ribulose15bisfosfat

Ribulose-1,5-bisphosphate, commonly abbreviated RuBP, is a five-carbon sugar phosphate that functions as the CO2 acceptor in the Calvin cycle of photosynthesis. It is a ketopentose with phosphate groups at the C1 and C5 positions. In the presence of the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), RuBP reacts with carbon dioxide to form an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA). This carboxylation step is the primary carbon-fixation reaction in C3 photosynthesis.

RuBP is regenerated within the Calvin cycle to sustain carbon fixation. It is synthesized from ribulose-5-phosphate by phosphoribulokinase, which uses ATP as a phosphate donor. The regeneration process consumes ATP and NADPH produced during the light reactions and converts triose phosphates back into RuBP for another round of CO2 fixation.

RuBP is found in plants, algae, and many photosynthetic bacteria. While RuBisCO catalyzes the desirable carboxylation of RuBP with CO2, it can also catalyze oxygenation of RuBP, leading to photorespiration under certain conditions and affecting photosynthetic efficiency. The molecule's central role in the carbon fixation pathway makes it a key substrate in understanding the biochemical basis of photosynthesis.