Xylulose5phosphate

Xylulose-5-phosphate, also written as D-xylulose-5-phosphate (Xu5P), is a phosphorylated five-carbon sugar and an intermediate of the pentose phosphate pathway (PPP). It is a ketose sugar phosphate with a phosphate group at the fifth carbon and is part of the cytosolic metabolism in many organisms, including plants, animals, and bacteria.

In the non-oxidative branch of the PPP, xylulose-5-phosphate participates in carbon shuffling reactions catalyzed by transketolase and transaldolase. One major transketolase reaction transfers a two-carbon unit from Xu5P to ribose-5-phosphate to form glyceraldehyde-3-phosphate (G3P) and sedoheptulose-7-phosphate. In a second transketolase step, Xu5P donates a two-carbon unit to erythrose-4-phosphate to yield glyceraldehyde-3-phosphate and fructose-6-phosphate. These products can re-enter glycolysis or serve as precursors for nucleotide synthesis. Xu5P is formed from ribulose-5-phosphate by ribulose-5-phosphate 3-epimerase, linking the oxidative and non-oxidative phases of the pathway.

Biological context: The PPP provides NADPH for reductive biosynthesis and defense against oxidative stress, as well as ribose-5-phosphate for nucleotide synthesis. Xylulose-5-phosphate participates as both a substrate for transketolase and a product of epimerization, helping to balance the cellular pools of five- and six-carbon sugars.

Regulatory aspects vary by organism; in some bacteria, xu5p levels influence carbon metabolism through regulatory networks, reflecting the integration of PPP flux with central metabolism.