RuBP

Ribulose-1,5-bisphosphate (RuBP) is a five-carbon sugar phosphate that serves as the CO2 acceptor in the Calvin cycle of photosynthesis. In the chloroplast stroma, RuBP is formed by the phosphorylation of ribulose-5-phosphate by the enzyme phosphoribulokinase, using ATP.

RuBP reacts with carbon dioxide, catalyzed by ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), to yield two molecules of 3-phosphoglycerate (3-PGA). This carboxylation step is the first major carbon-fixing reaction in carbon assimilation.

Oxygen can also react with RuBP in a competing oxygenase reaction, producing 2-phosphoglycolate and 3-PGA; this photorespiration pathway reduces net CO2 fixation, particularly under conditions of high O2 or low CO2.

Throughout the Calvin cycle, RuBP is regenerated from triose phosphates and other intermediates to sustain carbon fixation. The regeneration phase consumes ATP, with phosphoribulokinase and other enzymes converting intermediates back into RuBP, enabling continued CO2 fixation.

RuBP is localized in the chloroplast stroma of plants, algae, and many bacteria that perform photosynthesis. Its proper function is essential for efficient carbon fixation, and RuBisCO’s dual carboxylase/oxygenase activity makes RuBP handling a key point in photosynthetic efficiency.