Phosphopentose isomerase

The ribulose 5-phosphate is now converted to ribose 5-phosphate by isomerization, a reaction catalyzed by phosphopentose isomerase ... 

Finally, ribose-5-phosphate is converted into ribulose 5-phosphate by phosphopentose isomerase while xylulose 5-phosphate is converted into ribulose 5-phosphate by phosphopentose epimerase. Ribulose 5-phosphate is converted into ribulose 1,5-bisphosphate through the action of phosphoribulose kinase (Figure 20,11). The sum of these reactions is... 

Xylulose 5-phosphate can be formed from ribose 5-phosphate by the sequential action of phosphopentose isomerase and phosphopentose epimerase, and so the net reaction starting from ribose 5-phosphate is... 

Ribulose 5-phosphate ribose 5-phosphate Phosphopentose isomerase... 

A recurring intermediate. Phosphopentose isomerase interconverts the aldose ribose 5-phosphate and the ketose ribulose 5-phosphate. Propose a mechanism. 

The pentose phosphate pathway also catalyzes the interconversion of three-, four-, five-, six-, and seven-carbon sugars in a series of non-oxidative reactions. All these reactions occur in the cytosol, and in plants part of the pentose phosphate pathway also participates in the formation of hexoses from CO2 in photosynthesis. Thus, D-ribulose 5-phosphate can be directly converted into D-ribose 5-phosphate by phosphopentose isomerase, or to D-xylulose 5-phosphate by phosphopentose epimerase. D-Xylulose 5-phosphate can then be combined with D-ribose 5-phosphate to give rise to sedoheptulose 7-phosphate and glyceraldehyde-3-phosphate. This reaction is a transfer of a two-carbon unit catalyzed by transketolase. Both products of this reaction can be further converted into erythrose 4-phosphate and fructose 6-phosphate. The four-carbon sugar phosphate erythrose 4-phosphate can then enter into another transketolase-catalyzed reaction with the D-xylulose 5-phosphate to form glyceraldehyde 3-phosphate and fructose 6-phosphate, both of which can finally enter glycolysis. 

Figure 6.13 BLAST search results. Part of the results from a BLAST search of the nonredundant (nr) protein sequence database with the use of the sequence of ribose-5-phosphate isomerase (also called phosphopentose isomerase, pp. 572 and S81) from coli as a query. Among the 268 sequences found is the orthologous sequence from human beings, and the alignment between these sequences is shown (highlighted in yellow). The number of sequences with this level of similarity expected to be in the database by chance is 2 X 10 as shown by the E value (highlighted in red). Because this value is much less than 1, the observed sequence alignment is highly significant.

The answer is e. (Murray, pp 219-229. Scrivei pp 1521-1552. Sack, pp 121-138. Wilson, pp 287-317.) The pentose phosphate pathway generates reducing power in the form of NADPH in the oxidative branch of the pathway and synthesizes five-carbon sugars in the nonoxidative branch of the pathway. The pentose phosphate pathway also carries out the interconversion of three-, four-, five-, six-, and seven-carbon sugars in the nonoxidative reactions. The final sugar product of the oxidative branch of the pathway is ribulose-5-phosphate. The first step of the nonoxidative branch of the pathway is the conversion of ribulose-5-phosphate to ribose-5-phosphate or xylulose-5-phosphate in the presence of the enzymes phosphopentose isomerase and phosphopentose epimerase, respectively. Thus ribulose-5-phosphate is a key intermediate that is common to both the oxidative and nonoxidative branches of the pentose phosphate pathway. 

Ribulose-5-Phosphate <=> Ribose-5-Phosphate (catalyzed by Phosphopentose Isomerase)... 

Ribose-5-phosphate is a pentose phosphate pathway intermediate produced by phosphopentose isomerase-catalyzed rearrangement of ribulose-5-phosphate. -i-j ... 

See also Pentose Phosphate Pathway, Pentose Phosphate Pathway Intermediate, Phosphopentose Isomerase, The Importance of PRPP... 

Phosphogluconate Dehydrogenase Phosphopentose Isomerase Phosphopentose Epimerase Transketolase... 

Phosphopentose isomerase is an enzyme of the pentose phosphate pathway that catalyzes the reaction below ... 

Carbon shuffling. Ribose 5-phosphate labeled with " C at C-1 is added to a solution containing transketolase, transaldolase, phosphopentose epimerase, phosphopentose isomerase, and glyceraldehyde 3-phosphate. What is the distribution of the radioactive label in the erythrose 4-phosphate and fructose 6-phosphate that are formed in this reaction mixture ... 

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