Ribulose15bisphosphat

Ribulose-1,5-bisphosphate, commonly abbreviated RuBP, is a five-carbon sugar phosphate that carries two phosphate groups, typically at the 1 and 5 positions. It serves as the CO2 acceptor in the Calvin cycle of photosynthesis and is central to the carbon fixation process. In some texts it is referred to as ribulose bisphosphate or ribulose-1,5-bisphosphate.

In plants, algae, and cyanobacteria, RuBP is produced in the chloroplast stroma by phosphorylation of ribulose-5-phosphate through the action of the enzyme phosphoribulokinase, using ATP as the phosphate donor. RuBP is the substrate for the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). Carboxylation of RuBP by RuBisCO fixes carbon dioxide to form an elusive six-carbon intermediate that immediately cleaves into two molecules of 3-phosphoglycerate, a key 3-carbon sugar in the Calvin cycle.

The regenerated RuBP is essential for continuing the cycle. After fixation of CO2, the resulting triose phosphates can be routed toward the synthesis of sugars or reassembled into RuBP, a process that consumes ATP and reduces equivalents (NADPH) supplied by the light reactions. The efficiency of RuBP regeneration and RuBisCO's carboxylation activity are major determinants of photosynthetic carbon fixation rates.

Ribulose-1,5-bisphosphate is thus a central metabolic intermediate in photosynthesis, linking ATP- and NADPH-powered light reactions to the fixation of inorganic carbon and the production of carbohydrate in carbon-fixing organisms.