Ribose 1, 5-bisphosphate

This enzyme [EC 5.4.2.7], also known as phosphodeoxy-ribomutase, catalyzes the interconversion of D-ribose 1-phosphate and D-ribose 5-phosphate. This enzyme will also catalyze the interconversion of 2-deoxy-D-ribose 1-phosphate and 2-deoxy-D-ribose 5-phosphate. The bisphosphate compound is the cofactor thus, D-ribose 1,5-bisphosphate, 2-deoxy-D-ribose 1,5-bisphosphate, or even a-D-glucose 1,6-bisphosphate. 

C-labeled carbon dioxide is administered to a green plant, and shortly thereafter the following compounds are isolated from the plant 3-phosphoglycerate, glucose, erythrose-4-phosphate, sedoheptulose-l,7-bisphosphate, ribose-5-phosphate. In which carbon atoms will radioactivity be found ... 

T) Ribulose 1,5-bisphosphate (2 Carbon dioxide (3) 3-Phosphoglycerate (4 1,3-Bisphosphoglycerate (5 Clyceraldehyde 3-phosphate Dihydroxyacetone phosphate (7) Fructose 1,6-bisphosphate Fructose 6-phosphate (9) Erythrose 4-phosphate Sedoheptulose 1,7-bisphosphate ( Sedoheptulose 7-phosphate Xylulose 5-phosphate Ribose 5-phosphate Ribulose 5-phosphate ( ) Glucose 6-phosphate... 

Ribose 5-phosphate ) Xylulose 5-phosphate (1 Sedoheptulose 7-phosphate Glyceraldehyde 3-phosphate (1 Erythrose 4-phosphate Fructose 6-phosphate Fructose 1,6-bisphosphate Glycerone-3-phosphate... 

FIGURE 20-10 Third stage of C02 assimilation. This schematic diagram shows the interconversions of triose phosphates and pentose phosphates. Black dots represent the number of carbons in each compound. The starting materials are glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Reactions catalyzed by transaldolase ( and ) and transketolase ((3) and ) produce pentose phosphates that are converted to ribulose 1,5-bisphosphate—ribose... 

The pentose phosphates formed in the transketolase reactions—ribose 5-phosphate and xylulose 5-phos-phate—are converted to ribulose 5-phosphate (steps (7) and (3)), which in the final step ( ) of the cycle is phosphorylated to ribulose 1,5-bisphosphate by ribulose 5-phosphate kinase (Fig. 20-13). This is the third very exergonic reaction of the pathway, as the phosphate anhydride bond in ATP is swapped for a phosphate ester in ribulose 1,5-bisphosphate. 

D-Ribose 5-phosphate D-glycero-D-altro-octulox 1,8-bisphosphate 3 61" A 33 TPI 170 33... 

Fig. 1. The reductive pentose phosphate cycle (RPP). The solid lines indicate reactions of the RPP cycle. The number of lines per arrow indicates the number of times each reaction occurs for one complete turn of the cycle in which three molecules of COj are converted to one molecule of G3P. Each reaction of the cycle occurs at least once. The double dashed lines indicate the principal reactions removing intermediate compounds of the cycle for biosynthesis. Abbreviations RuBP, ribulose 1,5-bis-phosphate PGA, 3-phosphoglycerate DPGA, 1,3-diphosphoglycerate, FBP, fructose 1,6-bisphos-phate F6P, fructose 6-phosphate SBP, sedoheptulose 1,7-bisphosphate S7P, sedoheptulose 7-phosphate Xu5P, xylulose 5-phosphate R5P, ribose 5-phosphate Ru5P, ribulose 5-phosphate TPP, thiamine pyrophosphate. From Ref. 1.

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... 

Figure 20.11. Regeneration of Ribulose 1,5-Bisphosphate. Both ribose 5-phosphate and xylulose 5-phosphate are converted into ribulose 5-phosphate, which is then phosphorylated to complete the regeneration of ribulose 1,5-bisphosphate. 

Phosphofructokinase uses ATP to convert fructose 6-phosphate to fructose 1,6-bisphosphate, which is then cleaved by aldolase to yield dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate. The conversion of DHAP to a second molecule of glyc-eraldehyde 3-phosphate provides the molecules that are needed for the synthesis of ribose 5-phosphate. 

A range of deoxyadenosine bisphosphates with modified ribose units have been prepared and evaluated as ligands at the P2Yj receptor. The modications included deoxy-compounds, carbocyclic analogues, anhydrohexitol nucleosides, and morpholino-compounds. Improved routes have been reported to the... 

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