Category: metabolite
Aliases: R5P, D-Ribose-5-phosphate
Central intermediate of the pentose phosphate pathway (PPP). Produced by the oxidative branch (glucose-6-phosphate → 6-phosphogluconate → ribulose-5-phosphate → R5P) or via the non-oxidative branch (transketolase / transaldolase reshuffling of fructose-6-phosphate and glyceraldehyde-3-phosphate). R5P is converted by phosphoribosyl pyrophosphate synthetase (PRPS1/2) to PRPP — the activated ribose donor for purine, pyrimidine, and histidine biosynthesis and for NAD / NADP salvage. The PPP also produces NADPH for reductive biosynthesis and redox homeostasis. Because R5P is phosphorylated and highly polar, it is not a supplementable molecule — ergogenic / metabolic claims for 'ribose' refer to the non-phosphorylated D-ribose sugar, which enters the cell and is phosphorylated by ribokinase to R5P intracellularly.