1,6-bisphosphatase,
ribulose-5-phosphate kinase and glyceraldehyde 3-phosphate dehydrogenase reach their maxima.
The light driven pumping of protons from the stroma to the thylakoid lumen occurs with the concomitant
efflux of Mg2+ ions from the lumen to the stroma. The efflux of Mg2+ maintains electrical neutrality.
Both rubisco and fructose 1,6-bisphosphatase are Mg2+ activated, stimulating CO2 fixation. The rate
determining step of the Calvin cycle is fructose 1,6-bisphosphatase which make this enzyme a key
enzyme in Calvin cycle regulation.