Which enzyme catalyzes the regeneration of RuBP in the Calvin cycle?

Phosphoribulokinase catalyzes the regeneration of RuBP in the Calvin cycle. It uses ATP to phosphorylate ribulose 5-phosphate, producing ribulose 1,5-bisphosphate, or RuBP—the molecule that accepts carbon dioxide at the start of the cycle.

RuBP is consumed when Rubisco adds CO2 to it, producing unstable six-carbon intermediates that split into 3-phosphoglycerate. After several reduction and rearrangement reactions, the Calvin cycle produces ribulose 5-phosphate again. Phosphoribulokinase then restores RuBP, allowing carbon fixation to continue.

This enzyme is easy to confuse with Rubisco because both are central to the Calvin cycle. Rubisco performs the initial CO2-fixation reaction; phosphoribulokinase performs the ATP-dependent regeneration step. Fructose bisphosphatase and glyceraldehyde-3-phosphate dehydrogenase participate in other Calvin-cycle reactions but do not catalyze RuBP formation. In plants, phosphoribulokinase is regulated in response to light-linked changes in the chloroplast, including thioredoxin-mediated redox control.