Ribulose-1,5-bisphosphate

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Ribulose-1,5-bisphosphate (RuBP) is an important 5-carbon intermediate in the Calvin cycle taking place during photosynthesis. It is the substrate used by the enzyme to fix carbon dioxide to create a highly unstable 6 carbon phosphate which virtually instantaneously decays into two molecules of glycerate 3-phosphate . The hellishly unstable actual molecule created by the initial carboxylation was unknown until 1988 when it was isolated by Dupont scientists. The (G3P) is then reduced in the Calvin-Benson cycle.

Ribulose-1,5-bisphosphate is regenerated in the Calvin cycle using ATP from the light-dependent reactions of photosynthesis. The 6-carbon sugar produced is used to synthesize starch, amino acids, lipids, etc. This molecule of glucose that is produced from two triose sugar molecules is made using the glycolysis enzyme in reverse. Ribulose-1,2-C5-bisphosphate_carboxylase/oxygenase RuBisCO the enzyme which catalyzes the reaction of Ribulose-1,5-bisphosphate with either O2 or CO2 in photosynthesis is unusually weak at distinguishing the productive ie. reductive substrate (CO2), from the unproductive respiratory substrate, O2. This weakness in the enzyme is the cause of photorespiration such that healthy leaves in bright light may have zero net carbon fixation when the ratio of O2 to CO2 reaches a threshold at which oxygen is fixed instead of carbon. This phenomenon is primarily temperature dependent. High temperature decreases the concentration of CO2 dissolved in the moisture in the leaf tissues. See C4 vs C3 photosynthesis. C4 plants use a carbon dioxide pumping system to increase the concentration of CO2 in photosynthetic tissues. 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 (of the Calvin cycle). 5 G3P = 3 RuBP. 2 G3P = 1 C6H12O6 (glucose).