RPIA (Ribose-5-phosphate isomerase) belongs to the ribose 5-phosphate isomerase family. RPIA is an enzyme that catalyzes the conversion between ribose-5-phosphate (R5P) and ribulose-5-phosphate (Ru5P). RPI exists as two distinct proteins forms, termed RPIA and RPIB. 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.