ribose5phosphatein

Ribose-5-phosphate isomerase (Rpi) is an enzyme in the pentose phosphate pathway that catalyzes the reversible isomerization of ribose-5-phosphate to ribulose-5-phosphate. This reaction interconverts an aldose and a ketose, helping to balance the pool of five-carbon sugars used for nucleotide synthesis and carbohydrate metabolism.

There are two unrelated enzyme families that perform this reaction: RpiA and RpiB. RpiA is widespread across bacteria, archaea, and eukaryotes, while RpiB is smaller and less common, with distinct catalytic strategies. Although both classes catalyze the same chemical transformation, they differ in structure and the specific residues involved in catalysis. Both enzymes operate via enediol intermediate chemistry, but their active sites and overall folds are different.

Biologically, ribose-5-phosphate isomerase links the oxidative and non-oxidative branches of the pentose phosphate pathway to nucleotide biosynthesis. By converting ribose-5-phosphate to ribulose-5-phosphate, Rpi helps regulate flux through carbohydrate and nucleotide metabolism according to cellular demand. The enzyme is typically cytosolic and expressed in organisms that rely on pentose phosphate metabolism for ribose provision.

Clinical and biotechnological relevance varies by organism. In bacteria, loss of Rpi activity can impair growth, while in cancer cells overexpression of RPIA has been observed in some contexts, drawing interest as a potential metabolic target. In research and metabolic engineering, Rpi enzymes are studied for their roles in flux control and for applications in salvage pathways that recycle five-carbon sugars.