R5P

Ribose-5-phosphate (R5P) is a phosphorylated form of the sugar ribose and a key metabolic intermediate in cellular metabolism. It is generated in the oxidative phase of the pentose phosphate pathway from glucose-6-phosphate, a process that also produces NADPH. R5P can be formed from ribulose-5-phosphate by an isomerization reaction, linking the PPP to nucleotide biosynthesis and other metabolic needs.

In cells, R5P serves as a precursor for the synthesis of nucleotides and nucleic acids. It is the starting sugar for the production of phosphoribosyl pyrophosphate (PRPP) via PRPP synthetase, a critical donor of ribose units in the pathways that build purine and pyrimidine nucleotides. This connection makes R5P essential for DNA and RNA synthesis, as well as for the production of cofactors and signaling molecules that depend on nucleotide building blocks.

R5P also participates in the non-oxidative branch of the PPP, where it can be interconverted with other sugar phosphates such as glyceraldehyde-3-phosphate and fructose-6-phosphate through the actions of transketolase and transaldolase. This flexibility allows the cell to balance ribose availability with other glycolytic and biosynthetic demands, redirecting carbon skeletons as needed for energy production or macromolecule synthesis.

In summary, ribose-5-phosphate is a central metabolite linking the pentose phosphate pathway to nucleotide biosynthesis and broader cellular metabolism, with fluxes that adapt to the cell’s growth and redox requirements.