Ribose5phosphat

Ribose-5-phosphate, commonly abbreviated as R5P, is a five-carbon sugar phosphate (chemical formula C5H11O8P) that functions as a key intermediate in central carbon metabolism. It is the phosphorylated form of ribose and serves as a precursor for nucleotide biosynthesis and various cofactors.

R5P is produced and interconverted through the pentose phosphate pathway (PPP). In the oxidative phase, glucose-6-phosphate is oxidized to ribulose-5-phosphate with the concurrent generation of NADPH and CO2. Ribulose-5-phosphate is then isomerized by ribose-5-phosphate isomerase to ribose-5-phosphate. In the nonoxidative phase, transketolase and transaldolase reactions can generate ribose-5-phosphate from other sugar phosphates, enabling the cell to supply R5P as needed for anabolic processes.

A central role of ribose-5-phosphate is its conversion to 5-phosphoribosyl-1-pyrophosphate (PRPP) by ATP-dependent ribose-phosphate pyrophosphokinase. PRPP is a critical substrate for the de novo and salvage pathways of purine and pyrimidine nucleotides and for the biosynthesis of several vital cofactors and amino acids, including histidine in many organisms. Thus R5P links carbohydrate metabolism with nucleotide, cofactor, and amino acid biosynthesis.

Physiological significance varies with cellular demand. Rapidly proliferating cells and tissues with high nucleotide synthesis requirements tend to channel more flux through the PPP to produce R5P, while in other contexts the nonoxidative branch can adjust R5P supply without excessive NADPH production. Abnormalities in PPP flux can affect redox balance and nucleotide availability.