<Tr> <Th> PDBsum </Th> <Td> structure summary </Td> </Tr> <P> As shown in Figure 2 (left), RuBisCO is one of many enzymes in the Calvin cycle . </P> <P> During carbon fixation, the substrate molecules for RuBisCO are ribulose - 1, 5 - bisphosphate and carbon dioxide (distinct from the "activating" carbon dioxide). RuBisCO also catalyses a reaction between ribulose - 1, 5 - bisphosphate and molecular oxygen (O 2) instead of carbon dioxide (CO). </P> <P> When carbon dioxide is the substrate, the product of the carboxylase reaction is a highly unstable six - carbon phosphorylated intermediate known as 3 - keto - 2 - carboxyarabinitol - 1, 5 - bisphosphate, which decays virtually instantaneously into two molecules of glycerate - 3 - phosphate . The extremely unstable molecule created by the initial carboxylation was unknown until 1988, when it was isolated . The 3 - phosphoglycerate can be used to produce larger molecules such as glucose . Also, Rubisco side activities can lead to useless or inhibitory by - products; one such product is xylulose - 1, 5 - bisphosphate, which inhibits Rubisco activity . </P>

Rubisco can use ____ or co2 as a substrate