Ribulose 5-phosphate, pentose

D-Ribulose Formed in metabolic processes. Ribulose phosphate is an intermediate in pentose phosphate pathway. ... 

Some lactic acid bacteria of the genus Lactobacillus, as well as Leuconostoc mesenteroides and Zymomonas mobilis, carry out the heterolactic fermentation (Eq. 17-33) which is based on the reactions of the pentose phosphate pathway. These organisms lack aldolase, the key enzyme necessary for cleavage of fructose 1,6-bisphosphate to the triose phosphates. Glucose is converted to ribulose 5-P using the oxidative reactions of the pentose phosphate pathway. The ribulose-phosphate is cleaved by phosphoketolase (Eq. 14-23) to acetyl-phosphate and glyceraldehyde 3-phosphate, which are converted to ethanol and lactate, respectively. The overall yield is only one ATP per glucose fermented. 

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 isoinerase, EC 5.3.1.6 transketolase, EC 2.2.1.1 and transaldolase, EC 2.2.1.2.

What really happens is that six molecules of glucose 6-phosphate are converted to 6 CO2 + 6 ribulose 5-phosphates + 12 NADPH + 12 (see Table 20.3 in the text). The ribulose phosphates are then converted back to five molecules of glucose 6-phosphate by the action of transketolase and transaldolase. By these reactions three pentoses are converted to two hexoses and one triose. Thus six pentoses can be converted to four hexoses plus two trioses, and the latter can be converted to the fifth hexose. 

Ribulose phosphate epimerase, EC 5.1.3.4. Its official name is L-ribulose-5-phosphate-4-epimerase. This is a key enzyme in the pentose phosphate pathway. An epimer is a stereoisomer variant of a sugar differing in the configuration at only one carbon atom (see Chap. 11). 

Tranaketolase. The two pentose phosphates derived from ribulose phosphate, ribose-5-phosphate and xylulose-5-phosphate, participate in a transfer reaction in which two carbons are shifted from one to the other (V). Both the molecule split and the one formed are ketols, con-... 

The origin of ribose-5-phosphate from ribulose- phosphate has been discussed, as well as the degradation of these pentose phosphates to smaller fragments. The problem of forming ribose phosphate arises in at least two other conditions (1) the utilization of ribose added as a nutrient for microbial growth, and (2) the generation of ribose from nucleosides, either directly by the hydrolytic cleavage of a nucleoside, as in... 

This enzyme interconverts ribulose-5-P and ribose-5-P via an enediol intermediate (Figure 23.30). The reaction (and mechanism) is quite similar to the phosphoglucoisomerase reaction of glycolysis, which interconverts glucose-6-P and fructose-6-P. The ribose-5-P produced in this reaction is utilized in the biosynthesis of coenzymes (including N/ DH, N/ DPH, F/ D, and Big), nucleotides, and nucleic acids (DNA and RNA). The net reaction for the first four steps of the pentose phosphate pathway is... 

In the pentose plu phatc pathway for degrading sugars, ribulose 5-phosphate is converted to ribose 5-phosphale. Propose a mechanism for the isomerization. 

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 ... 

Two other pentose phosphates (ribose 5-phosphate and xylulose 5-phosphate), which are derived from ribulose 5-phosphate, are important for the subsequent reaction of the cycle. Two molecules of... 

The second part consists of a series of reactions in which ribulose 5-phosphate is reconverted back to glucose 6-phosphate (i.e. 5C 6C ) (Figure 6.20) (Appendix 6.8). Some key processes that depend on NADPH, and therefore the pentose phosphate pathway, are identified in Table 6.3 and presented in Figure 6.21. 

Phosphoric acid esters of the ketopentose D-ribulose (2) are intermediates in the pentose phosphate pathway (see p.l52) and in photosynthesis (see p.l28). The most widely distributed of the ketohexoses is D-fructose. In free form, it is present in fruit juices and in honey. Bound fructose is found in sucrose (B) and plant polysaccharides (e.g., inulin). 

The oxidative segment of the PPP converts glucose 6-phosphate to ribulose 5-phosphate. One CO2 and two NADPH+H" are formed in the process. Depending on the metabolic state, the much more complex regenerative part of the pathway (see B) can convert some of the pentose phosphates back to hexose phosphates, or it can pass them on to glycolysis for breakdown. In most cells, less than 10% of glucose 6-phosphate is degraded via the pentose phosphate pathway. 

This enzyme [EC 5.1.3.1] (also known as phosphoribu-lose epimerase, erythrose-4-phosphate epimerase, and pentose-5-phosphate 3-epimerase) catalyzes the interconversion of D-ribulose 5-phosphate and D-xylulose 5-phosphate. The enzyme can also act on D-erythrose 4-phosphate. 

In tissues that require primarily NADPH, the pentose phosphates produced in the oxidative phase of the pathway are recycled into glucose 6-phosphate. In this nonoxidative phase, ribulose 5-phosphate is first epimerized to xylulose 5-phosphate ... 

The first phase of the pentose phosphate pathway consists of two oxidations that convert glucose 6-phosphate to ribulose 5-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. 

The pentose phosphates are converted to ribulose 5-phosphate, then phosphorylated to ribulose 1,5-bisphosphate to complete the Calvin cycle. 

Although we have described metabolic transformations in plant cells in terms of individual pathways, these pathways interconnect so completely that we should instead consider pools of metabolic intermediates shared among these pathways and connected by readily reversible reactions (Fig. 20-37). One such metabolite pool includes the hexose phosphates glucose 1-phosphate, glucose 6-phosphate, and fructose 6-phosphate a second includes the 5-phosphates of the pentoses ri-bose, ribulose, and xylulose a third includes the triose phosphates dihydroxyacetone phosphate and glycer-aldehyde 3-phosphate. Metabolite fluxes through these... 

Calvin cycle 752 plastids 752 chloroplast 752 amyloplast 752 carbon-fixation reaction 753 ribulose 1,5-bisphosphate 753 3-phosphoglycerate 753 pentose phosphate pathway 753 reductive pentose phosphate pathway 753 C3 plants 754 ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) 754 rubisco activase 757... 

The oxidative portion of the pentose phosphate pathway consists of three reactions that lead to the formation of ribulose 5-phosphate, C02, and two molecules of NADPH for each molecule of glucose 6-phosphate oxidized (Figure 13.2). This portion of the pathway is particularly important in the liver and lactating mammary glands, which are active in the biosynthesis of fatty acids, in the adrenal cortex, which is active in the NADPH-dependent synthesis of steroids, and in erythrocytes, which require NADPH to keep glutathione reduced. 

---