<p>Ribose 5-phosphate isomerase, also known as phosphoriboisomerase, catalyses the reversible conversion of D-ribose 5-phosphate to D-ribulose 5-phosphate, the first step in the non-oxidative branch of the pentose phosphate pathway [<cite idref="PUB00028856"/>]. This reaction enables ribose to be synthesized from sugars, as well as the recycling of sugars during the degradation of nucleotides. There are two unrelated types of ribose 5-phosphate isomerases: type A (RpiA) is the most common and is found in most organisms, while type B (RpiB) is restricted to specific eukaryotic and prokaryotic species. <taxon tax_id="562">Escherichia coli</taxon> produces both RpiA and RpiB (also known as AlsB), although RpiA accounts for 99% of total RPI enzymes [<cite idref="PUB00020248"/>]. </p><p>This entry represents a subgroup of type A ribose-5-phosphate isomerase that is found in bacteria, archaea and eukaryotes (plants and Metazoa), but excluding those found in fungi. </p>