Ribulose-1,5-biphosphate (RuBP) is an important substrate involved in carbon fixation. The enzyme Rubisco catalyzes RuBP with carbon dioxide in order to synthesize a highly unstable 6-carbon intermediate known as 3-keto-2-carboxyarabinitol 1,5-bisphosphate, which decays virtually instantaneously into two molecules of glycerate 3-phosphate (3PG) (see Calvin Cycle).
Used in carbon fixation (of the Calvin cycle): 5-carbon RuBP + 1-carbon CO2 --> 6-carbon intermediate --> 2 3PG. Each (3-carbon) 3PG is then reduced to G3P in carbon reduction. 5 G3P = 3 RuBP. 2 G3P = 1 C6H12O6 (glucose).