Ribose-5-phosphate isomerase (Rpi) is an enzyme that catalyzes the conversion between ribose-5-phosphate (R5P) and ribulose-5-phosphate (Ru5P). It is a member or a larger class of isomerases which catalyze the interconversion of chemical isomers (in this case structural isomers of pentose). It plays a vital role in biochemical metabolism in both the pentose phosphate pathway and the Calvin cycle. The systematic name of this enzyme class is D-ribose-5-phosphate aldose-ketose-isomerase.

RpiA in human beings is encoded on the second chromosome on the short arm (p arm) at position 11.2. Its encoding sequence is nearly 60,000 base pairs long. The only known naturally occurring genetic mutation results in ribose-5-phosphate isomerase deficiency, discussed below. The enzyme is thought to have been present for most of evolutionary history. Knock-out experiments conducted on the genes of various species meant to encode RpiA have indicated similar conserved residues and structural motiffs, indicating ancient origins of the gene.

Rpi exists as two distinct proteins forms, termed RpiA and RpiB. Although RpiA and RpiB catalyze the same reaction, they show no sequence or overall structural homology.2 According to Jung et al., an assessment of RpiA using SDS-PAGE shows that the enzyme is a homodimer of 25 kDa subunits. The molecular weight of the RpiA dimer was found to be 49 kDa by gel filtration. Recently, the crystal structure of RpiA was determined. (please see http://www3.interscience.wiley.com/cgi-bin/fulltext/97516673/PDFSTAR)

Due to its role in the pentose phosphate pathway and the Calvin cycle, RpiA is highly conserved in most organisms, such as bacteria, plants, and animals. RpiA plays an essential role in the metabolism of plants and animals, as it is involved in the Calvin cycle which takes place in plants, and the pentose phosphate pathway which takes place in plants as well as animals.

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