Ru5P

Ru5P, or ribulose-5-phosphate, is a phosphorylated ketopentose that serves as an intermediate in the pentose phosphate pathway (PPP). It is formed in the oxidative phase of PPP from glucose-6-phosphate through a sequence of enzymatic steps that yields NADPH and carbon dioxide.

Ru5P can be directed into two primary routes: isomerization to ribose-5-phosphate (R5P) by ribulose-5-phosphate isomerase, or epimerization to xylulose-5-phosphate (Xu5P) by ribulose-5-phosphate 3-epimerase. Both R5P and Xu5P feed into the non-oxidative phase of PPP, where transketolase and transaldolase reactions rearrange carbon skeletons to generate glyceraldehyde-3-phosphate and fructose-6-phosphate, which can re-enter glycolysis or be diverted toward biosynthetic pathways.

Physiological role: Ru5P links redox balance and nucleotide biosynthesis. Through the PPP, it participates in producing NADPH for reductive biosynthesis and maintaining cellular redox state, while its downstream products provide ribose for nucleotide and nucleic acid synthesis.

Biochemical context: In bacteria, plants, and animals, Ru5P is a central node connecting carbon metabolism with anabolic processes. Its availability and flux are modulated by the cell’s demand for NADPH, ribose-5-phosphate, and glycolytic intermediates, allowing metabolic flexibility under varying physiological conditions.