Phosphoribose isomerase

B22. Bruns, F. H., Noltmann, E., and Vahlhaus, E., Uber den Stoffwechsel von Ribose-5-phosphat in Hamolysaten. I. Aktivitatsmessung und Eigenschaften der Phosphoribose-isomerase. II. Der Pentosephosphat n Cyclus in roten Blutzellen. Biochem. Z. 330, 483-496 (1958). 

Figure 6.4. Role of transketolase in the pentose phosphate pathway. Glucose 6-phosphate dehydrogenase, EC 1.1.1.49 phosphogluconate dehydrogenase, EC 1.1.1.44 ribulose-phosphate epimerase, EC 5.1.3.1 phosphoribose isomerase, EC 5.3.1.6 transketolase, EC 2.2.1.1 and transaldolase, EC 2.2.1.2.

In contrast to phosphohydrolases, the phosphatase activity of enzymes which non-hydrolytically catalyse the transfer of phosphate groups can be stimulated by vanadate. Vanadate can spontaneously form esters with unphosphorylated substrates such as sugars. These vanadate esters act as alternative substrates for mutases and isomerases, stimulating their phosphatase activity. Examples are phosphoglucomutase, which catalyses the mutation (phosphate shift) between glucose-1-phosphate and glucose-6-phosphate, and phosphoribose isomerase, which catalyses the isomerisation between ribose-5-phosphate and ribulose-5-phosphate.P ]... 

Fig, 17.6 Hexose monophosphate pathway. 1, glucose-6-P-dehydrogenase 2, 6-P-gluconolactonase 3,6-P-gluconate dehydrogenase 4, phosphoribose isomerase 5, phosphoketopentose epimerase 6, transketolase 7, transaldolase TPP, thiamine pyrophosphate. 

Transketolase catalyzes a reaction [Eq. (19)] between thiamine pyrophosphate and sedoheptulose-7-phosphate to produce thiamine pyrophosphate-glycolaldehyde and a five-carbon compound, ribose-5-phosphate (XI), labeled in carbon atoms 1, 2, and 3. Phosphoribose isomerase (Axelrod and Jang, 1954) converts this compound to ribulose-5-phosphate (I) [Eq. (20)]. 

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