Ribose phosphate epimerase

Ribulose 5-phosphate is capable of a reversible isomerization to other pentose phosphates-xylulose 5-phosphate and ribose 5-phosphate. These reactions are catalyzed by two respective enzymes, viz., pentose-phosphate epimerase and pentose-phosphate isomerase, according to the scheme below ... 

RIBOSE-5-PHOSPHATE EPIMERASE RIBOSEPHOSPHATE PYROPHOSPHO-KINASE... 

RIBOSE-5-PHOSPHATE EPIMERASE D-Ribose 1-phosphate, interconversion to D-ribose 5-phosphate,... 

The isomerization step (reactions 13 and 14 in Table 22.1) involves the conversion of both ribose-5-phosphate and xylulose-5-phosphate to ribulose-5-phosphate. Ribose-5-phosphate isomerase catalyzes the conversion of ribose-5-phosphate to ribulose-5-phosphate, and xylulose-5-phosphate epimerase catalyzes the conversion of xylulose-5-phosphate to iibulose-5-phosphate (Figure 22.15). The reverse of both these reactions takes place in the pentose phosphate pathway, catalyzed by the same enzymes. 

One-pot synthesis of o-glycero-D-altro-2-octulose 8-phosphate (98). Abbreviations RPE ribu-lose-5-phosphate epimerase RPI ribose-5-phosphate isomerase. 

Ribulose 5-phosphate is the substrate for two enzymes. Ribulose 5-phosphate 3-epimerase alters the configuration about carbon 3, forming another ketopentose, xylulose 5-phosphate. Ribose 5-phosphate ketoisom-erase converts ribulose 5-phosphate to the corresponding aldopentose, ribose 5-phosphate, which is the precursor of the ribose required for nucleotide and nucleic acid synthesis. Transketolase transfers the two-carbon... 

Ribulose-5-phosphate (3.13) can be converted to ribose-5-phosphate (3.14) and xylulose 5-phosphate (3.15), by the enzymes ribose-5-phosphate isomerase and ribulose 5-phosphate 3-epimerase, respectively. The two pentose-phosphate molecules, 3.14 and 3.15, are converted to a C3 and a C7 sugar-phosphate, glyceraldehyde 3-phosphate (3.4) and sedoheptulose-7-phosphate (3.16), respectively, via the action of atransketolase. 

Details of these reactions, showing the structures of the molecules involved, are given in Fig. 1. These reactions require xylulose 5-phosphate as well as ribose-5-phosphate. Xylulose 5-phosphate is an epimer (see Topic Jl) of ribulose 5-phosphate and is made by phosphopentose epimerase ... 

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... 

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. 

The contribution of this pathway to the supply of ribose 5-phosphate may be a minor one. The epimerase and isomerase are the same enzymes that participate in the pentose phosphate pathway. 

Carbon shuffling. Ribose 5-phosphate labeled with Cat C-1 is added to a solution containing transketolase, transal dolase, phosphopenlose epimerase, phosphopentosc isu-merase, 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 ... 

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. 

The nonoxidative portion of the pentose phosphate pathway consists of a series of rearrangement and transfer reactions that first convert ribulose 5-phosphate to ribose 5-phosphate and xylulose 5-phosphate, and then the ribose 5-phosphate and xyulose 5-phosphate are converted to intermediates of the glycolytic pathway. The enzymes involved are an epimerase, an isomerase, transketolase, and transaldolase. 

The epimerase and isomerase convert ribulose 5-phosphate to two other 5-carbon sugars (Fig. 29.7). The isomerase converts ribulose 5-phosphate to ribose 5-phos-phate. The epimerase changes the stereochemical position of one hydroxyl group (at carbon 3), converting ribulose 5-phosphate to xylulose 5-phosphate. 

The reactions catalyzed by the epimerase, isomerase, transketolase, and transaldolase are all reversible reactions under physiologic conditions. Thus, ribose 5-phosphate required for purine and pyrimidine synthesis can be generated from intermediates of the glycolytic pathway, as well as from the oxidative phase of the pentose phosphate pathway. The sequence of reactions that generate ribose 5-phos-phate from intermediates of glycolysis is indicated below. 

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