Ribulose5phosphate

Ribulose-5-phosphate, often abbreviated Ru5P, is a five-carbon sugar phosphate (a pentulose phosphate) with a ketone at the second carbon and a phosphate group at the fifth carbon. In biology it most commonly appears in the D-configuration and is a key intermediate in central carbon metabolism, notably in the pentose phosphate pathway and, in plants, in the Calvin cycle.

In the oxidative phase of the pentose phosphate pathway, glucose-6-phosphate is oxidized and decarboxylated to produce ribulose-5-phosphate, along with NADPH and CO2. Ribulose-5-phosphate can be isomerized to ribose-5-phosphate, which provides ribose-5-phosphate for nucleotide synthesis, or epimerized to xylulose-5-phosphate, which enters the non-oxidative steps of the pathway.

In the non-oxidative phase, transketolase and transaldolase reactions rearrange carbon skeletons among ribulose-5-phosphate, ribose-5-phosphate, and xylulose-5-phosphate to generate glyceraldehyde-3-phosphate and fructose-6-phosphate. These products can feed into glycolysis or other biosynthetic pathways, linking pentose phosphate metabolism to energy production and macromolecule synthesis.

In plants and algae, Ru5P is also a precursor in photosynthetic carbon fixation. It is phosphorylated by phosphoribulokinase to ribulose-1,5-bisphosphate, the CO2-acceptor substrate for Rubisco in the Calvin cycle.

Overall, ribulose-5-phosphate serves as a central hub connecting NADPH production, nucleotide biosynthesis, and carbohydrate interconversion in cellular metabolism.